The multiplexer protocol described in the present document operates between an UE and a TE and allows a number of simultaneous sessions over a normal serial asynchronous interface. Each session consists of a stream of bytes transferring various kinds of data; for instance, voice, fax, data, SMS, CBS, phonebook maintenance, battery status, GPRS, USSD etc. This permits, for example, SMS and CBS to be transferred to a TE when a data connection is in progress. Many other combinations are possible including digital voice. It is, for instance, possible to transfer digital voice in combination with SMS. The multiplexer allows a complete system to be partitioned in a flexible way between a UE and TE. The design of the multiplexer is flexible and independent of UE/TE platforms, and allows existing applications to work without any modifications. The multiplexer is designed, with special care for battery-powered devices, to include very important functionality such as power saving control and priorities. It is also specially designed to require minimum processing power and memory consumption. The multiplexer is defined as a single mode with different options based on the ISO HDLC standard (ISO/IEC 13239) although the basic option is not in accordance with HDLC. In the basic option, the multiplexer does not make use of any transparency mechanism or error recovery method. The advanced option uses the ISO HDLC standard transparency mechanism and gives the multiplexer an easy re-synchronisation method and the ability to operate over links which use DC1/DC3 (XON/XOFF) flow control. The advanced option also may include error-recovery for links subject to errors. In its basic option, the multiplexer is intended for use in situations where the link between UE and TE is of a very good quality and where the HDLC transparency mechanism (byte stuffing) can not be implemented in the UE. If an UE supports the HDLC transparency mechanism, it shall be used by the multiplexer. The ISO HDLC transparency mechanism must be used if loss of synchronisation may occur caused by, for example, data over-runs or under-runs. The error-recovery option should be used in situations where the link is subject to errors. The multiplexer is based on a control channel. On this channel, management information is exchanged, such as parameter negotiation, power saving control information, testing, flow control, close down etc. The multiplexer is optional, but when supported, it is activated with the AT+CMUX command described in TS 27.007.

The scope of the present document is to define a multiplexing protocol between a UE and a TE. The multiplexing protocol can be used to send any data, for instance voice, SMS, USSD, fax etc. The present document describes the protocol, but not the commands or data transported with it.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

For the purposes of the present document, the following abbreviations apply: ABM DLC DM ERM FCS MSC PSC SABM UAU UIH UI Additional abbreviations can be found in TR 21.905.

The multiplexer provides mechanisms for conveying streams of data between TE and UE over a single start-stop framed, serial link. Figure 1 shows the arrangement of the various protocol levels and functions. The multiplexer layer provides multiplexing of data arranged in octet streams with no other framing; if the structure of the data has to be conveyed, a convergence layer may be necessary. The present document defines some convergence layers, others may be added later.

The multiplexer provides a virtual connection between a process in the TE and a similar process in the UE. For example, a PC application supporting SMS functions could be connected to the SMS handler in the UE via a multiplexer channel. The present document uses start-stop transmission with eight-bit characters. Communication between the two multiplexing entities takes place using frames constructed as defined below. Each channel between TE and UE is called a Data Link Connection (DLC) and is established separately and sequentially. Each DLC may have individual flow control procedures for buffer management purposes and the aggregate link also has overall flow control mechanisms. DLCs have two modes of operation; Error-Recovery Mode (ERM) and non-error-recovery mode (non-ERM), the choice of mode is made when a DLC is established. DLCs using error recovery mode may exist on the same link as DLCs using non-error recovery mode. If the error-recovery mode (ERM) is to be used at least on one DLC, then the multiplexer must be configured with the ISO HDLC transparency mechanism. The use of error recovery mode is optional. Non-error recovery mode uses the UI frame or UIH frame to carry user data; error recovery mode uses the I frame. The multiplexer has three operating options, basic, advanced without error recovery and advanced with error recovery. The characteristics of the options are: