This Recommendation describes a standardized method for automatic mode negotiation, detection of bit alignment, and confirmation of subchannel connectivity for multimedia terminals on digital networks. This new protocol provides backward compatibility with existing standards and creates an extensible mechanism to negotiate future protocols.
The two primary benefits from implementing this Recommendation are:
1) improved reliability in completing calls and successfully establishing multimedia communications, since peculiar network characteristics (such as "restricted" networks) that interfere with call establishment and protocol negotiations are automatically handled by V.140 procedures; and
2) for terminals that support multiple communication modes, an automatic means of selecting modes.
Principal features of this Recommendation include the ability to find bit alignment, detect remote network type, and perform in-band testing of channel characteristics; a flexible and extensible capability exchange and mode selection facility is also built-in.
V.140 procedures apply to every channel of a multi-channel call and begin following establishment of the end-to-end digital connection and before any multimedia or other communication protocols are initiated. The procedures are divided into three phases:
• Phase 1 – Send/search for V.140 signature (note that V.8/V.8 bis, voice, and H.221 FAS, or any subset of these may besimultaneously transmitted). If such signature is detected, proceed to:
• Phase 2 – Characterize the digital connection (64 vs 56 kbit/s, detect octet/septet alignment), and diagnose any odd characteristics of the network, (i.e. it could be restricted, that is, it transfers only 7 of 8 bits to the far-end). Once this is complete,
• Phase 3 – Exchange mode capabilities (similar to V.8 bis) and select desired operation mode. Modes can include voice communications, multimedia communications, and channel aggregation protocols, but capabilities are kept simple, given that the purpose of this Recommendation is only to select a particular protocol, not to determine all of the parameters related to that protocol (which can typically be determined using the protocol itself).
The Phase 1 procedures have been designed to allow simultaneously signalling of other protocols, such as H.320, to minimize the amount of time lost in beginning communications if a terminal that implements V.140 finds itself communicating with a terminal that does not.
Following Phase 3, a terminal can immediately begin procedures associated with the selected operation mode.