1. 04/05/2013Bahman R. Alyaei1 Chapter 10 ISDN and DSL

2. 04/05/2013Bahman R. Alyaei2 1 PSTN Today The overview of the modern public switched telecommunications network is presented in Figure (see next slide). The structure and functionality of the network are only reviewed here because most of the elements in the figure are discussed in previous lectures and some new topics such as ISDN and ADSL will be explained in this lecture.

3. 04/05/2013Bahman R. Alyaei3 Overview of the public switched telecommunications network

4. 04/05/2013Bahman R. Alyaei4 Continue… Figure presents a simplified diagram of a Regional or National PSTN that has connections to the Global Internet and PSTN. The Network contains the Public Land Mobile Network (PLMN), which provides Wireless Access for Cellular Subscribers and is connected to the PSTN/ISDN Network at the Trunk Exchange Level.

5. 04/05/2013Bahman R. Alyaei5 Continue… Internet users are connected to the Global Internet via the hosts of their ISPs. Networks of National ISPs are connected and this interconnection is extended to the networks of ISPs of neighboring countries, and these networks together make up the Global Internet. Figure also shows two main methods for Accessing the Internet.

6. 04/05/2013Bahman R. Alyaei6 Continue… A Telephone or ISDN Network is used for dial-up connections and ADSL provides permanent higher rate Internet Service. Digital PABX is connected to a Local Exchange with a 1,544/2,048 kbps Digital Line that has the capacity of 23/30 Channels. For Data Communication via an Analog Network or Digital Network with Analog Subscriber Interfaces, a Modem is required.

7. 04/05/2013Bahman R. Alyaei7 Continue… Also in the Figure, one office site of a company has high-data-rate Access to its ISP. All employees have access to the Internet via the Company’s Private LAN. Leased Lines, which interconnect two offices in the Figure, are often the most economical solution for high-data-rate circuits that are needed, for example, for LAN interconnections.

8. 04/05/2013Bahman R. Alyaei8 2 The Internet The name ‘Internet’ is derived from the term ‘inter-networking’. Each of these networks is based on a special form of switching designed specifically for handling data, using a standard way of packaging and addressing the data: the so- called Internet Protocol (IP). The subject of IP and Data Networks is covered in detail in (Computer Networks).

9. 04/05/2013Bahman R. Alyaei9 Continue… A huge range of applications by Business and Residential Users now exploits the Internet. The most common of these applications are e-mails and other peer-to-peer applications such as Gaming, Information Gathering from Web Pages, and Internet Shopping. Figure in the next slide presents a simplified schematic diagram of the Internet.

10. 04/05/2013Bahman R. Alyaei10 The Internet Concept

11. 04/05/2013Bahman R. Alyaei11 Continue… Each of the component networks in the Internet constellation are owned and operated by Internet Service Providers (ISPs). Users and Providers of information (in the form of Web Pages) subscribe to an ISP for their Internet service. Users access their ISPs over Links provided by one of a range of networks such as PSTN, ISDN, Mobile, Leased Line, ADSL/broadband and Cable TV (CATV).

12. 04/05/2013Bahman R. Alyaei12 Continue… ISP Networks usually comprise a number of IP switching units known as ‘Routers’ located at various points in the country and linked by high capacity transmission circuits leased from a network operator (PSTN), so- called Leased Lines or Private Circuits. ISP Networks link with one another in order to obtain full interconnectivity between users.

13. 04/05/2013Bahman R. Alyaei13 Continue… The smaller ones connect to larger ISPs. The larger in turn connect to the largest ISPs, those with a National or International coverage–the ‘Backbone ISPs’. The interconnection of the largest ISPs in the United Kingdom is achieved by Jumpering cables between each of their Routers collocated in Central Buildings, known as Internet Exchange (INX).

14. 04/05/2013Bahman R. Alyaei14 Continue… The largest is located in the City of London: ‘Telehouse’. This provides participants with full interconnectivity within the UK and to links going to the United States, Asia and Mainland Europe.

15. 04/05/2013Bahman R. Alyaei15 2 Access to the Internet Internet can be access from: 1.Dial-up via the PSTN 2.Over ISDN. 3.Over ADSL. 4.Over a Cable Modem. 5.Leased line access.

16. 04/05/2013Bahman R. Alyaei16 3 Internet Access Through The Dial-up Network (PSTN) The original and still a common way of Accessing the Internet is through a Telephone Call, thus taking advantage of the full interconnectivity of the PSTN. The procedure is in two stages: 1.The first stage is a Telephony Call to the ISP. 2.The second stage is a Data Transfer session between the Computer and the Internet via the ISP.

17. 04/05/2013Bahman R. Alyaei17 Continue… Dial-Up Access to the Internet

18. 04/05/2013Bahman R. Alyaei18 Continue… For the first stage, a standard call is set up from the Computer, which is connected to a Telephone Line in the normal way. A Modem Card in the Computer makes it behave like a Telephone Instrument, providing the equivalent of lifting the receiver (‘off-hook’), detecting dial tone, and sending out the dialed digit tones. The Modem also enables Data to be passed over the standard Local Telephone Line.

19. 04/05/2013Bahman R. Alyaei19 Local-access network and digital local exchange site

20. 04/05/2013Bahman R. Alyaei20 Continue… The ISP is connected to the PSTN over a standard Local Line or by an ISDN (Integrated Services Digital Network) Local Line, but in either case standard Telephone Numbers are allocated. Once through to the ISP the second stage begins, the call connection through the PSTN now just acts as a two-way path between the Computer and the ISP.

21. 04/05/2013Bahman R. Alyaei21 Continue… An interchange of Data occurs so that the ISP can verify the identity of the user and establish a data session to the Internet. IP is used between the Computer and the ISP’s equipment (an IP router), passing over the transparent PSTN call connection path. The ISP then routes the Data between the Computer and the Internet for the duration of the session.

22. 04/05/2013Bahman R. Alyaei22 Continue… When the user indicates that the session is over, the Modem Card in the Computer creates the equivalent of the ‘on-hook’ condition so that the Telephone Exchange clears the call to the ISP in the normal way.

23. 04/05/2013Bahman R. Alyaei23 3.1 Modem It is a contraction of the words Modulator and Demodulator. The Modem converts a computer’s Digital Bit Stream into a signal (ASK, PSK, FSK) suitable for transmission over the Telephone Line and vice versa. Modems are available in two forms: 1.Internal Modems. 2.External Modems.

24. 04/05/2013Bahman R. Alyaei24 Continue… Typical direct-connection installation

25. 04/05/2013Bahman R. Alyaei25 Continue… Internet service over PSTN the dial-up network Analog- ASK, PSK, FSK

26. 04/05/2013Bahman R. Alyaei26 3.2 Modem Transmission Modes Three transmission modes of operation used by modems: 1.Simplex: Data are received or sent in one direction. 2.Half-Duplex: In which the communication channel is shared between sending and receiving stations in a manner that receiving or transmission of the data is performed alternatively.

27. 04/05/2013Bahman R. Alyaei27 Continue… 3.Full-Duplex: In which transmission receiving of the data is performed simultaneously. In two-wire telephone circuit, this is performed using FDM.

28. 04/05/2013Bahman R. Alyaei28 4 Internet Access Over ISDN ISDN stands for Integrated Services Digital Network. It is a design for a completely Digital Telephone and Telecom Networks. It is designed to carry Voice, Data, Images, and Slow Scan Video. The concept of the ISDN is that the Network of Digital Telephony Exchanges is enhanced to allow the Switching of Data Calls.

29. 04/05/2013Bahman R. Alyaei29 Continue… Accessing the Internet via ISDN is essentially the same as for the PSTN, except that the Interface of the ISDN Line to the Computer is Digital rather than Analogue, so the Modem Card does not convert the Computer output to Analogue and consequently higher speeds up to 64 kbps or even 128 kbps are possible. The ISDN Modem is also referred to as Terminal Adapter (TA) or Digital Modem.

30. 04/05/2013Bahman R. Alyaei30 Local-access network and digital local exchange site

31. 04/05/2013Bahman R. Alyaei31 Continue… Dial-up and ISDN Modem

32. 04/05/2013Bahman R. Alyaei32 Continue… Two forms of internationally standardized ISDN are available: 1.Basic Rate Interface (BRI) ISDN. 2.Primary Rate Interface (PRI) ISDN. In each of the above ISDN services, there are two types of Channels: 1.Bearer Channels (B-Channels) for Data Transmission purposes. 2.Delta Channel (D-Channel) for Signaling purposes.

33. 04/05/2013Bahman R. Alyaei33 4.1.1 Basic Rate Interface ISDN It consists of the following channels: 1.Two 64 kbps B-Channels that is equivalent to two Time Slots. 2.One 16 kbps Signaling or D-Channel. The BRI ISDN is designated “2B + D”. The User Rate is 144 kbps R b = 2 × 64 kbps + 16 kbps = 144 kbps and is normally carried over a Single Copper Pair.

34. 04/05/2013Bahman R. Alyaei34 4.1.2 Primary Rate Interface ISDN It consists of the following channels: 1.30 × 64 kbps B-Channels. 2.One 64 kbps signaling D-Channel. This PRI ISDN is designated ‘30B + D’. The user rate is 2 Mbps R b = 30 x 64 kbps + 64 kbps = 2 Mbps and this is carried over an Optical Fiber or Microwave Radio Link.

35. 04/05/2013Bahman R. Alyaei35 4.2 ISDN Exchanges The main task of the Telephone or ISDN exchange is to build up a Physical Connection between two parties. In practice, most PSTN Exchanges today are combined ISDN and Telephony Systems. When a subscriber wishes to have Basic Rate ISDN Service the existing Copper Line is shifted from the Analogue Line Card to an ISDN Line Card on the same Concentrator Switch-Block (requiring a Re-jumpering at the MDF).

36. 04/05/2013Bahman R. Alyaei36 ISDN Lines on a Digital Local Exchange

37. 04/05/2013Bahman R. Alyaei37 Continue… A Network Terminating Unit (NTU) is installed at the subscriber’s premises to terminate the Digital Line System and present a set of Sockets for the subscriber’s Data and Telephony Terminals. There is usually a Terminal Adaptor (ISDN Modem) associated with the NTU to provide a conversion of the Analogue Telephone to Digital so that it can use the ISDN Line.

38. 04/05/2013Bahman R. Alyaei38 Continue… In the case of PRI ISDN service the termination at the Subscriber Concentrator is at the 2 Mbps level, requiring an appropriate Digital Line Termination–as provided for Digital PABX. Figure also shows the arrangement for ISDN, with an indication of how the BORSCHT functions are spread between the ISDN Line Card and the NTU on the subscriber’s premises, instead of being solely in the Line Card as for Analogue Subscriber Lines.

39. 04/05/2013Bahman R. Alyaei39 Continue… The acronym of BORSCHT is: 1.Battery: -48 V dc power supply. 2.Overload: for protecting the switch. 3.Ringing current. 4.Supervision. 5.Codec: ADC and DAC. 6.Hybrid: 2-to-4 wire conversion. 7.Test: testing the end point of the local loop.

40. 04/05/2013Bahman R. Alyaei40 Continue… Non-Data Calls from ISDN subscribers are routed through the Switch Network as for Telephony with each B-Channel being treated separately. Data Calls are also normally handled on a single B-Channel basis, with the required extra features indicated through the call set- up across the network through the ISDN signaling system. ISDN Lines are given standard Telephony Numbers.

41. 04/05/2013Bahman R. Alyaei41 4.3 ISDN Waveform The Line Coding technique used by the BRI ISDN is called 2B1Q (2 Binary 1 Quarternary).

42. 04/05/2013Bahman R. Alyaei42 4 Over ADSL The use of ADSL enables a high-speed data service to be carried in addition to telephony over the copper pair of a standard telephone line. Figure on the next slide shows how the Internet is accessed via ADSL. At the subscriber premises, the internal telephone line and the computer are separately connected to the splitter within the ADSL termination equipment.

43. 04/05/2013Bahman R. Alyaei43 Access to the Internet via ADSL

44. 04/05/2013Bahman R. Alyaei44 Continue… This device directs the computer output to the high speed data channel on the line, and similarly in the reverse direction; while enabling the telephone signal to be carried in the normal way on the local line. The ADSL splitter and terminating equipment located at the exchange similarly separate the telephony and high speed data signal. The telephony is carried over a copper pair to the input of the local exchange switch.

45. 04/05/2013Bahman R. Alyaei45 Continue… The data stream carrying the Internet-bound traffic is connected to the DSLAM (digital subscriber line access multiplexer) equipment. DSLAM combines the data from many such lines onto a high speed transmission link to a data network. This network carries the Internet-bound data in an efficient way, interleaving it with data streams from other sources, to the connecting points of the various ISPs in the country. A corresponding process applies in the reverse direction from the Internet to the computer.

46. 04/05/2013Bahman R. Alyaei46 5 Over A Cable Modem Cable TV Networks are able to provide a high speed data link to an ISP over one of the Spare TV Channels carried through the Network. The technique requires a so-called Cable Modem, which converts the output of the Computer into a signal compatible with the TV distribution system. Figure on the next slide shows how the Internet is accessed via Cable Modem.

47. 04/05/2013Bahman R. Alyaei47 Access to the Internet via Cable Modem

48. 04/05/2013Bahman R. Alyaei48 Continue… The computer is connected to a Splitter- like device which segregates the received TV signal from the Broadband Data Stream at the subscriber’s premises, where both are carried as a composite TV signal over the Coaxial Cable from the Street Cabinet. The Street Cabinet therefore performs a similar aggregating function as the DSLAM.

49. 04/05/2013Bahman R. Alyaei49 6 Leased Line Access The private-circuit service network provides point-to-point un-switched links, known as ‘private circuits’ or ‘leased lines’, between business premises (e.g. offices, factories or warehouses). Although the private-circuit service provided to the business customers is non-switched, the network may use automatic digital cross- connection units (DXC). See the figure on the next slide.

50. 04/05/2013Bahman R. Alyaei50 Continue… A Private-Circuit Services Network (Leased Line Access)

51. 04/05/2013Bahman R. Alyaei51 Continue… The DXCs are technically exchange switch blocks but without the exchange call-control system. Connections once set up are held for the duration of the lease of the circuit, typically several years. Such private circuits are used for linking private telephone systems (i.e. voice) or alternatively, linking data terminals and computers (i.e. data) in different business premises.