CA2443079A1 - A method and apparatus for transferring data packets in communication networks - Google Patents

Abstract

A method and a transcoder for transcoding data communicated between a sender (100) and a receiver (102). Data units (202, 208) comprising information in a first format (F, IF) are received and coded into new data units (208, 210) in a second format (IF, F'), based on a current coding state (302, 502, 706) extracted from a previous coding of data. A new current coding state is extracted each time a data unit (202, 500, 700) is coded. If a received data unit is in an erroneous sequence order, the current coding state is saved and a new pseudo coding state (DS2', ES2', CS2') is eytracted for codng a next received data unit. Eventually received missing data (F2, S4) is coded into the second format (IF, F') based on the saved coding sate, and may then be used as valid data. In this way, delays are minimised and jitter buffers are not necessary, while still maintaining a high quality.

Description

A METHOD AND APPARATUS FOR TRANSFERRING DATA PACKETS IN
COMMUNICATION NETWORKS
TECHNICAL FIELD
The present invention relates generally to a method and apparatus for transferring data units from a sender to a receiver. In particular, the invention is concerned with compensating for lost or disordered data units received in a transcoder.
BACKGROUND OF THE INVENTION AND PRIOR ART
Recently, systems and solutions have been developed for providing packet-based transmission of digitally encoded information for a variety of services, such as telephony, video telephony, television and video distribution, in addition to Tnternet surfing and e-mail or the like. These services are delay sensitive to a varying extent and are often classified accordingly for selecting suitable transmission mechanisms. The most delay sensitive applications, such as telephony and video telephony, are sometimes referred to as real-time services demanding a total transmission time from sender to receiver of not more than approximately 200 miliseconds. Less delay sensitive applications, such as downloading of data, e.g., homepages from the Internet, are sometimes referred to as best effort services, for which a transmission time of a couple of seconds is mostly acceptable. Thus when transmission resources are shared; real-time data is typically given priority over best effort data.
The information to be transmitted is digitally encoded at the sending side and fitted into data units,.
e.g., data packets or frames, of a specific format according SUBSTITUTE SHEET (RULE 26) to a coding scheme. The data units are handled as transmission units along the transmission path which may for example comprise various networks, switches, gateways and routers. The encoded information is finally decoded when received at its destination in order to be presented or played to a user at the receiving end. However, due to standard discrepancies, the sending and receiving equipment are often only capable of using specific coding schemes being different from each other, requiring that the information encoded by the sender is re-coded somewhere before reaching the receiving party. This operation is often referred to as transcoding.
In addition, different networks and/or switching nodes along the transmission path may also use formats and coding schemes of different standards, resulting in further needs for transcoding. Thus, the transmitted data may have to be re-coded several times before reaching the final receiver. For real-time services, and to a certain extent also media streaming services, it is desirable to minimise the delays due to transcoding activities in nodes of the total transmission path. For example, if a delay of more than 200 miliseconds is introduced in a voice call between two parties, the dialogue flow becomes considerably disturbed.
Fig. 1 illustrates an exemplary communication scenario for transferring data between a sender 100 and a receiver 102. In this simplified example, asymmetric transmission in only one direction at a certain point of time is considered. For symmetric transmissions, each party 100 and 102 is of course able to act as both sender and receiver, such as for voice calls. The sender 100 is connected to an access network 104 which may be a Local Area SUBSTITUTE SHEET (RULE 26) Network (LAN), a wireless network or the like. The receiver 102 is likewise connected to another access network 106.
Each access network 104, 106 is further connected to respective gateway switches 108, 110 for communication over an intermediate communication system 112, which may comprise one or more public networks such as the Internet or a Public Switched Telephony Network (PSTN).
The sender 100 is capable of using a first coding scheme for encoding the data to be transmitted. The receiver 102, on the other hand, is capable of using a second coding scheme for decoding the received data, the. second coding scheme being different from the first. Moreover, the intermediate communication system 112 may use one or more further coding schemes in different parts thereof for transferring the communicated data. It is thus evident that the communicated data must be re-coded each time it enters a new domain using a coding scheme different from the one used bef ore .
For example, transcoding must be done between packet switched networks and circuit switched networks using, e.g., Time Division Multiplexing (TDM). The data is thus transferred over one or more transcoders for creating new data units from the received previous ones before being transmitted further in the transmission path. The gateways 108, 110, as well as other switching nodes within the intermediate communication system 112 in Fig. 1, may typically comprise such transcoders.
Furthermore, if one or more of the networks 104, 112, 106 use packet based transmission, there is a potential risk of the data packets being received at the transcoders in the wrong order. In packet based transmission, each data packet is treated individually and may take separate SUBSTITUTE SHEET (RULE 26) transmission paths and/or be subjected to different delays in intermediate nodes. Therefore, a buffer is often arranged in transcoders and receivers, sometimes referred to as a fitter buffer, to compensate for packets being received with different delays. The buffer enables the reception of a plurality of data packets arranged in a queue before being transcoded or decoded. The packets may then be re-ordered in the queue if necessary. Another method is to simply discard packets received in the wrong order, although resulting in a degradation of quality. The receiving side of an intermediate node between a packet domain and a TDM domain typically includes a fitter buffer.
fitter-buffering techniques entail the drawback of introducing unwanted delays in the transmission. It also requires a clock timing reference when TDM based transmission is involved. Furthermore, the final receiver 102 typically includes a fitter buffer or similar function for decoding, adding to the total latency of the transmitted data. A transcoding function should not add more latency than the time required to process the data and to perform the transcoding. However, if every data packet is sequentially transcoded as they arrive, the resulting quality will be degraded when arriving in the wrong order.
In particular, many coding schemes rely on historic data when decoding a particular chunk of data,~i.e., information from the previously decoded data is used fox decoding the next data. All data must therefore be decoded in the same sequence order as it was outputted from the sending encoder in order to acquire high quality at the receiving end.
It is an object of the present invention to overcome the drawbacks outlined above and achieve a high quality of received encoded data when being decoded at the SUBSTITUTE SHEET (RULE 26) receiving end, at the same time minimising transmission delays in intermediate transcoders.
SUMMARY OF THE INVENTION
These and other objects are achieved by a simple solution for minimising delays in transcoders and eliminating the need for fitter buffers, wherein data units arriving in an erroneous sequence order are compensated for by restoring late arriving data.
A method and a transcoder unit are provided for transcoding communicated data in a transmission path between a sender and a final receiver. Data units comprising coded information in a first format are received and coded into new data units in a second format, based on a current coding state extracted from a previous coding of data. A new current coding state is extracted each time a data unit is coded.
When receiving each data unit, it is detected whether the received data is in an erroneous sequence order, which may be done by reading information indicating a sequence order of the received data. If that is the case, the coding state extracted from the previous coding of data is saved and a new pseudo coding state is extracted for coding a next received data unit.
Tf missing data is eventually received, it is coded into the second format based on the saved coding state, and the coded missing data may be used as valid data.
The saved coding state may then be deleted. The saved coding state may also be deleted if the corresponding missing data is not received within a pre-set time period, which may be based on data flow characteristics. The saved coding state may also be deleted if the number of saved decoding states SUBSTITUTE SHEET (RULE 26) reaches a pre-set threshold value, which likewise may be based on data flow characteristics.
The inventive procedure may be used for decoding received data units, wherein the coding states are decoding states and that the second format is an intermediate format.
The received data units may then be decoded into a stream of intermediate data divided into intermediate data segments which are encoded into new data units.
The inventive procedure may also be used for encoding received data units, wherein the coding states are encoding states and the first format is an intermediate format. The received data units may then comprise decoded intermediate data which is encoded into new data units.
If the received data is in an erroneous sequence order, error concealed data may be derived from the previously received data unit and the current decoding state, wherein the new pseudo coding state can be extracted based on the derived error concealed data.
The inventive procedure may be implemented in a transcoder unit located in the transmission path between the sender and the final receiver, and having means for performing the procedure.
The inventive procedure may also be implemented in a computer program product directly loadable into a computer in the transcoder unit, comprising software code means for performing the procedure.
The inventive procedure may alternatively be implemented in a computer program product stored on a computer usable medium, comprising a readable program for causing a computer in the transcoder unit to perform the procedure.
SUBSTITUTE SHEET (RULE 26) BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described in more detail and with reference to the accompanying drawings, in which:
Fig. l is a schematic view of an exemplary communication system for transferring data.
Fig. 2 is a schematic view of the data flow in a transcoder.
Fig. 3 is a logical scheme illustrating the decoding of data packets arriving in the right order.
Fig. 4 is a logical scheme illustrating the decoding of data when a data packet arrives in the wrong order.
Fig. 5 is a logical scheme illustrating the encoding of intermediate data.
Fig. 6a is a logical scheme illustrating the encoding of data when a data unit is lost.
Fig. 6b is a logical scheme illustrating the encoding of data when missing data arrives late.
Fig. 7 is a logical scheme illustrating generally the transcoding of data when a data unit arrives in the wrong .
order.
DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 1 illustrates an exemplary communication system, described above, for transferring data between a sender 100 and a receiver 102, in which the present invention may be implemented. Of course, the invention may also be used in numerous other types of communication systems.
Fig. 2 is a schematic view of the data flow in an exemplary transcoder unit 200 for transcoding data according to the invention. Data 202, encoded according toga first coding scheme and having a first format F, is received by SUBSTITUTE SHEET (RULE 26) the transcoder unit 200 for transcoding the data into a second coding scheme having a second format F'. The transcoding process is in this example divided into two operations, a decoding operation 204 and an encoding operation 206. Between the two operations, the data is in a decoded intermediate format IF 208. However, it is possible within the scope of the invention to re-code or transcode the incoming data 202 of the first format F directly into the second format F' without using any intermediate format.
After being coded into the second format F', the data 210 is transmitted further on to the next node, not shown, in the transmission path. Furthermore, the transcoder comprises a memory 212 for storing coding states, which will be described in more detail below.
Typically, the incoming data is grouped into data units, e.g., packets or frames, encoded according to the first format F. The incoming data may be packet switched or circuit switched. Each incoming data unit includes a field or the like with information indicating a sequence order, such as a sequence number or a time stamp, which is read by the receiving transcoder unit 200. In the normal case, the received data units are decoded sequentially in operation 204 into the next format IF or F' if the data units are received in the correct order as they were originally transmitted and with no lost data.
An example of this is illustrated in Fig. 3, with further reference to Fig. 2, wherein F1, F2, F3... are data packets 202 of the first format F being received in the correct order at the transcoder unit 200. The data packets 202 are decoded separately in operation 204 into intermediate data units 300 having the intermediate format TF. When decoding each data packet 202, information from the SUBSTITUTE SHEET (RULE 26) previous decoding result, which is represented in Fig. 3 as a series of current historic decoding states DS 302, is used. A decoding state comprises variables or parameters, apart from the actual data, extracted from the performed decoding operation and reflecting some characteristics of the data.
Thus, a received data packet F1 is decoded 204 into an intermediate data unit IFl based on a current decoding state DSO being extracted from the decoding result of a previously decoded data packet, not shown. After using the decoding state DSO, a new decoding state DS1 is extracted from the latest decoding of F1. The next data packet F2 is then decoded 204 into IF2 based on the last decoding state DS1, and so on.
In this description, it is generally assumed that all incoming data packets have equal size, although in reality, the packet size may vary. In that case, the data may be rearranged into equally sized units, but the transcoding process can be handled using the same principles as in the simplified case of equal packet size.
The-decoding procedure illustrated in Fig. 3 involves data packets received in the correct sequence order. However, if a data packet is received having a sequence number higher than the previous sequence number incremented by one, it may be assumed that the missing packet is lost or delayed. Alternatively, the packet order may be indicated by a time stamp or any other order indicating information. If a data packet is thus missing, an error concealment mechanism may be invoked in order to minimise the resulting quality degradation at the receiving end. The error concealment mechanism may derive error concealed data based on the previous data packet and the SUBSTITUTE SHEET (RULE 26) latest decoding state extracted from the decoding of the previous data packet. Error concealment is a well-known procedure which attempts to create the most likely data when no real data is available. The error concealment is typically performed according to a prescribed standard for the data format, or if no standard exists, a proprietary mechanism may be used.
According to one aspect of the invention, an error concealment mechanism is invoked when data is received in the wrong sequence order, and the latest decoding state before the error concealment is saved for later use if the missing data packet eventually turns up, although in the wrong order. In that case, the missing data packet is decoded based on the saved decoding state according to the invention.
Fig. 4 illustrates an exemplary decoding procedure according to the invention when a data packet is received in the wrong sequence order. First, a data packet F1 is decoded 204 into the intermediate data unit IFl based on the latest decoding state DSO, wherein a new decoding state DS1 is extracted, as in the previous example of Fig. 3. However, next comes a data packet F3 having a sequence number 3 higher than the previous sequence number 1 incremented by one. Upon detecting that the sequence number of the received data packet F3 is erroneous, i.e. higher than expected, an error concealment mechanism 400 is invoked for deriving error concealed data 402 in the intermediate format and for extracting a new pseudo decoding state DS2', based on the previous decoding state DS1. The pseudo decoding state DS2' may be extracted in the same way as the other decoding states since the error concealment is a decoder function in.
SUBSTITUTE SHEET (RULE 26) which a decoding state can be extracted regardless of whether real data is available.
The data packet F3 is then decoded 204 into the intermediate data unit IF3 based on the new pseudo decoding state DS2'. Furthermore, the previous decoding state DS1 is saved in the memory 212 in the transcoder unit 200. A new decoding state DS3 is also extracted from the decoding 204 of data packet F3 for later use when decoding a data packet of the next sequence order, not shown.
In this example, the missing data packet F2 is received next and can be decoded 204 into the intermediate data unit IF2 based on the saved previous decoding state DS1. The used saved decoding state DS1 may thereafter be deleted from the memory 212 in the transcoder unit 200.
Finally, the restored intermediate data unit IF2 is inserted in the right sequence order between IF1 and IF3, replacing °the earlier derived error concealed data 402. However, if the missing data packet F2 is not received, e.g., within a pre-set time period, the error concealed data 402 may remain valid when the data of the intermediate format 208 is encoded 206 into the second format F' in the transcoder unit 200.
In this way, valid and decoded intermediate data units 300 can be arranged in a correct sequence order before being encoded again into the second format F', even if one or more data units in the first format F was received at the transcoder in an erroneous order. Furthermore, this procedure is conducted with a minimum of delay, maintaining a highest possible quality.
The example of Fig. 4 described above was concerned with one data packet received out of sequence. A
corresponding procedure may be conducted if multiple data SUBSTITUTE SHEET (RULE 26) packets are received out of sequence using multiple saved decoding states. Furthermore, a saved decoding state may be deleted from the memory 212 if it has remained unused for a pre-set time period. The pre-set delete time period may further be configurable or adaptive depending on data flow characteristics, such as the number of data packets of erroneous sequence order varying over time. Alternatively, a number of simultaneously saved decoding states may be maximised by a pre-set threshold value, which may likewise be configurable or adaptive depending on data flow characteristics. If the number of saved decoding states reaches the pre-set threshold value, a mechanism may be triggered for deleting individual saved decoding states from the memory 212.
After the decoding operation 204, the intermediate data units 300 are arranged into a continuous stream of data 208 which is fed into the encoding operation 206, see Fig.
2. The transcoder unit 200 then encodes the intermediate data 208 into new data units such as packets, frames or blocks, i.e., data portions of a predetermined size, according to the new second coding scheme or format F' depending on the availability of data. If no intermediate data 208 is currently available, the encoding process may simply wait until there is. The new data units in the second format F' are finally transmitted to the next node in the transmission path.
Fig. 5 illustrates an encoding procedure according to the invention for the intermediate data stream 208 comprising the intermediate data units IF1, IF2, IF3... from the previous decoding operation 204. First, the intermediate data is received and may be arranged into new intermediate data segments~Sl, S2, 53... 500, if necessary, to be encoded SUBSTITUTE SHEET (RULE 26) into the second format F'. The size of the new intermediate data segments 500, indicated by vertical dashed lines in the figure, is in this example different from the size of the intermediate data units 300 coming from the decoding operation 204. Each new intermediate data segment or unit 500 is then encoded 206 into a new data unit 506 of the second format F' based on a current encoding state ES 502 extracted from a previous encoding operation. An encoding state comprises variables or parameters, apart from the actual data, extracted from the performed encoding operation and which is used for encoding data.
Thus, an intermediate data segment Sl is encoded 206 into a new data unit F'1 based on a last encoding state ESO being extracted from the previously encoded new data packet, not shown. After using the encoding state ESO, a new encoding state ES1 is extracted from the latest encoding of F'1. The next intermediate data segment or unit S2 is then encoded 206 into F'2 based on the last decoding state DSL, and so on.
The intermediate data units 300 may be classified according to the nature of their contents such that "good data" has been created directly from payload of an incoming data packet or unit, and "error concealed data" has been derived from the latest decoding state by using the error concealment mechanism 400 when no incoming data packet or unit of a correct sequence order was available. In the example of Fig. 5, IF1. and.IF2 contains 'good data', whereas IF3 contains 'error concealed data', in the figure indicated with strokes. Consequently, segments S1 and S2 contain only 'good data' but segments S3 and S4 contain both 'good data' and 'error concealed data' as a result from the difference SUBSTITUTE SHEET (RULE 26) of size between the intermediate data units 300 and the intermediate data segments 500.
According to one aspect of the invention, the intermediate data segments 500 are treated differently depending on the classification of the data. Thus, .if a segment 500 contains at least some 'good data', the segment is encoded and a new encoding state is extracted accordingly. This is the case for all segments Sl - S4 shown in Fig. 5.
However, if an entire segment contains no 'good data', i.e., only 'error concealed data', the segment may simply be discarded and the latest encoding state extracted at least partly from 'good data' is saved. This is illustrated in Fig. 6a, wherein a segment S1 containing 'good data' is encoded into a new data unit F'1 and a new encoding state ES1 is extracted. The next segment S2 contains 'error concealed data' and no 'good data', which is indicated with strokes in the figure, and is therefore discarded 600. One new data unit is thereby considered to be lost. The latest encoding state ES1 is saved for later use, should the missing data eventually arrive.
With reference to Fig. 6b, a segment S1 containing 'good data' is encoded into a new data unit F'I and a new encoding state ES1 is extracted, just as in the example of Fig. 6a. The next arriving segment S2 contains 'error concealed data' and no 'good data', again indicated with strokes in the figure. According to an alternative embodiment, the segment S2 is first encoded into a pseudo data unit 602, based on the available error concealed data, which is then discarded 600. In this case, a new pseudo encoding state ES2' can. be extracted from the encoding of S2, to be used for encoding the next segment, not shown. In SUBSTITUTE SHEET (RULE 26) this example, the missing data eventually arrives later in segment S4, which may be encoded into a new valid data unit F'2 based on the earlier saved encoding state ES1. It should be noted that the sequence order of the segments S1 - S4 is selected from the division of the intermediate data stream for illustrative purposes, and is not to be confused with the sequence order of the actual data. Thus in this example, segment S4 contains data that should have been included in segment S2.
The restored new data unit F'2 may then be' transmitted to the next node in the transmission path.
Furthermore, it is possible to send F'2 in an erroneous sequence position, in this case after F'3, since the final receiver is often capable of restoring the correct data unit order before decoding.
The procedure described above in connection with Fig. 6b may also be performed fox multiple segments arriving out of sequence, using multiple saved encoding states.
Moreover, the same mechanisms as described above for saving multiple decoding states can generally also be applied for saving multiple encoding states.
The examples described above include the two separate operations of first decoding received data from a first format to an intermediate format, and then encoding the intermediate data into a second format for transmission.
However, the invention may also be used for re-coding incoming data of the first format directly into the second format without using an intermediate format.
Fig. 7 illustrates the most general case according to the invention of a re-coding or transcoding procedure when a data unit is received in the wrong sequence order.
Generally, F1, F2, F3... denote incoming data units 700 having SUBSTITUTE SHEET (RULE 26) a first format F. The data units 700 are coded separately in operation 702 into new data units 704, in the figure generally denoted as F'1, F'2, F'3..., having a second format F'. The expression "coding" is used here for denoting any of decoding, transcoding or encoding. When coding each incoming data unit 700, information from a previous coding result, which is represented in Fig. 7 as a series of current historic coding states CS 706, is used.
Thus, a data unit F1 of the first format F is coded 702 into a new data unit F'1 of the second format F' based on a current coding state CSO, wherein a new coding state CS1 is extracted. The next received data unit F3 includes a sequence number 3 higher than the previous sequence number 1 incremented by one. Upon detecting that the sequence number of the received data unit F3 is higher than expected, pseudo data 708 of the second format F' is created based on the current coding state CS1 for extracting a new pseudo coding state CS2'. The data unit F3 is then coded 702 into a new data unit F'3 based on the pseudo coding state CS2'. Furthermore, the previous coding state CSl is saved in the memory 212. A new coding state CS3 is also extracted from the coding 702 of data unit F3 into the new data unit F'3.
The missing data unit F2 is received next and can be coded 702 into a new data unit F'2 based on the saved previous coding state CS1. The used saved coding state CS1 may thereafter be deleted from the memory 212. Finally, the restored data unit F'2 is used as valid data and the earlier created pseudo data 708 is discarded. However, if the missing data unit F2 is never received within a pre-set time period, the pseudo data 708 may be used as valid data or may be discarded, depending on the implementation.
SUBSTITUTE SHEET (RULE 26) By using the invention as described above, delays in transcoders are minimised and the need for fitter buffers is eliminated, while still maintaining a high quality of transmitted data. Specifically, data units arriving in an erroneous sequence order are compensated for by restoring late arriving data according to the invention. The invention is primarily aimed to be used for speech transcoding, but may also be used for, e.g., fax transcoding, modem data or packet based video signals.
The inventive method may be implemented in a software code comprised in a computer program product, directly loadable into a computer in the transcoder unit 200 or stored on a computer usable medium.
Glhile the invention has been described with reference to specific exemplary embodiments, the description is only intended to illustrate the inventive concept and should not be taken as limiting the scope of the invention.
Various alternatives, modifications and equivalents may be used without departing from the spirit of the invention, which is defined by the appended claims.
SUBSTITUTE SHEET (RULE 26)

Claims (18)

1. A method of transcoding communicated data in a transmission path between a sender (100) and a final receiver (102), comprising the steps of:

- A) receiving data units (202,500,700) comprising coded information in a first format (F,IF), - B) coding (204, 206, 702) the received data units (202,500,700) into new data units (300,506,704) in a second format (IF,F'), based on a current coding state (302,502,706) extracted from a previous coding of data, and - C) extracting a new current coding state (302,502,706) each time a data unit (202,500,700) is coded, characterised by the further steps of:

- D) detecting whether the received data is in an erroneous sequence order, and if so:

- E) saving the coding state (302,502,706) extracted from the previous coding of data and extracting a new pseudo coding state (DS2',ES2',CS2') for coding a next received data unit (202, 500, 700) a

2. A method according to claim 1, characterised by the further steps of:
- F) receiving missing data (F2,S4), - G) coding the missing data (F2,S4) into the second format (IF,F') based on the saved coding state (302,502,706), and - H) using the coded missing data (IF2,F'2) as valid data.

3. A method according to claim 2, characterised in that the saved coding state (302,502,706) is deleted after step G).

4. A method according to claim 1, characterised in that the saved coding state (302,502,706) is deleted if the corresponding missing data is not received within a pre-set time period.

5. A method according to claim 4, characterised in that the pre-set time period is based on data flow characteristics.

6. A method according to claim 1, characterised in that the saved coding state (302,502,706) is deleted if the number of saved decoding states reaches a pre-set threshold value.

7. A method according to claim 6, characterised in that the pre-set threshold value is based on data flow characteristics.

8. A method according to any of claims 1 - 7, characterised in that the received data units (202) are decoded in step B), the coding states are decoding states (302) and that the second format is an intermediate format (IF).

9. A method according to claim 8, characterised in that the received data units (202) are decoded in step B) into a stream of intermediate data (208) divided into intermediate data segments (500).

10. A method according to claim 9, characterised in that the intermediate data segments (500) are encoded into new data units (506).

11. A method according to any of claims Z - 7, characterised in that the received data units (500) are encoded in step B), the coding states are encoding states (502) and the first format is an intermediate format (IF).

12. A method according to claim 11, characterised in that the received data units (500) comprise decoded intermediate data (208) which is encoded in step B) into new data units (506).

13. A method according to any of claims 1 - 12, characterised in that the new pseudo coding state (DS2',ES2',CS2') extracted in step E) is used for coding a next received data unit (F3,S3).

14. A method according to any of claims 1 - 13, characterised in that step A) comprises the further step of reading information indicating a sequence order of the received data (202,500,700,F2,S4).

15. A method according to any of claims 1 - 14, characterised in that if the received data is in an erroneous sequence order, error concealed data (402) is derived from the previously received data-unit and the current decoding state (302,DS1) and the new pseudo coding state (DS2',ES2',CS2'), is extracted in step E) based on the derived error concealed data (402).

16. A transcoder unit (200) located in the transmission path between the sender (100) and the final receiver (102), and having means for performing the method of any of claims 1 -15.

17. A computer program product directly loadable into a computer in a transcoder unit (200), comprising software code means for performing the method of any of claims 1 -15.

18. A computer program product stored on a computer usable medium, comprising a readable program for causing a computer in a transcoder unit (200) to perform the method of any of claims 1 - 15.