ISDN

__**Introduction**__


 * Integrated Services Digital Network (ISDN)** is comprised of digital telephony and data-transport services offered by regional telephone carriers. ISDN involves the digitization of the telephone network, which permits voice, data, text, graphics, music, video, and other source material to be transmitted over existing telephone wires. The emergence of ISDN represents an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities. 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 videoconferencing. ISDN is available to much of the USA and in most markets it is priced very comparably to standard analog phone circuits. It can provide speeds of roughly 128,000 bits-per-second over regular phone lines. In practice, most people will be limited to 56,000or 64,000 bits-per-second. Unlike DSL, ISDN can be used to connect to many different locations, one at a time, just like a regular telephone call, as long the other location also has ISDN.

An ISDN is controlled via D Channel, which support 3 layer protocol.The 3 layer are as follows: Layer 1 physical layer frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal),

Figure 1: ISDN Layer 1

The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical-layer frame are used as follows: F-Provides synchronization. L-Adjusts the average bit value. E-Used for contention resolution when several terminals on a passive bus contend for a channel. A-Activates devices. S-Unassigned. B1, B2, and D---Used for user data.

Layer 2 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) and Layer 3 consist of Message and Information Elements.

Figure 2: ISDN Layer 2

Refer the figure 2, 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.



Figure 3 : ISDN Layer 3

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