ISDN Signaling
ISDN uses Q.921 as its Band 2 signaling agreement and Q.931 as its Band 3 signaling
protocol.
Q.921
Layer 2 of the ISDN signaling agreement is Link Admission Procedure, D approach (LAPD).
LAPD is agnate to High-Level Abstracts Link Ascendancy (HDLC) and Link Admission Procedure,
Balanced (LAPB). As the amplification of the LAPD acronym indicates, this band is used
across the D approach to ensure that ascendancy and signaling advice flows and is
received properly. The LAPD anatomy architecture is actual agnate to that of HDLC. Like HDLC,
LAPD uses authoritative advice and unnumbered frames. The LAPD agreement is formally
specified in ITU-T Q.920 and ITU-T Q.921. The Terminal Endpoint Identifier (TEI)
field identifies either a distinct terminal or assorted terminals. A TEI of all 1s indicates a
broadcast.
Q.931
Two Band 3 blueprint are acclimated for ISDN signaling: ITU-T I.450 (also accepted as ITUT
Q.930) and ITU-T I.451 (also accepted as ITU-T Q.931). Together, these protocols support
user-to-user, circuit-switched (the B channels), and packet-switched (the D channel) connections.
A array of call-establishment, call-termination, information, and miscellaneous
messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION,
CANCEL, STATUS, and DISCONNECT. These letters are functionally similar
to those provided by the X.25 protocol.
Because ISDN bulletin types ability admission the activity of a BRI or PRI block configuration,
you should appraise the letters that are allotment of the Q.931 packet anatomy and
see how ISDN carries out the signaling function. Figure 4-7 illustrates the architecture of an
ISDN frame.
Chapter 4: Performing Alarm Signaling over Digital Voice Ports 195
IEs as Required
0 Bulletin Type
Flag Alarm Advertence Value
0 0 0 0 Breadth of Advertence Alarm Value
Protocol Discriminator
8 n 7 6 5 4 3 2 1
Figure 4-7 ISDN Anatomy Format
ISDN signaling takes abode in the D approach and uses a message-oriented agreement that
supports alarm ascendancy signaling and packet data. In its role as arresting carrier for the B channels,
the D approach directs the CO about-face to accelerate admission calls to accurate time slots
on the Cisco admission server or router.
Following are the apparatus of the ISDN anatomy that address these instructions:
■ Agreement discriminator: This is the agreement acclimated to encode the butt of the
layer.
■ Breadth of alarm advertence value: This defines the breadth of the abutting field. The alarm reference
might be one or two bytes (octet) long, depending on the admeasurement of the value
being encoded.
■ Flag: This is set to aught (0) for letters beatific by the affair that allocated the alarm reference
value. Otherwise, it is set to one (1).
■ Alarm advertence value: This is an approximate amount that is allocated for the continuance of
the specific session. This amount identifies the alarm amid the accessory advancement the
call and the ISDN switch.
■ Bulletin type: This identifies the bulletin blazon (for example, SETUP) that determines
what added advice is appropriate and allowed. The bulletin blazon might
be one or added octets. Back there is added than one octet, the aboriginal octet is coded as
eight 0s.
■ ISDN Advice Element (IE): Most D-channel letters accommodate added information
needed for alarm processing, such as the calling affair number, alleged party
number, and CID. The added advice in a bulletin is anesthetized in information
elements.
ISDN sends instructions in Band 3 letters that are put into Band 2 frames and are
finally time-multiplexed assimilate a average with either a BRI or a PRI Band 1 line-coding
specification.
A delineation of D-channel letters is apparent in Figure 4-8. These letters acquiesce complete
control over alarm enactment and clearing, arrangement maintenance, and the passing
of added call-related advice amid switches.
The added advice appropriate by an ISDN bulletin is anesthetized in IEs and varies
depending on the bulletin type, the activity actuality performed, and the affiliated equipment.
Mandatory and alternative IEs for D-channel letters are authentic in ITU-T Q.931.
An IE can be a distinct byte or several bytes, and by account the message, the about-face can
determine this information. For example, in octet 1 of the IE, if bit 8, or the extension
bit, is 0, the IE is of a capricious length. If the bit is 1, the IE is a distinct byte.
The advice independent in octet 3 is the coding accepted and the location. Tables 4-3
and 4-4 accommodate the accessible agreeable of these fields.
196 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Figure 4-8 ISDN Agreement Stack
Table 4-3 Coding Standard
Bit Sequence Meaning
00 ITU connected coding
11 Accepted specific to the area field
Table 4-4 Location
Bit Sequence Meaning
0000 User
0001 Clandestine arrangement confined the bounded user
0010 Public arrangement confined the bounded user
0011 Transit network
0100 Public arrangement confined the alien user
0101 Alien clandestine network
0111 All-embracing network
1010 Arrangement above the interworking point
Chapter 4: Performing Alarm Signaling over Digital Voice Ports 197
IEs as Required
0 Bulletin Type
Flag CRV
0 0 0 0 Breadth of CRV
Protocol Discriminator
8 7 6 5 4 3 2 1
The ISDN Agreement Stack
4
3
2
1
Octet
0 Location
IEs (Multiple Bytes)
Coding Standard
1
1
Length of IEs
0 IE Identifier
8 7 6 5 4 3 2 1
Typical Architecture of a Variable-Length IE:
A alleged cardinal is anesthetized to the PSTN by an IE. The IE contains bytes anecdotic the
numbering plan and the blazon of number. Typically, the calculation blazon is not changed.
However, there ability be times back a arrangement ambassador elects to accept a specific
gateway handle all all-embracing calls. If this affiliation to the PSTN is an ISDN PRI, the
IE charge acquaint the PSTN that the alleged cardinal is in all-embracing format.