ITU-T V.150.1

48:55 XID/LR field Octet 6: V.44 capability (C0)
56:63 XID/LR field Octet 7: V.44 data compression request (P0)
64:79 XID/LR field Octet 8 and 9: V.44 number of codewords in transmit direction (P1T)
80:95 XID/LR field Octet 10 and 11: V.44 number of codewords in receive direction (P1R)
96:103 XID/LR field Octet 12: V.44 maximum string length in transmit direction (P2T)
104:111 XID/LR field Octet 13: V.44 maximum string length in receive direction (P2R)
112:127 XID/LR field Octet 14 and 15: V.44 length of history in transmit direction (P3T)
128:143 XID/LR field Octet 16 and 17: V.44 length of history in receive direction (P3R)
Notes:
† If protocol support=Yes, the modem will attempt/accept the use of this protocol during V.42 detection
phase (which includes attempting/accepting the “alternate” – Annex A protocol). If protocol support=No, the
modem explicitly does not ever attempt/accept this protocol, and will be unable to operate in this mode. If
protocol support=unknown, the gateway has insufficient knowledge to make a prediction.
‡ If compression support=Yes, an originating modem will propose this compression method during XID/LR
negotiation. A terminating modem will choose one such proposed method during negotiation, if the
terminating modem itself supports that method. If several acceptable compressions were proposed, a
terminating modem must choose only one, and will “prefer” V.44 to V.42 bis (LAPM), or prefer V.42 bis to
MNP5 – if none of the proposed compressions are acceptable, No Compression is chosen.
If all fields in B0 to B15 are “unknown”, the PROF_XCHG message need not be sent, since the
non-reception of this message indicates the gateway’s lack of knowledge.
If compression support=No, the modem will not propose this method (if originating), and will never
make this choice (if terminating). Note that LAPM supports only (V.44, V.42 bis) and Annex
A/V.42 (1996) supports only (V.42 bis, MNP5), regardless of noted compression support; no
compression is supported if there is no protocol.
If compression support=Unknown, the gateway has insufficient knowledge to make a prediction.
If all the protocols that support a compression method are not supported, then the compression
method is not supported and should be notated as such. If all the protocols which support a
compression have unknown support, then the compression’s support is unknown and again should
be notated as such. Finally, repeating from above, if all protocols and compressions have unknown
support, the PROF_XCHG message need not be sent.
If a compression method is not supported or the support is unknown, the corresponding
Compression parameter fields must be set to zero. However, it is permissible to not send trailing
unsupported/unknown compression fields.
15.4.11 User Data (INFO messages)
This section defines the user-data message formats used for Modem Relay.
15.4.11.1 Octet without Formatting (I_OCTET)
If the CONNECT message indicates not to use a DLCI then the format of this messages is shown in
the following figure.
ITU-T Rec. V.150.1 (01/2003) – Prepublished version 44