Bearer Capability
The bearer capability IE identifies which capabilities a call is requesting from the network. This IE must be present in the Setup message or the call fails. This can be one of the largest IEs, ranging from 4 to 12 octets, and it contains information such as the type of service (ToS), rate multiplier (if necessary), user information Layer 1 protocol, and asynchronous control information, if needed (start and stop bits/parity). Figure 8-17 shows the fields that are contained in the bearer capability IE.
The bearer capability IE is no small affair because it is responsible for conveying the actual call characteristics to the called device and negotiates between both devices. Within the IE message, the third octet is most important because it specifies the information transfer capability. This identifies the type of traffic that is present on the call. Table 8-5 lists the information transfer capabilities and their coding formats (listed as bit 1 to bit 5 from right to left).
Figure 8-17 Bearer Capabilities IE Format
8 7 6 5 4 3 2 1
Figure 8-17 Bearer Capabilities IE Format
8 7 6 5 4 3 2 1
|
0 |
Bearer Capability Information Element Identifier 0 0 0 0 1 0 0 |
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|
Length of the Bearer Capability Contents |
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|
Ext. 1 |
Coding Standard |
Information Transfer Capability |
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|
Ext. 1 |
Transfer Mode |
Information Transfer Rate |
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|
Ext. 1 |
Rate Multiplier |
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|
Ext. 0/1 |
Layer 1 Ident 0 1 |
User Information Layer 1 Protocol |
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|
Ext. 0/1 |
Synch/ Asynch |
Negot. |
User Rate |
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|
Ext. 0/1 |
Intermediate Rate |
NIC on Tx |
NIC on Rx |
Flow Control on Tx |
Flow Control on Rx |
Spare 0 |
|
|
Ext. 0/1 |
Hdr/ no Hdr |
Multiframe |
Mode |
LLI Negot. |
Assignor/cc |
In-band Neg |
Spare 0 |
|
Ext. 0/1 |
Number of Stop Bits |
Number of Data Bits |
Parity |
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|
Ext. 1 |
Duplex Mode |
Modem Type |
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|
Ext. 1 |
Layer 2 Indent. 1 0 |
User Information Layer 2 Protocol |
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|
Ext. 0 |
Layer 3 Indent. 1 1 |
User Information Layer 3 Protocol |
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|
0 0 0 |
Additional Layer 3 Protocol Information (Most Significant Bits) |
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|
0 0 0 |
Additional Layer 3 Protocol Information (Most Significant Bits) |
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|
Information Transfer Capability |
Binary Value |
|
Speech |
00000 |
|
Unrestricted Digital Information |
00010 |
|
Restricted Digital Information |
10010 |
|
3.1 kHz Audio |
00001 |
|
Unrestricted Digital Information with Tones |
10001 |
|
Video |
00011 |
In Cisco IOS, the bearer capability is listed as a hexadecimal value, as shown in Example 8-2.
Example 8-2 Q.931 Call Setup Showing the Bearer Capability 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____
In this output, you can see that the bearer capability is 0x8890, which is equal to a 64-kbps data call (unrestricted digital information). Refer to Table 8-6 for a listing of the bearer capability codes and their definitions.
|
Bearer Capability |
Information Transfer Capability |
User Information Layer 1 Protocol |
|
0x8090A2 |
Speech |
G.711 |l-law Speech |
|
0x8090A3 |
Speech |
G.711 A-law |
|
0x9090A2 |
3.1 kHz Audio |
G.711 |l-law Speech |
|
0x9090A3 |
3.1 kHz Audio |
G.711 A-law |
|
0x8890A2 |
Unrestricted digital information |
G.711 |l-law Speech |
|
0x8890A3 |
Unrestricted digital information |
G.711 A-law |
|
0x8890 |
Unrestricted digital information |
64 kbps (64 kbps-data call) |
In the next octet, the transfer information rate specifies how many B channels are used on the call. This value is used in tandem with the rate multiplier. However, if only one B channel is to be used on the call, the rate multiplier is not used. The Layer 1 protocol field in octet 5 identifies the voice companding algorithm. This field is useful in troubleshooting, particularly if it is suspected that there is an A-law |i-law mismatch between devices.
Continue reading here: Channel ID
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