T30 Overview A P A C F A E Training培训课件.ppt

Phase B – Pre-message Procedure The pre-message procedure consists of the identification of capabilities and the commanding of the chosen conditions as well as the confirmation of acceptable conditions. i.e. Exchange ID; Negotiate transmit protocol and speed; Training(channel probing) DIS-digital identification signal(capabilities identification) DCS-digital command signal(the response of DIS, indicate what capability will been used in the transmit) CFR-confirmation to receive(the training is successful) FTT-failure to train(the training is failed) (CSI)-called subscriber identification (NSF)-non-standard facilities 11) preamble + (NSF)+(CSI) + DIS Low speed data. Binary @ HDLC frame structure @ V.21:300bps(FSK). Preamble The preamble shall precede all binary coded signalling whenever a new transmission of information begins in any direction (i.e. for each line turnaround). This preamble assures that all elements of the communication channel (e.g. echo suppressors) are properly conditioned so that the subsequent data may be passed unimpaired. This preamble shall be a series of flag sequences for 1 s +/- 15%. Flag sequence Format: 0111 1110 (in short: 1sec 0x7E) 11) preamble + (NSF)+(CSI) + DIS (CSI) - Called Subscriber Identification Optional frame (non-terminal) that provides the specific identity of the called subscriber by its international phone number . FCF(Facsimile Control Field) format - 0000 0010 . FIF(Facsimile Information Field) length - 20 bytes (NSF) -Non-Standard Facilities Optional frame (non-terminal) that may be used to identify specific user requirements which are not covered by T.30 number . FCF format - 0000 0100 . FIF length - min 3 bytes (the only standard portion of NSF) DIS - Digital Identification Signal Mandatory frame (terminal frame) that characterizes the capabilities of the called terminal . FCF format: 0000 0001 . FIF length - min 3 bytes (basic capabilities), 10 bytes defined as of 1996 (op