(SDSL)    is a Digital Subscriber LineSymmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line(DSL) variant w...
generally only be used over short distances, typically less than 3mi (5.5 km)[William Stallings book].At the telephone exc...
How ADSL worksOn the wireFrequency plan for ADSL. The red area is the frequency range used bynormal voice telephony (PSTN)...
to the standard. However, this requires matching vendor-supplied equipmenton both ends of the line, and will likely result...
Ericsson has several devices that support non-standard upstream speeds ofup to 2 Mbit/s in ADSL2 and ADSL2+.Installation i...
^ a b ADSL2 Annex L is also known as RE-ADSL2, where RE stands forReach Extended. With this ADSL standard, the power of th...
of which can be either upstream or downstream (dynamically). --DaveSymonds 06:11, 21 September 2005 (UTC)Yep, just looked ...
The site only mentions the word "upload" once, maybe more informationshould be provided about uploading on ADSL, the uploa...
..."or lower signal to noise (SNR)ratios"... Shouldnt this be lower Signal toNoise Ratios(SNR)? Cruxit 15:59, 14 June 2006...
I agree, it should be Asymmetric digital subscriber line. But what links here isoverwhelmingly in favour of the capitaised...
possibly even higher), there isnt a single definitive answer to your question.Ive gotten 1.5Mb/s; 1500/56 = 26. Guy Harris...
simply wired parallel. This kind of installation is not even allowed here. If youwant more than one telefone you use a sma...
It is great to have all this tech info about the range of possible speeds. But itwould be good to also have a table of the...
The latest standardization efforts (G.SHDSL.bis) tend to allow for flexiblychanging the amount of bandwidth dedicated to e...
Voice and dataSome variants of DSL connections, like ADSL and very high speed DSL(VDSL), typically work by dividing the fr...
central offices and customers on conventional twisted pair cabling.[1]U.S. telephone companies promote DSL to compete with...
States) the speech is generally digitized into a 64 kbit/s data stream in theform of an 8 bit signal using a sampling rate...
DSL. This allowed Internet Service Providers to offer efficient service toconsumers, who benefitted greatly from the abili...
to drop significantly over the years, thus making ADSL more economical whencompared to dial up access. Telephone companies...
Tranceiver, or ATU-R, or more commonly known as a DSL modem, is hookedup to a phone line. Modems actually modulate and dem...
[]EquipmentThe customer end of the connection consists of a Terminal Adaptor or inlaymans terms "DSL modem." This converts...
DSL implementations may create bridged or routed networks. In a bridgedconfiguration, the group of subscriber computers ef...
Very High Speed Digital Subscriber Line (VDSL)Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version ofVDS...
speed, since it exceeds the narrowband speeds of 56k telephone lines). Morebroadly, ISDN is a set of protocols for establi...
144 kbit/s rate is broken down into two 64 kbit/s bearer channels (Bchannels) and one 16 kbit/s signalling channel (D chan...
[]Data ChannelThe bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampledat 8 kHz with G.711 encoding. ...
as long (100 ms per digit) tone sequences. This translated to much faster callsetup times, which is greatly desired by car...
However, most modern non-VoIP PBXs use ISDN-PRI (Primary RateInterface) circuits. These are connected via T1 lines with th...
France Télécom offers ISDN services under their product name Numeris (2B+D), of which a professional Duo and home Itoo ver...
[]International occurrenceA study [2] of the German Department of Science shows the following spreadof ISDN-channels per 1...
types of services at the same time. It is primarily used within networkbackbones and employs ATM.Another alternative ISDN ...
interface is provided to the customer. In other locations, the NT-1 device ismaintained by the telco, and the S/T interfac...
ISDN is commonly used in radio broadcasting. Since ISDN provides a highquality connection this assists in delivering good ...
10:50:24.37 21/1/24 T SETUPCall Reference : 000062-localBearer Capability : CCITT, Speech, Circuit mode, 64 kbit/sChannel ...
10:50:28.43 21/1/24 T RR0000 02 01 01 be ....10:50:28.43 21/1/24 T CONNECT_ACKCall Reference : 000062-local0000 00 01 a6 b...
0000 02 01 01 c1 ....10:51:36.37 21/1/24 R RR0000 02 01 01 ad ....10:51:36.37 21/1/24 T RR0000 02 01 01 c1 ....ProtocolsDS...
Asynchronous Transfer Mode ATMAsynchronous Transfer Mode (ATM) is a cell relay, packet switching networkand data link laye...
ATM has proven very successful in the WAN scenario and numeroustelecommunication providers have implemented ATM in their w...
in new WAN implementions.[]Recent developmentsInterest in using native ATM for carrying live video and audio has increased...
considered a fast optical network link, and many PDH links in the digitalnetwork were considerably slower, ranging from 1....
applications with respect to echo cancellation. Most of the interestedEuropean parties eventually came around to the argum...
On slow links (2 Mbit/s and below). ATM still makes sense, and this is why somany ADSL systems use ATM as an intermediate ...
[]Using cells and virtual circuits for traffic engineeringAnother key ATM concept is that of the traffic contract. When an...
To maintain network performance it is possible to police virtual circuits againsttheir traffic contracts. If a circuit is ...
Network Node Interface or Private Network-to-Network Interface (PNNI)protocol. PNNI uses the same shortest path first algo...
The 8-bit CRC is used to correct single-bit header errors and detect multi-bitheader errors. When multi-bit header errors ...
InternetThe Internet is a worldwide, publicly accessible series of interconnectedcomputer networks that transmit data by p...
MIT in 1950, after becoming interested in information technology. At MIT, heserved on a committee that established Lincoln...
backbone using computers called "fuzzballs" by their inventor, David L. Mills.The following year, NSF sponsored the develo...
was eventually replaced in popularity by the Mosaic web browser. In 1993, theNational Center for Supercomputing Applicatio...
describe how to exchange data over the network. Indeed, the Internet isessentially defined by its interconnections and rou...
subnetworks such as:GEANTGLORIADThe Internet2 Network (formally known as the Abilene Network)JANET (the UKs national resea...
Rey, California, but is overseen by an international board of directors drawnfrom across the Internet technical, business,...
some glitches such as mojibake (incorrect display of foreign languagecharacters, also known as kryakozyabry) still remain....
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  1. 1. (SDSL) is a Digital Subscriber LineSymmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line(DSL) variant with E1-like data rates (72 to 2320 Kbit/s). It runs over one pairof copper wires, with a maximum range of about 3 kilometers or 1.86 miles.The main difference between ADSL and SDSL is that SDSL has the sameupstream data rate as downstream (symmetrical), whereas ADSL always hassmaller upstream bandwidth (asymmetrical). However, unlike ADSL, it cantco-exist with a conventional voice service on the same pair as it takes overthe entire bandwidth. It typically falls between ADSL and T-1/E-1 in price, andit is mainly targeted at small and medium businesses who may host a serveron site, (e.g. a Terminal Server or Virtual Private Network) and want to useDSL, but dont need the higher performance of a leased line.SDSL was never properly standardized until Recommendation G.991.2 (ex-G.shdsl) was approved by ITU-T. SDSL is often confused with G.SHDSL; inEurope, G.SHDSL was standardized by ETSI using the name SDSL. ThisETSI variant is compatible with the ITU-T G.SHDSL standardized regionalvariant for Europe.SDSL equipment usually only interoperates with devices from the samevendor, though devices from other vendors using the same DSL chipset maybe compatible. Most new installations use G.SHDSL equipment instead ofSDSL.Asymmetric Digital Subscriber Line (ADSL)Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, adata communications technology that enables faster data transmission overcopper telephone lines than a conventional voice band modem can provide. Itdoes this by utilizing frequencies that are not used by a voice telephone call. Asplitter - or micro filter - allows a single telephone connection to be used forboth ADSL service and voice calls at the same time. Because phone linesvary in quality and were not originally engineered with DSL in mind, it can
  2. 2. generally only be used over short distances, typically less than 3mi (5.5 km)[William Stallings book].At the telephone exchange the line generally terminates at a DSLAM whereanother frequency splitter separates the voice band signal for the conventionalphone network. Data carried by the ADSL is typically routed over thetelephone companys data network and eventually reaches a conventionalinternet network. In the UK under British Telecom the data network in questionis its ATM network which in turn sends it to its IP network IP Colossus.ExplanationA gateway is commonly used to make an ADSL connection. The modem inthe picture is also a wireless access point, hence the antenna. Thedistinguishing characteristic of ADSL over other forms of DSL is that thevolume of data flow is greater in one direction than the other, i.e. it isasymmetric. Providers usually market ADSL as a service for consumers toconnect to the Internet in a relatively passive mode: able to use the higherspeed direction for the "download" from the Internet but not needing to runservers that would require high speed in the other direction.There are both technical and marketing reasons why ADSL is in many placesthe most common type offered to home users. On the technical side, there islikely to be more crosstalk from other circuits at the DSLAM end (where thewires from many local loops are close to each other) than at the customerpremises. Thus the upload signal is weakest at the noisiest part of the localloop, while the download signal is strongest at the noisiest part of the localloop. It therefore makes technical sense to have the DSLAM transmit at ahigher bit rate than does the modem on the customer end. Since the typicalhome user in fact does prefer a higher download speed, the telephonecompanies chose to make a virtue out of necessity, hence ADSL. On themarketing side, limiting upload speeds limits the attractiveness of this serviceto business customers, often causing them to purchase higher cost DigitalSignal 1 services instead. In this fashion, it segments the digitalcommunications market between business and home users
  3. 3. How ADSL worksOn the wireFrequency plan for ADSL. The red area is the frequency range used bynormal voice telephony (PSTN), the green (upstream) and blue (downstream)areas are used for ADSL.Currently, most ADSL communication is full duplex. Full duplex ADSLcommunication is usually achieved on a wire pair by either frequency divisionduplex (FDD), echo canceling duplex (ECD), or time division duplexing (TDD).FDM uses two separate frequency bands, referred to as the upstream anddownstream bands. The upstream band is used for communication from theend user to the telephone central office. The downstream band is used forcommunicating from the central office to the end user. With standard ADSL(annex A), the band from 25.875 kHz to 138 kHz is used for upstreamcommunication, while 138 kHz – 1104 kHz is used for downstreamcommunication. Each of these is further divided into smaller frequencychannels of 4.3125 kHz. During initial training, the ADSL modem tests whichof the available channels have an acceptable signal-to-noise ratio. Thedistance from the telephone exchange, noise on the copper wire, orinterference from AM radio stations may introduce errors on somefrequencies. By keeping the channels small, a high error rate on onefrequency thus need not render the line unusable: the channel will not beused, merely resulting in reduced throughput on an otherwise functional ADSLconnection.Vendors may support usage of higher frequencies as a proprietary extension
  4. 4. to the standard. However, this requires matching vendor-supplied equipmenton both ends of the line, and will likely result in crosstalk issues that affectother lines in the same bundle.There is a direct relationship between the number of channels available andthe throughput capacity of the ADSL connection. The exact data capacity perchannel depends on the modulation method used.ModulationADSL initially existed in two flavors (similar to VDSL), namely CAP and DMT.CAP was the de facto standard for ADSL deployments up until 1996,deployed in 90 percent of ADSL installs at the time. However, DMT waschosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also calledG.dmt and G.lite respectively). Therefore all modern installations of ADSL arebased on the DMT modulation scheme.ADSL standards‫الصورة المرفقة هامة وتوضح دون لود واللبلود ستريم المختلفة‬Annexes J and M shift the upstream/downstream frequency split up to 276kHz (from 138 kHz used in the commonly deployed annex A) in order to boostupstream rates. Additionally, the "all-digital-loop" variants of ADSL2 andADSL2+ (annexes I and J) support an extra 256 Kbit/s of upstream if thebandwidth normally used for POTS voice calls is allocated for ADSL usage.While the ADSL access utilizes the 1.1 MHz band, ADSL2+ utilizes the 2.2MHz band.The downstream and upstream rates displayed are theoretical maxima. Notealso that because Digital subscriber line access multiplexers and ADSLmodems may have been implemented based on differing or incompletestandards some manufacturers may advertise different speeds. For example,
  5. 5. Ericsson has several devices that support non-standard upstream speeds ofup to 2 Mbit/s in ADSL2 and ADSL2+.Installation issuesDue to the way it uses the frequency spectrum, ADSL deployment presentssome issues. It is necessary to install appropriate frequency filters at thecustomers premises, to avoid interferences with the voice service, while at thesame time taking care to keep a clean signal level for the ADSL connection.In the early days of DSL, installation required a technician to visit thepremises. A splitter was installed near the demarcation point, from which adedicated data line was installed. This way, the DSL signal is separatedearlier and is not attenuated inside the customer premises. However, thisprocedure is costly, and also caused problems with customers complainingabout having to wait for the technician to perform the installation. As a result,many DSL vendors started offering a self-install option, in which they shipequipment and instructions to the customer. Instead of separating the DSLsignal at the demarcation point, the opposite is done: the DSL signal is"filtered off" at each phone outlet by use of a low pass filter, also known asmicro filter. This method does not require any rewiring inside the customerpremises.A side effect of the move to the self-install model is that the DSL signal can bedegraded, especially if more than 5 voice band devices are connected to theline. The DSL signal is now present on all telephone wiring in the building,causing attenuation and echo. A way to circumvent this is to go back to theoriginal model, and install one filter upstream from all telephone jacks in thebuilding, except for the jack to which the DSL modem will be connected. Sincethis requires wiring changes by the customer and may not work on somehousehold telephone wiring, it is rarely done. It is usually much easier toinstall filters at each telephone jack that is in use.Footnotes and references
  6. 6. ^ a b ADSL2 Annex L is also known as RE-ADSL2, where RE stands forReach Extended. With this ADSL standard, the power of the lowerfrequencies used for transmitting data is boosted up to increase the reach ofthis signal up to 7 kilometers (23,000 ft). The upper frequency limit for RE-ADSL2 is reduced to 552 kHz to keep the total power roughly the same asannex A. Since RE-ADSL2 is intended for use on long loops there isnt much(any) usable bandwidth above 552 kHz anyway. Although this standard hasbeen ratified by the ITU, not all local loop network maintainers allow thisprotocol to be used on their network, lest the extra power on the lowerfrequencies cause problems for existing services due to crosstalk.vs. DSLIs this better than DSL? What abuot cable? Whats the difference? Is it big orsmall?That is not true. The theoretical maximum for G.992.1 for example is 13Mbps(224 downstream carriers * 15bit/carrier * 4kHz symbol frequency).--69.105.224.209Can someone elucidate DSLAM? I suppose its DSL access apparatus, butabbreviations should be defined before use.Its explained here --Tolien 3 July 2005 20:17 (UTC)I understand that "Annex A" is for ADSL over analog lines while "Annex B" isfor ISDN lines. Could someone enter this into the article with some info onhow likely you are to need one or the other? Or other compatibilities or non-compatibilities to watch out for? Thanks. ---Ransom/CGSpectrum allocation not quite rightthe spectrum breakdown is not entirely accurate in this article. CAP didallocate spectrum the way that the diagram indicates it, but DMT doesnt - itdefines 247 (from memory - this figure might be a bit out) subchannels, each
  7. 7. of which can be either upstream or downstream (dynamically). --DaveSymonds 06:11, 21 September 2005 (UTC)Yep, just looked it up: DMT splits the channel into 247 sub channels, each4kHz wide. This is the standard way its done now. --Dave Symonds 06:15, 21September 2005 (UTC)FaxesOut of interest, how does ADSL affect faxes? can you send or receive a faxon an ADSL line? - Ta bu shi da yu 05:34, 14 January 2006 (UTC)A fax is just a low-speed modem, so it uses the voice band. This band isprotected by a splitter from the DSL signal. Biot 09:10, 14 January 2006(UTC)Thats what I though. Cheers Biot! - Ta bu shi da yu 13:53, 15 January 2006(UTC)AsynchronousADSL is a synchronous protocol at the lowest data layer. Amusingly enough, itruns ATM (async transfer mode) on top of this layer, but it still usedsynchronized clocks at the sending and receiving end.That is, I think, true of most layers atop which ATM runs. Guy Harris 23:22, 25November 2006 (UTC)Upload speeds"Upstream rates start at 64 Kbit/s and typically reach 256 Kbit/s but can go ashigh as 1024 Kbit/s." Is this also true for ADSL2 and ADSL2+? Or do theyallow higher upstream rates? --osmosis 10:26, 26 February 2006 (UTC)Upload
  8. 8. The site only mentions the word "upload" once, maybe more informationshould be provided about uploading on ADSL, the uploading speed, and ifuploading is affected by downloading. etc.--82.152.138.225 01:29, 8 March2006 (UTC)What I think you are referring to is "link saturation", where the maximumupload speed obtainable reduces during a high speed download, or viceversa. But this behavior is not restricted to ADSL. --osmosis 12:24, 8 March2006 (UTC)Actually, can ADSL upload and download data at the same time, or is iteffectively one or the other? --geoff_o 20:35, 11 April 2006 (UTC)I thought I should point out that in Japan, download speeds for ADSL go ashigh as 47Mbps, and upload speeds up to 5Mbps. Without the propertechnical background, I wouldnt want to alter this article, but I thought it mightbe useful info to add. For evidence of this see the NTT page, in English, onthis: [ ‫فقط الضعضاء المسجلين والمفعلين يمكنهم رؤية الوصل ت - إذا كنت مسجل فى المنتدى اتصل لبالدارة‬‫]لتفعيل ضعضويتك . إضغط هنا للتسجيل‬I believe that the 47/5 Mbps line is probably a bonded ADSL2+ line, in otherwords, two ADSL2+ lines used together for the connection. This is covered inthe ADSL2/2+ ITU-standard. But I cannot support this claim with anyevidence, its only a guess because the speeds would match pretty nicely.This is not correct - in Japan "adsl2++" is used , which extends the adslspectrum to 3.75MHz, thus roughly doubling the downstream rate to 50Mbps.5 Mbps upstream is with triple upstream - the upstream band is three timesthe US bandwidth of regular adsl. ~~===ADSL backbone networkI was wondering, if its appropriate to mention that the shift from ATMbackbone networks to Ethernet is also because of the future possibilities ofusing the same backbone network for other services like POTS or mobilephone networks?
  9. 9. ..."or lower signal to noise (SNR)ratios"... Shouldnt this be lower Signal toNoise Ratios(SNR)? Cruxit 15:59, 14 June 2006 (UTC)Updating ADSL and DSL entriesCarlosRibeiro 17:23, 21 October 2006 (UTC). Ive dropped a note at the DSLhistory. I moved some notes on DSL installation that applied specifically toADSL here. It refers to some history on the usage of splitters, that is notlonger absolutely required but is still of interest, both historically and also tounderstand some practical aspects of the technology.Is it an analogue or digital technology?Is it correct to describe it as a digital technology since it modulates analoguecarriers?I.I.A —The preceding unsigned comment was added by 195.130.120.24 (talk)09:03, 21 December 2006 (UTC).Annex MThe page here states the download speed is 28mbit, but the [[ITU G.992.5Annex M] page says 24mbit... which is it? 125.236.153.184 09:05, 31 January2007 (UTC)CapitalizationReading [1] I think it means this article is incorrectly capitalised, while Digitalsubscriber line is correctly titled. Does anyone agree that only the first wordshould be capitalized? Disagree? Think both styles are correct for theirrespective article? Jim.henderson 20:29, 19 March 2007 (UTC)
  10. 10. I agree, it should be Asymmetric digital subscriber line. But what links here isoverwhelmingly in favour of the capitaised version. I will give it a week thenmove it. -- RHaworth 10:14, 15 October 2007 (UTC)S=1/2?My USR router specs says that with S=1/2, it can do 12Mbps, up from 8. Whatdoes S=1/2 mean and how does it extend the download speed? Any infoappreciated, cheers! 84.9.32.226 14:47, 25 June 2007 (UTC)OFDM system comparison tableFeel free to add an ADSL column to the OFDM#OFDM system comparisontable. Mange01 11:46, 17 July 2007 (UTC)[HistoryI came to this page to find out when ADSL was invented and rolled out. Ifsomeone knows, can they add it to the article? It would also be good to knowwho invented it. exterminator 11:06, 8 August 2007 (UTC)How Much Faster is ADSL compared to Dialup?I came here hoping to find a definitive answer, because Verizons claims makeno sense ("768 kbit/s DSL is 21 times faster than 56k dialup"). 768/56 == 13which is nowhere near Verizons claim. (Its even less when you consider 56Kmodems use compression to increase effective thoughput to ~150 kbps).) Idlike to find an actual study thats not biased by salesmanship. - Theaveng17:37, 14 September 2007 (UTC)Given that ADSL can run from 768Kb/s (or possibly even lower) to 6Mb/s (or
  11. 11. possibly even higher), there isnt a single definitive answer to your question.Ive gotten 1.5Mb/s; 1500/56 = 26. Guy Harris (talk) 19:21, 20 November 2007(UTC)Typical marketing flim-flam, probably. Remember that the max tech speed ofthis segment of the SYSTEM that you are using is only one factor in theresponse you experience. Your computer is a factor. Latencies anywherealong the way are a factor, that remains no matter how fast the peak speeds.As a practical matter, you can hope that DSL will be about ten times fasterthan dial-up, in real life. -69.87.200.195 (talk) 11:17, 24 March 2008 (UTC)Does DSL use V.42, V.44, or some other data compression?My dialup modem uses V.44 to compress **** 6-to-1 (effective throughput of300 kbit/s) and executables like flash programs 3-to-1 (effective throughput of150 kbit/s). Does DSL use a similar technology to squash data on the fly? -Theaveng (talk) 18:30, 20 November 2007 (UTC)[Installation issuesI think this part is about how it is done in the USA? This can be very differentdepending on local telephone installations in other parts of the world.Example: ADSL is very in wide use in Germany now and local installation isusually done by the customer, but I have never heard of a microfilter. Whenyou order ADSL you get a spitter from your provider and you install it yourselfby pluging it into the telephone jack. The splitter is the only device directlyconnected to the public network. The splitter itself has two outputs, one for aDSL modem and one for a telephone or an ISDN network termination device.Since the combination of ADSL+ISDN is very common in Germany , there arecombined spliiter/termination boxes (called "NTSplit" or something),sometimes there is even a small ISDN PBX inside that box.I think microfilters make sense if you have multiple telefone jacks that are
  12. 12. simply wired parallel. This kind of installation is not even allowed here. If youwant more than one telefone you use a small PBX (or ISDN devicesconnectes to a S0 bus) with the ADSL splitter in front.Christian —Preceding unsigned comment added by 80.133.139.40 (talk)15:51, 15 January 2008 (UTC)Filters are not necessary for ISDN lines, which do not have a large ringercondenser producing echoes. ISDN is rare in the USA, and almost nonexistent in American homes. Are the German DSL splitters separately markedwith a phone socket and a modem socket?Yes, the splitter has 3 Connections: An input for the old phone jack (thesplitter is the only device connected to the public network) and 2 Outputs: Oneis for the modem, the other one is for telephone equipment.Is the German arrangement different when DSL is applied to an old analogline?No, as far as I know all providers in Germany use ANNEX-B DSL ("DSL overISDN") even when there is no ISDN but an analog phone line. One of thereasons is historic, for several years you could only get DSL in combinationwith ISDN. Now you can also get it with analog, but they use the sameequipment.Most splitters have a small switch ("ISDN - ANALOG") so the same devicecould be used for ANNEX-A on an analog line, giving some more bandwidth,but the telcos dont use that and the devices are shipped configured to ISDN(ANNEX-B). I think they also want to avoid customer confusion. (Christian)OSI ModelWhere in the OSI model do (A)DSL and ADSL+ fit? In the link layer? —Preceding unsigned comment added by 194.251.240.114 (talk) 12:42, 7February 2008 (UTC)768Kbps
  13. 13. It is great to have all this tech info about the range of possible speeds. But itwould be good to also have a table of the most common marketed speeds. Inthe US 2008 at the low end for Verizon that is 768 Kbps / 128 Kbps. It wouldbe interesting to see a table of the most common low-end DSL speeds currentaround the world, and the average price. -69.87.200.195 (talk) 11:11, 24March 2008 (UTC)Single-Pair High-speed Digital Subscriber Lineingle-Pair high-speed digital subscriber line (SHDSL) is a telecommunicationstechnology for Digital Subscriber Line (DSL) subscriber lines. It describes atransmission method for signals on copper pair lines, being mostly used inaccess networks to connect subscribers to Telephone exchanges or POPAccess Points.G.SHDSL was standardized in February 2001 internationally by ITU-T withrecommendation G.991.2.G.SHDSL features symmetrical data rates from 192 kbit/s to 2,304 kbit/s ofpayload in 64 kbit/s increments for one pair and 384 kbit/s to 4,608 kbit/s in128 kbit/s increments for two pair applications. The reach varies according tothe loop rate and noise conditions (more noise or higher rate meansdecreased reach) and may be up to 3,000 meters. The two pair feature mayalternatively be used for increased reach applications by keeping the data ratelow (halving the data rate per pair will provide similar speeds to single pairlines while increasing the error/noise tolerance).The payload may be either clear channel (unstructured), T1 or E1 (full rate orfractional), n x ISDN Basic Rate Access (BRA), Asynchronous Transfer Mode(ATM) or dual bearer mode (i.e. a mixture of two separate streams (e.g. T1and packet based) sharing the payload bandwidth of the G.shdsl loop).In Europe, a variant of G.SHDSL was standardized by ETSI using the nameSDSL. This ETSI variant is not compatible with the ITU-T G.SHDSLstandardized regional variant for Europe and must not be confused with theusage of the term SDSL in North America.
  14. 14. The latest standardization efforts (G.SHDSL.bis) tend to allow for flexiblychanging the amount of bandwidth dedicated to each transport unit to providedynamic rate repartitioning of bandwidth demands during the uptime of theinterface and optionally provides for extended data rates by using a differentmodulation method (32-TCPAM instead of 16-TCPAM, where TCPAM isTrellis-Coded Pulse Amplitude Modulation). Also, a new payload type isintroduced: packet based, e.g. to allow for Ethernet-frames to be transportednatively. (Currently, they may only be framed in ATM or T1/E1/...).G.SHDSL.bis can deliver a minimum of 2 Mbit/s and a maximum of 5.69Mbit/s over distances of up to 2.7 km (9 KftDigital subscriber line"DSL" redirects here. For other uses, see DSL (disambiguation).A DSL ModemComparing DSL & Dial-UpDSL or xDSL, is a family of technologies that provide digital data transmissionover the wires of a local telephone network. DSL originally stood for digitalsubscriber loop, although in recent years, the term digital subscriber line hasbeen widely adopted as a more marketing-friendly term for ADSL, which is themost popular version of consumer-ready DSL. DSL uses high frequency,while regular telephone uses low frequency on the same telephone line.Typically, the download speed of consumer DSL services ranges from 256kilobits per second (kbit/s) to 24,000 kbit/s, depending on DSL technology,line conditions and service level implemented. Typically, upload speed islower than download speed for Asymmetric Digital Subscriber Line (ADSL)and equal to download speed for the rarer Symmetric Digital Subscriber Line(SDSL)
  15. 15. Voice and dataSome variants of DSL connections, like ADSL and very high speed DSL(VDSL), typically work by dividing the frequencies used in a single phone lineinto two primary bands. The ISP data is carried over the high frequency band(25 kHz and above) whereas the voice is carried over the lower frequencyband (4 kHz and below). (See the ADSL article on how the high frequencyband is sub-divided). The user typically installs a DSL filter on each phone.This filters out the high frequencies from the phone line, so that the phoneonly sends or receives the lower frequencies (the human voice). The DSLmodem and the normal telephone equipment can be used simultaneously onthe line without interference from each other.History and scienceDigital subscriber line technology was originally implemented as part of theISDN specification, which is later reused as IDSL. Higher speed DSLconnections like HDSL and SDSL have been developed to extend the rangeof DS1 services on copper lines. Consumer oriented ADSL is designed tooperate also on a BRI ISDN line, which itself is a form of DSL, as well as onan analog phone line.DSL, like many other forms of communication, stems directly from ClaudeShannons seminal 1948 scientific paper: A Mathematical Theory ofCommunication. Employees at Bellcore (now Telcordia Technologies)developed ADSL in 1988 by placing wideband digital signals above theexisting baseband analog voice signal carried between telephone company
  16. 16. central offices and customers on conventional twisted pair cabling.[1]U.S. telephone companies promote DSL to compete with cable modems. DSLservice was first provided over a dedicated "dry loop", but when the FCCrequired the incumbent local exchange carriers ILECs to lease their lines tocompeting providers such as Earthlink, shared-line DSL became common.Also known as DSL over Unbundled Network Element , this allows a singlepair to carry data (via a digital subscriber line access multiplexer [DSLAM])and analog voice (via a circuit switched telephone switch) at the same time.Inline low-pass filter/splitters keep the high frequency DSL signals out of theusers telephones. Although DSL avoids the voice frequency band, thenonlinear elements in the phone would otherwise generate audibleintermodulation products and impair the operation of the data modem.Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km(1.25 miles) of unshielded twisted pair copper wire. The latest standard,ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from theDSLAM. Distances greater than 2 km (1.25 miles) significantly reduce thebandwidth usable on the wires, thus reducing the data rate. By using an ADSLloop extender, these distances can be increased substantially.In 2007, Dr. John Papandriopoulos, a University of Melbourne engineeringresearcher, patented algorithms that can potentially boost DSL line speeds toa maximum of 250 Mbit/s. [2][]OperationThe local loop of the public switched telephone network (PSTN) was initiallydesigned to carry POTS voice communication and signaling, since theconcept of data communications as we know it today did not exist. Forreasons of economy, the phone system nominally passes audio between 300and 3,400 Hz, which is regarded as the range required for human speech tobe clearly intelligible. This is known as voiceband or commercial bandwidth.At the local telephone exchange (United Kingdom) or central office (United
  17. 17. States) the speech is generally digitized into a 64 kbit/s data stream in theform of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, accordingto the Nyquist theorem, any signal above 4,000 Hz is not passed by the phonenetwork (and has to be blocked by a filter to prevent aliasing effects).The laws of physics, specifically the Shannon limit, cap the speed of datatransmission. For a long time, it was believed that a conventional phone linecouldnt be pushed beyond the low speed limits (typically under 9600 bit/s). Inthe 1950s, 4 MHz television signals were often carried between studios onordinary twisted pair telephone cable, suggesting that the Shannon Limitwould allow transmitting many megabits per second. However, these cableshad other impairments besides Gaussian noise, preventing such rates frombecoming practical in the field. In the 1980s techniques were developed forbroadband communications that allowed the limit to be greatly extended.The local loop connecting the telephone exchange to most subscribers iscapable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS.Depending on the length and quality of the loop, the upper limit can be tens ofmegahertz. DSL takes advantage of this unused bandwidth of the local loopby creating 4312.5 Hz wide channels starting between 10 and 100 kHz,depending on how the system is configured. Allocation of channels continuesat higher and higher frequencies (up to 1.1 MHz for ADSL) until new channelsare deemed unusable. Each channel is evaluated for usability in much thesame way an analog modem would on a POTS connection. More usablechannels equates to more available bandwidth, which is why distance and linequality are a factor (the higher frequencies used by DSL travel only shortdistances). The pool of usable channels is then split into two differentfrequency bands for upstream and downstream traffic, based on apreconfigured ratio. This segregation reduces interference. Once the channelgroups have been established, the individual channels are bonded into a pairof virtual circuits, one in each direction. Like analog modems, DSLtransceivers constantly monitor the quality of each channel and will add orremove them from service depending on whether they are usable.One of Lechleiders[1] contributions to DSL was his insight that an asymmetricarrangement offered more than double the bandwidth capacity of synchronous
  18. 18. DSL. This allowed Internet Service Providers to offer efficient service toconsumers, who benefitted greatly from the ability to download large amountsof data but rarely needed to upload comparable amounts. ADSL supports twomodes of transport: fast channel and interleaved channel. Fast channel ispreferred for streaming multimedia, where an occasional dropped bit isacceptable, but lags are less so. Interleaved channel works better for filetransfers, where transmission errors are impermissible, even thoughresending packets may increase latency.Because DSL operates at above the 3.4 kHz voice limit, it cannot be passedthrough a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placedonly for POTS service. A DSL signal cannot pass through a properly installedand working load coil, nor can voice service be maintained past a certaindistance without such coils. Some areas that are within range for DSL serviceare disqualified from eligibility because of load coil placement. Because of thisphone companies are endeavoring to remove load coils on copper loops thatcan operate without them, and conditioning lines not to need them through theuse of fiber to the neighborhood or node FTTN.The commercial success of DSL and similar technologies largely reflects theadvances made in electronics, that, over the past few decades, have beengetting faster and cheaper even while digging trenches in the ground for newcables (copper or fiber optic) remains expensive. Several factors contributedto the popularization of DSL technology:Until the late 1990s, the cost of digital signal processors for DSL wasprohibitive. All types of DSL employ highly complex digital signal processingalgorithms to overcome the inherent limitations of the existing twisted pairwires. Due to the advancements of VLSI technology, the cost of theequipment associated with a DSL deployment (a DSLAM at one end and aDSL modem at the other end) lowered significantly.A DSL line can be deployed over existing cable. Such deployment, evenincluding equipment, is much cheaper than installing a new, high-bandwidthfiber-optic cable over the same route and distance. This is true both for ADSLand SDSL variations.In the case of ADSL, competition in Internet access caused sub******ion fees
  19. 19. to drop significantly over the years, thus making ADSL more economical whencompared to dial up access. Telephone companies were pressured intomoving to ADSL largely due to competition from cable companies, which useDOCSIS cable modem technology to achieve similar speeds. Demand forhigh bandwidth applications, such as video and file sharing, also contributedto popularize ADSL technology.Most residential and small-office DSL implementations reserve lowfrequencies for POTS service, so that with suitable filters and/or splitters theexisting voice service continues to operate independent of the DSL service.Thus POTS-based communications, including fax machines and analogmodems, can share the wires with DSL. Only one DSL "modem" can use thesubscriber line at a time. The standard way to let multiple computers share aDSL connection is to use a router that establishes a connection between theDSL modem and a local Ethernet, Powerline, or Wi-Fi network on thecustomers premises.Once upstream and downstream channels are established, they are used toconnect the subscriber to a service such as an Internet service provider.Dry-loop DSL or "naked DSL," which does not require the subscriber to havetraditional land-line telephone service, started making a comeback in the USin 2004 when Qwest started offering it, closely followed by Speakeasy. As aresult of AT&Ts merger with SBC,[2] and Verizons merger with MCI,[3] thosetelephone companies are required to offer naked DSL to consumers.Even without the regulatory mandate, however, many ILECs offer naked DSLto consumers. The number of telephone landlines in the US has dropped from188 million in 2000 to 172 million in 2005, while the number of cellularsubscribers has grown to 195 million. [3]. This lack of demand for landlineservice has resulted in the expansion of naked DSL availability.[]Typical setup and connection proceduresThe first step is the physical connection. On the customer side, the DSL
  20. 20. Tranceiver, or ATU-R, or more commonly known as a DSL modem, is hookedup to a phone line. Modems actually modulate and demodulate a signal,where the DSL Transceiver is actually a radio signal transmit and receive unit.The telephone company(telco) connects the other end of the line to a DSLAM,which concentrates a large number of individual DSL connections into a singlebox. The location of the DSLAM depends on the telco, but it cannot be locatedtoo far from the user because of attenuation, the loss of data due to the largeamount of electrical resistance encountered as the data moves between theDSLAM and the users DSL modem. It is common for a few residential blocksto be connected to one DSLAM. When the DSL modem is powered up, it goesthrough a sync procedure. The actual process varies from modem to modembut can be generally described as:The DSL Transceiver does a self-test.The DSL Transceiver checks the connection between the DSL Transceiverand the computer. For residential variations of DSL, this is usually theEthernet (RJ-45) port or a USB port; in rare models, a FireWire port is used.Older DSL modems sported a native ATM interface (usually, a 25 Mbit serialinterface). Also, some variations of DSL (such as SDSL) use synchronousserial connections.The DSL Transceiver then attempts to synchronize with the DSLAM. Data canonly come into the computer when the DSLAM and the modem aresynchronized. The synchronization process is relatively quick (in the range ofseconds) but is very complex, involving extensive tests that allow both sidesof the connection to optimize the performance according to the characteristicsof the line in use. External, or stand-alone modem units have an indicatorlabeled "CD", "DSL", or "LINK", which can be used to tell if the modem issynchronized. During synchronization the light flashes; when synchronized,the light stays lit, usually with a green color.Modern DSL gateways have more functionality and usually go through aninitialization procedure that is very similar to a PC starting up. The systemimage is loaded from the flash memory; the system boots, synchronizes theDSL connection and establishes the IP connection between the local networkand the service provider, using protocols such as DHCP or PPPoE. Thesystem image can usually be updated to correct bugs, or to add newfunctionality.
  21. 21. []EquipmentThe customer end of the connection consists of a Terminal Adaptor or inlaymans terms "DSL modem." This converts data from the digital signals usedby computers into a voltage signal of a suitable frequency range which is thenapplied to the phone line.In some DSL variations (for example, HDSL), the terminal adapter is directlyconnected to the computer via a serial interface, using protocols such as RS-232 or V.35. In other cases (particularly ADSL), its common for the customerequipment to be integrated with higher level functionality, such as routing,firewalling, or other application-specific hardware and software. In this case,the entire equipment is usually referred to as a DSL router or DSL gateway.Some kinds of DSL technology require installation of appropriate filters toseparate, or "split", the DSL signal from the low frequency voice signal. Theseparation can be done either at the demarcation point, or can be done withfilters installed at the telephone outlets inside the customer premises. Eitherway has its practical and economical limitations. See ADSL for moreinformation about this.At the exchange, a digital subscriber line access multiplexer (DSLAM)terminates the DSL circuits and aggregates them, where they are handed offonto other networking transports. In the case of ADSL, the voice component isalso separated at this step, either by a filter integrated in the DSLAM or by aspecialized filtering equipment installed before it. The DSLAM terminates allconnections and recovers the original digital information.[]Protocols and configurationsMany DSL technologies implement an ATM layer over the low-level bitstreamlayer to enable the adaptation of a number of different technologies over thesame link.
  22. 22. DSL implementations may create bridged or routed networks. In a bridgedconfiguration, the group of subscriber computers effectively connect into asingle subnet. The earliest implementations used DHCP to provide networkdetails such as the IP address to the subscriber equipment, withauthentication via MAC address or an assigned host name. Laterimplementations often use PPP over Ethernet or ATM (PPPoE or PPPoA),while authenticating with a userid and password and using PPP mechanismsto provide network details.DSL also has *******ion ratios which need to be taken into consideration whendeciding between broadband technologies.[]DSL technologiesThe line length limitations from telephone exchange to subscriber are morerestrictive for higher data transmission rates. Technologies such as VDSLprovide very high speed, short-range links as a method of delivering "tripleplay" services (typically implemented in fiber to the curb networkarchitectures). Technologies likes GDSL can further increase the data rate ofDSL.Example DSL technologies (sometimes called xDSL) include:ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to providedata flow that is slightly higher than dual channel ISDN.High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSLtechnology that uses a higher frequency spectrum of copper, twisted paircables.Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow isequal in both directions.Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardisedreplacement for early proprietary SDSL.Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greaterin one direction than the other.Rate-Adaptive Digital Subscriber Line (RADSL)
  23. 23. Very High Speed Digital Subscriber Line (VDSL)Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version ofVDSLEtherloop Ethernet Local LoopUni Digital Subscriber Line (UDSL), technology developed by TexasInstruments, backwards compatible with all DMT standardsGigabit Digital Subscriber Line (GDSL), based on binder MIMOtechnologies[4].[]Transmission methodsTransmission methods vary by market, region, carrier, and equipment.2B1Q: Two-binary, one-quaternary, used for IDSL and HDSLCAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL,used for HDSLDMT: Discrete multitone modulation, the most numerous kind, also known asOFDM (Orthogonal frequency-division multiplexing)List of device bandwidths‫يمكنكم رؤيتها فى الرالبط ليت أحدهم حولها الى لبى دى اف لهنها هامة لهنها جميلة وهامة وموضحة‬[ ‫فقط الضعضاء المسجلين والمفعلين يمكنهم رؤية الوصل ت - إذا كنت مسجل فى المنتدى اتصل لبالدارة لتفعيل‬‫]ضعضويتك . إضغط هنا للتسجيل‬‫وتشمل كل ما أضعرفه ضعن طرق توصيل الهنترهنت‬Integrated Services Digital NetworkISDNIntegrated Services Digital Network or Isolated Subscriber Digital Network(ISDN), originally "Integriertes Sprach- und Datennetz" (German for"Integrated Speech and Data Net"), is a circuit-switched telephone networksystem, designed to allow digital transmission of voice and data over ordinarytelephone copper wires, resulting in better voice quality than an analog phone.It offers circuit-switched connections (for either voice or data) in increments of64 kbit/s. Another major use case is Internet access, where ISDN typicallyprovides a maximum of 128 kbit/s (which can be considered to be broadband
  24. 24. speed, since it exceeds the narrowband speeds of 56k telephone lines). Morebroadly, ISDN is a set of protocols for establishing and breaking circuitswitched connections, and for advanced call features for the user. It wasintroduced in the late 1980s.[1]In a videoconference, ISDN provides simultaneous voice, video, and ****transmission between individual desktop videoconferencing systems andgroup (room) videoconferencing systems.ISDN elementsIntegrated Services refers to ISDNs ability to deliver at minimum twosimultaneous connections, in any combination of data, voice, video, and fax,over a single line. Multiple devices can be attached to the line, and used asneeded. That means an ISDN line can take care of most peoples completecommunications needs at a much higher transmission rate, without forcing thepurchase of multiple analogue phone lines.Digital refers to its purely digital transmission, as opposed to the analogtransmission of plain old telephone service (POTS). Use of an analogtelephone modem for Internet access requires that the Internet serviceproviders (ISP) modem converts the digital ******* to analog signals beforesending it and the users modem then converts those signals back to digitalwhen receiving. When connecting with ISDN there is no analogue conversion.Network refers to the fact that ISDN is not simply a point-to-point solution likea leased line. ISDN networks extend from the local telephone exchange to theremote user and includes all of the telecommunications and switchingequipment in between.The purpose of the ISDN is to provide fully integrated digital services to theusers. These services fall under three categories: bearer services,supplementary services and teleservices.[]Basic Rate InterfaceThe entry level interface to ISDN is the Basic Rate Interface (BRI), a 144kbit/s service delivered over a pair of standard telephone copper wires. The
  25. 25. 144 kbit/s rate is broken down into two 64 kbit/s bearer channels (Bchannels) and one 16 kbit/s signalling channel (D channel).BRI is sometimes referred to as 2B+DThe Interface specifies three different network interfaces:The U interface is a two-wire interface between the exchange and theNetwork Terminating Unit which is usually the demarcation point in non-NorthAmerican networks.The T interface is a serial interface between a computing device and aTerminal Adapter, which is the digital equivalent of a modem.The S interface is a four-wire bus that ISDN consumer devices plug into; the S& T reference points are commonly implemented as a single interface labeledS/T on an NT1The R interface defines the point between a non-ISDN device and a terminaladapter (TA) which provides translation to and from such a device.BRI-ISDN is very popular in Europe, but very uncommon in North America.[]Primary Rate InterfaceThe other ISDN service available is the Primary Rate Interface (PRI) which iscarried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 Bchannels of 64 kbit/s, one D channel of 64 kbit/s and a timing and alarmchannel of 64 kbit/s. In North America PRI service is delivered on one or moreT1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has23 B channels and 1 D channel for signalling (Japan uses a circuit called aJ1, which is similar to a T1).PRI-ISDN is popular throughout the world, especially for connection of PSTNcircuits to PBXsEven though many network professionals use the term "ISDN" to refer to thelower-bandwidth BRI circuit, in North America by far the majority of ISDNservices are in fact PRI circuits serving PBXs.
  26. 26. []Data ChannelThe bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampledat 8 kHz with G.711 encoding. B-Channels can also be used to carry data,since they are nothing more than digital channels.Each one of these channels is known as a DS0 (dee-ess-zero).[]Signalling ChannelThe signalling channel (D) uses Q.931 for signalling with the other side of thelink.[]Consumer and industry perspectivesThere are two points of view into the ISDN world. The most commonviewpoint is that of the end user, who wants to get a digital connection into thetelephone/data network from home, whose performance would be better thanan ordinary analog modem connection. The typical end-users connection tothe Internet is related to this point of view, and discussion on the merits ofvarious ISDN modems, carriers offerings and tarriffing (features, pricing) arefrom this perspective. Much of the following discussion is from this point ofview, but it should be noted that as a data connection service, ISDN has beenmostly superseded by DSL.There is a second viewpoint: that of the telephone industry, where ISDN is acore technology. A telephone network can be thought of as a collection ofwires strung between switching systems. The common electrical specificationfor the signals on these wires is T1 or E1. On a normal T1, the signalling isdone with A&B bits to indicate on-hook or off-hook conditions and MF andDTMF tones to encode the destination number. ISDN is much better becausemessages can be sent much more quickly than by trying to encode numbers
  27. 27. as long (100 ms per digit) tone sequences. This translated to much faster callsetup times, which is greatly desired by carriers who have to pay for line timeand also by callers who become impatient while their call hops from switch toswitch.It is also used as a smart-network technology intended to add new services tothe public switched telephone network (PSTN) by giving users direct accessto end-to-end circuit-switched digital services.[]Canada and The United StatesISDN-BRI (Basic Rate Interface) has never gained popularity as a telephoneaccess technology in Canada and the US and today remains a niche product.The service was seen as a solution in search of a problem, and the extensivearray of options and features were difficult for most customers to understandand utilize. ISDN has long been known by several derogatory acronymshighlighting these issues, such as It Still Does Nothing, InnovationsSubscribers Dont Need, and I Still Dont kNow. . One of the few places whereBRI still exists is in videoconferencing. High-end videoconferencing made bycompanies such as Sony, Polycom and Tandberg bond up to 6 B-channelstogether (using a BRI circuit for every 2 channels) to provide digital, circuit-switched video connections to almost anywhere in the world. This is veryexpensive, and is being replaced by IP-based conferencing, but where cost isno ******, and perfect quality required, BRI is the preferred choice.Part of the difficulty is that as BRI was coming into service, the concept ofwhat the word broadband meant was being revised upward to mean at least256 kbit/s incoming to the customer.[citation needed] As ADSL grew inpopularity in the United States, the consumer market for BRI imploded. Itsonly remaining positive element is that while ADSL has a functional distancelimitation, BRI does not have any such limit and may be acceptable insituations where the customer is too remote for ADSL to work. This is furtherstymied by many North American CLECs having given up on BRI and will notinstall it as a new option for remote/rural exchanges.
  28. 28. However, most modern non-VoIP PBXs use ISDN-PRI (Primary RateInterface) circuits. These are connected via T1 lines with the central officeswitch, replacing older analog two-way and Direct Inward Dialing (DID) trunks.PRI is capable of delivering Automatic Number Identification (ANI) in bothdirections so that the telephone number of an extension, rather than acompanys main number, can be sent. It is still commonly used in recordingstudios, when a voice-over actor is in one studio, but the director andproducer are in a studio at another location. The ISDN protocol deliverschannelized, not-over-the-Internet service, powerful call setup and routingfeatures, faster setup and tear down, superior audio fidelity (as compared toPOTS service) and, at higher densities, lower cost.[]JapanIn Japan, it became popular to some extent from around 1999 to 2001, butnow that ADSL has been introduced, the number of subscribers is in decline.NTT, a dominant Japanese telephone company, provides an ISDN servicewith the names INS64 and INS1500, which are much less recognized thanISDN.[]United KingdomIn the UK, British Telecom (BT) provides ISDN2e (BRI) as well as ISDN30(PRI). Until April 2006, they also offered Home Highway and BusinessHighway, which are BRI ISDN-based services that offer integrated analogueconnectivity as well as ISDN. Later versions of the Highway products alsoincluded built-in USB sockets for direct computer access. Home Highway hasbeen bought by many home users, usually for Internet connection, althoughnot as fast as ADSL, because it was available before ADSL and in placeswhere ADSL does not reach. Virgin Media also use ISDN lines for customer ofVirgin Broadband who live in cable tv areas.[]France
  29. 29. France Télécom offers ISDN services under their product name Numeris (2B+D), of which a professional Duo and home Itoo version is available. ISDN isgenerally known as RNIS in France and has widespread availability. Theintroduction of ADSL is reducing ISDN use for data transfer and Internetaccess, although it is still common in more rural and outlying areas, and forapplications such as business voice and point-of-sale terminals.[]GermanyIn Germany, ISDN is very popular with an installed base of 25 millionchannels (29% of all subscriber lines in Germany as of 2003 and 20% of allISDN channels worldwide). Due to the success of ISDN, the number ofinstalled analog lines is decreasing. Deutsche Telekom (DTAG) offers bothBRI and PRI. Competing phone companies often offer ISDN only and noanalog lines. Because of the widespread availability of ADSL services, ISDNis today primarily used for voice and fax traffic, but is still very popular thanksto the pricing policy of German telecommunication providers. Today ISDN(BRI) and ADSL/VDSL are often bundled on the same line., mainly becausethe combination of ADSL with an analog line has no cost advantage over acombined ISDN-ADSL line.[]IndiaIn India, ISDN was very popular until the introduction of ADSL. BharatSanchar Nigam Limited, the largest communication service provider in Indiaand a state owned company, is offering both ISDN BRI and PRI servicesacross the country over its ISDN network. After the introduction of ADSLbroadband technology with static IPs, the data transfer load is taken up byADSL. But ISDN still plays a very big role as a backup network for point-to-point leased line customers and low cost reliable data network fororganisations located all over India, such as banks, E-seva centres, LifeInsurance Corporation of India, and so on.
  30. 30. []International occurrenceA study [2] of the German Department of Science shows the following spreadof ISDN-channels per 1000 inhabitants in the year 2005:Norway 401Denmark 339Germany 333Switzerland 331Japan 240UK 160Finland 160Sweden 135Italy 105France 85Spain 58USA 47[]ConfigurationsIn ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta").B channels are used for data (which may include voice), and D channels areintended for signaling and control (but can also be used for data).There are two ISDN implementations. Basic Rate Interface (BRI), also calledBasic Rate Access (BRA) in Europe — consists of two B channels, each withbandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s.Together these three channels can be designated as 2B+D. Primary RateInterface (PRI), also called Primary Rate Access (PRA) in Europe — containsa greater number of B channels and a D channel with a bandwidth of 64kbit/s. The number of B channels for PRI varies according to the nation: inNorth America and Japan it is 23B+1D, with an aggregate bit rate of 1.544Mbit/s (T1); in Europe, India and Australia it is 30B+1D, with an aggregate bitrate of 2.048 Mbit/s (E1). Broadband Integrated Services Digital Network(BISDN) is another ISDN implementation and it is able to manage different
  31. 31. types of services at the same time. It is primarily used within networkbackbones and employs ATM.Another alternative ISDN configuration can be used in which the B channelsof an ISDN basic rate interface are bonded to provide a total duplexbandwidth of 128 kbit/s. This precludes use of the line for voice calls while theinternet connection is in use.Using bipolar with eight-zero substitution encoding technique, call data istransmitted over the data (B) channels, with the signalling (D) channels usedfor call setup and management. Once a call is set up, there is a simple 64kbit/s synchronous bidirectional data channel between the end parties, lastinguntil the call is terminated. There can be as many calls as there are bearerchannels, to the same or different end-points. Bearer channels may also bemultiplexed into what may be considered single, higher-bandwidth channelsvia a process called B channel bonding.The D channel can also be used for sending and receiving X.25 data packets,and connection to X.25 packet network, this is specified in X.31. In practice,X.31 was only commercially implemented in UK, France and Japan.[]Reference pointsA set of reference points are defined in the ISDN standard to refer to certainpoints between the telco and the end user ISDN equipment.R - defines the point between a non-ISDN device and a terminal adapter (TA)which provides translation to and from such a deviceS - defines the point between the ISDN equipment (or TA) and a NetworkTermination Type 2 (NT-2) deviceT - defines the point between the NT-2 and NT-1 devices11 Most NT-1 devices can perform the functions of the NT-2 as well, and so theS and T reference points are generally collapsed into the S/T reference point.² Inside North America, the NT-1 device is considered customer premisesequipment(CPE) and must be maintained by the customer, thus, the U
  32. 32. interface is provided to the customer. In other locations, the NT-1 device ismaintained by the telco, and the S/T interface is provided to the customer. InIndia, service providers provide U interface and an NT1 may be supplied byService provider as part of service offering[]Types of communicationsAmong the kinds of data that can be moved over the 64 kbit/s channels arepulse-code modulated voice calls, providing access to the traditional voicePSTN. This information can be passed between the network and the userend-point at call set-up time. In North America, ISDN is now used mostly asan alternative to analog connections, most commonly for Internet access.Some of the services envisioned as being delivered over ISDN are nowdelivered over the Internet instead. In Europe, and in Germany in particular,ISDN has been successfully marketed as a phone with features, as opposedto a POTS phone (Plain Old Telephone Service) with few or no features.Meanwhile, features that were first available with ISDN (such as Three-WayCall, Call Forwarding, Caller ID, etc.) are now commonly available for ordinaryanalog phones as well, eliminating this advantage of ISDN. Anotheradvantage of ISDN was the possibility of multiple simultaneous calls (one callper B channel), e.g. for big families, but with the increased popularity andreduced prices of mobile telephony this has become less interesting as well,making ISDN unappealing to the private customer. However, ISDN is typicallymore reliable than POTS, and has a significantly faster call setup timecompared with POTS, and IP connections over ISDN typically have some 30–35ms round trip time, as opposed to 120–180ms (both measured withotherwise unused lines) over 56k or V.34 modems, making ISDN morepleasant for telecommuters.Where an analog connection requires a modem, an ISDN connection requiresa terminal adapter (TA). The function of an ISDN terminal adapter is oftendelivered in the form of a PC card with an S/T interface, and single-chipsolutions seem to exist, considering the plethora of combined ISDN- andADSL-routers.
  33. 33. ISDN is commonly used in radio broadcasting. Since ISDN provides a highquality connection this assists in delivering good quality audio for transmissionin radio. Most radio studios are equipped with ISDN lines as their main form ofcommunication with other studios or standard phone lines.[]Sample callThe following is an example of a Primary Rate (PRI) ISDN call showing theQ.921/LAPD and the Q.931/Network message intermixed (i.e. exactly whatwas exchanged on the D-channel). The call is originating from the switchwhere the trace was taken and goes out to some other switch, possibly anend-office LEC, who terminates the call.The first line format is <time> <D-channel> <Transmitted/Received><LAPD/ISDN message ID>. If the message is an ISDN level message, then adecoding of the message is attempted showing the various InformationElements that make up the message. All ISDN messages are tagged with anID number relative to the switch that started the call (local/remote). Followingthis optional decoding is a dump of the bytes of the message in <offset><hex> ... <hex> <ascii> ... <ascii> format.The RR messages at the beginning prior to the call are the keep alivemessages. Then you will see a SETUP message that starts the call. Eachmessage is acknowledged by the other side with a RR.10:49:47.33 21/1/24 R RR0000 02 01 01 a5 ....10:49:47.34 21/1/24 T RR0000 02 01 01 b9 ....10:50:17.57 21/1/24 R RR0000 02 01 01 a5 ....10:50:17.58 21/1/24 T RR0000 02 01 01 b9 ....
  34. 34. 10:50:24.37 21/1/24 T SETUPCall Reference : 000062-localBearer Capability : CCITT, Speech, Circuit mode, 64 kbit/sChannel ID : Implicit Interface ID implies current span, 21/1/5, ExclusiveCalling Party Number : 8018023000 National number User-provided, notscreened Presentation allowedCalled Party Number : 3739120 Type: SUBSCRB0000 00 01 a4 b8 08 02 00 3e 05 04 03 80 90 a2 18 03 .......>........0010 a9 83 85 6c 0c 21 80 38 30 31 38 30 32 33 30 30 ...l.!.8018023000020 30 70 08 c1 33 37 33 39 31 32 30 0p..373912010:50:24.37 21/1/24 R RR0000 00 01 01 a6 ....10:50:24.77 21/1/24 R CALL PROCEEDINGCall Reference : 000062-localChannel ID : Implicit Interface ID implies current span, 21/1/5, Exclusive0000 02 01 b8 a6 08 02 80 3e 02 18 03 a9 83 85 .......>......10:50:24.77 21/1/24 T RR0000 02 01 01 ba ....10:50:25.02 21/1/24 R *****INGCall Reference : 000062-localProgress Indicator : CCITT, Public network serving local user,In-band information or an appropriate pattern is now available0000 02 01 ba a6 08 02 80 3e 01 1e 02 82 88 .......>.....10:50:25.02 21/1/24 T RR0000 02 01 01 bc ....10:50:28.43 21/1/24 R CONNECTCall Reference : 000062-local0000 02 01 bc a6 08 02 80 3e 07 .......>.
  35. 35. 10:50:28.43 21/1/24 T RR0000 02 01 01 be ....10:50:28.43 21/1/24 T CONNECT_ACKCall Reference : 000062-local0000 00 01 a6 be 08 02 00 3e 0f .......>.10:50:28.44 21/1/24 R RR0000 00 01 01 a8 ....10:50:35.69 21/1/24 T DISCONNECTCall Reference : 000062-localCause : 16, Normal call clearing.0000 00 01 a8 be 08 02 00 3e 45 08 02 8a 90 .......>E....10:50:35.70 21/1/24 R RR0000 00 01 01 aa ....10:50:36.98 21/1/24 R RELEASECall Reference : 000062-local0000 02 01 be aa 08 02 80 3e 4d .......>M10:50:36.98 21/1/24 T RR0000 02 01 01 c0 ....10:50:36.99 21/1/24 T RELEASE COMPLETECall Reference : 000062-local0000 00 01 aa c0 08 02 00 3e 5a .......>Z10:50:36.00 21/1/24 R RR0000 00 01 01 ac ....10:51:06.10 21/1/24 R RR0000 02 01 01 ad ....10:51:06.10 21/1/24 T RR
  36. 36. 0000 02 01 01 c1 ....10:51:36.37 21/1/24 R RR0000 02 01 01 ad ....10:51:36.37 21/1/24 T RR0000 02 01 01 c1 ....ProtocolsDSS1 (ETSI "Euro-ISDN", also used in many non-European countries)DSS2 (Digital Subscriber Signalling System No. 2)ETS 300 specification at ETSINI-1 (US National ISDN Phase 1)NI-2 (US National ISDN Phase 2)4ESS (Lucent 4ESS specific protocol defined in AT&T TR 41459)INS-NET 64/1500 (Japanese national/NTT carrier-specific protocol)DACS used in the UK by British Telecom it uses non standard D channelsignalling for Pair gainQSIGRemote operations service element protocol (ROSE)Q.931FTZ 1 TR 6 (obsolete German national protocol)TS.013/TS.014 (obsolete Australian national protocol)VN2/VN3/VN4 (obsolete French national protocols)Specifications defining the physical layer and part of the data link layers ofISDN:ISDN BRI: ITU-T I.430.ISDN PRI: ITU-T I.431.From the point of view of the OSI architecture, an ISDN line has a stack ofthree protocolsphysical layerdata link layernetwork layer (the ISDN protocol, properly)
  37. 37. Asynchronous Transfer Mode ATMAsynchronous Transfer Mode (ATM) is a cell relay, packet switching networkand data link layer protocol which encodes data traffic into small (53 bytes; 48bytes of data and 5 bytes of header information) fixed-sized cells. ATMprovides data link layer services that run over Layer 1 links. This differs fromother technologies based on packet-switched networks (such as the InternetProtocol or Ethernet), in which variable sized packets (known as frames whenreferencing Layer 2) are used. ATM is a connection-oriented technology, inwhich a logical connection is established between the two endpoints beforethe actual data exchange begins.The standards for ATM were first developed in the mid 1980s. The goal wasto design a single networking strategy that could transport real-time video andaudio as well as image files, **** and email. Two groups, the InternationalTelecommunications Union and the ATM Forum were involved in the creationof the standards. ATM has been used primarily with telephone and IPnetworks.ATM AddressingA Virtual Channel (VC) denotes the transport of ATM cells which have thesame unique identifier, called the Virtual Channel Identifier (VCI). Thisidentifier is encoded in the cell header. A virtual channel represents the basicmeans of communication between two end-points, and is analogous to anX.25 virtual circuit.[1]A Virtual Path (VP) denotes the transport of ATM cells belonging to virtualchannels which share a common identifier, called the Virtual Path Identifier(VPI), which is also encoded in the cell header. A virtual path, in other words,is a grouping of virtual channels which connect the same end-points, andwhich share a traffic allocation. This two layer approach can be used toseparate the management of routes and bandwidth from the setup ofindividual connections.[Successes and failures of ATM technology
  38. 38. ATM has proven very successful in the WAN scenario and numeroustelecommunication providers have implemented ATM in their wide-areanetwork cores. Many ADSL implementations also use ATM. However, ATMhas failed to gain wide use as a LAN technology, and its complexity has heldback its full deployment as the single integrating network technology in theway that its inventors originally intended. Since there will always be bothbrand-new and obsolescent link-layer technologies, particularly in the LANarea, not all of them will fit neatly into the synchronous optical networkingmodel for which ATM was designed. Therefore, a protocol is needed toprovide a unifying layer over both ATM and non-ATM link layers, as ATM itselfcannot fill that role. IP already does that; therefore, there is often no point inimplementing ATM at the network layer.In addition, the need for cells to reduce jitter has declined as transport speedsincreased (see below), and improvements in Voice over IP (VoIP) have madethe integration of speech and data possible at the IP layer, again removing the incentive for ubiquitous deployment of ATM. MostTelcos are now planning to integrate their voice network activities into their IPnetworks, rather than their IP networks into the voice infrastructure.Many technically sound ideas from ATM were adopted by MPLS, a genericLayer 2 packet switching protocol. ATM remains widely deployed, and is usedas a multiplexing service in DSL networks, where its compromises fit DSLslow-data-rate needs well. In turn, DSL networks support IP (and IP servicessuch as VoIP) via PPP over ATM and Ethernet over ATM (RFC 2684).ATM will remain deployed for some time in higher-speed interconnects wherecarriers have already committed themselves to existing ATM deployments;ATM is used here as a way of unifying PDH/SDH traffic and packet-switchedtraffic under a single infrastructure. However, ATM is increasingly challengedby speed and traffic shaping requirements of converged networks. Inparticular, the complexity of SAR imposes a performance bottleneck, as thefastest SARs known run at 10 Gbit/s and have limited traffic shapingcapabilities. Currently it seems likely that gigabit Ethernet implementations(10Gbit-Ethernet, Metro Ethernet) will replace ATM as a technology of choice
  39. 39. in new WAN implementions.[]Recent developmentsInterest in using native ATM for carrying live video and audio has increasedrecently. In these environments, low latency and very high quality of serviceare required to handle linear audio and video streams. Towards this goalstandards are being developed such as AES47 (IEC 62365), which provides astandard for professional uncompressed audio transport over ATM. This isworth comparing with professional video over IP.[]ATM conceptsWhy cells?The motivation for the use of small data cells was the reduction of jitter (delayvariance, in this case) in the multiplexing of data streams; reduction of this(and also end-to-end round-trip delays) is particularly important when carryingvoice traffic.This is because the conversion of digitized voice back into an analog audiosignal is an inherently real-time process, and to do a good job, the codec thatdoes this needs an evenly spaced (in time) stream of data items. If the nextdata item is not available when it is needed, the codec has no choice but toproduce silence or guess - and if the data is late, it is useless, because thetime period when it should have been converted to a signal has alreadypassed.Now consider a speech signal reduced to packets, and forced to share a linkwith bursty data traffic (i.e. some of the data packets will be large). No matterhow small the speech packets could be made, they would always encounterfull-size data packets, and under normal queuing conditions, might experiencemaximum queuing delays.At the time ATM was designed, 155 Mbit/s SDH (135 Mbit/s payload) was
  40. 40. considered a fast optical network link, and many PDH links in the digitalnetwork were considerably slower, ranging from 1.544 to 45 Mbit/s in the USA(2 to 34 Mbit/s in Europe).At this rate, a typical full-length 1500 byte (12000-bit) data packet would take77.42 µs to transmit. In a lower-speed link, such as a 1.544 Mbit/s T1 link, a1500 byte packet would take up to 7.8 milliseconds.A queueing delay induced by several such data packets might be severaltimes the figure of 7.8 ms, in addition to any packet generation delay in theshorter speech packet. This was clearly unacceptable for speech traffic, whichneeds to have low jitter in the data stream being fed into the codec if it is toproduce good-quality sound. A packet voice system can produce this in anumber of ways:Have a playback buffer between the network and the codec, one large enoughto tide the codec over almost all the jitter in the data. This allows smoothingout the jitter, but the delay introduced by passage through the buffer would besuch that echo cancellers would be required even in local networks; this wasconsidered too expensive at the time. Also, it would have increased the delayacross the channel, and conversation is difficult over high-delay channels.Build a system which can inherently provide low jitter (and minimal overalldelay) to traffic which needs it.Operate on a 1:1 user basis (i.e., a dedicated pipe).ATM was designed to implement a low-jitter network interface. However, to beable to provide short queueing delays, but also be able to carry largedatagrams, it had to have cells. ATM broke up all packets, data, and voicestreams into 48-byte chunks, adding a 5-byte routing header to each one sothat they could be reassembled later. The choice of 48 bytes was, as is all toooften the case, political instead of technical.[2] When the CCITT wasstandardizing ATM, parties from the United States wanted a 64-byte payloadbecause having the size be a power of 2 made working with the data easierand this size was felt to be a good compromise between larger payloadsoptimized for data transmission and shorter payloads optimized for real-timeapplications like voice; parties from Europe wanted 32-byte payloads becausethe small size (and therefore short transmission times) simplify voice
  41. 41. applications with respect to echo cancellation. Most of the interestedEuropean parties eventually came around to the arguments made by theAmericans, but France and a few allies held out until the bitter end. With 32bytes, France would have been able to implement an ATM-based voicenetwork with calls from one end of France to the other requiring no echocancellation. 48 bytes (plus 5 header bytes = 53) was chosen as acompromise between the two sides, but it was ideal for neither and everybodyhas had to live with it ever since. 5-byte headers were chosen because it wasthought that 10% of the payload was the maximum price to pay for routinginformation. ATM multiplexed these 53-byte cells instead of packets. Doing soreduced the worst-case queuing jitter by a factor of almost 30, removing theneed for echo cancellers.[]Cells in practiceDifferent types of services are supported by ATM via ATM Adaptation Layers(AAL). Standardized AALs include AAL1, AAL2, and AAL5, and the rarelyused AAL3 and AAL4. AAL1 is used for constant bit rate (CBR) services andcircuit emulation. AAL2 through AAL4 are used for variable bit rate (VBR)services, and AAL5 for data. Which AAL is in use for a given cell is notencoded in the cell. Instead, it is negotiated by or configured at the endpointson a per-virtual-connection basis.Since the time ATM was designed, networks have become much faster. A1500 byte (12000-bit) full-size Ethernet packet takes only 1.2 µs to transmit ona 10 Gbit/s optical network, removing the need for small cells to reduce jitter.Some consider that this removes the need for ATM in the network backbone.Additionally, the hardware for implementing the service adaptation for IPpackets is expensive at very high speeds. Specifically, the cost ofsegmentation and reassembly (SAR) hardware at OC-3 and above speedsmakes ATM less competitive for IP than Packet Over SONET (POS). SARperformance limits mean that the fastest IP router ATM interfaces are OC12 -OC48 (STM4 - STM16), while (as of 2004) POS can operate at OC-192(STM64) with higher speeds expected in the future.
  42. 42. On slow links (2 Mbit/s and below). ATM still makes sense, and this is why somany ADSL systems use ATM as an intermediate layer between the physicallink layer and a Layer 2 protocol like PPP or Ethernet.At these lower speeds, ATMs ability to carry multiple logical circuits on asingle physical or virtual medium is useful, although other techniques exist,such as PPP and Ethernet VLANs, which are optional in VDSLimplementations. DSL can be used as an access method for an ATM network,allowing a DSL termination point in a telephone central office to connect tomany internet service providers across a wide-area ATM network. In theUnited States, at least, this has allowed DSL providers to provide DSL accessto the customers of many internet service providers. Since one DSLtermination point can support multiple ISPs, the economic feasibility of DSL issubstantially improved.[]Why virtual circuits?ATM is a channel-based transport layer, using Virtual circuits (VCs). This isencompassed in the concept of the Virtual Paths (VP) and Virtual Channels.Every ATM cell has an 8- or 12-bit Virtual Path Identifier (VPI) and 16-bitVirtual Channel Identifier (VCI) pair defined in its header. Together, theseidentify the virtual circuit used by the connection. The length of the VPI variesaccording to whether the cell is sent on the user-network interface (on theedge of the network), or if it is sent on the network-network interface (insidethe network).As these cells traverse an ATM network, switching is achieved by changingthe VPI/VCI values. Although the VPI/VCI values are not necessarilyconsistent from one end of the connection to the other, the concept of a circuitis consistent (unlike IP, where any given packet could get to its destination bya different route than the others).Another advantage of the use of virtual circuits is the ability to use them as amultiplexing layer, allowing different services (such as voice, Frame Relay,n*64 channels , IP).
  43. 43. []Using cells and virtual circuits for traffic engineeringAnother key ATM concept is that of the traffic contract. When an ATM circuit isset up each switch is informed of the traffic class of the connection.ATM traffic contracts are part of the mechanism by which "Quality of Service"(QoS) is ensured. There are four basic types (and several variants) whicheach have a set of parameters describing the connection.CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which isconstant.VBR - Variable bit rate: an average cell rate is specified, which can peak at acertain level for a maximum interval before being problematic.ABR - Available bit rate: a minimum guaranteed rate is specified.UBR - Unspecified bit rate: traffic is allocated to all remaining transmissioncapacity.VBR has real-time and non-real-time variants, and is used for "bursty" traffic.Non-real-time is usually abbreviated to vbr-nrt.Most traffic classes also introduce the concept of Cell Delay VariationTolerance (CDVT) which defines the "clumping" of cells in time.Traffic contracts are usually maintained by the use of "Shaping", acombination of queuing and marking of cells, and enforced by "Policing".[]Traffic shapingTraffic shaping is usually done at the entry point to an ATM network andattempts to ensure that the cell flow will meet its traffic contract.[]Traffic policing
  44. 44. To maintain network performance it is possible to police virtual circuits againsttheir traffic contracts. If a circuit is exceeding its traffic contract, the networkcan either drop the cells or mark the Cell Loss Priority (CLP) bit (to identify acell as discardable farther down the line). Basic policing works on a cell by cellbasis, but this is sub-optimal for encapsulated packet traffic (as discarding asingle cell will invalidate the whole packet). As a result, schemes such asPartial Packet Discard (PPD) and Early Packet Discard (EPD) have beencreated that will discard a whole series of cells until the next frame starts. Thisreduces the number of redundant cells in the network, saving bandwidth forfull frames. EPD and PPD work with AAL5 connections as they use the frameend bit to detect the end of packets.[]Types of virtual circuits and pathsVirtual circuits and virtual paths can be built statically or dynamically. Staticcircuits (permanent virtual circuits or PVCs) or paths (permanent virtual pathsor PVPs) require that the provisioner must build the circuit as a series ofsegments, one for each pair of interfaces through which it passes.PVPs and PVCs are conceptually simple, but require significant effort in largenetworks. They also do not support the re-routing of service in the event of afailure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs orSPVCs), in contrast, are built by specifying the characteristics of the circuit(the service "contract") and the two endpoints.Finally, switched virtual circuits (SVCs) are built and torn down on demandwhen requested by an end piece of equipment. One application for SVCs is tocarry individual telephone calls when a network of telephone switches areinter-connected by ATM. SVCs were also used in attempts to replace localarea networks with ATM.[]Virtual circuit routingMost ATM networks supporting SPVPs, SPVCs, and SVCs use the Private
  45. 45. Network Node Interface or Private Network-to-Network Interface (PNNI)protocol. PNNI uses the same shortest path first algorithm used by OSPF andIS-IS to route IP packets to share topology information between switches andselect a route through a network. PNNI also includes a very powerfulsummarization mechanism to allow construction of very large networks, aswell as a call admission control (CAC) algorithm that determines whethersufficient bandwidth is available on a proposed route through a network tosatisfy the service requirements of a VC or VP.[]Call admission and connection establishmentA connection has to be established for two parties to be able to send cells toeach other. In ATM this is called a VC ("Virtual Connection"). It can be a PVC("Permanent Virtual Connection"), which is created administratively, or anSVC("Switched Virtual Connection"), which is created as needed by thecommunicating parties. SVC creation is done by "signaling" in which therequesting party indicates the address of the receiving party, the type ofservice requested, and traffic parameters if applicable to the selected service."Call admission" is then done by the network to confirm that the requestedresources are available, and that a route exists for the connection.‫ واليكم الصورة فى المرفقا ت‬ATM.PNGGFC = Generic Flow Control (4 bits) (default: 4-zero bits)VPI = Virtual Path Identifier (8 bits UNI) or (12 bits NNI)VCI = Virtual channel identifier (16 bits)PT = Payload Type (3 bits)CLP = Cell Loss Priority (1-bit)HEC = Header Error Correction (8-bit CRC, polynomial = X8 + X2 + X + 1)The PT field is used to designate various special kinds of cells for operations,administration, and maintenance (OAM) purposes, and to delineate packetboundaries in some AALs.Several of ATMs link protocols use the HEC field to drive a CRC-BasedFraming algorithm, which allows the position of the ATM cells to be found withno overhead required beyond what is otherwise needed for header protection.
  46. 46. The 8-bit CRC is used to correct single-bit header errors and detect multi-bitheader errors. When multi-bit header errors are detected, the current andsubsequent cells are dropped until a cell with no header errors is found.In a UNI cell the GFC field is reserved for a local flow control/submultiplexingsystem between users. This was intended to allow several terminals to sharea single network connection, in the same way that two ISDN phones canshare a single basic rate ISDN connection. All four GFC bits must be zero bydefault.The NNI cell format is almost identical to the UNI format, except that the 4-bitGFC field is re-allocated to the VPI field, extending the VPI to 12 bits. Thus, asingle NNI ATM interconnection is capable of addressing almost 212 VPs ofup to almost 216 VCs each (in practice some of the VP and VC numbers arereserved).[]ReferencesP.S. Neelakanta; A ****book on ATM Telecommunications, Principles andimplementation. CRC Press. 2000 ISBN 0-8493-1805-X.[]Further readingAmos E. Joel, Jr., Asynchronous Transfer Mode (IEEE Press, 1993)Martin De Prycker, Asynchronous Transfer Mode. Solutions for BroadbandISDN (Prentice-Hall, 1993)Tom Golway, Planning and Managing ATM Networks. New York: Manning,1997. ISBN 132621894.P.S. Neelakanta A ****book on ATM Telecommunications, Principles andimplementation. CRC Press. 2000 ISBN 0-8493-1805-X.‫ومعلوما ت ضعن الهنترهنت وضعتها فى النهاية لن معظمنا قد ل يحتاجها ولكننى أحببت أن أحيط لبالموضوع هنوضعا‬‫ما وطبعا فيما سبق قمت لباختصار سريع‬‫وتجدو هذا الجزء فى الرالبط‬[ ‫فقط الضعضاء المسجلين والمفعلين يمكنهم رؤية الوصل ت - إذا كنت مسجل فى المنتدى اتصل لبالدارة لتفعيل‬‫]ضعضويتك . إضغط هنا للتسجيل‬
  47. 47. InternetThe Internet is a worldwide, publicly accessible series of interconnectedcomputer networks that transmit data by packet switching using the standardInternet Protocol (IP). It is a "network of networks" that consists of millions ofsmaller domestic, academic, business, and government networks, whichtogether carry various information and services, such as electronic mail,online chat, file transfer, and the interlinked web pages and other resources ofthe World Wide Web (WWW).TerminologyThe Internet and the World Wide Web are not one and the same. The Internetis a collection of interconnected computer networks, linked by copper wires,fiber-optic cables, wireless connections, etc. In contrast, the Web is acollection of interconnected ********s and other resources, linked by hyperlinksand URLs. The World Wide Web is one of the services accessible via theInternet, along with various others including e-mail, file sharing, online gamingand others described below. However, "the Internet" and "the Web" arecommonly used interchangeably in non-technical settings.HistoryMain article: History of the InternetCreationThe USSRs launch of Sputnik spurred the United States to create theAdvanced Research Projects Agency, known as ARPA, in February 1958 toregain a technological lead.[1][2] ARPA created the Information ProcessingTechnology Office (IPTO) to further the research of the Semi AutomaticGround Environment (SAGE) program, which had networked country-wideradar systems together for the first time. J. C. R. Licklider was selected tohead the IPTO, and saw universal networking as a potential unifying humanrevolution.Licklider moved from the Psycho-Acoustic Laboratory at Harvard University to
  48. 48. MIT in 1950, after becoming interested in information technology. At MIT, heserved on a committee that established Lincoln Laboratory and worked on theSAGE project. In 1957 he became a Vice President at BBN, where he boughtthe first production PDP-1 computer and conducted the first publicdemonstration of time-sharing.At the IPTO, Licklider recruited Lawrence Roberts to head a project toimplement a network, and Roberts based the technology on the work of PaulBaran,[citation needed] who had written an exhaustive study for the U.S. AirForce that recommended packet switching (as opposed to circuit switching) tomake a network highly robust and survivable. After much work, the first twonodes of what would become the ARPANET were interconnected betweenUCLA and SRI International in Menlo Park, California, on October 29, 1969.The ARPANET was one of the "eve" networks of todays Internet. Followingon from the demonstration that packet switching worked on the ARPANET,the British Post Office, Telenet, DATAPAC and TRANSPAC collaborated tocreate the first international packet-switched network service. In the UK, thiswas referred to as the International Packet Stream Service (IPSS), in 1978.The collection of X.25-based networks grew from Europe and the US to coverCanada, Hong Kong and Australia by 1981. The X.25 packet switchingstandard was developed in the CCITT (now called ITU-T) around 1976. X.25was independent of the TCP/IP protocols that arose from the experimentalwork of DARPA on the ARPANET, Packet Radio Net and Packet Satellite Netduring the same time period. Vinton Cerf and Robert Kahn developed the firstde******ion of the TCP protocols during 1973 and published a paper on thesubject in May 1974. Use of the term "Internet" to describe a single globalTCP/IP network originated in December 1974 with the publication of RFC 675,the first full specification of TCP that was written by Vinton Cerf, Yogen Dalaland Carl Sunshine, then at Stanford University. During the next nine years,work proceeded to refine the protocols and to implement them on a widerange of operating systems.The first TCP/IP-wide area network was made operational by January 1, 1983when all hosts on the ARPANET were switched over from the older NCPprotocols to TCP/IP. In 1985, the United States National Science Foundation(NSF) commissioned the construction of a university 56 kilobit/second network
  49. 49. backbone using computers called "fuzzballs" by their inventor, David L. Mills.The following year, NSF sponsored the development of a higher-speed 1.5megabit/second backbone that became the NSFNet. A key decision to use theDARPA TCP/IP protocols was made by Dennis Jennings, then in charge ofthe Supercomputer program at NSF.The opening of the network to commercial interests began in 1988. The USFederal Networking Council approved the interconnection of the NSFNET tothe commercial MCI Mail system in that year and the link was made in thesummer of 1989. Other commercial electronic e-mail services were soonconnected, including OnTyme, Telemail and Compuserve. In that same year,three commercial Internet Service Providers were created: UUNET, PSINETand CERFNET. Important, separate networks that offered gateways into, thenlater merged with, the Internet include Usenet and BITNET. Various othercommercial and educational networks, such as Telenet, Tymnet, Compuserveand JANET were interconnected with the growing Internet. Telenet (latercalled Sprintnet) was a large privately funded national computer network withfree dial-up access in cities throughout the U.S. that had been in operationsince the 1970s. This network was eventually interconnected with the othersin the 1980s as the TCP/IP protocol became increasingly popular. The abilityof TCP/IP to work over virtually any pre-existing communication networksallowed for a great ease of growth, although the rapid growth of the Internetwas due primarily to the availability of commercial routers from companiessuch as Cisco Systems, Proteon and Juniper, the availability of commercialEthernet equipment for local-area networking and the widespreadimplementation of TCP/IP on the UNIX operating system.GrowthAlthough the basic applications and guidelines that make the Internet possiblehad existed for almost a decade, the network did not gain a public face untilthe 1990s. On August 6, 1991, CERN, which straddles the border betweenFrance and Switzerland, publicized the new World Wide Web project. TheWeb was invented by English scientist Tim Berners-Lee in 1989.An early popular web browser was ViolaWWW, based upon HyperCard. It
  50. 50. was eventually replaced in popularity by the Mosaic web browser. In 1993, theNational Center for Supercomputing Applications at the University of Illinoisreleased version 1.0 of Mosaic, and by late 1994 there was growing publicinterest in the previously academic, technical Internet. By 1996 usage of theword Internet had become commonplace, and consequently, so had its use asa synecdoche in reference to the World Wide Web.Meanwhile, over the course of the decade, the Internet successfullyaccommodated the majority of previously existing public computer networks(although some networks, such as FidoNet, have remained separate). Duringthe 1990s, it was estimated that the Internet grew by 100% per year, with abrief period of explosive growth in 1996 and 1997.[3] This growth is oftenattributed to the lack of central administration, which allows organic growth ofthe network, as well as the non-proprietary open nature of the Internetprotocols, which encourages vendor interoperability and prevents any onecompany from exerting too much control over the network.University students appreciation and contributionsNew findings in the field of communications during the 1960s, 1970s and1980s were quickly adopted by universities across North America.Examples of early university Internet communities are Cleveland FreeNet,Blacksburg Electronic Village and NSTN in Nova Scotia.[4] Students took upthe opportunity of free communications and saw this new phenomenon as atool of liberation. Personal computers and the Internet would free them fromcorporations and governments (Nelson, Jennings, Stallman).Graduate students played a huge part in the creation of ARPANET. In the1960s, the network working group, which did most of the design forARPANETs protocols, was composed mainly of graduate students.Todays InternetAside from the complex physical connections that make up its infrastructure,the Internet is facilitated by bi- or multi-lateral commercial contracts (e.g.,peering agreements), and by technical specifications or protocols that
  51. 51. describe how to exchange data over the network. Indeed, the Internet isessentially defined by its interconnections and routing policies.As of March 31, 2008, 1.407 billion people use the Internet according toInternet World Stats.Internet protocolsIn this con****, there are three layers of protocols:At the lower level (OSI layer 3) is IP (Internet Protocol), which defines thedatagrams or packets that carry blocks of data from one node to another. Thevast majority of todays Internet uses version four of the IP protocol (i.e. IPv4),and, although IPv6 is standardized, it exists only as "islands" of connectivity,and there are many ISPs without any IPv6 connectivity.[5] ICMP (InternetControl Message Protocol) also exists at this level. ICMP is connectionless; itis used for control, signaling, and error reporting purposes.TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)exist at the next layer up (OSI layer 4); these are the protocols by which datais transmitted. TCP makes a virtual "connection", which gives some level ofguarantee of reliability. UDP is a best-effort, connectionless transport, in whichdata packets that are lost in transit will not be re-sent.The application protocols sit on top of TCP and UDP and occupy layers 5, 6,and 7 of the OSI model. These define the specific messages and data formatssent and understood by the applications running at each end of thecommunication. Examples of these protocols are HTTP, FTP, and SMTP.Internet structureThere have been many analyses of the Internet and its structure. Forexample, it has been determined that the Internet IP routing structure andhyper**** links of the World Wide Web are examples of scale-free networks.Similar to the way the commercial Internet providers connect via Internetexchange points, research networks tend to interconnect into large
  52. 52. subnetworks such as:GEANTGLORIADThe Internet2 Network (formally known as the Abilene Network)JANET (the UKs national research and education network)These in turn are built around relatively smaller networks. See also the list ofacademic computer network organizations.In network diagrams, [ ‫فقط الضعضاء المسجلين والمفعلين يمكنهم رؤية الوصل ت - إذا كنت مسجل فى المنتدى اتصل لبالدارة‬‫]لتفعيل ضعضويتك . إضغط هنا للتسجيل‬the Internet is often represented by a cloud symbol, into and out of whichnetwork communications can pass.ICANNhe Internet Corporation for Assigned Names and Numbers (ICANN) is theauthority that coordinates the assignment of unique identifiers on the Internet,including domain names, Internet Protocol (IP) addresses, and protocol portand parameter numbers. A globally unified namespace (i.e., a system ofnames in which there is at most one holder for each possible name) isessential for the Internet to function. ICANN is headquartered in Marina del
  53. 53. Rey, California, but is overseen by an international board of directors drawnfrom across the Internet technical, business, academic, and non-commercialcommunities. The US government continues to have the primary role inapproving changes to the root zone file that lies at the heart of the domainname system. Because the Internet is a distributed network comprising manyvoluntarily interconnected networks, the Internet, as such, has no governingbody. ICANNs role in coordinating the assignment of unique identifiersdistinguishes it as perhaps the only central coordinating body on the globalInternet, but the scope of its authority extends only to the Internets systems ofdomain names, IP addresses, protocol ports and parameter numbers.On November 16, 2005, the World Summit on the Information Society, held inTunis, established the Internet Governance Forum (IGF) to discuss Internet-related issues.LanguageFor more details on this topic, see English on the Internet.Further information: UnicodeThe prevalent language for communication on the Internet is English. Thismay be a result of the Internets origins, as well as Englishs role as a linguafranca. It may also be related to the poor capability of early computers, largelyoriginating in the United States, to handle characters other than those in theEnglish variant of the Latin alphabet.After English (30% of Web visitors) the most requested languages on theWorld Wide Web are Chinese (17%), Spanish (9%), Japanese (7%), French(5%) and German (5%).[6]By continent, 38% of the worlds Internet users are based in Asia, 27% inEurope, 18% in North America, and 10% in Latin America and the Caribbean.[7]The Internets technologies have developed enough in recent years,especially in the use of Unicode, that good facilities are available fordevelopment and communication in most widely used languages. However,
  54. 54. some glitches such as mojibake (incorrect display of foreign languagecharacters, also known as kryakozyabry) still remain.Internet and the workplaceThe Internet is allowing greater flexibility in working hours and location,especially with the spread of unmetered high-speed connections and Webapplications.The Internet viewed on mobile devicesThe Internet can now be accessed virtually anywhere by numerous means.Mobile phones, datacards, handheld game consoles and cellular routers allowusers to connect to the Internet from anywhere there is a cellular networksupporting that devices technology.Common uses of the InternetE-mailFor more details on this topic, see E-mail.The concept of sending electronic **** messages between parties in a wayanalogous to mailing letters or memos predates the creation of the Internet.Even today it can be important to distinguish between Internet and internal e-mail systems. Internet e-mail may travel and be stored unencrypted on manyother networks and machines out of both the senders and the recipientscontrol. During this time it is quite possible for the ******* to be read and eventampered with by third parties, if anyone considers it important enough. Purelyinternal or intranet mail systems, where the information never leaves thecorporate or organizations network, are much more secure, although in anyorganization there will be IT and other personnel whose job may involvemonitoring, and occasionally accessing, the e-mail of other employees notaddressed to them.The World Wide Web

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