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Integrated Services Digital Network
Integrated Services Digital Network (ISDN) refers to a set of digital services that are becoming available to end users. ISDN involves the digitization of the telephone network so that voice, data, text, graphics, music, video, and other source material can be provided to end users from a single end-user terminal over existing telephone wiring. Proponents of ISDN imagine a worldwide network much like the present telephone network, except that digital transmission is used and a variety of new services are available.
ISDN is an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities. Standardizing subscriber services attempts to ensure a level of international compatibility. Standardizing the user/network interface stimulates development and marketing of these interfaces by third-party manufacturers. Standardizing network and internetwork capabilities helps achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another.
ISDN applications include high-speed image applications (such as Group IV facsimile), additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and video conferencing. Voice, of course, will also be a popular application for ISDN.
Many carriers are beginning to offer ISDN under tariff. In North America, large local-exchange carriers (LECs) are beginning to provide ISDN service as an alternative to the T1 connections (digital carrier facilities provided by telephone companies) that currently carry bulk wide-area telephone service (WATS) services.
ISDN components include terminals, terminal adapters (TAs), network-termination devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a terminal adapter. The ISDN TA can either be a stand-alone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, and V.35.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device, typically found in digital private branch exchanges (PBXs), that performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists; it is a single device that combines the functions of an NT1 and an NT2.
A number of reference points are specified in ISDN. These reference points define logical interfaces between functional groupings such as TAs and NT1s. ISDN reference points include the following:
A sample ISDN configuration is shown in Figure 10-1. This figure shows three devices attached to an ISDN switch at the central office. Two of these devices are ISDN-compatible, so they can be attached through an S reference point to NT2 devices. The third device (a standard, non-ISDN telephone) attaches through the R reference point to a TA. Any of these devices could also attach to a NT1/2 device, which would replace both the NT1 and the NT2. And, although they are not shown, similar user stations are attached to the far right ISDN switch.
Figure 10-1 : Sample ISDN Configuration
The ISDN Basic Rate Interface (BRI) service offers two B channels and one D channel (2B+D). BRI B-channel service operates at 64 kbps and is meant to carry user data; BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances. The D channel signaling protocol comprises Layers 1 through 3 of the OSI reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps. The BRI physical layer specification is International Telecommunication Union Telecommunication Standardization Sector (ITU-T) (formerly the Consultative Committee for International Telegraph and Telephone [CCITT]) I.430.
ISDN Primary Rate Interface (PRI) service offers 23 B channels and one D channel in North America and Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs at 64 kbps). ISDN PRI in Europe, Australia, and other parts of the world provides 30 B plus one 64-kbps D channel and a total interface rate of 2.048 Mbps. The PRI physical-layer specification is ITU-T I.431.
ISDN physical-layer (Layer 1) frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal). Both physical-layer interfaces are shown in Figure 10-2.
Figure 10-2 : ISDN Physical-Layer Frame Formats
The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical-layer frame are used as follows:
Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions can result if two terminals transmit simultaneously. ISDN therefore provides features to determine link contention. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit position. The TE expects the next E bit to be the same as its last transmitted D bit.
Terminals cannot transmit into the D channel unless they first detect a specific number of ones (indicating "no signal") corresponding to a preestablished priority. If the TE detects a bit in the echo (E) channel that is different from its D bits, it must stop transmitting immediately. This simple technique ensures that only one terminal can transmit its D message at one time. After successful D message transmission, the terminal has its priority reduced by requiring it to detect more continuous ones before transmitting. Terminals cannot raise their priority until all other devices on the same line have had an opportunity to send a D message. Telephone connections have higher priority than all other services, and signaling information has a higher priority than nonsignaling information.
Layer 2 of the ISDN signaling protocol is Link Access Procedure, D channel, also known as LAPD. LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB) (see Chapter 11, "Synchronous Data Link Control and Derivatives," and Chapter 12, "X.25," for more information on these protocols). As the expansion of the LAPD acronym indicates, it is used across the D channel to ensure that control and signaling information flows and is received properly. The LAPD frame format (see Figure 10-3) is very similar to that of HDLC and, like HDLC, LAPD uses supervisory, information, and unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.
Figure 10-3 : LAPD Frame Format
The LAPD flag and control fields are identical to those of HDLC. The LAPD address field can be either one or two bytes long. If the extended address bit of the first byte is set, the address is one byte; if it is not set, the address is two bytes. The first address field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The
C/R bit indicates whether the frame contains a command or a response. The terminal end-point identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all ones indicates a broadcast.
Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly CCITT) I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together, these protocols support user-to-user, circuit-switched, and packet-switched connections. A variety of call establishment, call termination, information, and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT. These messages are functionally similar to those provided by the X.25 protocol (see Chapter 12, "X.25," for more information). Figure 10-4, from ITU-T I.451, shows the typical stages of an ISDN circuit-switched call.
Figure 10-4 : ISDN Circuit-Switched Call Stages
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