CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- ISDN PRIs are often being used for connecting PSTN circuits to Private Branch Exchange
(PBX) systems, which are telephone exchanges that serving small offices.
- Physical layer always includes some specifications about encoding and framing to allow network
devices to send and receive bits over the media. The details normally can be ignored. However,
some understanding on encoding and framing is important for ISDN PRI configuration, as when
configuring PRI connections on Cisco routers, the encoding and framing information and options
(as provided by the Telco) must be configured correctly for proper operation.
- PRI Encoding In physical layer, encoding defines how to represent 1s and 0s with different
energy levels, eg: +5 volt = 1 and –5 volt = 0.
The available encoding schemes for T1 ISDN PRI are Alternate Mark Inversion (AMI) and
Binary 8 with Zero Substitution / Bipolar 8 with Zero Substitution (B8ZS).
The only encoding scheme for E1 ISDN PRI is High-Density Bipolar 3 (HDB3).
- PRI Framing PRI lines send and receive a serial stream of bits. In ISDN physical layer,
framing defines how to interpret a serial stream of bits to identify the individual component
channels of a bit stream – this bit is part of the D channel, or the 1st B channel, or …
The available framing formats for T1 ISDN PRI are Super Frame (SF) and Extended Super
Frame (ESF). SF is the older format.
The available framing formats for E1 ISDN PRI are CRC4 and NO-CRC4.
Encoding AMI and B8ZS (T1). HDB3 (E1).
Framing SF and ESF (T1). CRC4 and NO-CRC4 (E1).
- Alternate Mark Inversion (AMI) is a T1 encoding scheme that encodes 0s as 01 during each bit
cell, and 1s as 11 or 00 alternately during each bit cell. The sending device must maintain ones
density but not independently of the data stream. It is contrast with B8ZS.
Binary 8 with Zero Substitution (B8ZS) is another T1 encoding scheme which is being
interpreted at the remote end of a connection. It uses a special substitution code whenever 8
consecutive 0s are transmitted over a T1 circuit. It is contrast with AMI.
- T1 = 24 different 64kbps DS0 channels + a 8kbps management channel (1.544Mbps)
E1 = 32 different 64kbps DS0 channels (2.048Mbps)
T1 = 23B+1D. E1 = 30B+1D + 1 used for synchronization.
- ISDN BRI uses a single encoding scheme (2B+D) and a single format for framing. Hence, it has
no encoding and framing configuration options as in PRI.
- ISDN E channel (Echo channel) is a 64kbps control channel used for circuit switching.
Information of this channel can be found in the 1984 ITU-T ISDN specification, but it was
dropped since the 1988 version of the specification.
- ISDN H channel (High-speed channel) is a full-duplex ISDN primary rate channel which
performs the same function as B channel, but operates at rates exceeding DS0 (64kbps). Below
shows its implementations and the corresponding transmission rates:
H0 384kbps (6 B channels)
H10 1472kbps (23 B channels)
H11 1536kbps (24 B channels)
H12 1920kbps (30 B channels)
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com