WO1996018244A1

WO1996018244A1 - Transcoder and method for use in a communication system

Info

Publication number: WO1996018244A1
Application number: PCT/US1995/013556
Authority: WO
Inventors: Valy Lev; Kamala Urs; Dan Coombes; Hooman Kashef
Original assignee: Motorola Inc.
Priority date: 1994-12-08
Filing date: 1995-10-10
Publication date: 1996-06-13
Also published as: AU3964095A; AR001052A1; IL115561A0

Abstract

A site controller (102) instructs a transcoder (121) to operate in a partial transcoding mode (401). While in the partial transcoding mode, the transcoder continuously monitors for a synchronization pattern. If the synchronisation pattern is detected within a predetermined period of time, the transcoder switches operation to a transparent mode (402). While in the transparent mode, the transcoder transparently passes compressed digital voice. If the synchronization pattern is not detected within the predetermined period of time, the transcoder switches operation to a full transcoding mode (400). Regardless of the operating mode, the transcoder continues to monitor for the synchronization pattern. Additionally, the transcoder transmits the synchronization pattern in either the partial transcoding mode or transparent mode. By detecting the synchronization pattern generated by the transcoder, other transcoders (122-123) can also switch to the transparent mode, thereby avoiding multiple format conversions.

Description

TRANSCODER AND METHOD FOR USE IN A COMMUNICATION

SYSTEM

Field of the Invention

The present invention relates generally to communication systems and, in particular, to a transcoder and method for use in such communication systems.

Background of the Invention

Communication systems are known to comprise mobile units (i.e., hand-held portable or in-car mobile radiotelephones) that wirelessly communicate digitally-represented voice signals within sites controlled by site controllers. Often, site controllers communicate with a switching center so that multiple sites can communicate with each other via the switching center. Current switching centers typically route voice signals in non-compressed digital voice formats, e.g., a pulse-code modulation (PCM) format. In contrast, mobile units typically transmit and receive voice signals in the form of compressed digital voice, e.g., data packets created through the use of a vector-sum excited linear predictive (VSELP) voice coder. Transcoders are provided to perform the conversions, when required, between the compressed digital format and the non- compressed digital format and vice versa. A typical example occurs when a land-based telephone, using a non-compressed digital format, communicates with a mobile unit via the switching center. In this situation, compressed digital voice from the mobile unit is converted into the non-compressed digital format by a transcoder associated with the appropriate site controller. The non-compressed digital voice is then routed, via the switching center, to the land- based telephone. When communicating from the telephone to the mobile unit, the process is reversed.

Transcoders are also used for calls between two mobile units. In this case, compressed digital voice from a first mobile is converted to a non-compressed format by a transcoder and sent through a site controller to a switching center. The switching center, in turn, routes the non-compressed voice possibly to the same site controller or another site controller, which re-converts the non-compressed voice, via another transcoder, to the compressed format. The re- compressed digital voice is then transmitted to the second mobile via the site controller. As known in the art, conversions by a transcoder add delay to the transmission of the message and slightly degrade the resulting quality of the voice. Thus, calls requiring more than one conversion, as in the present example, may be subjected to prohibitive delays and audio degradation.

An attractive solution to this problem is to allow compressed digital voice signals to be transparently passed through the transcoders, and hence through the switching center, when a mobile-to-mobile call is established. In this manner, the delays and audio degradation inherent in multiple conversion can be avoided. Assuming that transcoders can operate in this manner, i.e., transparently passing compressed voice, neither current switching centers or site controllers provide the necessary control for such operations.

The problem described above is exacerbated when voice messages are required not only to span multiple transcoders, but also to span multiple switching centers. Even if the transcoders affiliated with each switching center are capable of passing compressed digital voice, operations associated with the inter-switching center communication link must be similarly controlled. For example, the echo-cancellation found on most inter-switching center communication links would need to be disabled when passing compressed digital voice. Current methods for controlling the operations of switching centers do not address the need to pass compressed voice between switching centers, i.e., a mobile-to-mobile call is treated similarly to any other type of call. Therefore, a need exists for a method that overcomes the prior art difficulties associated with multiple transcoder format conversions required for mobile-to- mobile calls. Additionally, mobile-to-mobile calls that span multiple switching centers must also be able to avoid multiple transcoder format conversions.

Brief Description of the Drawings

FIG. 1 is a box diagram of a communication system having a single switching center configuration.

FIG. 2 is a box diagram of a communication system having a multiple switching center configuration.

FIG. 3 is a box diagram of a transcoder in accordance with the present invention.

FIG. 4 is a state diagram illustrating the operating modes of a transcoder in accordance with the present invention.

FIG. 5 illustrates input/output data format relationships for a transcoder in accordance with the present invention.

FIG. 6 illustrates the data formats shown in FIG. 5.

FIG. 7 is a flow chart of a method for controlling operating modes of a transcoder in accordance with the present invention.

FIG. 8 illustrates a protocol stack used to pass control information between switching centers, site controllers, and transcoders. Description of a Preferred Embodiment

The present invention describes a transcoder for use in a communication system and a method for controlling the transcoder such that multiple transcoder format conversions are avoided in mobile-to-mobile calls. In either a single or multiple switching center configuration, a site controller, typically in response to a call request from a first mobile unit, instructs a transcoder to operate in a partial transcoding mode. While in the partial transcoding mode, the transcoder continuously monitors outbound voice data words, via a synchronization detector, for a synchronization pattern. When detected, the synchronization pattern indicates that the call in progress is mobile-to-mobile. If the synchronization pattern is detected within a predetermined period of time, the transcoder switches operation to a transparent mode. If the synchronization pattern is not detected within the predetermined period of time, the transcoder switches operation to a full transcoding mode. Regardless of the operating mode, the transcoder continues to monitor for the synchronization pattern.

Additionally, the transcoder transmits the synchronization pattern via inbound voice data words while operating in either the partial transcoding mode or transparent mode. By detecting the synchronization pattern generated by the transcoder, other transcoders can switch operating modes when necessary. When transcoders switch from the partial transcoding mode to the transparent mode, and vice versa, a state change notification is sent by the trancoder to the switching center via the site controller, thereby informing the switching center of the mode change. Using these methods, the present invention overcomes difficulties of multiple transcoder format conversions associated with prior art methods of mobile-to-mobile communications.

The present invention can be more fully described with reference to FIGS. 1-9. FIG. 1 shows a communication system (100) having a single switching center configuration that may be used to implement the present invention. For example, the communication system (100) may comprise a system in conformance with Groupe Speciale Mobile (GSM) standards. Such a system is typically used to provide communications over a single geographic region, i.e., a city. The communication system (100) comprises a switching center (101) in communication with site controllers (102-104) via transcoders (121- 123). The site controllers (102-104) control the operations of base stations (106-108), which are in wireless communication, via wireless communication resources (111-112), with a plurality of mobile units (116-117). The switching center (101) may comprise a Digital

Multiplex System Mobile Switching Center (DMS MSC) by Northern Telecom. The site controllers (102-104) and the transcoders (121-123) may comprise Base Station Controllers (BSCs) and transcoders, respectively, manufactured by Motorola, Inc. Additionally, the mobile units (116-117) may comprise digital MicroTAC® portable radiotelephones by Motorola, Inc.

Typically, voice information is passed between the switching center (101) and the transcoders (121-123), via a wired voice link (125), in a non-compressed digital format. In a preferred embodiment, the present invention incorporates the use of a full-rate non-compressed digital format (i.e., a 64 Kbit pulse code modulation (PCM) format) and a reduced-rate non-compressed digital format (i.e., a 56 Kbit PCM format), as described below. Within the switching center (101), the non-compressed digital voice is routed through a digital switch matrix (124), as known in the art. In contrast, the mobile units (116-

117) transceive voice information in the form of compressed digital voice. For example, mobile units in accordance with GSM standards use a regular pulse excitation linear predictive coder (RPE-LPC) having a 13 Kbit format. The wireless communication resources (111-112), which may comprise radio frequency (RF) carriers divided in time in to time slots, convey the compressed digital voice between the mobile units (116-117) and the base stations (106-107). In turn, the compressed digital voice is sent between the base stations (106-108), site controllers (102-104), and transcoders (121-123) via wired voice links (126). The transcoders (121-123) operate partially under the control of the site controllers (102-104). Additionally, the present invention allows the switching center (101) to be informed of the operating modes of the transcoders (121-123). To this end, wired control links (127,129) are provided between the switching center (101) and the site controllers (102-104) as well as the site controllers (102-104) and the transcoders (121-123). A wired control link (128) is also provided between the site controllers (102-104) and the base stations (106-108). The wired voice links (125-126) and the wired control links (127-129) typically comprise Public Switched Telephone Network (PSTN) resources, such as Tl telephone lines, as known in the art. Also, the transcoders (121-123) may be provided as part of the site controllers (102-104) (i.e., individual circuit boards within the controller hardware), or they may be provided as separate physical entities apart from the site controllers (102-104).

The site controllers (102-104) determine when calls are originated and terminated by mobile units (116-117), and thus partially control the operating mode of the transcoders (121-123). As discussed in greater detail below, the transcoders (121-123) also control each others operating modes through the use of a synchronization pattern. To maintain continuity throughout the system (100), the operating mode of each transcoder (121-123) is stored in memory (131) of the switching center (101). In addition to single switching center configurations, the present invention also addresses cases of mobile-to-mobile calls spanning multiple switching centers.

FIG. 2 shows a communication system (200) having a multiple switching center configuration that may be used to implement the present invention. Similar to the system (100) of FIG. 1, the communication system (200) may comprise a system in conformance with GSM standards. Such a system typically provides communications over a variety of geographic locations, i.e., cities in different states. The communication system (200) comprises switching centers (201-203) in communication with site controllers (206-208) via transcoders (221-223). The site controllers (206-208) control the operations of base stations (211-213), which are in wireless communication, via wireless communication resources (216-217), with a plurality of mobile units (218-219). Each of the above-listed elements of the system (200), including wired voice links (225-226), wired control links (227-229), digital switch matrices (236-238), and memories (231-233), are essentially identical to those listed with respect to FIG. 1, with the following exceptions and additions.

The switching centers (201-203) communicate via wired voice links (241) and wired control links (242), i.e., Tl telephone lines. In the present invention, the wired voice links (241) convey non- compressed as well as compressed digital voice between the switching centers (201-203). As known in the art, echo cancelers (239) are also provided for use with the wired voice links (241). Echo cancelers (239) are used to mitigate the effects of echoing known to occur in transmission lines. However, the present invention anticipates that the echo cancelers (239) will be disabled when passing compressed digital voice, as described in further detail below.

Home location registers (HLRs) (204) and visiting location registers (VLRs) (205) are also provided. Although only one home location register (204) and visiting location register (205) are shown in FIG. 2 for clarity, the switching centers (201-203) each have access to associated home and visiting location registers. Additionally, the home location registers (204) and visiting location registers (205) may communicate with each other, as known in the art. Each of the home location registers (204) and visiting location registers (205) include information regarding the service provisions (e.g., features) and location for their associated mobile units (218-219). Additionally, each of the home location registers (204), visiting location registers (205), and memories (231-233) contain so-called global title translation tables. The global title translation tables associate mobile units, based on mobile unit identification information, with unique address information of the switching centers (201-203), home location registers (204), and visiting location registers (205). FIG. 3 illustrates a box diagram of a transcoder (300) in accordance with the present invention. A format converter (301) is provided for performing format conversions to outbound and inbound voice data words, as described in detail below. A first transceiver (302), coupled to the format converter (301) and capable of receiving the outbound voice data words from, and transmitting the appropriately converted inbound voice data words to, a switching center is provided. Similarly, a second transceiver (303), coupled to the format converter (301) and capable of receiving the inbound voice data words from, and transmitting the appropriately converted outbound voice data words to, a site controller is also provided. The first and second transceivers (302-303) may comprise wireline interface transceivers, as known in the art.

A synchronization detector (304) and a synchronization generator (305) are also provided. The synchronization detector (304) monitors outbound voice data words received by the first transcoder (302) for a synchronization pattern to derive a synchronization status (i.e., whether or not the synchronization pattern has been detected). The synchronization status is sent to a controller (306). As discussed below, the controller (306) controls an operating mode of the format converter (301), and hence, the transcoder (300), based on the synchronization status and a predetermined period of time measured by a timer (307). Additionally, the controller (306) controls the synchronization generator (305) based on the operating mode. The site controllers (102-104, 206-208), using wired control links (127, 227)

(not shown in FIG. 3), may also control the transcoder (300) operating mode via instructions to the controller (306).

The operating modes of a transcoder in accordance with the present invention are shown in FIG. 4 in the form of a state diagram. The transcoder may operate in one of three operating modes as shown; a full transcoding mode (400), a partial transcoding mode (401), and a transparent mode (402). The full transcoding mode (400) is the default mode, i.e., transcoders are powered-up and remain in the full transcoding mode (400) initially. While in the full transcoding mode (400), the transcoder can switch to the partial transcoding mode (401) in response to either of two events: an instruction from a site controller or detection of the synchronization pattern.

Once in the partial transcoding mode (401), expiration of a predetermined period of time, initiated upon entering the partial transcoding mode (401), causes the transcoder to switch back to the full transcoding mode (400). However, if the synchronization pattern is detected before the expiration of the predetermined period of time, the transcoder switches to the transparent mode (402). Once in the transparent mode (402), an instruction from the site controller or a detection loss of the synchronization pattern causes the transcoder to revert to the partial transcoding mode (401). As before, the predetermined period of time is initiated when the partial transcoding mode (401) is entered and operation proceeds as described above. Each of the operating modes (400-402) is characterized by the types of data formats used by the transcoder (that is, the format converter) while in each mode (400-402). FIG. 5 illustrates the data format relationships of the transcoders when operating in the full transcoding, partial transcoding, or transparent mode. In FIG. 5, the three columns represent the three operating modes, i.e., full transcoding, partial transcoding, or transparent. The two rows represent the direction of the digital voice being processed by the transcoder; outbound voice comprises outbound voice data words from a switching center destined for a site controller and mobile unit, inbound voice comprises inbound voice data words from a mobile unit via a site controller destined for a switching center. The inbound and outbound rows are further subdivided into transcoder inputs and outputs for that direction. Thus, regardless of the operating mode, inbound input (i.e., from a site controller to a switching center) takes the form of compressed digital voice. When operating in the full transcoding mode, the inbound compressed digital voice is converted into full-rate non-compressed digital voice. When operating in the partial transcoding mode, the inbound compressed digital voice is converted into reduced-rate non-compressed digital voice with the synchronization pattern added. When operating in the transparent mode, the inbound compressed digital voice is passed through the transcoder in the form of bit-stuffed compressed digital voice with the synchronization pattern added. The added synchronization pattern is generated by a synchronization generator and added to the inbound voice data words prior to transmission to the switching center, as described below.

In a similar fashion, outbound output (i.e., from a switching center to a site controller) takes the form of compressed digital voice, regardless of the operating mode. When operating in the full transcoding mode, the compressed digital voice is produced by converting full-rate non-compressed digital voice input to the transcoder. While not normally expected, the synchronization pattern can be optionally added to the full-rate non-compressed digital voice. In the partial transcoding mode, reduced-rate non- compressed digital voice with the added synchronization pattern is converted. In the transparent mode, the compressed digital voice is extracted from the bit-stuffed compressed digital voice and added synchronization pattern.

FIG. 6 illustrates the differences between the data formats described with reference to FIG. 5. In the preferred embodiment, an inbound/outbound word of compressed digital voice (600), represented by the symbol CV, occupies two bits. When operating a transcoder in the partial transcoding mode (401), the inbound/outbound word of compressed digital voice (600) is converted into reduced-rate non- compressed digital voice (601), shown as seven bits of PCM data, and vice versa. When operating a transcoder in the full transcoding mode (400), the inbound/outbound word of compressed digital voice (600) is converted into full-rate non-compressed digital voice (603), shown as eight bits of PCM data, and vice versa. Finally, when operating a transcoder in the transparent mode (402), the inbound/outbound word of compressed digital voice (600) is converted into bit-stuffed compressed digital voice (602), shown as the inbound/outbound word of compressed digital voice (600) prefixed by a five bit pattern of stuff bits, and vice versa.

The stuff bits are set to the pattern "10000" so that even if the bit-stuffed compressed digital voice (602) is mistakenly decoded as non-compressed digital voice (601, 603), the linear properties of PCM are exploited such that the bit-stuffed compressed digital voice (602) will appear to be low-amplitude noise, thereby minimizing any negative audio quality effects. Using the bit-stuffed compressed digital voice (602) to mimic the properties (i.e., length) of non- compressed digital voice (601, 603), the present invention allows compressed digital voice (600) to transparently pass through a switching center, thereby eliminating the need for multiple transcoder conversions.

Usage of the synchronization pattern is also illustrated if FIG. 6. The synchronization pattern is chosen to be a particular pattern of bits, which pattern is continuously transmitted and received in a serial fashion via the least-significant bit of inbound and outbound voice data words. For example, the synchronization pattern can be the 24-bit binary string "000011111101101111110000", although many other patterns are possible. When a first transcoder receives an inbound voice data word in the compressed digital format (600), and converts them into either the reduced-rate non-compressed digital format (601) or the bit-stuffed compressed digital format (602), a single bit (shown as S) of the synchronization pattern is shifted into the least-significant bit position. After 24 inbound data words have been thus converted and routed to a switching center, the pattern is thus repeatedly sent to other transcoders destined to receive the inbound voice data words.

Once passed through the switching center, the inbound voice data words appear as outbound voice data words to a second transcoder, which transcoder monitors the least-significant bit of the outbound voice data words. As each outbound voice data word is received by the second transcoder, operating in any of the three possible modes, the least-significant bit can be appended to previously received least-significant bits. For each new bit thus received, the resulting accumulated bits can be compared against the known synchronization pattern. As shown, a transcoder operating in the full transcoding mode, expecting full-rate non-compressed voice (603), still monitors the least-significant bit for the synchronization pattern. This is done so that the transcoder receiving the outbound voice data words can detect a change in operating mode of the originating transcoder. The present invention relies on the "noise¬ like" characteristics, i.e., random probability distribution, of the least-significant bit of non-compressed digital voice to prevent false detections of the synchronization pattern when operating in the full transcoding mode. By selecting a highly correlative and repetitive synchronization pattern, the chances of a false detection can be minimized.

In light of the previous discussion, overall operation of the present invention can be described with reference to the flowchart shown in FIG. 7. At step 700, a first transcoder operates in the default full transcoding mode. As discussed previously, the first transcoder, while in the full transcoding mode, continuously monitors the least-significant bit of received outbound voice data words for the synchronization pattern.

At step 701, the first transcoder determines if its associated site controller has instructed it to switch to the partial transcoding mode. Typically, the site controller will so instruct the first transcoder in response to detecting a call request from a first mobile unit. If the site controller has not instructed the first transcoder to switch modes at step 701, it is determined if the synchronization pattern has been detected, as described above, at step 702. Assuming a 24-bit synchronization pattern and a received outbound voice data word rate of 8 KHz, it would take approximately 3 ms. to accumulate and detect a single iteration of the synchronization pattern. The synchronization pattern to be detected, if present, is generated by a second transcoder when it is determined that the call is intended for a second mobile unit. In a similar manner as described above, the second transcoder would be notified by its associated site controller to switch modes and begin transmission of the synchronization pattern. Thus, if the synchronization pattern has not been detected, operation of the transcoder continues at step 700 in the full transcoding mode.

If, however, an instruction to change operating modes is received at step 701, or the synchronization pattern is detected at step 702, the transcoder switches to the partial transcoding mode at step 703. Once in the partial transcoding mode, the first transcoder instructs a synchronization generator to transmit the synchronization pattern, as described above, as an indication to other transcoders that a mobile-originated call is underway. Upon entering the partial transcoding mode, a predetermined period of time is initiated at step 704. It is then determined at step 705 if the synchronization pattern has been detected two more times. This is done to insure that a false detections will not cause the first transcoder to switch into the transparent mode. If he synchronization pattern has not been detected two more times, it is determined if the predetermined period of time has expired at step 706. If, at step 706, the predetermined period of time has not yet expired, processing returns to step 705. If, however, the predetermined period of time has expired, it is assumed that the first detection of the synchronization pattern at step 702 was incorrect, and the first transcoder switches back to the full transcoding mode at step 700. The length of the predetermined period of time is based on the time required to detect two iterations of the synchronization pattern. Thus, allowing for processing delays, the predetermined period of time can be set to, for example, 7 ms. or more, dependent upon desired system performance.

If the synchronization pattern is detected a second time at step 705, a first state change notification is sent to the switching center, via the site controller, at step 707. The first state change notification serves to notify the switching center that the first transcoder has switched to transparent mode operation, as shown in step 708. Knowing that the first transcoder is operating in the transparent mode, i.e., transeiving bit-stuffed compressed digital voice with the switching center, the switching center knows to disable echo cancellation in the event that the call spans more than a single switching center.

Once operating in the transparent mode, it is assumed that the call is mobile-to-mobile so long as the synchronization pattern is continuously detected. At step 709, the first transcoder again determines if an instruction has been received from the site controller to switch operating modes. This could occur if the site controller determines that a special service, such as call waiting, or conference calling is to implemented. Such services require the transcoder to operate in a transcoding mode.

If the site controller has not instructed the first transcoder to switch modes, it is determined at step 711 if the synchronization pattern has been lost, i.e., not detected. In a preferred embodiment, the loss of a single iteration of the synchronization pattern causes the synchronization status to indicate lost synchronization. Once again assuming a 24-bit synchronization pattern and 8 KHz word rate, it would take approximately 3 ms. to lose a single iteration If the site controller has instructed the first transcoder to switch modes or if synchronization has been lost, a second state change notification is sent to the switching center, via the site controller, at step 710. The second state change notification serves to notify the switching center that the first transcoder has switched to from transparent mode operation to partial transcoding operation, as shown in step 703. Knowing that the first transcoder is operating in the transparent mode, i.e., transeiving bit-stuffed compressed digital voice with the switching center, the switching center knows to disable echo cancellation in the event that the call spans more than a single switching center.

Having returned to the partial transcoding mode, the first transcoder will either revert to the full transcoding mode or to the transparent mode, as described in the previous steps. If, for example, the call has been terminated or is no longer mobile-to- mobile, the first transcoder will switch to the full transcoding mode; if the synchronization pattern was only missed a single time due to noise or similar circumstance, the synchronization pattern should be detected once again, and transparent operation will resume. Assuming the 3 ms. detection/loss time of a single iteration of the synchronization pattern, it can be seen that transitions from the full transcoding mode to the transparent mode, and vice versa, require at least 12 ms. In this manner, the present invention allows transcoders to switch to transparent operation and back quickly enough such that any disruptions to continuous voice quality will be nearly imperceptible.

FIG. 8 illustrates a protocol stack used to pass control information as described above. In the context of the present invention, the term protocol stack is used to describe protocols adhering to the Open System Interconnection (OSI) model promulgated by the International Standards Organization (ISO). In particular, FIG. 8 illustrates a protocol stack (800) used to pass control information between switching centers, site controllers, and transcoders

The protocol stack (800) used to pass control information between switching centers, site controllers, and transcoders comprises four layers in compliance with known control signaling standards. The physical and link layers of the protocol stack comprises the Message Transfer Part (MTP) (804). The network layer comprises the Signaling Connection Control Part (SCCP) (803). The physical, link, and network layers (903-804) described are in conformance with the so-called Signaling System Seven (SS7) protocol commonly used in communication systems of the type described above. The application layer comprises two separate parts: the Base Station System Application Part (BSSAP) (802) and the Base Station System Management Application Part (BSSMAP) (801). The BSSAP and BSSMAP layers (801-802) are used to pass the control information, as described above, between switching centers, site controllers, and transcoders. For example, a state change notification from a transcoder to a switching center, via a site controller, would be passed in the BSSAP and BSSMAP layers (801- 802). The present invention describes a transcoder for use in a communication system and a method for controlling the transcoder such that multiple transcoder format conversions are avoided in mobile-to-mobile calls. This is accomplished by the use of a synchronization pattern, transmitted between transcoders. Once a transcoder detects the synchronization pattern, it is assumed that a mobile-to-mobile call is in progress, thus allowing the transcoder to switch operating modes such that compressed voice is transparently passed through the system. Likewise, loss of the synchronization pattern indicates that transparent operation is not longer needed, thus allowing the transcoder to switch to a transcoding mode.

We claim:

Claims

1. A transcoder for transcoding outbound voice data words from a switching center and for transcoding inbound voice data words from a site controller, the transcoder comprising:

a format converter for converting the outbound voice data words from a non-compressed digital format to a compressed digital format and the inbound voice data words from the compressed digital format to the non-compressed digital format when operating in either of a partial transcoding mode and a full transcoding mode, and for converting the outbound voice data words from a bit-stuffed compressed digital format to the compressed digital format and the inbound voice data words from the compressed digital format to the bit-stuffed compressed digital format when operating in a transparent mode;

a controller, operably coupled to the format converter, for controlling an operating mode of the format converter based in part on a synchronization status, wherein the operating mode is one of the partial transcoding mode, the full transcoding mode, and the transparent mode; and

a synchronization detector, operably coupled to the controller, for determining the synchronization status, wherein the synchronization detector monitors the outbound voice data words for a synchronization pattern to determine the synchronization status.

2. The transcoder of claim 1, and further comprising:

a first transceiver, operably coupled to the format converter and the synchronization detector, for receiving the outbound voice data words from, and transmitting the inbound voice data words to, the switching center; and

a second transceiver, operably coupled to the format converter, for receiving the inbound voice data words from, and transmitting the outbound voice data words to, the site controller.

3. The transcoder of claim 2, wherein the control means further comprises timer means for determining expiration of a predetermined period of time, wherein the controller determines the operating mode based in part on expiration of the predetermined period of time.

4. The transcoder of claim 3, and further comprising:

a synchronization generator, operably coupled to the controller and the first transceiver, for generating the synchronization pattern, wherein the synchronization pattern is transmitted as part of the inbound voice data words when the voice converter is operating in either of the partial transcoding mode and the full transcoding mode.

5. In a communication system that comprises at least one switching center, at least one site controller in communication with the at least one switching center, and at least two transcoders in communication with the at least one switching center and the at least one site controller, a method for controlling operating modes of a first transcoder of the at least two transcoders, the method comprising the steps of:

responsive to an instruction from the at least one site controller, operating the first transcoder in a partial transcoding mode;

continuously monitoring, by the first transcoder while operating in the partial transcoding mode, outbound voice data words received from the at least one switching center for a synchronization pattern; and

when the first transcoder, while operating in the partial transcoding mode, does not detect the synchronization pattern after a predetermined period of time, operating the first transcoder in a full transcoding mode, wherein the first transcoder continuously monitors for the synchronization pattern while in the full transcoding mode. predetermined period of time, the transcoder switches operation to a transparent mode (402). While in the transparent mode, the transcoder transparently passes compressed digital voice. If the synchronization pattern is not detected within the predetermined period of time, the transcoder switches operation to a full transcoding mode (400). Regardless of the operating mode, the transcoder continues to monitor for the synchronization pattern. Additionally, the transcoder transmits the synchronization pattern in either the partial transcoding mode or transparent mode. By detecting the synchronization pattern generated by the transcoder, other transcoders (122-123) can also switch to the transparent mode, thereby avoiding multiple format conversions.

6. The method of claim 5, the step of operating the first transcoder in the partial transcoding mode further comprising the step of transmitting, by the first transcoder, the synchronization pattern in the least-significant bit of inbound voice data words sent to the at least one switching center.

7. The method of claim 5, the step of continuously monitoring for the synchronization pattern further comprising the step of serially monitoring, by the first transcoder, a least-significant bit of the outbound voice data words for the synchronization pattern.

8. The method of claim 7, further comprising the step of continuously monitoring for the synchronization pattern, wherein the synchronization pattern is transmitted by a second transcoder of the at least two transcoders.

9. The method of claim 5, further comprising the step of:

when the first transcoder, while operating in the partial transcoding mode, detects the synchronization pattern, operating the first transcoder in a transparent mode, wherein the first transcoder continuously monitors for the synchronization pattern while in the transparent mode.

10. The method of claim 9, further comprising the step of:

when the first transcoder switches operation from the partial transcoding mode to the transparent mode, sending a first state change notification from the first transcoder to the at least one site controller.

11. The method of claim 10, further comprising the step of:

sending the first state change notification from the at least one site controller to the at least one switching center.

12. The method of claim 9, the step of operating the first transcoder in the transparent mode further comprising the step of serially transmitting, by the first transcoder, the synchronization pattern in a least-significant bit of inbound voice data words sent to the at least one switching center.

13. The method of claim 9, the step of operating the first transcoder in the transparent mode further comprising the step of serially monitoring, by the first transcoder, a least-significant bit of the outbound voice data words for the synchronization pattern.

14. The method of claim 9, further comprising the step of:

while the first transcoder is operating in the transparent mode, instructing, by the at least one site controller, the first transcoder to operate in the partial transcoding mode.

15. The method of claim 9, further comprising the step of:

when the first transcoder, while operating in the transparent mode, does not detect the synchronization pattern, operating the first transcoder in the partial transcoding mode.

16. The method of claim 15, further comprising the step of:

when the first transcoder switches operation from the transparent mode to the partial transcoding mode, sending a second state change notification from the first transcoder to the at least one site controller.

17. The method of claim 16, further comprising the step of:

sending the second state change notification from the at least one site controller to the at least one switching center.