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Method of reverting to tandem operation between transcoders of a communication system
| Details |
Inventors: Aftelak, Steven Basil;
Assignee: Motorola, Inc. (Schaumburg, IL)
Primary Examiner: Rao; Seema S.
Assistant Examiner:
Attorney, Agent or Firm: Creps; Heather L.
To facilitate the reversion of a communication to tandem operation (200), a first transcoder (20), having previously changed back to tandem operation (206), inverts bits (208) of a double-encoded frame format that correspond to synchronisation bits in a non-tandem, single-encoded frame format to generate errors, in relation to the synchronisation bits, at a second transcoder (34). Upon detection (210) of a predetermined number of errors in the synchronisation bits during a predetermined time, the second transcoder (34) reverts to tandem operation, as shown in the flow diagram of FIG. 2. |
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DETAILED DESCRIPTION In telephony systems generally, speech quality must be maintained at an adequate level to ensure meaningful and discernible communication.
Indeed, in mobile communication networks, such as the Global System for Mobile (GSM) communication, it is becoming increasingly important to improve speech quality to satisfy the demands placed on the system by subscribers (particularly in relation to mobile unit to mobile unit calls) and to support and enhance the market penetration of such systems.
In mobile communication systems, a double encoding process currently takes place.
Specifically, a mobile unit will first encode speech for transmission to base station equipment over a radio frequency link, for example.
Subsequently, the fixed infrastructure equipment will further encode the signals for transmission onward into the system to ensure efficient and robust communication over an air-interface (typically to another mobile unit).
In relation to GSM, for example, a mobile unit encodes a speech communication at a rate of 16 kbit/s, which includes 13 kbit/s of sampled and encoded speech and 3 kbit/s of ancillary information, such as parity check and correction bits (and the like) and synchronisation information; that are framed into what is known as a TRAU (Transcoder Rate Adaption Unit) frame.
This 16 kbit/s speech is multiplexed into a time-slot containing three other speech calls to produce a channel of 64 kbit/s on a landline, and this channel is communicated by a base station controller (BSC) (in a downlink) to at least one fixed base transceiver station (BTS).
As will be understood, the BTS serves a cell that is typically partitioned into distinct sectors each administered by individual transceivers, while a BSC serves a group of cells.
A transcoder (which provides a transposition in the coding scheme received by the BTS) de-multiplexes the channel and then encodes each speech communication as a 64 kbit/s pulse code modulated (PCM) format for transparent and sequential transmission through a first Mobile Switching Centre (MSC), a second MSC and then to a second transcoder for PCM decoding for onward routing to a BSC, BTS and ultimately, perhaps, to another subscriber unit
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