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ISDN ( INTEGRATED SERVICES DIGITAL NETWORK) INTRODUCTION: ISDN was one of the largest and most ambitious projects launched in the mid 1980’s. Rapid development in Semiconductor, Computer and Communication Technologies were leading to a merger of various fields and, increasingly same digital techniques and digital equipment were being employed for data, voice and video transmission. This led to the concept of ISDN- a world wide single digital telecommunication network that can provide all these services over the existing telephone twisted pair wire ISDN had following objectives: A world wide all digital Network It will provide access to both leased and dial-up telephone services It will be governed by a single set of standards It will be a natural evolution of the existing Public switched Telephone Network (PSTN) and will take a decade or more to be fully implemented It will provide a wide variety of services including voice, data, video, monitoring etc, using a common outlet ( RJ-45 connector) similar to telephone jack thereby providing portability of equipment and applications. ISDN was designed to provide a wide variety of Telecommunication Services through an all digital network including Access to dial up and leased telephone lines Data transmission capability via access to Packet Switching Network Video Conferencing and Videotext Alarm monitoring Telemetry Teletext Fax All modern Telephone services such as Call Forwarding, Call Waiting, Caller ID, Call Transfer, Conference Call, Credit Call Calling and so on It is instructive to note that despite of all the hype about ISDN, it has yet to achieve the universal deployment it hoped for because of cost factor. AT & T and MCI did try to push National ISDN aggressively a few years back. However, it is fading away in favor of more recent developments. Still, there a re a lot of installations in Europe and North America with ISDN deployed and a basic knowledge of ISDN operation is necessary in order to gain an appreciation of more recent high-speed technologies. We shall look at the basic features of ISDN and move on to more advanced Broadband technologies. Hyder Khoja Page 1 4/30/2017 ISDN CHARACTERISTICS AND ARCHITECTURE The ISDN is an all-digital Network, which means information is carried in digital form right from the customer premises to destination including the local loop. Also important to note is that it uses existing twisted pair telephone wiring. This implies that the conversion from analog to digital is performed in the Telephone set. ISDN Provides both the Circuit Mode and Packet Mode Services ISDN Allows multitasking capabilities over multiple channels, all over a single pair of twisted wire and over the existing local loop.. This allows a user to (i) Simultaneously make a telephone call and receive another one on the same line. (ii) Carry out a telephone conversation on one channel and transmit data on the 2nd channel at the same time on a single link This multitasking capability of ISDN allows it to provide Integrated Services Simultaneously including voice, data, video etc For example, ISDN allows a researcher to have telephone conversation with his assistant from a remote location, see him on the screen and also discuss a document that appears in a separate window of the screen or a specialist in North America to discuss an X-RAY of a patient some where in Asia with his family Doctor These kinds of applications require very High BW, Sophisticated equipment and complex Communication Technologies. ISDN uses complex time Division Multiplexing schemes to multiplex several services on a single link and sophisticated signaling Protocols to control the link ISDN uses a separate Packet mode channel for Signaling functions. This channel can also be used for special services such as Telemetry and Alarm Monitoring because of the intermittent nature of these services. National ISDN called ISDN-1 provides Standards for Vendor Interoperability. Broadband ISDN provides Standards for very High Speed Networking with technologies such as ATM and SONET. ISDN INTERFACES /SERVICES AND CHANNELS: ISDN defines two types of Interfaces to provide the services mentioned above. (a) A BASIC RATE INTERFACE commonly known as BRI (b) A PRIMARY RATE INTERFACE commonly known as PRI. BASIC RATE INTERFACE (BRI): A BRI consists of 2 Bearer channels called BChannels each running at 64Kbps and used to carry Voice or Data ( in Circuit Mode and Packet Mode ). And a CONTROL channel Called D- channel running at 16 Kbps used primarily for Signaling and Control. The D channel only operates in the Packet Mode. Notice also that the D-Channel can also be used for special services such as telemetry and alarm monitoring when not used by B-channel This set up is popularly known as 2B+D running at a combined rate of 144Kbps as shown in the Diagram below. Draw the diagram in the space provided. Hyder Khoja Page 2 4/30/2017 PRIMARY RATE SERVICE (PRI): (a) Provides 23 B channels and a D- Channel each running at 64Kbps (b) B-Channels are used for to carry voice or data and can operate in either the circuit mode or packet mode. © D- Channel operates only in the packet mode and is used for Call set-up and Clearing, Signaling and control of B-Channels on time multiplexed basis (d) D- Channel can also be used for special services such as Telemetry, file transfer, email, fax etc when idle (e) This set-up is commonly known as (23B+D) and offers a combined rate of 1.544 Mbps, which coincides with the T1 or DS1 rate. Thus ISDN was designed to be compatible with all existing technological infrastructure in order to allow a natural evolution. Fig below shows BRI applications. Draw these pictures in the space provided. (Refer to the pictures in your notes written for Telecommunications Institute). Fig below shows PRI applications. Notice that prior to implementing ISDN, a large business organization may have been using separate connections for the various services such as : A bunch of analog trunks for its switched public Telephone requirements A separate analog Tie trunk for leased lines A separate Packet mode line for Data A separate Centrex line etc With ISDN implementation ( as shown in the fig), a business organization gets the same range of services through an integrated digital access. MegaLink PRI is providing the access to various Networks such as PSTN, Public Switched Data networks (such as FR or ATM) and other services Notice also that with ISDN, BW can be shared between applications, for example, if outgoing channels are idle, they can be used to carry in coming traffic and vice versa. Common PRI CPES include LAN Servers, HOSTS, Concentrators and PBXS Hyder Khoja Page 3 4/30/2017 The Table shows ISDN channels and their applications ( HIOKI) H-channels are used for High Speed transmissions such as High Quality Audio, Teleconferencing, Video conferencing and high Speed Data ISDN INTERFACES, FUNCTIONAL GROUPINGS AND REFERENCEPOINTS: A key element of ISDN is a small set of compatible multipurpose user to Network Interfaces developed to support a wide range of applications. Fig below shows a complete ISDN Network Architecture. Draw the picture in the space provided. As shown in the fig, the ISDN reference configuration consists of functional groupings and reference points at which physical interfaces may exist. The functional groupings are a set of functions that may be required at an Interface, whereas, the reference points are employed to divide the groupings into distinct entities. Following functional groupings have been defined: TE ( Terminal Equipment): TE1 and TE2 NT (Network Termination): NT1 and NT2 Reference Points: R, S, T, U We shall now consider each one of these functional groupings and Reference points separately and try to understand what they really mean. TE FUNCTIONAL GROUPING: This simply means what kind of user equipment can be connected to an ISDN Network. ISDN defines two kinds-TE1 and TE2 TE1: These are ISDN compatible equipment with 4-wire digital interface built in. These Equipment can be connected directly to an ISDN Network with out requiring any physical or Protocol conversion. These include ISDN digital telephones, data terminals and integrated voice and data workstations. They connect directly to the “S” interface. Thus “S” interface in ISDN compatible Interface. TE2: These are non-ISDN compatible equipment such as regular analog telephone and workstations that use RS-232, 422, 423, V.35 interfaces. These equipment must be connected through a device called TERMINAL ADAPTER TA . The TA performs physical and protocol conversions to enable regular devices to communicate over the ISDN Network. The Interface between the TE2and TA is called “R” interface. Thus “R” stands for any interface that is not compatible with “s” interface. The function of TA is to convert “R” interface to “S” interface. NT1: (Network termination 1): is a device that makes the 4-Wire ISDN internal wiring compatible with the 2-Wire local loop. It performs physical layer ( Layer 1) functions such as physical and electrical termination of the loop, line monitoring, timing and bit multiplexing etc. It interfaces the CPE 4-Wire digital ISDN wiring to the 2-Wire ISDN Network Subscriber’s loop. A BRI connection would need an NT1. Hyder Khoja Page 4 4/30/2017 NT2: (Network Termination 2): include devices that perform switching and data concentration functions equivalent to layer1, 2, and 3 of the OSI model ( physical, data link, and Network). These include PBXS, LAN Servers, Terminal controllers, concentrators, Multiplexers etc. Typically a PRI line would require an NT2 device. This interface is called a “T” interface. This device may be just an extra card installed in a PBX or a Multiplexer or a LAN Server. In North America, a 3rd party and not the communication Carrier provide this device. Notice that the point between NT2 and NT1 is called the “T” interface. For BRI service, NT2 wouldn’t exist. In that case the point between the TE and NT1 is called an S/T interface. Fig shows that NT1 interfaces 4-Wire digital wiring of the customer to 2-Wire street wiring. This 2-Wire loop terminates at the Local Termination (LT) inside the Local Exchange (LE). The Communication Carrier will provide NT1. The Street wiring is called the “U” interface. Fig shows an interface point “V” inside the ISDN local exchange, between LT and ET. ET is called the Exchange Termination. The LT complements the functions of NT1 on the LE side. ET is the ISDN circuit mode Switch and PH (Packet Handler) is the ISDN packet Switch which interfaces with the public data switching Network such as Frame Relay or ATM or X.25 etc. The ISDN BRI Physical Layer Protocol : Defines the physical interfaces, electrical connector, data encoding scheme, bit timing and synchronization techniques, signaling capabilities for activation and deactivation of terminals as well as procedures to gain an orderly access to D-channel The BRI has two modes of operation- point –to-point and multipoint. In point – to – point operation the maximum distance between the TE and NT is 1000 meters. The BRI is a serial, Synchronous, full duplex connection. The BRI electrical interface between the NT and TE is an 8 pin interface called RJ-45 connector similar to our telephone jack. The fig shows this connector as well as the pin layout and pin function ISDN BRI runs at 192 kbps. But the user BW is only 144 kbps. Rest of the BW is used for framing. ISDN BRI uses PSEUDO-TERNARY line encoding over the S/T interface between TE and NT. This Scheme is shown below. (Draw picture in the space below). A “1” is coded as 0V and a “0” is coded as +1 or –1 alternately. Why? __________________________ The ISDN BRI uses Two Binary one Quaternary (2B1Q) encoding over the “u” interface (that is over the street wiring) between the NT and LE. As shown in the fig below. Draw fig in the space provided. It encodes two bits as follows: 00= -3V, 01= -1V, 10= +3V, 11= +1V. Encode the bit pattern 100111100010101 in the space below. Hyder Khoja Page 5 4/30/2017 It is a FDX, 160Kbps, 4 Wire, synchronous link between an NT and LE. It uses echo cancellers to provide FDX operation. User BW is still 144kbps. Maximum distance between NT and LE is 18KM. ISDN PRI physical layer protocol: The PRI provides a point to point connection between the ISDN Network and a device that requires a large amount of BW such as a PBX, Communication controller, a LAN Server, a concentrator or Multiplexer and Video conferencing system. PRI provides 23-B channels for user application and a D- channel for control. In north America, PRI operates at 1.544MBPS and can be carried over a T1 circuit. Thus T1 is the physical layer of PRI. Frame structure is exactly the same as T1 line-193 bit frame including the framing bit (248 +1) transmitted 8000 times per second. Frame duration is 125 microseconds Uses AMI- B8ZS encoding. Notice that NT1 functions are built in and therefore a PRI typically stops at NT2. It is usually a card inserted into the devices mentioned above. Since 23 channels share the PRI trunk BW, ISDN defines layer 1,2,3 functions for the operation of the D channel in order to provide control of these devices. We shall look at the layer 2 and layer 3 functions of the D channel in the next section. Remember that both BRI and PRI - B channels require only physical layer services, but D- channel (BRI and PRI both) require layer 1,2 and 3 services. In Europe, PRI is transmitted over E lines at 2.048 MBPS with 32 and provides 32 channels. Time slot “0” is used for error control and frame alignment and time slot 16 is used for D-channel operation. ISDN Data link Control Protocol for D-channel: ISDN uses LAP-D (Link Access Procedure for D Channel) to provide Data link services. Lap-D is very similar to HDLC studied in earlier semester and so it will not be elaborated here. Please consult your notes for Principles of Telecom course. ISDN Network Layer Protocol for the D channel: ISDN network layer services are used for call establishment and termination and to control the call in progress. It uses SS#7 call control messages as shown in the table below. The procedure is similar to X.25 and TCP network layer functions. Study the table shown. It is called ISUP (ISDN User Part). Hyder Khoja Page 6 4/30/2017 Hyder Khoja Page 7 4/30/2017