Method of local routing and transcoder therefor

A transcoder of a communication system is arranged to selectively embed (104) a data pattern into a call (102) to produce a slow-rate data channel in the call emanating from a switch. The data pattern includes a call identity that uniquely identifies how the call is routed through the communication system. In the event that transcoder detects (106) this call identity during subsequent routing of the call back to the switch, the transcoder identifies that the call can be routed directly through the switch and therefore instructs (112) the communication system to execute a local routing operation in which control information is routed throughout the communication system while traffic information generated in the call is routed directly through the switch, as illustrated in FIG. 4.

BACKGROUND TO THE INVENTION 
This invention relates, in general, to a method of routing a communication 
at a local level and its associated signalling scheme, and is 
particularly, but not exclusively, applicable to the routing of a 
communication within a cellular communicaton network. 
SUMMARY OF THE PRIOR ART 
The cost of obtaining interconnection between fixed site infrastructure of 
telecommunication systems is a factor that influences the ability to 
provide a viable, inexpensive telephony system. Indeed, rather than the 
cost of installing the infrastructure (which can be recouped steadily over 
the lifetime of the system) or the cost of a licence for the limited 
air-interface in a radio communication system, it is the recurring costs 
associated with hiring high-capacity, robust and high-quality links, such 
as satellite links and high data rate Megastreams, that effects service 
costs. This problem is becoming increasingly apparent in relation to 
cellular communication systems, such as the Global System for Mobile (GSM) 
communication, in which the percentage of mobile-to-mobile calls and 
mobile-to-fixed network calls within the same local (geographic) area is 
increasing with market penetration. In particular, leasing costs for such 
high-quality links can be excessive in systems where there is a large 
volume of low-tariff calls, such as within an in-building environment 
where calls could otherwise potentially be routed on a local basis rather 
than by routing a call between fixed base transceiver stations (BTSs) and 
fixed but physically separate base site controllers (BSCs). 
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 at the base station to ensure 
efficient and robust communication over an air-interface. 
In relation to GSM, for example, a mobile unit encodes a speech 
communication at a rate of 16 kbit/s, which includes 13kbit/s of sampled 
speech and 3 kbit/s of ancillary information, such as parity check and 
correction bits (and the like) and synchronisation information. 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) 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. 
With specific regard to the encoding operation of the GSM system (which is 
used solely for the purposes of explanation), a TRAU (Transcoder Rate 
Adaptation Unit) frame of information has a duration of 20 milliseconds 
(ms), while speech is sampled at a rate of 8000 samples per second. 
Therefore, bearing in mind that each sample is an 8-bit word, each TRAU 
frame consists of one-hundred and sixty 8-bit samples. Subsequently, 
transcoder operation codes these one-hundred and sixty samples as an 8-bit 
PCM word to provide 1280 bits of PCM information per frame (equivalent to 
64 kbit/s). As will be understood, the structure of the 8-bit PCM frame is 
indicative of a signal level, with the Least Significant Bit (LSB) being 
of relatively little importance in the re-construction of encoded 
information when compared with the relative importance of successive bits. 
As such, the Most Significant Bit (MSB) has the greatest effect on the 
re-construction of encoded information, since its bit-value in an 8-bit 
binary word is indicative of a level one-hundred and twenty-eight times 
greater than the bit-value of the LSB. 
RELATED PATENT APPLICATIONS 
The present invention is related and complementary to the simultaneously 
filed, co-pending UK patent application (No. ) entitled "TRANSCODER AND 
METHOD FOR A NON-TANDEM CODING OPERATION", which other application is also 
filed in the name of Motorola Limited and which is incorporated herein by 
reference. In this other co-pending application, non-tandem operation 
between transcoders of a communication network is achieved by identifying 
transcoder compatibility. Generally (but without specific limitation), the 
co-pending application discloses the concept that a first transcoder is 
arranged to initially double encode information by transcoding, for 
example, single encoded speech with pulse code modulation. Periodically, 
every consecutive word in a double encoded frame has its least significant 
bit substituted with sequential bits of a predetermined data pattern 
(indicative of transcoder compatibility) to produce an embedded slow-rate 
data channel. A second transcoder, arranged initially to receive double 
encoded frames, searches for the predetermined data pattern, and is 
arranged to re-configure itself to a vo-coder by-pass mode (i.e. 
non-tandem operation) if the predetermined data pattern is found. A 
feedback mechanism causes the first transcoder to adopt a vo-coder by-pass 
mode, whereby single encoded speech, for example, is routed directly 
through the first and second transcoders without the need of applying 
double encoding and its associated decoding. 
SUMMARY OF THE INVENTION 
According to a first aspect of the present invention there is provided a 
method of modifying the routing of a call between a first and a second 
communication unit of a communication system having a transcoder coupled 
to receive information from the first communication unit via a switch, the 
method comprising the steps of: a) producing an encoded signal by having 
the transcoder embed into the information a call identity identifying how 
the call is routed through the communication system; b) communicating the 
encoded signal into the communication system; c) at the transcoder, 
receiving a signal from the communication system and attempting to detect 
the call identity in the signal; and d) in the event that the call 
identity is detected, causing the call to be routed to the second 
communication unit only via the switch that has been thus identified as 
serving both the first and second communication units. Typically the call 
comprises traffic channel information and associated control channel 
information, but only traffic channel information is routed directly via 
the switch to the second communication unit in step d). 
In a second aspect of the present invention there is provided a transcoder 
coupled to receive information via a switch from a first communication 
unit of a communication system arranged to route a call between the first 
communication unit and a second communication unit, the transcoder unit 
comprising: a) means for producing an encoded signal by embedding a call 
identity identifying how the call is routed through the communication 
system; b) means for communicating the encoded signal into the 
communication system; c) means for receiving a signal from the 
communication system and for attempting to detect the call identity in the 
signal; and d) means, responsive to the detection of the call identity, 
for causing the call to be routed to the second communication unit only 
via the switch thus identified as serving both the first and second 
communication units. 
In a preferred embodiment, the transcoder further comprises: memory for 
storing a predetermined bit sequence; and means for modifying control 
information for the call to include the predetermined bit sequence that 
indicates that the call between the first and second communication units 
has been routed only via the switch. The transcoder further includes means 
for detecting the predetermined bit sequence in the signal received; and 
means for reverting the routing of the call through the communication 
system in the event that the predetermined bit sequence is undetected. 
The present invention significantly reduces the recurring costs of 
interconnection between BTS and BSC sites by locally routing the call 
traffic within the BTS site, both for mobile-to-mobile calls and 
mobile-to-fixed network (subscriber) calls. 
An exemplary embodiment of the present invention will now be described with 
reference to the accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIG. 1 in which there is shown a block diagram of a 
communication system 10 according to a preferred embodiment of the present 
invention. A cellular portion of the communication system 10 can be 
considered to include a MSC 12 coupled via a first transcoder 14 (and 
through a communication resource 15 capable of supporting both traffic 
channel and control channel information) to a BSC 16. The BSC 16 is 
coupled to a shared circuit resource manager 18 which controls the 
operation of a first cross-point switch driver 20 (or the like), which in 
turn operates a first switch 22 that couples a traffic channel resource 24 
to the BSC 16. The first switch 22 is also coupled via the traffic channel 
resource 24 to a second switch 26 that operates under the control of a 
second cross-point switch driver 28 that in turn is coupled to a BTS 30. 
The BTS 30, which may be a picocellular BTS, is also coupled via the 
traffic channel resource 24 to the second switch 26 and hence to the BSC 
16 and MSC 12. The term traffic channel resource is used to describe, 
generally, a multitude of traffic channels having associated header 
information bits. The BSC 16 is additionally coupled to the BTS 30 by a 
control channel resource 32 that is used to regulate information flow over 
the traffic channel resource and for general system control, as will be 
understood. 
Generally, in relation to FIG. 1 only (and as will be appreciated), dotted 
interconnections represent dedicated control (signalling) channel 
resources, while solid interconnections are indicative of traffic channel 
resources and dashed interconnections support both control and traffic 
channel resources. Furthermore, the shared channel resource manager 18 
will be implemented, typically, as a software function in a BSC, although 
it has been shown as a functionally distinct block in FIG. 1. Similarly, 
the cross-point switch drivers may be co-located within either a BSC, a 
BTS or a switch. 
With respect to an interconnected landline portion of communication system 
10, the MSC 12 is coupled via a second transcoder 34 (and through both a 
communication resource 36 capable of supporting both traffic channel and 
control channel information and a traffic channel resource 38) to a 
Private Branch Exchange (PBX) controller 40. The PBX 40 is coupled to the 
first switch 22 via a traffic channel resource 38, or to a third 
transcoder 42 via the communication resource 36. The third transcoder 42 
is further connected to the second switch 26 via another traffic channel 
resource 44. The PBX 40 (which administers the control of PBX signalling 
within a local landline system and which therefore performs a function 
analogous to that of BSC 16) is also coupled via a control channel 
resource 46 to the shared circuit resource manager 18. The third 
transcoder 42 is coupled to an echo cancellor 48 via the traffic channel 
resource 44 and the communication resource 36, while a PBX 50 is connected 
to the traffic channel resource 44 and the communication resource 36. 
Generally, as will be appreciated, the interconnections between, on the one 
hand, the second switch 26, the BTS 30 and the third transcoder 42 and, on 
the other hand, the first switch 22, the BSC 16 and the PBX 40 can be 
achieved over a common link, such as provided by a dedicated E1 link (or 
the like). 
To implement the present invention in which a call is locally routed 
whenever possible, two distinct operating phases are required, namely: (i) 
identification of which calls can be locally routed; and (ii) 
implementation of local traffic routing. 
The mechanism used to identify when to apply local routing is based on a 
technique in which a transcoder embeds a data pattern (that contains a 
unique call identity) into, typically, every consecutive word of a 
selected double-encoded frame. Typically, the data pattern is embedded in 
an uplink by altering at least one bit of some words in the selected frame 
to replicate (over time) the data pattern as an embedded slow-rate data 
channel. It is therefore usual that the bits selected for alteration in 
the word are of relatively minor importance (significance) in relation to 
the reproduction of any information relayed in the word. 
In the event that the transcoder embedding the data pattern in the uplink 
also receives in a downlink the same data pattern (and hence the unique 
call identity), the system realises that local routing is possible. 
Alternatively, the communication system contains sufficient intelligence 
to relate a particular transcoder in an uplink from a BTS of a cellular 
communication system with a different transcoder coupled to a PBX that is 
served by a switch commonly shared between the BTS and the PBX. In both 
instances, the switch 26 coupled to the BTS can be instructed by the 
cross-point switch driver (in response to the shared circuit resource 
manager) to route calls locally and thereby eliminate the requirement to 
send traffic channel information between, principally, the second switch 
26 (or the third transcoder 42) and the first switch 22 (or the PBX 40). 
However, control information must still be communicated between these 
otherwise isolated circuits to maintain proper system operation and to 
ensure that the local routing mechanism can be terminated if a need 
arises. Specifically, signalling for call set-up, tear-down and 
supplementary services must be routed directly (and transparently) between 
the MSC 12 and either the fixed network (i.e. PBX 50) or BTS 30. Once a 
circuit has been identified for local routing, the traffic (e.g. speech or 
data communications) can be routed directly between channel coders within 
a BTS (for mobile to mobile calls) or between a BTS and an external fixed 
network. 
As will be understood, when local routing has been activated by the 
selective closure of the first and second switches (22 and 26, 
respectively), header information in a locally routed traffic channel is 
sufficient to allow the system to locally route a call, and therefore the 
necessity for an additional dedicated control channel (at a local level) 
may be dispensed with. 
Turning now to FIG. 2 which illustrates an exemplary format for an embedded 
data pattern used as a communication protocol in the communication system 
of FIG. 1. If we consider, for exemplary purposes, that a frame of 
information generated by a transcoder contains one-hundred and sixty 8-bit 
samples, then the embedded data pattern should ideally contain no more 
than one-hundred and sixty bits (and probably many less than this). 
Typically, each of the bits in the data pattern is, preferably, 
sequentially substituted for respective bits of each 8-bit sample of a 
selected frame to modify its information content and to produce the 
slow-rate embedded data channel. The data pattern contains the unique call 
identify 60, which unique call identity is typically preceded by a 
synchronisation pattern 62, voice coding data bits 64 and signalling state 
bits 66. Additionally, the data pattern will contain spare bits 68 that 
may be used as an addressee field for the transcoder originating the 
information transfer and parity check bits 70 for error detection and 
correction purposes. 
The call identity 60 could be compiled from the combination of an MSC 
number, a BSS number and an air-interface or fixed line circuit number. 
The voice coding data bits 64 identify a frame type (such as frame having 
either a GSM frame format, a GSM extended frame format, or a half-rate or 
a full-rate format) and may contain four or more bits to allow sufficient 
scope for defining frame types likely to be encountered. The signalling 
state bits 66 represent an additional indication of transcoder intention, 
and may be used to identify that the encoding scheme is either double 
encoded PCM, is speech 16 kbit/s speech encoded or is about to change 
between these two extremes. As such 2-bits of information would be 
sufficient, with values accordingly assigned from logical 00 to logical 
11. In terms of the predetermined data pattern, a technique such as HDLC 
(High-speed Data Link Control line) framing may be used where maximum 
protection is required. 
With specific regard to implementation of the present invention in a GSM 
environment, the method used to identify when to implement a local routing 
scheme is based on sending the data pattern in-band on the least 
significant bits of a selected frame of the 64 kbit/s voice circuits that 
are communicated to the MSC. The in-band (embedded data channel) will be 
ostensibly undetectable to normal voice calls that are initially active 
and are not subject presently subject to local routing. In cases where a 
transcoder subsequently identifies the call identity in the data pattern 
as being the call identity that it originally encoded, the transcoder 
applies a predetermined bit sequence to a selected number (say, four) of 
the most significant bits of the 64 kbit/s voice circuits to indicate that 
local routing is active and to set up the local routing mode within other 
interconnected and associated equipment in the system. 
FIG. 3 shows, in some respects, a more detailed block diagram of the 
communication system of FIG. 1, including an indication of a preferred 
TRAU frame communication protocol 80 emanating from a transcoder XCDR. As 
can be seen, the transcoder contains a processor 82 that is coupled to 
memory 84 containing information 86 associated with the data pattern and 
the predetermined bit sequence 88. The processor 82, in response to 
entering the local routing mode, encodes its PCM output with the 
predetermined bit sequence. The processor 82 is also arranged to 
orchestrate recovery of any encoded data pattern that is receives. The 
transcoder XCDR continues transmitting the predetermined bit sequence on 
the four MSBs to the MSC, whereupon the MSC routes this predetermined bit 
sequence onward within the communication system. As such, a circuit in the 
transcoder that is actively participating in a locally routed call should 
continue to receive the predetermined bit sequence, but in the event that 
this circuit ceases to receive the predetermined bit sequence (because the 
MSC has changed the call routing or has terminated the call) the 
transcoder will identify the change in status (condition) in the circuit 
and will notify the BTS to revert to normal routing of the call. This 
structure therefore ensures that the communication system quickly reverts 
to normal routing operation, although slight tuning of timing parameters 
for handling the termination of local routing may be necessary in order to 
avoid irrelevant processing during call clear-down. 
It will be understood and appreciated that some processing of the TRAU 
frames is required because the uplink and downlink framing is not 
identical, and the transcoder is normally flow controlled by the BTS 
channel coding process. Specifically, the 3 kbit/s ancillary (or header) 
information is adapted to ensure that up-link and down-link communications 
are appropriately structured to guarantee proper routing and recognition. 
Additionally, a preferred embodiment of the present invention contemplates 
the use of the channel coding process to implement flow control function 
normally dealt with by transcoders of the cellular or fixed networks, i.e. 
the synchronisation of the TRAU frame generation process is usually 
controlled by the channel coder and implemented in the transcoder. The 
channel coder must now be able to perform both function and so is adapted 
accordingly, and transcoders within the cellular and fixed site portions 
of the system therefore need to be in controlled communication, as will be 
understood. 
FIG. 4 is a flow diagram illustrating local routing according to a 
preferred embodiment of the present invention. The process begin a block 
100. At block 102 a transcoder receives information, and then embeds (at 
104) the data pattern into PCM encoded data, for example. The data pattern 
can be embedded every frame at the expense of signal quality, or 
periodically if some delay in the establishment of local routing is 
acceptable. The transcoder then determines 106 whether the unique call 
identity embedded in the data pattern in up-link is returned in the 
downlink. If the unique call identity is missing, then normal routing 108 
of the communication is required, and the process returns to block 102. In 
the event that the same unique call identity is received by the transcoder 
in the downlink, local routing is executed at 110. In this operating mode, 
the transcoder provides a control signal to the MSC by placing the 
predetermined bit sequence (block 112) on a control channel resource. 
Provided that the transcoder does not notify the BTS of local routing 
release 114, local routing continues 116, else the local routing ends 118 
and the system returns to normal operation at block 100. 
It will be appreciated that the embedding of the call identity is 
complementary to the embedding of information envisaged in the 
aforementioned co-pending UK patent application entitled "TRANSCODER AND 
METHOD FOR A NON-TANDEM CODING OPERATION", and that words within a 
selected frame not previously required to embed information could be 
modified to extend the embedded data channel to contain an indication of 
both non-tandem and local routing capabilities. Alternatively, an 
alternative bit in each word of a selected frame could be used. 
Although the communication system 10 of FIG. 1 is shown to comprise a 
cellular radio communication portion and an interconnected landline 
portion, it will however be appreciated that the present invention may be 
implemented within a purely cellular system where there is no 
interconnection to a PBX. In this case, mobile units 94 and 96 are served 
by a single BTS, as shown in FIG. 3. 
The present invention therefore advantageously provides a mechanism for 
locally routing a call through a serving BTS, which mechanism is easily 
implemented and which does not require significant modification of 
existing infrastructure. As will be appreciated, the present invention 
therefore improves overall system capacity and efficiency by avoiding 
having to transmit traffic channel information to circuits external to a 
local call, i.e. BTS to BSC and BSC to MSC traffic channel resources are 
released for use in other non-local communications. 
It will, of course, be appreciated that the above description has been 
given by way of example only, and that modifications in detail, such as 
the application of the general principal to data communication and 
particularly in the event that the communication resource between 
transcoders (MSCs) is both of sufficiently high quality and is 
sufficiently robust, may be made within the scope of the present 
invention. Furthermore, although the detailed description of a preferred 
embodiment specifically refers to the substitution of the least 
significant bits (LSBs) of a periodic frame, it will be understood that 
the bit that is actually substituted need only have relatively minor 
significance in the overall re-construction of the word and that, 
therefore, other low order bits (e.g. the next lowest order bit that is 
also fairly insignificant in relation to an 8-bit (256 level) word) may 
also be used for the purpose of embedding the predetermined data sequence. 
In this respect, the significance of a particular bit in relation to a 
particular word is dependent upon the length and structure of the word in 
question (which length and structure is determined by the particular 
application), and that it is therefore only important to select a bit that 
is of sufficiently minor importance so as not to corrupt significantly the 
information relayed in the original word. Additionally, as will be 
appreciated, although the transcoders of FIG. 1 are shown located between 
the MSC and BSC (and, indeed, are usually proximinal to the MSC in many 
systems), the present invention is not limited to this structure and 
therefore contemplates the positioning of the transcoders in other 
locations within the illustrated infrastructure, particularly in a 
position between the BTS and BSC.