Channel ID

The next IE you will see in an output is the channel ID. The channel ID (as with all of the output in a Q.931 debug), is listed in hexadecimal format, as shown in Example 8-3.

Example 8-3 Q.931 Call Setup Showing the Channel ID Value

TX -> SETUP pd = 8 callref = 0x04 Bearer Capability i = 0x8890 Channel ID i = 0x83

Called Party Number i = 0x80, '9195551212' RX <- CALL_PROC pd = 8 callref = 0x84

Channel ID i = 0x89 RX <- CONNECT pd = 8 callref = 0x84 TX -> CONNECT_ACK pd = 8 callref = 0x04____

NOTE

Cisco IOS will default to a descending B channel selection order.

Figure 8-18 shows the actual channel ID IE.

Figure 8-18 Channel ID IE Format

Binary Identification of a Channel ID IE

Octet 1

Length of IE Content

Interface ID

Interface Type

Spare

D-Channel Indicator

Channel Selection

Interface Identifier

Coding Standard

Number Map

Channel Type

Channel Number/Slot Map

Bit 8

The purpose of this IE is to identify which channel is for bearer services that the signaling is controlling. It is possible to send multiple channels during the call setup phase to provide a selection in the event that a channel is unavailable at the call destination.

Within the message format there are several sections that allow for the channel selection to occur:

• Octet 1 = This is the binary representation of the Channel ID IE. LSB first, it reads 00011000.

• Octet 2 = Indicates the actual length of the channel identification.

• Octet 3 = The last four octets are generally considered to be associated with octet 3:

— The information channel selection identifies which B-Channel(s) is supposed to be used with an ISDN BRI circuit.

— The D channel indicator specifies whether the requested channel is actually the D channel. If it is, this field is flagged as a 1.

— The P/E field states whether the channel specified is preferred or exclusive. If it is exclusive, no other channels can be used.

— The interface specifies whether the circuit is BRI or PRI. BRI is indicated with a 0 and PRI is indicated with a 1.

— Interface ID is associated with ISDN non-facility associated signaling (NFAS) circuits.

— The channel type specifies if the channel is a B channel or an H channel (used with B-ISDN).

— The number/map bit identifies whether the following octet has a channel number (BRI) or a channel map (PRI).

— The final octet specifies the B channel associated with the PRI circuit. The value identified equals the timeslot on the circuit.

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