Introduction to wi fi and wlan


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Introduction to wi fi and wlan

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Introduction to wi fi and wlan

  1. 1. Introduction to Wi-Fiand WLANIEEE 802.11 standardsAn overview about the IEEE 802.11 standards for Wi-Fi andWLAN applications and the associated WLAN equipment andthe use of WiFi hotspots.Orlando Moreno,
  2. 2. 2Executive SummaryWireless connectivity for computers is now well established and virtually all newlaptops contain a Wi-Fi capability. Of the WLAN solutions that are available theIEEE 802.11 standard, often termed Wi-Fi has become the de-facto standard. Withoperating speeds of systems using the IEEE 802.11 standards of around 54 Mbpsbeing commonplace, Wi-Fi is able to compete well with wired systems. As a resultof the flexibility and performance of the system, Wi-Fi "hotpots" are widespreadand in common use. These enable people to use their laptop computers as they waitin hotels, airport lounges, cafes, and many other places using a wire-less link ratherthan needing to use a cable.In addition to the 802.11 standards being used for temporary connections, and fortemporary Wireless Local Area Network, WLAN applications, they may also beused for more permanent installations. In offices WLAN equipment may be used toprovide semi-permanent WLAN solutions. Here the use of WLAN equipmentenables offices to be set up without the need for permanent wiring, and this canprovide a considerable cost saving. The use of WLAN equipment allows changesto be made around the office without the need to re-wiring.As a result the Wi-Fi, IEEE 802.11 standard is widely used to provide WLANsolutions both for temporary connections in hotspots in cafes, airports, hotels andsimilar places as well as within office scenarios.Orlando Moreno 407.808.0322
  3. 3. 3IEEE 802.11 StandardsThere is a plethora of standards under the IEEE 802 LMSC (LAN / MAN Standards Committee).Of these even 802.11 has a variety of standards, each with a letter suffix. These cover everythingfrom the wireless standards themselves, to standards for security aspects, quality of service andthe like: 802.11a - Wireless network bearer operating in the 5 GHz ISM band with data rate up to 54 Mbps 802.11b - Wireless network bearer operating in the 2.4 GHz ISM band with data rates up to 11 Mbps 802.11e - Quality of service and prioritisation 802.11f - Handover 802.11g - Wireless network bearer operating in 2.4 GHz ISM band with data rates up to 54 Mbps 802.11h - Power control 802.11i - Authentication and encryption 802.11j - Interworking 802.11k - Measurement reporting 802.11n - Wireless network bearer operating in the 2.4 and 5 GHz ISM bands with data rates up to 600 Mbps 802.11s - Mesh networking 802.11ac - Wireless network bearer operating below 6GHz to provide data rates of at least 1Gbps per second for multi-station operation and 500 Mbps on a single link 802.11ad - Wireless network bearer providing very high throughput at frequencies up to 60GHz 802.11af - Wi-Fi in TV spectrum white spaces (often called White-Fi)Of these the standards that are most widely known are the network bearer standards, 802.11a,802.11b, 802.11g and now 802.11n.802.11 Network bearer standardsAll the 802.11 Wi-Fi standards operate within the ISM (Industrial, Scientific and Medical)frequency bands. These are shared by a variety of other users, but no license is required foroperation within these frequencies. This makes them ideal for a general system for widespreaduse.There are a number of bearer standards that are in common use. These are the 802.11a, 802.11b,and 802.11g standards. The 802.11n standard is the latest providing raw data rates of up to 600Mbps.Each of the different standards has different features and they were launched at different times.The first accepted 802.11 WLAN standard was 802.11b. This used frequencies in the 2.4 GHzIndustrial Scientific and Medial (ISM) frequency band, this offered raw, over the air data rates ofOrlando Moreno 407.808.0322
  4. 4. 411 Mbps using a modulation scheme known as Complementary Code Keying (CCK) as well assupporting Direct-Sequence Spread Spectrum, or DSSS, from the original 802.11 specification.Almost in parallel with this a second standard was defined. This was 802.11a which used adifferent modulation technique, Orthogonal Frequency Division Multiplexing (OFDM) and usedthe 5 GHz ISM band. Of the two standards it was the 802.11b variant that caught on. This wasprimarily because the chips for the lower 2.4 GHz band were easier and cheaper to manufacture.The 802.11b standard became the main Wi-Fi standard. Looking to increase the speeds, anotherstandard, 802.11g was introduced and ratified in June 2003. Using the more popular 2.4 GHzband and OFDM, it offered raw data rates of 54 Mbps, the same as 802.11b. In addition to this, itoffered backward compatibility to 802.11b. Even before the standard was ratified, many vendorswere offering chipsets for the new standard, and today the vast majority of computer networkingthat is shipped uses 802.11g.Then in January 2004, the IEEE announced it had formed a new committee to develop an evenhigher speed standard. With much of the work now complete, 802.11n is beginning to establishitself in the same way as 802.11g. The industry came to a substantive agreement about thefeatures for 802.11n in early 2006. This gave many chip manufacturers sufficient information toget their developments under way. As a result it is anticipated that before long, with ratificationof 802.11n expected in 2007, that some cards and routers will find their way into the stores. 802.11a 802.11b 802.11g 802.11n Date of standard July July 1999 June 2003 Oct 2009 approval 1999 Maximum data rate 54 11 54 ~600 (Mbps) CCK or CCK, DSSS, or CCK, DSSS, or Modulation OFDM DSSS OFDM OFDM RF Band (GHz) 5 2.4 2.4 2.4 or 5 Number of spatial 1 1 1 1, 2, 3, or 4 streams Channel width (MHz) 20 20 20 20, or 40 nominal Summary of major 802.11 Wi-Fi StandardsBandwidths of nominal 20 MHz are usually quoted, although the actual bandwidth allowed is generally22 MHz.Orlando Moreno 407.808.0322
  5. 5. 5802.11 NetworksThere are two types of WLAN network that can be formed: infrastructure networks; and ad-hocnetworks.The infrastructure application is aimed at office areas or to provide a "hotspot". The WLANequipment can be installed instead of a wired system, and can provide considerable cost savings,especially when used in established offices. A backbone wired network is still required and isconnected to a server. The wireless network is then split up into a number of cells, each servicedby a base station or Access Point (AP) which acts as a controller for the cell. Each Access Pointmay have a range of between 30 and 300 meters dependent upon the environment and thelocation of the Access Point.The other type of network that may be used is termed an Ad-Hoc network. These are formedwhen a number of computers and peripherals are brought together. They may be needed whenseveral people come together and need to share data or if they need to access a printer withoutthe need for having to use wire connections. In this situation the users only communicate witheach other and not with a larger wired network. As a result there is no Access Point and specialalgorithms within the protocols are used to enable one of the peripherals to take over the role ofmaster to control the network with the others acting as slaves.IEEE 802.11 standards summaryWi-Fi has established itself in a number of areas for networking laptop computers. Although it isnot possible to consistently achieve the maximum data rates, and 802.11 is affected byinterference, it is nevertheless a particularly useful technology. Nowadays WLAN equipment isvery cheap, with access points widely available from computer suppliers, and this makes theimplementation of Wi-Fi, 802.11 very easy and affordable. This is being proved by the rapidlyincreasing popularity or all the variants of 802.11 Wi-Fi.IEEE 802.11a802.11a the new Wi-Fi standard providing data rates of 54 Mbps at 5 GHzThe IEEE 802.11a standard is capable of producing a high level of performance, and being in aband which is used less than the levels of interference are less allowing high levels ofperformance.The 802.11a standard is alphabetically the first of the variety of 802.11 standards that are inwidespread use today. Although 802.11a was ratified at the same time as 802.11b, it nevercaught on in the same way despite the fact that it offered a much higher data transfer rate. Thereason for this was that it operated in the 5 GHz ISM band rather than the 2.4 GHz band, and thisOrlando Moreno 407.808.0322
  6. 6. 6made chips more expensive. 802.11 was also, possibly ahead of its time. With the introduction ofwireless LAN technology, people were happier to settle for any connection, and even one with alower speed. Nevertheless 802.11 did achieve a significant amount of use and it also forced upthe speed of other 802.11 technologies running at 2.4 GHz.802.11a specification802.11a boasts an impressive performance. It is able to transfer data with raw data rates up to 54Mbps, and has a good range, although not when operating at its full data rate. Parameter Value Date of standard approval July 1999 Maximum data rate (Mbps) 54 Typical data rate (Mbps) 25 Typical range indoors (Meters) ~30 Modulation OFDM RF Band (GHz) 5 Number of spatial streams 1 Channel width (MHz) 20 Summary of 802.11 Wi-Fi StandardsThe 802.11a standard uses basic 802.11 concepts as its base, and it operates within the 5GHzIndustrial, Scientific and Medical (ISM) band enabling it to be used worldwide in a licence freeband. The modulation is Orthogonal Frequency Division Multiplexing (OFDM) to enable it totransfer raw data at a maximum rate of 54 Mbps, although a more realistic practical level is inthe region of the mid 20 Mbps region. The data rate can be reduced to 48, 36, 24, 18, 12, 9 then 6Mbit/s if required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4 to pointto point.Orlando Moreno 407.808.0322
  7. 7. 7Note on OFDM:Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a largenumber of close spaced carriers that are modulated with low rate data. Normally these signalswould be expected to interfere with each other, but by making the signals orthogonal to eachanother there is no mutual interference. This is achieved by having the carrier spacing equal tothe reciprocal of the symbol period. This means that when the signals are demodulated they willhave a whole number of cycles in the symbol period and their contribution will sum to zero - inother words there is no interference contribution. The data to be transmitted is split across all thecarriers and this means that by using error correction techniques, if some of the carriers are lostdue to multi-path effects, then the data can be reconstructed. Additionally having data carried ata low rate across all the carriers means that the effects of reflections and inter-symbolinterference can be overcome. It also means that single frequency networks, where alltransmitters can transmit on the same channel can be implemented.802.11a RF signalThe OFDM signal used for 802.11 comprises 52 subcarriers. Of these 48 are used for the datatransmission and four are sued as pilot subcarriers. The separation between the individualsubcarriers is 0.3125 MHz. This results from the fact that the 20 MHz bandwidth is divided by64. Although only 52 subcarriers are used, occupying a total of 16.6 MHz, the remaining space isused as a guard band between the different channels.A variety of forms of modulation can be used on each of the 802.11a subcarriers. BPSK, QPSK,16-QAM, and 64 QAM can be used as the conditions permit. For each set data rate there is acorresponding form of modulation that is used. Within the signal itself the symbol duration is 4microseconds, and there is a guard interval of 0.8 microseconds. Data rate (Mbps) Modulation Coding rate 6 BPSK 1/2 9 BPSK 3/4 12 QPSK 1/2 18 QPSK 3/4 24 16-QAM 1/2 36 16-QAM 3/4 48 64-QAM 1/2Orlando Moreno 407.808.0322
  8. 8. 8 Data rate (Mbps) Modulation Coding rate 54 64-QAM 3/4As with many data transmission systems, the generation of the signal is performed using digitalsignal processing techniques and a baseband signal is generated. This is then upconverted to thefinal frequency. Similarly for signal reception, the incoming 802.11a signal is converted down tobaseband and converted to its digital format after which it can be processed digitally.Although the use of OFDM for a mass produced systems such as 802.11a may appear to beparticularly complicated, it offers many advantages. The use of OFDM provides a significantreduction in the problems of interference caused by multipath effects. The use of OFDM alsoensures that there is efficient use of the radio spectrum.IEEE 802.11b802.11b the new Wi-Fi standard providing data rates of 11 Mbps at 2.4 GHzIEEE 802.11b was the first wireless LAN standard to be widely adopted and built-in to manylaptop computers and other forms of equipment. The standard for 802.11b was ratified by theIEEE in July 1999 and the idea for wireless networking quickly caught on with many Wi-Fihotspots being set up so that business people could access their emails and surf the Internet asrequired when they were travelling.It was only after 802.11 was ratified and products became available that Wi-Fi took off in a largeway. Wi-Fi hotspots were set up in many offices, hotels and airports and the idea of usingportable laptop computers while travelling became far easier.Although the IEEE 802.11a standard was introduced at the same time, it did not catch on in thesame way even though it was capable of higher speeds. The main reason for this was that itoperated in the 5 GHz ISM band rather than the 2.4 GHz of 802.11b, and this made it moreexpensive.802.11b specification802.11b boasts an impressive performance. It is able to transfer data with raw data rates up to 11Mbps, and has a good range, although not when operating at its full data rate. Parameter Value Date of standard approval July 1999Orlando Moreno 407.808.0322
  9. 9. 9 Parameter Value Maximum data rate (Mbps) 11 Typical data rate (Mbps) 5 Typical range indoors (Meters) ~30 Modulation CCK (DSSS) RF Band (GHz) 2.4 Channel width (MHz) 20 Summary of 802.11b Wi-Fi Standard SpecificationWhen transmitting data 802.11b uses the CSMA/CA technique that was defined in the original802.11 base standard and retained for 802.11b. Using this technique, when a node wants to makea transmission it listens for a clear channel and then transmits. It then listens for anacknowledgement and if it does not receive one it backs off a random amount of time, assuminganother transmission caused interference, and then listens for a clear channel and thenretransmits the data.RF modulation for 802.11bThe RF signal format used for 802.11b is CCK or complementary Code Keying. This is a slightvariation on CDMA (Code Division Multiple Access) that uses the basic DSSS (Direct SequenceSpread Spectrum) as its basis. In view of the fact that the original 802.11 specification useCDMA / DSSS, it was easy to upgrade any existing chipset and other investment to provide thenew 802.11b standard. As a result 802.11b chipsets appeared relatively quickly onto the market.802.11b data ratesAlthough 802.11b cards are specified to operate at a basic rate of 11 Mbps, the system monitorsthe signal quality. If the signal falls or interference levels rise, then it is possible for the system toadopt a slower data rate with more error correction that is more resilient. Under these conditionsthe system will first fall back to a rate of 5.5 Mbps, then 2, and finally 1 Mbps. This scheme isknown as Adaptive rate Selection (ARS).Although the basic raw data rates for transmitting data seem very good, in reality the actual datarates achieved over a real time network are much smaller. Even under reasonably good radioconditions, i.e. good signal and low interference the maximum data rate that might be expectedwhen the system uses TCP is about 5.9 Mbps. This results from a number of factors. One is theuse of CSMA/CA where the system has to wait for clear times on a channel to transmit andOrlando Moreno 407.808.0322
  10. 10. 10another is associated with the use of TCP and the additional overhead required. If UDP is usedrather than TCP then the data rate can increase to around 7.1 Mbps.Some 802.11b systems advertise that they support much higher data rates than the basic 802.11bstandard specifies. While more recent versions of the 802.11 standard, namely 802.11g, and802.11n specify much higher speeds, some proprietary improvements were made to 802.11b.These proprietary improvements offered speeds of 22, 33, or 44 Mbps and were sometimeslabeled as "802.11b+". These schemes were not endorsed by the IEEE and in any case they havebeen superseded by later versions of the 802.11 standard.802.11e for QoSThe new standard to provide Quality of Service, QoS for 802.11 Wi-Fi applicationsWi-Fi technology based on the 802.11 standard is now widespread in its use. Not only is it usedto provide real wireless LAN (WLAN) functionality, but it is also widely used to providelocalized mobile connectivity in terms of "hotspots". A variety of flavors of the IEEE 802.11 areavailable: 802.11a, 802.11b, 802.11g, and these different standards provide different datathroughput speeds and operate on different bands.One of the major shortfalls for the developing applications for Wi-Fi is that it is not possible toallocate a required quality of service for the particular application. Now with IEEE 802.11e theQuality of Service or QoS problem is being addressed.The need for QoSThe issue of Quality of Service, QoS on 802.11 Wi-Fi is of particular importance in someapplications, and accordingly 802.11e is addressing it. For surfing applications such as internetweb browsing of sending emails, delays in receiving responses or sending data does not have amajor impact. It results in slow downloads, or small delays in emails being sent. While it mayhave a small annoyance to the user, there is no real operational impact on the service beingprovided. However for applications such as voice or video transmission such as Voice over IP,VoIP, there is a far greater impact and this creates a much greater need for 802.11e. Delays, jitterand missing packets result in the system loosing the data and the service quality becoming poor.Accordingly for these time sensitive applications it is necessary to be able to prioritize the traffic.This can only be done by allocating a service priority level to the packets being sent, and this isnow all being addressed by IEEE standard 802.11e.MAC layerThe way in which data is transmitted and controlled has a major impact on the way that QoS isachieved. This is largely determined by the way the Medium Access Control (MAC) layeroperates. Within 802.11 there are two options for the MAC layer. The first is a centralizedcontrol scheme that is referred to as the Point Coordination Function (PCF), and the second is aOrlando Moreno 407.808.0322
  11. 11. 11contention based approach called Distributed Coordination Function (DCF). Of these fewmanufacturers of chips and equipment have implemented PCF and the industry seems to haveadopted the DCF approach.The PCF mode supports time sensitive traffic flows to some degree. Wireless Access Pointsperiodically send beacon frames to communicate network management and identification whichis specific to that WLAN. Between the sending of these frames, PCF splits the time frame into acontention free period and a contention period. If PCF is enabled on the remote station, it cantransmit data during the contention free polling periods. However the main reason why thisapproach has not been widely adopted is because the transmission times are not predictable.The other scheme, DCF uses a scheme called Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA). Within this scheme the MAC layer sends instructions for the receiver tolook for other carriers transmitting. If it sees none then it sends its packet after a given intervaland awaits an acknowledgement. If one is not received it then it knows its packet was notsuccessfully received. It then waits for a given time interval and also checks the channel beforeretrying to send its data packet.In more exact terms the transmitter uses a variety of methods to determine whether the channel isin use, monitoring the activity looking for real signals and also determining whether any signalsmay be expected. This can be achieved because every packet that is transmitted includes a valueindicating the length of time that transmitting station expects to occupy the channel. This is notedby any stations that receive the signal, and only when this time has expired may they considertransmitting.Once the channel appears to be idle the prospective transmitting station must wait for a periodequal to the DCF Inter-Frame Space (DIFS). If the channel has been active it must first wait for atime consisting of the DIFS plus a random number of back off slot times. This is to ensure that iftwo stations are waiting to transmit, then they do not both transmit together, and then repeatedlytransmit together.A time known as a Contention Window (CW) is used for this. This is a random number of back-off slots. If a transmitter intending to transmit senses that the channel becomes active, it mustwait until the channel comes free, waiting a random period for the channel to come free, but thistime allowing a longer CW.While the system works well in preventing stations transmitting together, the result of using thisaccess system is that if the network usage level is high, then the time that it takes for data to besuccessfully transferred increases. This results in the system appearing to become slower for theusers. In view of this WLANs may not provide a suitable QoS in their current form for systemswhere real time data transfer is required.Introducing QoSThe problem can be addressed by introducing a Quality of Service, QoS identifier into thesystem. In this way those applications where a high quality of service is required can tag theirOrlando Moreno 407.808.0322
  12. 12. 12transmissions and take priority over the transmissions carrying data that does not requireimmediate transmission and response. In this way the level of delay and jitter on data such as thatused for VoIP and video may be reduced.To introduce the QoS identifier, it has been necessary to develop a new MAC layer and this hasbeen undertaken under the standard IEEE 802.11e. In this the traffic is assigned a priority levelprior to transmission. These are termed User Priority (UP) levels and there are eight in total.Having done this, the transmitter then prioritizes all the data it has to waiting to be sent byassigning it one of four Access Categories (AC).In order to achieve the required functions, the re-developed MAC layer takes on aspects of boththe DCF and PCF from the previous MAC layer alternatives and is termed the HybridCoordination Function (HCF). In this the modified elements of the DCF are termed theEnhanced Distributed Channel Access (EDCA), while the elements of the PCF are termed theHCF Controlled Channel Access (HCCA).EDCAOf these the EDCA provides a mechanism whereby traffic can be prioritized but it remains acontention based system and therefore it cannot guarantee a give QoS. In view of this it is stillpossible that transmitters with data of a lower importance could still pre-empt data from anothertransmitter with data of a higher importance.When using EDCA, a new class of interframe space called an Arbitration Inter Frame Space(AIFS) has been introduced. This is chosen such that the higher the priority the message, theshorter the AIFS and associated with this there is also a shorter contention window. Thetransmitter then gains access to the channel in the normal way, but in view of the shorter AIFSand shorter contention window, this means that the higher the chance of it gaining access to thechannel. Although, statistically a higher priority message will usually gain the channel, this willnot always be the case.HCCAThe HCCA adopts a different technique, using a polling mechanism. Accordingly it can provideguarantees about the level of service it can provide, and thereby providing a true Quality ofService level. Using this, the transmitter is able to gain access to a radio channel for a givennumber of packets, and only after these have been sent is the channel released.The control station which is normally the Access Point is known as the Hybrid Coordinator(HC). It takes control of the channel. Although it has an IFS, it has what is termed a PointCoordination IFS. As this is shorter than the DIFS mentioned earlier, it will always gain controlof the channel. Once it has taken control it polls all the stations or transmitters in the network. Todo this it broadcasts as particular frame indicating the start of polling, and it will poll each stationin turn to determine the highest priority. It will then enable the transmitter with the highestOrlando Moreno 407.808.0322
  13. 13. 13priority data to transmit, although it will result in longer delays for traffic that has a lowerpriority.SummaryThere may still be a number of problems to overcome before QoS is fully implemented on Wi-Fi. One is the possibility of people "hi-jacking" services when there is no real need. Nevertheless802.11e is a major step in the right direction, and already vendors of WiFi products are adoptingthe standard. As such this makes it an important step forward in ensuring that 802.11 Wi-Fimeets the growing demands being placed upon it.IEEE 802.11g802.11g Wi-Fi standard providing 54 Mbps data transfer rates at 2.4 GHzAfter the introduction of Wi-Fi with the 802.11a and 802.11b standards, the 802.11b standardbecame the most popular operating in the 2.4 GHz ISM band. This standard proved to be themost popular despite the faster operating speed of the a variant of the standard because the costof producing chips to operate at 2.4 GHz were much less than ones to run at 5 GHz.In order to provide the higher speeds of 802.11a while operating on the 2.4 GHz ISM band, anew standard was introduced. Known as 802.11g, it soon took over from the b standard. Evenbefore the standard was ratified, 802.11g products were available on the market, and before longit became the dominant Wi-Fi technology.With the possible of interoperability with 802.11b, the 802.11g standard soon took off, offering ahigher level of performance than its predecessor.802.11g specificationsThe 802.11g standard provided a number of improvements over the 802.11b standard which wasits predecessor. The highlights of its performance are given in the table below. 802.11g Date of standard approval June 2003 Maximum data rate (Mbps) 54 Modulation CCK, DSSS, or OFDM RF Band (GHz) 2.4Orlando Moreno 407.808.0322
  14. 14. 14 802.11g Channel width (MHz) 20 Summary of 802.11g Wi-Fi Specification802.11g OperationLike 802.11b, its predecessor, 802.11g operates in the 2.4 GHz ISM band. It provides amaximum raw data throughput of 54 Mbps, although this translates to a real maximumthroughput of just over 24 Mbps.Although the system is compatible with 802.11b, the presence of an 802.11b participant in anetwork significantly reduces the speed of a net. In fact it was compatibility issues that took upmuch of the working time of the IEEE 802.11g committee.A variety of modulation schemes can be sued by 802.11g. For speeds of 6, 9, 12, 18, 24, 36, 48,and 54 Mbps Orthogonal frequency Division Multiplexing (OFDM) is used, but for 5.5 and 11Mbps it uses Complementary Code Keying (CCK), and then for 1 and 2 Mbps it usesDBPSK/DQPSK+DSSS.The maximum range that can be achieved by 802.11g devices is slightly greater than that ofthose using 802.11b, but the range at which the full 54 Mbps can be achieved is much shorterthan the maximum range of an 802.11 device. Only when signal levels and interference levelsare low can the maximum specified performance be achieved.IEEE 802.11 n Standard802.11 n standard, the new Wi-Fi standard providing increased data ratesOnce Wi-Fi standards including 802.11a, 802.11b, and 802.11g were established, workcommenced on looking at how the raw data speeds provided by Wi-Fi, 802.11 networks could beincreased still further. The result was that in January 2004, the IEEE announced that it hadformed a new committee to develop the new high speed, IEEE 802.11 n standard.The industry came to a substantive agreement about the features for 802.11n in early 2006. Thisgave many chip manufacturers sufficient information to get their developments under way. Thedraft is expected to be finalized in November 2008 with its formal publication in July 2009.However many products are already available on the market. Manufacturers are now releasingproducts based on the early or draft versions of the specifications assuming that the changes willonly be minor in their scope.Orlando Moreno 407.808.0322
  15. 15. 15With the improved performance offered by 802.11n, the standard soon became widespread withmany products offered for sale and use. Although initially few Wi-Fi hotspots offered thestandard, 802.11n devices were compatible and able to work with the 802.11b and 802.11g basedhotspots.Basic specification for the IEEE 802.11 n standardThe idea behind the IEEE 802.11 n standard was that it would be able to provide much betterperformance and be able to keep pace with the rapidly growing speeds provided by technologiessuch as Ethernet. The new 802.11 n standard boasts an impressive performance, the main pointsof which are summarized below: Parameter IEEE 802.11 n Standard Date of standard approval Anticipated Nov 2008 Maximum data rate (Mbps) 248 Typical throughput (Mbps) 74 RF Band (GHz) 2.4 or 5 Modulation CCK, DSSS, or OFDM Number of spatial streams 1, 2, 3, or 4 Channel width (MHz) 20, or 40 IEEE 802.11n standard salient featuresTo achieve this a number of new features that have been incorporated into the IEEE 802.11nstandard to enable the higher performance. The major innovations are summarized below: Changes to implementation of OFDM Introduction of MIMO MIMO power saving Wider channel bandwidth Antenna technology Reduced support for backward compatibility under special circumstances to improve data throughputAlthough each of these new innovations adds complexity to the system, much of this can beincorporated into the chipsets, enabling a large amount of the cost increase to be absorbed by thelarge production runs of the chipsets.Orlando Moreno 407.808.0322
  16. 16. 16OFDM implementation: It has been necessary to change the way in which the OFDMmodulation scheme is implemented to improve the data throughput of the single signal path. Byadapting the way it is set-up, the data rate can be increased from the 54 Mbps data rate achievedfor 802.11a and g to 65 Mbps.Use of MIMO in IEEE 802.11 n: MIMO or Multiple Input Multiple Output is a technique thatexploits multipath propagation. Normally when a signal is transmitted from A to B the signalwill reach the receiving antenna via multiple paths, causing interference. MIMO uses thismultipath propagation to increase the data rate by using a technique known as spatial divisionmultiplexing. The data is split into a number of what are termed spatial streams and these aretransmitted through separate antennas to corresponding antennas at the receiver. Doubling thenumber of spatial streams doubles the raw data rate, enabling a far greater utilization of theavailable bandwidth. The current 802.11n standard allows for up to four spatial streams.IEEE 802.11 n power saving: One of the problems with using MIMO is that it increases thepower of the hardware circuitry. More transmitters and receivers need to be supported and thisentails the use of more current. While it is not possible to eliminate the power increase resultingfrom the use of MIMO in 802.11n, it is possible to make the most efficient use of it. Data isnormally transmitted in a "bursty" fashion. This means that there are long periods when thesystem remains idle or running at a very slow speed. During these periods when MIMO is notrequired, the circuitry can be held inactive so that it does not consume power.Increased bandwidth: An optional mode for the new 802.11 chips is to run using a double sizedchannel bandwidth. Previous systems used 20 MHz bandwidth, but the new ones have the optionof using 40 MHz. The main trade-off for this is that there are less channels that can be used forother devices. There is sufficient room at 2.4 GHz for three 20 MHz channels, but only one 40MHz channel can be accommodated. Thus the choice of whether to use 20 or 40 MHz has to bemade dynamically by the devices in the net.Antenna technology for 802.11n: For 802.11n, the antenna associated technologies have beensignificantly improved by the introduction of beam forming and diversity.Beam forming focuses the radio signals directly along the path for the receiving antenna toimprove the range and overall performance. A higher signal level and better signal to noise ratiowill mean that the full use can be made of the channel.Diversity uses the multiple antennas available and combines or selects the best subset from alarger number of antennas to obtain the optimum signal conditions. This can be achieved becausethere are often surplus antennas in a MIMO system. As 802.11n supports any number ofantennas between one and four, it is possible that one device may have three antennas whileanother with which it is communicating will only have two. The supposedly surplus antenna canbe used to provide diversity reception or transmission as appropriate.Backward compatibility switching: While 802.11n provides backward compatibility for devicesin a net using earlier versions of 802.11, this adds a significant overhead to any exchanges,thereby reducing the data transfer capacity. To provide the maximum data transfer speeds whenOrlando Moreno 407.808.0322
  17. 17. 17all devices in the net at to the 802.11n standard, the backwards compatibility feature can beremoved. When earlier devices enter the net, the backward compatibility overhead and featuresare re-introduced. As with 802.11g, when earlier devices enter a net, the operation of the wholenet is considerably slowed. Therefore operating a net in 802.11n only mode offers considerableadvantages.802.11n Access Point operational modesIn view of the features associated with backward compatibility, there are three modes in whichan 802.11n access point can operate: Greenfield (only 802.11 n) - maximum performance Mixed (both 802.11 a, b, g, and n) Legacy (only 802.11 a, b, and g)IEEE 802.11 n summaryThe new IEEE 802.11 n standard provides a major improvement in the speed at which data canbe transferred over a wireless network. While this may not be needed for many small networkswhere small files are being transferred, the amount of data being passed over most networks isincreasing with many more large files, including photos, video clips (and videos), etc. beingtransferred. With the levels of data only set to increase, the new 802.11n standard will be able tomeet the challenge of providing the required capacity for wireless or Wi-Fi networks.IEEE 802.11ac Gigabit Wi-Fi802.11ac gigabit Wi-Fi standard to provide data throughput rates of up to 1Gbps at frequenciesup to 6 GHz.The IEEE802.11ac Wi-Fi standard has been developed to raise the data throughput ratesattainable on Wi-Fi networks up to rates of around 1 Gbps. The implementation of Gigabit Wi-Fiis needed to ensure that Wi-Fi standards keep up with the requirements of users.The new IEEE 802.11 ac Gigabit Wi-Fi standard is in development and is anticipated to bereleased by about 2012.IEEE 802.11ac Gigabit Wi-Fi highlightsThe IEEE 802.11ac Gigabit Wi-Fi standard utilizes a number of techniques that have beenutilized within previous IEEE 802.11 standards and builds on these technologies, while addingsome new techniques to ensure that the required throughput can be attained.Orlando Moreno 407.808.0322
  18. 18. 18 OFDM: The IEEE 802.11ac standard utilizes OFDM that has been very successfully used in previous forms of 802.11. The use of OFDM is particularly applicable to wideband data transmission as it combats some of the problems with selective fading. MIMO and MU-MIMO: In order to achieve the required spectral usage figures to attain the data throughput within the available space, the spectral usage figure of 7.5 bps/Hz is required. To achieve this, MIMO is required, and in the case of IEEE 802.11ac Wi-Fi, a form known as Multi- User MIMO, or MU MIMO is implemented. Note on MIMO: Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannons Law. MIMO provides a way of utilizing the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel. MU-MIMO enables the simultaneous transmission of different data frames to different clients. The use of MU-MIMO requires that equipment is able to utilise the spatial awareness of the different remote users. It also needs sophisticated queuing systems that can take advantage of opportunities to transmit to multiple clients when conditions are right. Error correction coding: The advances in chip manufacturing technology have enabled designers to take advantage of additional levels of processing power when compared to previous implementations of the 802.11 standards. This has enabled the use more sensitive coding techniques that depend on finer distinctions in the received signal. IN addition to this more aggressive error correction codes that use fewer check bits for the same amount of data have been utilized within the 802.11ac format Increased channel bandwidth: The previous versions of 802.11 standards have typically used 20 MHz channels, although 802.11n used up to 40 MHz wide channels. The 802.11ac standard uses channel bandwidths up to 80 MHz. To achieve this it is necessary to adapt automatic radio tuning capabilities so that higher-bandwidth channels are only used where necessary to conserve spectrum802.11ac summaryOrlando Moreno 407.808.0322
  19. 19. 19The IEEE 802.11ac Gigabit Wi-Fi offers significant advantages over the previous incarnations ofthe 802.11 standard. It offers backwards compatibility with previous versions and this willenable it to be introduced in the existing Wi-Fi ecosystem with the minimum of disruption.IEEE 802.11ad Microwave Wi-Fi802.11ad Microwave Wi-Fi standard to provide data throughput rates of up to 6Gbps atfrequencies around 60 GHz.The IEEE 802.11ad standard is aimed at providing data throughput speeds of up to 6Gbps. Toachieve these speeds frequencies in the region of 60GHz are used to achieve the levels ofbandwidth needed.Using frequencies in the millimeter range IEEE 802.11ad microwave Wi-Fi has a range that ismeasured of a few meters. The aim is that it will be used for very short range (across a room)high volume data transfers such as HD video transfers. When longer ranges are needed standardssuch as 802.11ac can be used.Development of the IEEE 802.11ad microwave Wi-Fi standard is currently under way, andfurther information is anticipated in the coming months.White-fi is a term being used to describe the use of a Wi-Fi technology within the TV unusedspectrum, or TV white space. The IEEE 802.11af working group has been set up to define astandard to implement this.With a number of administrations around the globe taking a more flexible approach to spectrumallocations, the idea of low power systems that are able to work within portions of spectrum thatmay need to be kept clear of high power transmitters to ensure coverage areas do not overlap isbeing seriously investigated.When using systems like white-fi, IEEE 802.11af that use TV white space, the overall systemmust not cause interference to the primary users. With processing technology developing further,this is now becoming more of a possibility.Benefits of IEEE 802.11af, White-FiThere are many benefits for a system such as IEEE 802.11af from using TV white space. Whilethe exact nature of the IEEE 802.11af system has not been fully defined, it is still possible to seemany of the benefits that can be gained: Propagation characteristics: In view of the fact that the 802.11af white-fi system operating the the TV white spaces would use frequencies below 1 GHz, this would allow for greater distancesOrlando Moreno 407.808.0322
  20. 20. 20 to be achieved. Current Wi-Fi systems use frequencies in the ISM bands - the lowest band is 2.4 GHz and here signals are easily absorbed. Additional bandwidth: One of the advantages of using TV white space is that additional otherwise unused frequencies can be accessed. However, it will be necessary to aggregate several TV channels to provide the bandwidths that Wi-Fi uses on 2.4 and 5.6 GHz, to achieve the required data throughput rates.Looking at these benefits, it is believed that the White-Fi system offers sufficient advantages toenable development to be undertaken.IEEE 802.1af white-fi technologiesIn order for white-fi 802.11af to be able to operate, it is necessary to ensure that the system doesnot create any undue interference with existing television transmissions. To achieve this there area number of technologies and rules that may be utilised. Cognitive radio: One way in which a white-fi system would be able to operate is to use cognitive radio technology; Note on Cognitive Radio: With pressure on radio spectrum increasing all the time, it is necessary to utilise the available spectrum as efficently as possible. One method of helping to achieve this is utlise radio technology that is able to sense the environment and configure itself accordingly - Cognitive Radio. The technology is heavily dependent upon Software Defined Radio technology as the radio needs to be configurable according to the previaling radio environment.Using this technology, it will be possible for the white-fi, IEEE 802.11af system to detect transmissionsand move to alternative channels. Geographic sensing: Another mthod that is favoured by many is geographic sensing. Although details are not fully defined, having a geographic database and a knowledge of what channels are available there is another way of allowing the system to avoid used channels.SummaryWork has only recently started on the IEEE 802.11af standard for white-fi applications in whitespace TV spectrum. A number of trials have been undertaken with success, although to deploy ascheme of this nature will require careful definition and implementation.Orlando Moreno 407.808.0322
  21. 21. 21802.11 channel basicsThere is a total of fourteen channels defined for use by Wi-Fi 802.11 for the 2.4 GHz ISM band.Not all of the channels are allowed in all countries: 11 are allowed by the FCC and used in whatis often termed the North American domain, and 13 are allowed in Europe where channels havebeen defined by ETSI. The WLAN / Wi-Fi channels are spaced 5 MHz apart (with the exceptionof a 12 MHz spacing between the last two channels).The 802.11 WLAN standards specify a bandwidth of 22 MHz and a 25 MHz channel separation,although nominal figures for the bandwidth of 20 MHz are often given. The 20 / 22 MHzbandwidth and channel separation of 5 MHz means that adjacent channels overlap and signals onadjacent channels will interfere with each other.The 22 MHz channel bandwidth holds for all standards even though 802.11b WLAN standardcan run at variety of speeds: 1, 2, 5.5, or 11 Mbps and the newer 802.11g standard can run atspeeds up to 54 Mbps. The differences occur in the RF modulation scheme used, but the WLANchannels are identical across all of the applicable 802.11 standards.When using Wi-Fi to provide WLAN solutions for offices, general use hotspots, or for anyWLAN applications, it is necessary to ensure that parameters such as the channels are correctlyset to ensure the required performance is achieved.Wi-Fi WLAN channel frequenciesThe table given below provides the frequencies for the total of fourteen WLAN / Wi-Fi channelsthat are available around the globe. Not all of these WLAN / Wi-Fi channels are available for usein all countries. Lower Frequency Center Frequency Upper Frequency Channel Number MHz MHz MHz1 2 401 2 412 2 4232 2 404 2 417 2 4283 2 411 2 422 2 4334 2 416 2 427 2 4385 2 421 2 432 2 443Orlando Moreno 407.808.0322
  22. 22. 22 Lower Frequency Center Frequency Upper Frequency Channel Number MHz MHz MHz6 2 426 2 437 2 4487 2 431 2 442 2 4538 2 436 2 447 2 4589 2 441 2 452 2 46310 2 451 2 457 2 46811 2 451 2 462 2 47312 2 456 2 467 2 47813 2 461 2 472 2 48314 2 473 2 484 2 495WiFi channel overlap and selectionThe channels used for WiFI are separated by 5 MHz in most cases but have a bandwidth of 22MHz. As a result channels overlap and it can be seen that it is possible to find a maximum ofthree non-overlapping channels. Therefore if there are adjacent pieces of WLAN equipment thatneed to work on non-interfering channels, there is only a possibility of three. There are fivecombinations of available non overlapping channels are given below: Wi-Fi Channel overlap and which ones can be used as sets.Orlando Moreno 407.808.0322
  23. 23. 23From the diagram above, it can be seen that Wi-Fi channels 1, 6, 11, or 2, 7, 12, or 3, 8, 13 or 4,9, 14 (if allowed) or 5, 10 (and possibly 14 if allowed) can be used together as sets. Often WiFirouters are set to channel 6 as the default, and therefore the set of channels 1, 6 and 11 is possiblythe most widely used.As some energy spreads out further outside the nominal bandwidth, if only two channels areused, then the further away from each other the better the performance.It is found that when interference exists, the throughput of the system is reduced. It thereforepays to reduce the levels of interference to improve the overall performance of the WLANequipment.WLAN / Wi-Fi Channel availabilityIn view of the differences in spectrum allocations around the globe and different requirementsfor the regulatory authorities, not all the WLAN channels are available in every country. Thetable below provides a broad indication of the availability of the different Wi-Fi channels indifferent parts of the world. Europe North America Channel Number Japan (ETSI) (FCC)1 Yes Yes Yes2 Yes Yes Yes3 Yes Yes Yes4 Yes Yes Yes5 Yes Yes Yes6 Yes Yes Yes7 Yes Yes Yes8 Yes Yes Yes9 Yes Yes Yes10 Yes Yes YesOrlando Moreno 407.808.0322
  24. 24. 24 Europe North America Channel Number Japan (ETSI) (FCC)11 Yes Yes Yes12 Yes No Yes13 Yes No Yes14 No No 802.11b onlyThis chart is only provides a general view, and there may be variations between differentcountries. For example some countries within the European zone Spain have restrictions on thechannels that may be used (France: channels 10 - 13 and Spain channels 10 and 11) use of Wi-Fiand do not allow many of the channels that might be thought to be available, although theposition is likely to change.802.11 spectrum and spectral maskIn any radio frequency system it is not possible to confine all the energy to a specific bandwidth.Some energy will always be present beyond the bandwidth provided. Instead bandwidths aredefined in terms of a spectral mask and the output from any transmitters must fall within thelevels defined by the mask.The 802.11 transmissions have a spectral mask defined as the energy from the transmitter willextend beyond the 22 MHz Wi-Fi channels allocated (i.e. +/- 11 MHz from fc the centrefrequency). The spectral mask defines the maximum levels that may emanate from thetransmitter over a given spectrum.At 11 MHz from the centre of the channel, the energy must be 30 dB lower than the maximumsignal level, and at 22 MHz away, the energy must be 50 dB below the maximum level. Furtheraway from the centre frequency, the energy levels fall further but some energy is still present andcould result in interference on some channels.WLAN / Wi-Fi channels in practiceWhile interference may appear to be a major possible problem, with the signals from differentaccess points interfering with one another, in reality this does not appear to be the problem thatmight be anticipated. In one study that was undertaken, measurements of usage and interferenceOrlando Moreno 407.808.0322
  25. 25. 25levels were undertaken at Heathrow airport in the UK where usage is high and interference wasanticipated to be a problem. To the surprise of many there was little evidence of interference,possibly as a result of the limited ranges experienced. However offices where WLANS are inwidespread use may experience problems, and occasional home use where most Wi-Fi accesspoints are all set to the same channel may experience some interference. However in general,with a little care, Wi-Fi / WLAN should not experience interference and if they do then the Wi-Fi channels can be changed.Orlando Moreno 407.808.0322