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Telephony
 

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    Telephony Telephony Document Transcript

    • TELEPHONY AS IT EXISTS TODAY DIFFERENCE BETWEEN CIRCUIT & PACKET SWITCHED NETWORKS DIFFERENCE BETWEEN DIGITISED & PACKETISED VOICE
    • 1. In Fig 1 a. Ear transducer 1 audio signal to Ear 1a b. Mouth audio signal 2a to transducer in mouth piece 2 c. Electrical signal from transducers 1 and 2 carried over 2 wire twisted pair to either local exchange (in case of direct extension lines) or to EPABX (in case of extension). d. These analogue signals are carried to the Codecs 6. e. These codecs digitise the analogue signals and these are switched in the PCM TDM switches to connect to extensions of the EPABX or the DELs connected to the LEX.. f. Or they may be connected through the trunks of these exchanges to other Local Exchanges (LEX) in the telephone network. g. The analogue (DEL) connections to the EPABX from the codex at the LEX is terminated on the CO (Central Office) trunk cards (5) in the EPABX through twisted pair copper connections to carry the analogue signals between the LEX and the EPABX. h. For digital circuits between the EPABX and the LEX, 2 twisted pair copper wires are used - one for Tx (transmit) and one for Rx (receipt). These could be through the basic rate interface (BRI) which have 2 B (bearer) channels and one signalling channels D. Or they may be connected through primary rate interface (PRI) which have 30 B channels and 2 D channels. i. When digital links are being used between the EPABX and the LEX, the digitised voice signals switched through the PCM TDM switch in the EPABX is transmitted digitally without conversion which happens if analogue trunks are used. j. The PCM TDM switches are designed for totally unblocked communication between all extensions. Thus if there are 100 extensions in the EPABX, there will be 50 simultaneous communications between these extensions. k. For unblocked communications through and beyond the EPABX a diversity of usage for external communications known as Erlang loading is used. This means that one trunk is provided for n extensions, where n could be 10 or 8 or 6 depending on how busy the exchange is. When this is done, every time the outside line is attempted to be seized by an extension, it gets a dial tone which means it is free to dial. This is called unblocked communication. l. In the LEX, there are usually large number of direct extensions, of the order of thousands (typically 5000 or 10000). Using the same Erlang loading principle to evacuate the traffic from a 10000 line exchange in an unblocked fashion we would need around 1000 trunks going out to the transport layer of the PSTN system. This is usually provided through Link Access Protocol (LAP) like V5.2 which provides 16 E1 circuits over a 4W digital circuit between the Access Network (AN) and the Local Exchange (LE or LEX). To evacuate 10000 DELs in an unblocked fashion you would need 2 – V5.2 .
    • m. From Fig 1 we see that from and after the local exchange the voice is totally digitised. n. Fig 1a shows the schematic of a total telephone network as it exits today 2. Fig 2 shows how the Local Exchange LEX is connected using V5.2 LAPs, and PHOM (public higher order MUX) in the LEX and TAX buildings through the TAX and the transport layer to other cities. 3. This schematic drawing also shows how point-to-point (p2p) leased lines are built up over PSTN and how it denies physical access from the PSTN. 4. Such p2p leased lines connect all locations of a MLO (multi- locational organisation) into a real private data network as shown in Fig 3a, or each location of the MLO to the nearest POP (point-of-presence) of the TSP IP Backbone to form a VPN as shown in Fig. 3b. A typical TSP IP Backbone topology is shown in Fig1 and its network architecture in Fig2 of VPN.ppt shown in URL http://www.slideshare.net/pankajmitra . This presentation also explains the security vulnerability of VPN networks. 5. In the circuit switched PSTN network the digitised voice is sent through TDM channels each 64 kbps, one per circuit, achieved through time slicing or time division multiplexing. 6. The BRI link provides 2 B and 1D channel or one ISDN connection. 7. The PRI link provides 30B + 2D channels or one E1 channel. 8. The V5.2 LAP interface provides 16 E1 circuits. 9. The HOM combines these to larger number of 64 kbps channels. 10. In circuit switched networks for each voice or fax or data communication an end-to-end virtual circuit is set up each of 64 kbps . The different types of communications can be carried out simultaneously one on each channel.
    • 11. Since an end -to-end communication channel is available for each communication taking place, the digitised voice, fax, or data is sent along these digital channels as they are without any need for packet formation. 12. Packet formation is required when there are no end –to-end channels for each communication, and various users are sharing a common network (no dedicated circuits or channels) . The packets are small and have a header containing the destination details and a trailer containing the sending end details. 13.Therefore, over a single point-to-point (p2p) link several and different communications can travel together in the form of packets with header and trailer addresses, sent from different sources and to different destinations. 14. In the above example the flow of communications can be simultaneously in both directions. 15.The concept of packetisation was developed to send data over shared data networks to make better utilisation of link and network bandwidth and avoid idling which would happen in the case of dedicated point-to-point links for data only networks, since data communications is bursty in nature. 16.In the PSTN circuit switched network the WAN bandwidth utilisation is ensured using the Erlang loading and user diversity. Thus in dial-up data circuits the problem of unutilised data communications is non – existent as the circuit will be called up only when the data communications are required. However, even in the connected state, the very nature of computer communication entails under-utilisation of the connected bandwidth.
    • 17.The problem of under-utilisation could occur in pure data p2p networks if this was used in the circuit switched or TDM mode. The problem gets substantially alleviated if packetisation is resorted to as then several sources could be in communication with several destinations at the same time over the same link. 18. In the circuit switched network the voice routes through the EPABX and / or the LEX. 19. The fundamental difference between the PSTN circuit switched system and the packet switched data networks is that the communications (voice or data) do not route through the LEX. 20.The voice packets along with the data packets are sent to the router in the customer premises and get connected to other organisation locations either through direct p2p leased circuits or through a service providers IP backbone (as in the case of VPN networks). 21. The voice packetisation takes place at the user end in either of the following ways. a. Using IP PBX where analogue voice through the copper cables get digitised through codecs and switched in the EPABX. The output streams moving out of the EPBAX get packetised in the IP trunk cards and these are then fed into the router for onward transmission through the IP WAN. b. Using standard EPABXs whose analogue trunks are fed into the voice packetising cards in the routers . The voice packets are then sent by the router along with data packets through the IP WAN. c. Using IP phones connected to the LAN which caries the voice packets created in the IP phones along with the data packets from the different computers in the LAN through the router to and across the WAN.
    • d. The IP WANs could be private using p2p links between enterprise locations or shared using TSP IP backbones (VPN) or Internet. e. The above options apply to users within an enterprise. f. For individual subscribers the only way he can use VoIP is through the Internet Connection he / she has through dial-up or broadband connection. g. Majority of the individual telephone subscribers today are on PSTN with DEL (direct extension lines) 22. There re two basic types of packet switched networks a. Connectionless - IP, Ethernet, UDP, RTP b. Connection oriented – X.25, Frame Relay, ATM, MPLS, TCP 23. TCP / IP protocol suit is the prevalent standard data communications in packet switched networks today. They are not very effective for VoIP, although all voice over the Internet is based on this protocol with its attendant shortcomings. 24. When individual enterprises are setting up their VoIP over their own private networks, they may use UDP or RTP / SIP. 25. Similarly if TSPs wish to they can for their VPN subscribers use UDP or RTP/SIP for VoIP and TCP / IP for the data communications. The MPLS connection oriented network can have multi-protocol suits encapsulated in one label. 26. Broadband access networks over existing telephone cables of the PSTN networks of various TSPs are arranged through DSLAMs fed from the IP Backbones of the TSP and an ADSL modem placed at the customer premises (CPE). The Internet Feed to the DSLAM is through TCP/IP from the NIXI (the national internet exchange interface) in the city where the DSLAM is situated. See figure 2 of VPN.ppt mentioned in (17) above.
    • 27. The metered telephony through the Broadband access network over the same copper cable is separated using voice filters and run as analogue signals into the Local Exchange of the PSTN. Thus the metered telephony over the broadband connection is still circuit switched through the PSTN. 28. These broadband connections may be extended for IPTV using a set-top box associated with the ADSL modems in the customer premises and use IGMP (internet group management protocol) for live TV, and RTSP (real time streaming protocol) for VOD (Video on Demand) and NPVR (network based personal video recorder). 29. The total Internet and IPTV signals are carried over the ADSL2+ which facilitates a maximum download speed of 24 mbps and an upload speed of 1.4 mbps. These speeds deteriorate with distance of the customer premises from the DSLAM located in the LEX (local exchange) building. At the average distance adequate bandwidth is available for downloading up to two video signals (Broadcast TV, VOD, gaming) compressed in MPEG4 (H.264) besides facilitating Internet connection up to 2 mbps. The entire DSLAM feed for Internet, and IPTV signals are received from the IP backbone of the TSP. 30. The metered telephony through voice filters are fed from the PSTN network. 31. VoIP if used and permitted by the TSP over their Broadband connections (many of them do not allow this) has to route through the IP Backbone of the TSP / Internet (if it is meant for destinations beyond the TSP IP Backbone).
    • 32. Fig 4 shows the key elements of a GSM network structure for mobile telephone. Fig 5 shows a typical layout of the CDMA one or the IS-95 network. The first generation (1G) GSM network used analogue network access from the mobile phone to the BTS. To make use of the frequency spectrum available for maximum number of cell phones covered by the BTS (Towers) and the BSC, FDMA (frequency division multiple access) was used. The 2G GSM and the IS-95 uses digital network access from the mobile phone. To make the best use of the frequency spectrum available 2G GSM uses TDMA (time division multiple access), and the IS-95 network uses CDMA (code division multiple access). 33. For comparison of these access methods, imagine a cocktail party, where couples are talking to each other in a single room. The room represents the available bandwidth. In GSM, a speaker takes turns talking to a listener. The speaker talks for a short time and then stops to let another pair talk. There is never more than a pair of speakers talking in the room, no one has to worry about two conversations mixing. In CDMA, any pair of speakers can talk at any time; however each uses a different language. Each listener can only understand the language of their partner. As more and more couples talk, the background noise (representing the noise floor) gets louder, but because of the difference in languages, conversations do not mix. In FDMA, each speaker pair speaks in a different pitch audible only to conversing pair. 34. From the BSC (base switching centre) to the MSC (main switching centre), the communication is TDM for both the GSM and the IS-95 systems. The MSC is connected to the PSTN network through TDM circuits and to the PSTN access network also through TDM to the TAX (trunk automatic exchange) in the city and through the transport layer through TDM / SDH to the TAX of other cities.
    • 35. For your ready reference, GSM has a market share of 80 to 85% and the IS-95 a share of 10 to 15% globally. 36. The above dissertation shows that the mobile telephony is entirely circuit switched up to 2.5 G. 37. For 3G mobile also the telephony is circuit switched. The only additional feature is that the voice and data communication will be simultaneous and data communications will be over a larger bandwidth to support TV and video transmission. 38. Thus as long as the basic communications systems need circuit switching, we will have to continue with the PSTN network. 39. In India we have 500+million mobile subscribers and 40+ million fixed line subscribers. The entire 540+million telephone users are on circuit switching. All these use digitised voice through the various switches and access and transport networks. But they do not use packetised voice. 40. 4G mobile has been designed with packetised voice, and hence VoIP. When this is implemented by any TSP they will need to relay their core network from their central switching station to the base switching stations to an IP backbone, and these will be connected to the IP Backbone of the TSP networks. It is like setting up a parallel IP network to the existing circuit switched core network. 41. A lot will depend on how the market takes to 4G. Firstly 4G handsets are going to be very expensive and certainly not affordable to a large majority of the present mobile phone subscribers – at least certainly not in India. Secondly there the issues of additional bandwidth and QoS for any VoIP telephony. Further the facilities that can be provided by 3.5G which is an extension of 3G with the same type of infrastructure already provides what 4G is promising.
    • 42. We should always view technology on its relevance and not as a stand alone novelty and newness. 43. To move from circuit switching to packet switching it is necessary to build TSP IP Backbones to the same extent of coverage as their present PSTN networks – a mammoth task, to say the least. 44. The interesting fact to remember is that TSP IP backbones are laid out over point-to-point TDM or SDH circuits connecting all the POPs (points-of-presence) of the IP network through full mesh or tri-node topology (for re-routing traffic against temporary link failures). TDM or SDH are essentially synchronous communications networks and circuit switched. 45. Thus we need TDM or SDH circuits to form IP backbones of TSPs for running all IP communications. 46. Packetisation of data and the asynchronous IP communications were made to take care of better bandwidth utilisation by sharing the bandwidth for multiple communicators, to overcome the bursty nature of data communications. It was not designed for or intended for real time communications like voice, fax, and video. Such real time communications are best carried out over synchronous communications networks or circuit switched networks. 47. The whole concept of VoIP was born with the desire for unified communication. Some technologists and researchers felt this could be done by sending voice packets along with data packets in queue. Since voice is a real time communications VoIP came up with problems which are being attempted to be resolved by constant research and development in this field.
    • 48. It is important to note that there are other alternatives for unifying communications in a more bandwidth efficient manner – combining circuit and packet switching over the same network by allocating bandwidth for each. The former may be used for all real time communications like voice and fax, and the latter for data and all IP services. 49. This alternative principle has been used in 3G/3.5G mobile telephony, and very successfully. 50. In my view the VoIP initiative for 4G is a retrograde step and should be seriously reviewed and the line taken in 3G/3.5G be accommodated in the 4G / LTE programme, particularly since no significant advantage is gained by moving into VoIP in place of circuit switched voice, and there are many more problems to be overcome. 51. Step in (50) will reduce the implementation cost for 4G by the MSPs (Mobile phone service providers) as then they will not need to IP Trunks at all BSCs and MSC to connect to their IP core network for communicating with their present 2G, 2.5G, 3G, and 3.5G subscribers with the onset of 4G. 52. Finally in Fig. 6 (Inset Fig.1), I have given a Venn diagram which gives the inter-relationship between the different communications options available to individual and enterprise users.
    • Fig 1 LEX 1a 1 Analogue 4 3 2a 2 P 6 C 7 M 1a 1 T 8 4 P3 4 3 C D 6 2a 2 Analogue M M T D Digital BRI M 5 Or PRI 10 EPABX 9 1. Transducer in ear piece of telephone instrument a. Audio signal from ear transducer to ear 2. Transducer in mouth piece of telephone handset a. Audio signal from mouth to mouth transducer 3. CO trunk card in EPABX 4. Codecs which convert analogue signals into digital signals for use in the TDM / PCM switches. 5. Digital trunk BRI / PRI 6. V5.2 LAP interfaces 7. Higher Order Multiplexers (HOM) 8. Link to HOM at trunk automatic exchange (TAX) building. These could be cable PCM, digital micro wave, or Fibre Optic links (OFC) 9. IP Trunk for interfacing with IP networks to communicate with VoIP phones 10. Link to IP Network
    • Fig. 1a Total Telephone System 1a 1 3 Analogue 4 P 2 6 2a C 7 M 1 8 P 3 Analogue 4 T 1a 3 4C 2a 2 D 6 M T M D Digital BRI M 5 Or PRI EPABX 9 LEX 10 TSP TSP IP BACKBONE TDM / SDH TRANSPORT INTERNET NETWORK 10 9 1a 1 3 Analogue 4 P 2a 2 C 6 7 M 1 Analogue 8 4 T 1a 3 4C 3 P 6 2a 2 M D T M D Digital BRI M 5 Or PRI EPABX LEX
    • Fig 2 GENERAL SCHEMATIC OF PUBLIC SWITCHTED TELEPHONE NETWORK (PSTN) SHOWING TRANSPORT AND ACCESSNETWORK ALSO LEASED LINE BUILD UP (IN RED) CITY A CITY B TM TM HM INTER-CITY CARIER HM TAX TAX NTM NTM PHOM PHOM HOM LEX LEX FOM LEX LEX MDF at LEX Building Primary Cables Street Pillar Boxes Secondary Cables Building DBs E E E E P P P P B B B B A A A A Point-to-point Point-to-point X X X X Leased lines Leased lines Channel Splitter Channel Splitter Data Router V Data Router V D D P P S S
    • Fig. 3 RPDN – ROUTED IP NETWORK L R4 L R1 R3 L Private Network R2 L R – Router at each organisation location L - LAN hub / Switch at each organisation location Fig. 3a L R 4 L R TTSP IP BACKBONE R L 3 FOR SHARED NETWORKS & 1 VPN R 2 L Fig. 3a
    • TDMA / FDMA Digi CS 8 7 P 1 1 TDM TDM 16 5 9 4 6 S 8 17 9 20 T IP N TDM TDMA / FDMA Digi CS 8 7 5 9 6 1 1 16 4 8 17 21 9 IP 22 Fig 4 - 2.5G (GPRS) System CDMA 16 8 7 P 2 6 1 1 S 5 17 20 7 8 9 T N CDMA 8 7 16 1 6 8 17 1 2 5 7 21 9 22 Fig.5 - 2.5G cdmaOne (IS95)System
    • Fig 6