Multiplex communications – Wide area network – Packet switching
Patent
1993-06-22
1994-12-20
Safourek, Benedict V.
Multiplex communications
Wide area network
Packet switching
370 8513, H04L 1266
Patent
active
053751191
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention pertains to a method for handling data transmission frames of variable length with a channel controller and for writing them to a cyclic buffer memory wherein the data transmission frames consist of a header field containing at least control data, a data field and an end field of variable length.
In the interface between the PCM and GSM networks, communication takes place by using TRAU frames which are defined in the GSM specifications and which possess the following properties:
- recurrence interval 20 ms
- contains data relating to one traffic communication (voice/data channel)
- reserves one PCM time slot
- transparent to the PCM channel
- contain 320 bits of control information and data.
In order for the channel decoder receiving the TRAU frames to be capable of operating in compliance with the GSM specifications, the frames must arrive at the channel decoder correctly phased. Owing to time slips of clocks, maintenance of the correct phasing requires that the TRAU frame be shortenable or extendable by four bits, i.e. the length of the TRAU frames be in fact 320.+-.4 bits. The channel decoder is informed of whether the TRAU frame concerned has been extended or shortened by control bits located at the very beginning of the frame.
The situation described above is problematic, because TRAU frames may nave a length varying four bits in both directions while the length of the buffer section is invariable. Several extended or shortened TRAU frames then cause the frame to slip within the buffer section, whereby the controller interrupts the channel decoder at the wrong time, either too late (TRAU frame shortened) or too early (TRAU frame extended), i.e. when the buffer section is full. The only right moment for interrupt would be when the buffer has a complete TRAU frame. For instance, in a GSM base station slip causes unnecessary delay in the speech signal.
SUMMARY OF THE INVENTION
The present invention provides a solution to problems in the interface between the PCM and GSM networks caused by non-ideal behaviour of the clocks controlling said networks. To achieve this, the method of the invention is characterised in that the buffer memory is divided into more than one logical section, each section comprising a header field and a data field, and that at least the following steps are performed by means of the channel controller and a channel decoder in respect of an incoming frame:
- reading an amount of information corresponding to the length of one buffer section by means of the channel controller from the beginning of the frame,
- writing the frame section which has been read to one section of the buffer memory, and interrupting the channel decoder, whereby the channel decoder reads the frame section from the buffer memory to its own storage space,
- recognising the real length of the frame indicated in the header field of the frame by means of the channel decoder,
- writing the required length, calculated using the indication of the length of the frame, by the channel decoder to the header field of a second buffer section,
- writing the frame section coming from the channel in a corresponding manner by the channel controller to an area of the buffer, and interrupting the channel encoder correspondingly, whereby the channel decoder reads the frame sections to its own memory,
- making the last interrupt related to the frame on the basis of the indicated real length of the frame when the last section of the frame is in the buffer.
The last interrupt related to the frame occurs, due to the extension/shortening of one buffer section, exactly when a complete frame is in the buffer. The buffer memory is preferably divided into two sections, each having a header and a data field.
In this way, a buffer of `flexible` length is realised, whereby slip of the TRAU frame within the buffer is avoided, and the channel encoder is interrupted at the correct moment. In the services provided by the GSM network, this can be noticed as elimination of speech delays caused by
REFERENCES:
patent: 4410889 (1983-10-01), Bryant et al.
patent: 5001708 (1991-03-01), Williams et al.
Nokia Telecommunications Oy
Safourek Benedict V.
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