Layer 1 includes specifications for encoding and framing. Without these specifications, the devices attached
to the network would not know how to send and receive bits across the medium. You need to be aware of
encoding and framing at Layer 1 when configuring an ISDN PRI, because you need to pick between two
options for each when configuring a PRI on a Cisco router. ISDN BRI uses a single encoding scheme and a
single option for framing. Because of this, there are no configuration options for either framing or encoding
in a router.
ISDN PRI in North America is based on a digital T1 circuit. T1 circuits use two different encoding schemes:
Alternate Mark Inversion (AMI) and Binary 8 with Zero Substitution (B8ZS). You will configure one
or the other for a PRI; all you need to do is make the router configuration match what the telephone
company is using. For PRI circuits in Europe, Australia, and other parts of the world that use E1s, the only
choice for line coding is High-Density Bipolar 3 (HDB3).
Framing, at ISDN Layer 1, defines how a device decides which bits are part of which channel. As with
encoding, PRI framing is based on the underlying T1 or E1 specifications. The two T1 framing options
define 24 64-kbps DS0 channels as well as an 8-kbps management channel used by the telephone company.
This gives you a total speed of 1.544 Mbps regardless of which framing methods are used. With E1s,
framing defines 32 64-kbps channels, for a total of 2.048 Mbps, regardless of the type of framing used.
The two options for framing on T1s are Extended Super Frame (ESF) and Super Frame (SF). In most
cases T1s use the newer ESF. For E1s, the line uses CRC-4 framing or the original line framing defined for
E1s.
As soon as the framing details are known, the PRI can assign some channels as B channels and one channel
as the D channel. For PRIs based on T1s, the first 23 DS0 channels are the B channels, and the last DS0
channel is the D channel, giving you 23B+D. With PRIs based on E1 circuits, the D channel is channel 15.
The channels are counted from 0 to 31. Channel 31 is unavailable because it is used for framing overhead.
This leaves channels 0 through 14 and 16 through 30 as the B channels, which results in a total of 30B+D.