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+ +	4.1		+ ISDN Concepts
	+ 4.1.3		+ ISDN 3-layer model and + protocols

+ + + + + + +

+ + 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: +

+ + + The ISDN network layer is defined in ITU-T + Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These + standards specify user-to-user, circuit-switched, and packet-switched + connections.

+ + + 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
+ +
+
+
+
+

+ + + +

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