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138 Chapter 6: Using ISDN and DDR Technologies

SPID requirements are dependent on both the software revision and the switch. Many switch
manufacturers are moving away from SPIDs, as they have already done in Europe. SPIDs are
required only in the U.S., and then are used only by certain switches. 5ESS, DMS-100, and NI-1
support the use of SPIDs; however, it is not necessary to configure them unless it is required by
the LEC.

ISDN Layer 3

ISDN Layer 3 does not impose the use of any network layer protocol for the B channels. The
use of the D channel is defined in Q.931 and specified in ITU I.451 and Q.931 + Q.932.

Q.931 is used between the TE and the local ISDN switch. Inside the ISDN network, the
Signalling System 7 (SS7) Internal Signalling Utility Protocol (ISUP) is used. Link Access
Procedure on the D channel (LAPD) is the ISDN data link layer protocol for the D channel. The
data link protocol for the B channel, however, can be any of the available protocols because the
information can be passed transparently to the remote party. HDLC, PPP, or Frame Relay
encapsulations can be used to pass data over the B channel.

LAPD

As mentioned, LAPD is the data link layer protocol for the D channel. It defines the framing
characteristics for payload transmission, as illustrated in Figure 6-6.

Figure 6-6

The LAPD Frame

The following list defines the subfields of the Address portion of the LAPB frame.

SAPI--Service access point identifier (6 bits)

C/R--Command/response bit (1 bit)

EA--Extended addressing bits

TEI--Terminal Endpoint Identifier

Flag

Address

SAPI
6 bits

C/R

1 bit

TEI

7 bits

1 bit

Ctrl

Data

FCS

Flag

Variable