WO1999066760A1

WO1999066760A1 - Method for correcting transmission errors in a communication link

Info

Publication number: WO1999066760A1
Application number: PCT/DE1999/001609
Authority: WO
Inventors: Klaus Hünlich
Original assignee: Siemens Aktiengesellschaft
Priority date: 1998-06-15
Filing date: 1999-06-01
Publication date: 1999-12-23
Also published as: DE19826584A1

Abstract

The invention relates to a method for correcting transmission errors in a communication link, preferably an ATM communication link, comprising the following steps: (a) reception of the data transmitted via the communication link; (b) continuous detection and storage of prediction data derived from the received data; (c) detection of data failure in the data stream received on the reception side of the communications link; and (d) replacement of failed received data with the prediction data if a data failure is detected. In this way, in case of data failure lost information is replaced with statistically modeled prediction data, which ensures the greatest possible intelligibility during voice communication and, in particular, prevents clicking sounds or such like.

Description

METHOD FOR CORRECTING TRANSFER ERRORS IN A COMMUNICATION CONNECTION

The invention relates to a method and a device for correcting transmission errors in a communication connection.

In the known asynchronous transfer mode (ATM), data are transmitted in cells of 53 bytes (48 bytes of user data, 5 bytes of control data) regardless of the information they represent (voice communication, data communication, multimedia). The network resources of the communication system are shared between connections with different quality of service and bit rate requirements.

Various connection types are possible in an ATM network, which differ in their bit rate statistics. A connection with a constant bit rate or deterministic bit rate has a fixed transmission bit rate (cell rate) over the entire connection duration. This bit rate must be constantly made available by the network. This type of connection is particularly suitable for real-time applications such as voice communication or video telephony, for which strict requirements are placed on line delay fluctuations, and which has a comparatively constant transmission rate.

Another connection type is the available bit rate, whereby the data is transmitted depending on the network capacity currently available. This type of connection is not suitable for real-time applications, but as inexpensive data transmission such as e-mail. With the connection type variable bit rate, the data to be transmitted of a virtual connection are transmitted at a time-varying transmission rate. Examples of such a connection type are video connections in which the video signals are encoded with a variable bit rate, voice communication with pause suppression and certain data transmission services.

Variable bit rate connections where the average bit rate is well below the maximum bit rate are suitable for statistical multiplexing. Many connections with a variable bit rate are routed via a common line or via a common switching matrix, whereby it is not necessary to reserve the maximum bit rate for each individual connection, since many uncorrelated connections with a lower average bit rate compared to the maximum bit rate have the available transmission capacity share on average. It is thus possible to "overbook" the line to a certain extent. The network infrastructure can thus be better used overall.

The ATM adaptation layer (AAL) extends the service offered by the ATM layer so that the special requirements of the protocols can be met. An important task of AAL is also to adapt the different data formats of the higher protocol layers to the ATM cell format.

There are different types of ATM adaptation layers (type 0 to type 5) that are specified by international standardization. The ATM adaptation layer type 2 defined by the International Telecommunications Union in Recommendation 1.363.2 is intended for the bandwidth-efficient transmission of services of low bit rate and short data packets of variable length in delay-sensitive applications, ie real-time applications. The data stream of several AAL type 2 users can be be supported. The typical application of the AAL type 2 layer is the simultaneous transmission of several telephone calls with pause suppression.

The AAL type 2 sublayer is shown schematically in FIG. It consists of a common sub-layer (CPS) and a service-specific convergence sub-layer (SSCS). The SSCS sublayer is not mandatory and can also be omitted depending on the service used.

The structure of a CPS packet is shown in FIG. A header section contains channel identification data (CID), length identification data (LI), user-user assignment data (UUI) and error correction data

(HEC). The user information field can accommodate, for example, 64 bytes, via which, for example, five voice communication channels of 12 bytes each can be transmitted.

In an ATM communication connection, a so-called traffic control must ensure that, despite the common transmission of data cells of different origins, different bit rates and different bit rate statistics, the required transmission quality of the ATM layer is ensured. In particular, the likelihood of cell loss occurring should be kept low. Nevertheless, cell losses can never be completely ruled out in an ATM communication connection.

In speech communication, the loss of individual cells has hitherto been corrected either by repeating the last information or by showing an empty cell (idle). In many cases, this continues until valid data is received again. Both methods generally result in cracking noises in the data stream, which increase with the frequency of the losses and can impair the quality of the connection. The invention is therefore based on the object of proposing a method for correcting transmission errors in a communication connection, in which the impairment of the transmission quality caused by the transmission errors is minimized.

In particular, crackling noises that occur in a voice communication connection are to be avoided by cell losses. The task is solved by the method defined in claim 1 and the device defined in claim 13. Advantageous developments of the invention are explained in the subclaims.

The method according to the invention for correcting transmission errors in a communication connection, in particular an ATM communication connection, has the following method steps: (a) receiving the data transmitted by the communication connection,

(b) continuous determination and storage of prediction data derived from the received data

(c) detection of data failures of the data stream received at the reception side of the communication link, (d) replacement of failed reception data with the prediction data if a data failure is detected.

Failed cells are replaced by the prediction data, which are continuously derived from the last received data. The prediction data are modeled on the statistical characteristics of the most recently received data, whereby a better intelligibility of voice data, it is sufficient ^¬. In particular, crackling noises generated by empty cells are avoided. Failed received data can preferably be replaced in m units of AAL2 packets. Other data lengths of the respective prediction data, for example 12 bytes, are also possible.

According to a preferred exemplary embodiment of the invention, voice information is transmitted in subchannels of the ATM adaptation layer 2 (AAL 2).

In method step (b), the received data are preferably weighted and a statistical vector S is determined from the weighted values, it being possible for the received data to be weighted in a two-stage method.

A linear prediction method is preferably used as the prediction method. An example of this is the low delay code excited linear prediction method.

Prediction data can preferably be stored for a period of approximately 3 ms. A shorter or longer period of time is also possible within the scope of the invention, depending on the area of application and the frequency of cell failures.

The invention is described below on the basis of preferred exemplary embodiments with reference to the drawing

FIG. 1 is a schematic block diagram of the device according to the invention for correcting transmission errors in an ATM communication link; Figure 2 is a schematic representation of the structure of the Type 2 ATM adaptation layer; and Figure 3 is a schematic representation of a CPS packet of the AAL type 2 sublayer.

First, the device according to the invention for correcting transmission errors in an ATM communication connection is explained with reference to FIG. 1. From a receiving device (not shown) of the ATM communication link, the data cells arrive at the data loss detection unit 1, in which the received data stream is checked. If the information of the previous cell is completely passed on, a new ATM cell must be present each time. If no new cell has arrived within a predefined tolerance time interval, a signal failure message is forwarded to the multiplexer 5.

A two-stage statistics unit 2, 3 is connected to the data loss reception unit 1 and receives the received data and determines the statistical weights of the data stream. The statistical vectors V characterizing the respective received data are determined from these weighted values and forwarded to the prediction unit 4. The prediction unit 4 determines from the statistical vectors V a prediction or extrapolation of the data following the last received data, for example over a period of 3 ms. The prediction unit 4 preferably uses a linear prediction method. An example of this is the Low Delay Code Excited Linear Prediction (LD-CELP), which is used in the

CCITT recommendation G.728 of the International Telecommunication Union from 1992 is described in detail.

The prediction data for a fixed time interval are stored in a memory, which can be assigned to the prediction unit 4 or the multiplexer 5, for example. Does the multiplexer 5 receive from the data loss detection Unit 1 a message that a data failure has been detected, the multiplexer 5 replaces the failed cell with a cell of the prediction data.

If, for example, subchannels of ATM adaptation Layer 2 (FIGS. 2 and 3) are used for transmission and the information length of a subchannel element that forms part of the CPS user data (FIG. 3) is limited, then 12 bytes of information are also required in order to contain the content of a Subchannel element to be replaced approximately. If one uses the method mentioned and described in Recommendation G.728 of the International Telecommunication Union, the steps of weighting and formation of the statistical vector V have to be used twice in succession. You then have 5 by 5 bytes of information in order to in this case, according to the procedure in G.728, 25 missing cycles of 125 μs each, i.e. correct a total of 3.125 ms using the forecast data.

The multiplexer 5 preferably contains a buffer memory for the time compensation of the transit times between the received direct data stream and the correction data.

Due to the predictive modeling of the data, the corrections of the failed data carried out by the method according to the invention correspond to the natural course, for example of a voice communication, much better than known methods, in which either empty cells are inserted or previous cells are repeated. As a result, the quality of the communication link is improved and crackling noises or the like can be avoided.

It should be noted that the invention is not restricted to use with the AAL 2 layer and also not to the use of the linear prediction method described in ITU recommendation G.728. The invention can of course also use other types of transmission and suitable prediction methods.

Claims

claims

1. A method for correcting transmission errors in a communication connection, comprising the method steps: (a) receiving the data transmitted by the communication connection,

(c) detection of data failures of the data stream received at the receiving end of the communication link,

(d) Replacement of failed reception data with the prediction data if a data failure is detected.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that the communication link is an ATM communication link.

3. The method of claim loder 2, that the forecast data are derived as a function of statistical properties of the received data.

4. The method of claim 1, 2 or 3, that the replacement of failed received data is carried out in units of AAL2 packets.

5. The method according to any one of claims 1 to 4, characterized in that voice information is transmitted over the ATM communication link.

6. The method according to claim 5, so that the speech information is transmitted in subchannels of the ATM adaptation layer 2 (AAL 2).

7. The method of claim 6, d a d u r c h g e k e n n z e i c h n e t that the information length of a subchannel element, which is replaced by a prediction sub-element in the event of a data failure, is 12 bytes.

8. The method according to any one of claims 1 to 7, so that the received data are weighted in method step (b) and a statistical vector V is determined from the weighted values.

9. The method of claim 8, d a d u r c h g e k e n n z e i c h n e t that the weighting of the received data takes place in a two-stage process.

10. The method according to any one of claims 1 to 9, so that a linear prediction method is used in step (b).

11. The method according to claim 10, which also means that a low delay code excited linear prediction prediction method is used.

12. The method according to any one of claims 1 to 11, characterized in that in method step (b) prediction data are stored for a period of approximately 3 ms.

13. Device for correcting transmission errors in a communication connection, comprising:

a receiving unit for receiving data transmitted through the communication link, a statistical unit (2, 3) for the continuous determination of statistical properties characterizing the received data,

- a prediction unit (4) connected to the statistics unit (2, 3) for deriving forecast data as a function of the results supplied by the statistics unit (2, 3),

a storage device for storing the prediction data,

- A data loss detection unit (1) for detecting data failures in the supplied by the receiving device

Reception data,

- A multiplexer (5) for replacing failed reception data with the prediction data, if the data loss detection unit (1) detects a data loss.

14. The apparatus of claim 13, so that the multiplexer (5) has a buffer memory for the prediction data for the timely insertion of the prediction data into the received data stream.

15. The apparatus of claim 13 or 14, so that the statistical unit (2, 3) determines a statistical vector characterizing the received data.