Fax Transmission

GSM technology provides two types of teleservices for fax: alternate speech/facsimile group 3 (TS 61) and automatic facsimile group 3 (TS 62). TS 61 is used when the user wants to switch between the voice call and the fax machine. It uses an in-call modification procedure via the user interface. The fax service can be provided on both transparent and nontransparent connections. Both mobile originated and terminated fax calls are supported. The bearer service information transfer mode for fax is circuit, duplex, synchronous, and symmetric.For GSM, the group 3 facsimile service is adapted from the fixed network specification defined in ITU-T F.160. This service specification is comprised of two parts: the control protocol described in ITU-T T.30, and the document transmission coding described in ITU-T T.4.

GSM terminal equipment can be connected to a two-wire basic fax machine with the use of the fax adapter function. The fax adapter function converts the analog signals coming from the two-wire fax machine into a serial digital stream having the ISDN-specific R interface as the output. This R interface output needs a standard synchronous TAF (GSM 07.03) to connect to the GSM MS. A personal computer (PC), emulating a fax machine, can be connected directly to a GSM phone with a commercially available PCMCIA fax card supporting GSM phones. In the PCMCIA fax card, the TAF and fax adapter functions are combined.

The MSC transfers the data link to the proper IWF. Different types of IWF are possible for the different type of transit networks. The transit network subsequently transports the fax information to the receiving fax machine. Both fax machines communicate with each other using the standard fax protocol defined in ITU-T T.30. It should be noted that depending on the implementation, the R reference point might not exist explicitly. In this case the LAPB protocol and consequently the LAPB entities operating across this interface may be omitted. L2RBOP and RLP protocol stacks are used at the radio interface.

As the fax data is synchronous, RA1' is used to transform the data rates supported on the air interface while adding some control bits. The air interface rate could be 3.6, 6, or 12 Kbps, depending on the bearer service subscribed by the fax user. The data is sent over the air with FEC. In the BSS, fax data is transformed into the intermediate rates of 8 or 16 Kbps by the RA1 function. The intermediate rate is transformed into an A interface data stream of 64 Kbps by the RA2 function. A similar view can be established for the transparent fax with the absence of L2R and RLP functions.