以前都是代码为王，主要在于实现功能，没有怎么仔细研究传真协议，对于我来说，就是发送AT指令，判断回应，这次又要做CLASS2的传真协议。所以就看看各种区别。

首先FCLASS? =?查询MODEM，可以得到如下返回值

0：数据模式；1：CLASS1传真；2：CLASS2传真；2.0：CLASS2.0传真

T系列建议 远程信息处理业务的终端设备和协议

T.30
Group 3 Facsimile Protocol
   
VOCAL Technologies software library includes software for complete Group 3 facsimile control. The basic requirements for Group 3 facsimile refer to V.21 channel 2 (300 bps half-duplex modulation), V.27ter (2400 bps and 4800 bps half duplex modulation), T.4 1-dimensional Huffman coding of images and T.30 as the underlying control procedure. The image is sent with a modulation from the "High Speed" set of modulations, V.27ter as the minimum required support.
Other Group 3 facsimile "high speed" modulations include V.29 (7200 bps and 9600 bps) and V.17 (7200 bps, 9600 bps, 12000 bps and 14400 bps). (Although V.29 specifies a 4800 bps modulation, it is not relevant to Group 3 facsimile.) V.33 is no longer specified as an option under T.30.

T.30 specifies an additional 14 modulation rates between 2400 bps and 33600 bps (in increments of 2400 bps) that can be used from the V.34 modulation. This is referred to as V.34fax. As with V.34 data modulation procedures, V.8 startup is required. V.34fax includes procedures for full and half duplex data channel operation; half duplex is mandatory for V.34fax support.

In non-V.34fax, V.21 is used to transport T.30 control information frames. V.34 half-duplex procedures define a 1200 bps and 2400 bps control channel. In V.34fax, this control channel is used to transport T.30 control information frames. The use of error correction mode for V.34fax is mandatory.

Advanced features of Group 3 include other image compression algorithms such as T.4 2-dimensional coding, T.6, JPEG gray scale and color modes. Image format conversion, although not specified by T.30, is an optional feature of facsimile service class 2 command sets. Binary file transfer (BFT), is a mechanism to transport a T.454 format file under error correction
 

ITU-T T.31 Asynchronous Fax DCE Control -- Service Class 1
ITU-T T.32 Asynchronous Fax DCE Control -- Service Class 2

我的理解就是T.30是MODEM和传真机或者MODEM直接的传真传输协议，而T.31/T.32则是PC和MODEM之间的控制协议。

T.31对应CLASS1和CLASS1.0协议；T.32对应CLASS2和CLASS2.0协议，而1/1.0 2/2.0这两组协议之间的差别比较大，而1/1.0以及2/2.0之间的差别不大