Cisco Cisco IOS Software Release 12.4(11)T

An ISDN interface must be configured for integrated services mode to enable H.320 primary and 
secondary call type checking. Enabling integrated services allows data, voice, and video call traffic to 
occur from a single ISDN BRI or PRI interface. When an interface is in integrated service mode:

ISDN performs call type checking for the incoming call. The call is rejected by ISDN if no voice or 
data dial peer is matched for an incoming call. 

The voice option for the isdn incoming-voice command, which causes all calls to bypass the modem 
and be handled as voice, is not available.

By default, the integrated services option is disabled from the supported interfaces.

An H.320 call consists of 1 to 16 ISDN B-channels. The first B-channel in an H.320 session is the 
primary B-channel and all additional B-channels are handled as secondary B-channels. Secondary 
B-channels are distinguished from primary B-channels by the call number received in the Q.931 ISDN 
setup message. The secondary called numbers for H.320 B-channels can be exchanged between the 
terminals using H.242 format (dynamic method), or can be configured statically (static method). 

An H.320 primary B-channel is different from the secondary B-channels in the following ways: 

A primary B-channel is the first ISDN call made in an H.320 call. 

The primary B-channel always carries voice. Depending on the audio codec selected, the remaining 
available bandwidth is used for video.

The primary B-channel carries the H.221 in-band-signaling. The secondary B-channels also contain 
bit-rate allocation signal (BAS), and only the appropriate values for a secondary leg. For more 
information on values for secondary B-channels, see ITU H.221 Annex A, Table A-5. 

Only the primary call with each H.320 session is passed to the session application. Secondary 
B-channels are handled by the H.320 B-channel aggregator.

Secondary B-channels only provide more B-channels for additional video bandwidth. 

During inbound dial peer matching, the list of H.320 sessions is searched before the incoming voice dial 
peer lookup. If the new called number matches a called number associated with an existing H.320 call 
session (dynamic or static), the leg is added to the existing H.320 call session as a secondary B-channel. 

The B-channel aggregator is responsible for handling call setup of additional B-channels for H.320 calls. 
It also allocates dynamic called numbers from the voice class called number pool to the gateway and 
frees them back up again.