ITU-T V.150.1

There are two phases in the process to determine the configuration of the TCX. The first phase is
the capability exchange phase. This occurs during the call set-up. These messages are defined in
clause 15.2.2 and are exchanged between the On-Ramp and Off-Ramp gateways. The gateways
declare whether they are of the None, Single or Double Trans-Compression types.
For the case where both G1 and G2 declare they are Double Trans-Compression Gateways, their
initial configuration can be Single-to-Single configuration or if both Gateway’s indicate their desire
they can mutually choose the Symmetric Double Trans-Compression configuration. This option is
indicated as part of the Gateways compression capability message during the call set-up phase (See
clause 15.2.10).
As a result of the Trans-Compression capability exchange the gateways shall select the mode of
operation as defined in Table 1.
TABLE 1/V.150.1
Trans-Compression Mode Selection
Trans-Compression Mode
Capabilities Exchanged
Resultant Trans-Compression Mode
Selected by Gateways
G1 G2 G1 G2
No No No No
No Single No No
No Double No Double
Single No No No
Single Single Single Single
Single Double Single Single
Double No Double None
Double Single Single Single
Double Double Single/Double* Single/Double*
* Note: If both Gateways indicate their ability and desire to support
Double-Double configuration it shall do so, otherwise they shall use the
Single-Single configuration
13.6 XID/LR Profiles for use in Modem Relay Connection Scenario MR1
Modem relay connection Scenario MR1 requires both M1 and M2 to negotiate identical
compression parameters. This section defines an optional method to provide the ability to predict
trans-compression parameters based upon knowledge gained from a previous connection or by
other means. This will allow the Gateways to optimize the protocol XID/LR exchange for improved
connect time, compression efficiency, and connectivity.
The gateways may exchange their knowledge as described in 22.2.5, and use this information to
agree on optimal and consistent XID/LR command and response sequences that each will send
when the particular physical connection comes up.
The advantage of using this prior knowledge is that an actual synchronous end-to-end exchange is
not necessary, thus minimizing the timing problems associated with such exchanges.
ITU-T Rec. V.150.1 (01/2003) – Prepublished version 22