ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and network layers of the OSI reference model:

• The ISDN BRI and PRI physical layer specifications are defined in ITU-T I.430 and I.431, respectively.
• The ISDN data link specification is based on LAPD and is formally specified in the following:
  • ITU-T Q.920
  • ITU-T Q.921
  • ITU-T Q.922
  • ITU-T Q.923

BRI service is provided over a local copper loop that traditionally carries analog phone service. While there is only one physical path for a BRI, there are three separate information paths, 2B+D. Information from the three channels is multiplexed into the one physical path.

ISDN physical layer, or Layer 1, frame formats differ depending on whether the frame is outbound or inbound. If the frame is outbound, it is sent from the terminal to the network. Outbound frames use the TE frame format. If the frame is inbound, it is sent from the network to the terminal. Inbound frames use the NT frame format.

Each frame contains two sample frames each containing the following:

• 8 bits from the B1 channel
• 8 bits from the B2 channel
• 4 bits from the D channel
• 6 bits of overhead

ISDN BRI frames contain 48 bits. Four thousand of these frames are transmitted every second. Each B channel, B1and B2, have a capacity of 8*4000 = 64 kbps, while channel D has a capacity of 4*4000 = 16 kbps. This accounts for 144 kbps of the total ISDN BRI physical interface bit rate of 192 kbps. The remainder of the data rate are the overhead bits that are required for transmission.

The overhead bits of an ISDN physical layer frame are used as follows:

• Framing bit – Provides synchronization
• Load balancing bit – Adjusts the average bit value
• Echo of previous D channel bits – Used for contention resolution when several terminals on a passive bus contend for a channel
• Activation bit – Activates devices
• Spare bit – Unassigned

Note that the physical bit rate for the BRI interface is 48*4000 = 192 kbps. The effective rate is 144 kbps = 64 kbps + 64 kbps + 16 kbps (2B+D).

Layer 2 of the ISDN signaling channel is LAPD. LAPD is similar to HDLC. LAPD is used across the D channel to ensure that control and signaling information is received and flows properly.

The LAPD flag and control fields are identical to those of HDLC. The LAPD address field is 2 bytes long. 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 command/response bit (C/R), indicates whether the frame contains a command or a response. The second byte contains the terminal endpoint identifier (TEI). Each piece of terminal equipment on the customer premises needs a unique identifier. The TEI may be statically assigned at installation, or the switch may dynamically assign it when the equipment is started up. If the TEI is statically assigned during installation, the TEI is a number ranging from 0 to 63. Dynamically assigned TEIs range from 64 to 126. A TEI of 127, or all 1s, indicates a broadcast.

Web Links Integrated Services Digital Netowrk (ISDN) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ isdn.htm