GB2341750A

GB2341750A - AAL2 minicells

Info

Publication number: GB2341750A
Application number: GB9921345A
Authority: GB
Inventors: Eva-Maria Lanig; Stefan Schneeberger
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1998-09-18
Filing date: 1999-09-09
Publication date: 2000-03-22
Anticipated expiration: 2019-09-09
Also published as: DE19842789A1; DE19842789C2; GB2341750B; GB9921345D0

Abstract

In the method in accordance with the invention for transmitting items of speech information, items of speech information (ni) of a connection (V1) are entered on the send side into AAL2 mini cells (AAL2-Z) which are transported by means of ATM cells (ATM-Z). In a first conversion device (IWU-A), each octet of the AAL2 mini cells is entered into a time slot (ts) of a PCM frame (R), and the items of speech information in the PCM frame are thereupon switched further by means of a PCM switching device (CN). In this way, AAL2 mini cells are transmitted and switched transparently in PCM format, as a result of which a migration from existing to future communication systems is facilitated.

Description

I # 2341750 Method for transmitting items of speech information in a

communication system.

The invention relates to a method for transmitting items of speech information in a communication system, which method provides, one after the other, a transmission in AAL2 mini cells and a transmission in PCM frames.

In communication systems, different transmission formats are used for the data to be transmitted, which data is, among other things, items of speech information, packet data. Transmission in the form of PCM frames is known from the narrow-band communication systems for speech transmission. This form of transmission was used both in fixed networks and in second-generation mobile radio networks, such as the GSM (global system for mobile communications) mobile radio system. In the GSM mobile radio system, code conversion units are additionally used in order to convert a speech coding defined for the radio transmission in the access network into the speech coding of the PCM transmission.

For a future mobile radio system, for example according to the UMTS-standard (universal system for mobile communications), it is intended that the speech information in the access network, i.e. adjoining the radio interface, be transmitted in the form of AAL2 mini cells. AAL2 mini cells (ATM adaption layer type 2) are data packets of variable length which can be transported within ATM cells (asynchronous transfer mode) and are particularly suitable for uses with a low data rate and increased demands on the delay time of the transmission.

ATM switching devices which carry out a switching of AAL2 mini cells of variable or fixed lengths are known from WO 95/34977 and US 5,301,189.These AAL2 mini cells are better suited to speech transmission than ATM cells. The introduction of such an AAL2 mini cell-based ATM switching device, however, requires great financial expenditure. In the mobile radio systems which exist at the present time, the complete switching devices have to be exchanged in order to adapt them to the demands of the transmission of ATM cells, or code conversion devices have to be provided, which carry out a code conversion between PCM transmission and ATM transmission. Various evolution scenarios from existing to future communication systems are known from DTR/SMG-032330U of 25.8.98, pages 39-50.

Embodiments of the invention therefore aim to present a method for transmitting items of speech information, which method renders possible an economical transition from existing to future communication systems.

The invention is defined in the independent claim attached hereto, to which reference should now be made.

Further, preferred features may be found in the attached dependent claims.

Items of speech information of a connection are entered on the send side into AAL2 mini cells which are transported by means of ATM cells. In a first conversion device, each octet of the AAL2 mini cells is entered unaltered, i.e. without code conversion or suchlike, into a time slot of a PCM frame, and the items of speech information in the PCM frame are thereupon switched further by means of a PCM switching device.

The AAL2 mini cells are therefore transported transparently in the PCM format and can thus be switched even with a PCM switching device. The expenditure on code conversion and reconversion which would usually result when coupling ATM and PCM components of a communication network is thu s dispensed with. ATM switching centres are not required either, but in the sense of a step-by-step migration, existing PCM switching devices can continue to be used.

Preferably the connection is described by a virtual connection identifier (VCI virtual channel identifier) for the ATM cells and a channel identifier (CID channel identifier) for the AAL2 mini cells.

Thus, the addressing possibilities of the ATM format can be retained for the PCM transmission as well.

Consequently, the virtual connection identifier and the channel identifier can describe the time slot in the PCM frame that is assigned to the connection. With this unique assignment, the conversion into the PCM format is facilitated. In this connection, one octet per 125 ps is entered into the PCM frame by the conversion device. one connection therefore occupies one PCM channel.

There is complete transparency of the conversion if the items of signalling information of the AAL2 mini cells are also entered octet by octet into the time slot of the PCM frame. The items of signalling information can be used on the receive side for the reconversion into AAL2 mini cells or another format.

Advantageously, after the switching in the PCM switching device, each octet of the time slot of the connection in the PCM frame is evaluated in a second conversion device and again entered into an AAL2 mini cell. In this way, in the case of two subscribers connected in an access network with ATM transmission, a doubled code conversion expenditure is avoided, the delay times fall considerably and a saving can be made on processor capacities for the code conversion.

In a preferred arrangement, after the last octet of an AAL2 mini cell has been entered, an octet having the value 11011 is entered into the corresponding time slot of the following PCM frame. For example, up to the arrival of the next AAL2 mini cell of the connections, the values 11011 can be inserted, as a result of which within the transmission in the PCM format the start of an AAL2 mini cell can reliably be recognised, for the first octet of the signalling information (header) of an AAL2 mini cell cannot take on the value "0". As soon as the next AAL2 mini cell arrives, the octets, starting again immediately with this mini cell, are to be entered into the respective time slot of the PCM frame. In this way, even in the event of compressed speech, an overflow into the conversion device is prevented from occurring.

The first conversion device is, for example, part of a switching device and connects a wide-band ATM switching portion to the narrow-band PCM switching device. Within such combined switching devices, therefore, switching can be carried out on a narrow band basis, but despite this the connection to ATM access networks by means of the ATM switching portion can take place without problem. The conversion in accordance with the invention simplifies these combined switching devices considerably.

Within a communication system, code conversion units ensure the matching of different speech codings.

Thus, advantageously, there is carried out with the connection setup a signalling between code conversion units of the communication system that compares the speech coders used, whereupon in the case of a positive comparison, the code conversion and reconversion for a connection are disabled (tandem free operation). In this way, the conversion in accordance with the invention can be carried out even in the case of conversion devices which are far apart.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a diagrammatic representation of the interfaces of a mobile radio system according to the prior art;

Figure 2 shows a diagrammatic representation of the interfaces of a communication system with different configuration levels; Figure 3 shows the connection of a conversion device into the communication system; Figure 4 shows a diagrammatic representation of the conversion of the items of speech information; and Figure 5 shows a combined switching device with wide-band portion and narrow-band portion.

Figure 1 shows devices and interfaces of a G5M mobile radio network. Between a mobile station MS and is a base station BTS is a radio interface according to a TDMA method (time division multiple access). By way of a PCM (pulse code modulation) interface Abi.,, the base station BTS is connected to a base station controller BSC, which carries out functions for the control and radio resource management of the base stations BTS.

The base station controller BSC is connected by way of a further PCM interface A,ub to a code conversion unit TRAU which performs functions for coding and channel matching between the base station BTS and a mobile switching centre MSC. The code conversion unit TRAU is connected to the mobile switching centre MSC, likewise by way of a PCM interface. The networking of a plurality of mobile switching centres MSC with each other or the transition to other communication networks take place according to the STM (synchronous transfer mode) transmission method.

The communication system according to Figure 2 is a future UMTS communication system, in which the radio interface is also of a new design, so an advantageous combination of FDMA (frequency division multiple access), CDMA (code division multiple access) and -G- optional TDMA methods for subscriber separation on the radio interface comes into use. Starting from the network configuration shown in Figure 1, a transmission on an ATM mini cell basis according to an AAL2 (ATM adaptation layer type 2) protocol is now striven for, (see in this respect ITU-T Recommendation 1.363.2 B ISDN ATM Adaptation layer type 2 specification, 8th

19th September 1997, Toronto, CA, pages 3-11). In order to do this, the base station BTS and the base station controller BSC as parts of a base station system BSS are expanded for the generation of AAL2 mini cells for speech transmission and the transport in ATM cells. This solution can also be set up in parallel with the existing GSM mobile radio system.

Starting from the base station controller BSC, a conversion between PCM format and ATM format according to DE 197 12 259 can take place. If the ATM cells are switched further, they can also be converted later in a mobile switching device MSC, or the entire switching takes place in ATM format. Different migration steps towards a future communication system, for example a LTMTS communication system, are therefore possible. In the following, only the alterations in the switching network CN in which PCM switching devices are used are discussed.

According to Figure 3, there is inserted between the base station system BSS and switching network CN a first conversion device IWU-A, which carries out the conversion from ATM format to PCM format. A corresponding second conversion device IWU-B carries out the reconversion from PCM format to ATM format.

Because a connection V1 for speech transmission has a two-sided flow of information, each of the conversion devices IWU-A, IWU-B is responsible for both conversion directions.

In the base station system ESS, as shown in Figure 4, a speech frame with 260 bit items of speech information ni is entered into AAL2 mini cells AAL2-Z, which are in turn transported inside ATM cells ATM-Z.

An ATM cell ATM-Z has a fixed length of 53 octets, while an AAL2 mini cell AAL2-Z contains a length indicator LI and can thus have a variable length. The length indicator LI is, like a channel identifier CID, a user indicator UUI and error control information HEC, part of the signalling information si within the AAL2 mini cell AAL2-Z. The length of the content of the speech information ni can vary. An ATM cell ATM-Z contains in the signalling portion a virtual connection identifier M. The channel identifier CID and the connection identifier WI characterise the connection is V1 sufficiently for a later switching to the further subscriber.

In the first conversion device IWU-A, each octet of the AAL2 mini cells AAL2-Z is entered into a time slot ts of a PCM frame R. One time slot ts per frame R is assigned to the connection V1, and in this time slot ts is written octet by octet the information from the AAL2 mini cells AAL2-Z of the connection V1. The items of speech information ni are thereupon transported in the PCM frame R and are switched further by means of a PCM switching device in the switching network W, for example to a second conversion device IWU-B. Not only the items of speech information ni but also the items of signalling information si of the AAL2 mini cells AAL2--Z are transported transparently in the PCM frame R in this way. If no information is available, octets having the value 1,011 are inserted until the next AAL2 mini cell AAL2-Z for the connection V1 arrives. In order to separate the AAL2 mini cells AAL2-Z, at least one octet with the value 1,011 is likewise entered.

The removal of the AAL2 mini cells AAL2-Z in the second conversion device IW-B is definitely possible without additional signalling measures, because, according to 1.363.2, the first octet of an AAL2 mini cell AAL2-2 cannot take on the value 11011. The first 6 bits of the second octet form a length indicator LI, the value of which is smaller by 1 than the useful load of the AAL2 mini cell AAL2-Z. In this way, the end of the AAL2 mini cell AAL2-Z is also easy to establish.

The AAL2 mini cell AAL2-Z which can thus be extracted can be entered by the second conversion device IWU-B into outgoing ATM cells ATM-Z. In this connection, a different channel identifier CID can also be used for the outgoing AAL2 mini cells AAL2-Z.

In an alternative embodiment of the invention according to Figure 5, the conversion device IWU-A is is not a separate component of the communication system but part of a switching device MSC and connects a wide band ATM switching portion ATM-V to the narrow-band PCM switching device CN, the latter being able to bring about a connection to the fixed network PSTN. Within such combined switching devices MSC, items of speech information ni are switched on a narrow-band basis.

The connection of the base station system BSS takes place by means of the ATM switching portion ATM-V. The coordination of the conversion and reconversion of the formats in one and the same device is very easily possible.

Because a transparent conversion is possible only if the remote station can also evaluate the items of information, a signalling of the speech codings used is necessary. Thus, there is advantageously carried out with the connection setup a signalling between code conversion units TRAU of the communication system that compares the speech coders used, whereupon in the case of a positive comparison, the code conversion and reconversion for a connection are disabled. The signalling takes place according to ETS 02.53, 03.53 and 04.53 Digital Telecommunication service system phase 2+ (tandem free operation), service description, stage 1-3. In this way, the conversion in accordance with the invention can be carried out even in the case of conversion devices IW-A and IW-B which are far apart.

Claims

Claims 1. Method for transmitting items of speech information in a

communication system, in which method - items of speech information of a connection are 5 entered on the send side into AAL2 mini cells, - the AAL2 mini cells are transported by means of ATM cells, - in a first conversion device, each octet of the AAL2 mini cells is entered into a time slot of a PCM frame, and - the items of speech information in the PCM frame are switched further by means of a PCM switching device.
2. A method according to claim 1, in which the connection is described by a virtual connection identifier for the ATM cells and a channel identifier for the AAL2 mini cells.
3. A method according to claim 2, in which the virtual connection identifier and the channel identifier describe the time slot in the PCM frame that is assigned to the connection.
4. A method according to any of the preceding claims, in which one octet per 125 ps is entered into the PCM frame by the conversion device.
5. A method according to claim 3, in which the items of speech information and items of signalling information of the AAL2 mini cells are also entered octet by octet into the time slot of the PCM frame.
6. A method according to any of the preceding claims, in which after the last octet of an AAL2 mini cell has been entered, an octet having the value 11011 is entered into the corresponding time slot of the following PCM frame.
7. A method according to any of the preceding claims, in which after the switching in the PCM switching device, each octet of the time slot of the connection in the PCM frame is evaluated in a second conversion device and the octet relating to items of useful and signalling information is in turn entered into an AAL2 mini cell.
8. A method according to any of claims 1 to 6, in which the first conversion device is part of a switching device and connects a wide-band ATM switching portion to the narrow-band PCM switching device.
9. A method according to any of the preceding claims, in which there is carried out a signalling between code conversion units of the communication system that compares the speech coders used, whereupon in the case of a positive comparison, the code conversion and reconversion for a connection are is disabled.
10. A method of transmitting items of speech information in a communication system, the method being substantially as herein described with reference to Figures 2 to 5 of the accompanying drawings.