WO2000013381A1

WO2000013381A1 - Transmission of atm adaptation layout protocol packets in an atm telecommunications network

Info

Publication number: WO2000013381A1
Application number: PCT/AU1999/000622
Authority: WO
Inventors: Leslie Graf
Original assignee: Telefonaktiebolaget L M Ericsson
Priority date: 1998-08-28
Filing date: 1999-07-30
Publication date: 2000-03-09
Also published as: AUPP556998A0

Abstract

A method of transmitting data within a telecommunication network (20) operating in synchronous transfer mode, the method comprising the steps of: formatting the data into one or more ATM adaptation layer packets (4), mapping each packet (4) into one or more consecutive STM time slots (44) transmitting an STM data stream (40) containing each mapped packet, and transmitting packet identifier information (CIC1, CIC2, CIC3, ...) identifying the one or more time slots (44) containing each said packet.

Description

TRANSMISSION OF ATM ADAPTATION LAYOUT PROTOCOL PACKETS IN AN ATM TELECOMMUNICATIONS NETWORK

This invention relates generally to the transmission of data within telecommunication networks operating in synchronous transmission mode. The invention is suitable for use in the transmission of variable bit rate data over existing narrow band ISDN networks, and it will be convenient to hereinafter disclose the invention in relation to that exemplary application. It is to be appreciated, however, that the invention is not limited to the application.

Asynchronous transfer mode, or ATM, is one of the general class of digital packet-switching technologies that relay and route traffic by means of an address contained within a data packet. ATM packet switching technology is particularly attractive for data traffic because it exploits communication channels much more efficiently than do synchronous transfer mode, or STM, technologies commonly used to transmit digitised voice signals. STM services are not routed by address, but rather over dedicated physical paths established either when a dialled call is set up or when a private line is installed. STM services transfer information between parties synchronously by time division multiplexing. Information is transmitted in frames, which are divided into time slots corresponding to voice channels. The time slots are multiplexed together with a synchronisation word to form the transmitted signal. Each time slot is synchronised to the transmission word. With each time slot representing one voice channel, that call's digitised voice traffic is guaranteed access to the assigned slot for as long as the call lasts. Accordingly, time slots within an STM frame cannot be shared amongst calls. Even when no voice information is being exchanged between the calling and the called party, this time slot remains unavailable for other users.

ATM enables a user to effectively make use of these unused time slots, or bandwidth, by giving users access to the whole data link at random intervals for random lengths of time. ATM packet switched technology does this by giving up the possibility of identifying data on the basis of its particular time slot within a frame, and transmits data in packets having a header containing an address representing the destination of the message.

ATM uses very short, fixed length packets, called cells. These cells are 53 bytes long and consist of a 5 byte header (containing the address) and a 48 byte information field. The header comprises several fields, including a virtual path identifier (VPI) and a virtual channel identifier (VCI). ATM switching equipment within the telecommunication network can route all ATM cells on the basis of the virtual path identifier until the ATM cell arrives at its final destination, where the virtual channel identifier is used to distribute information from a particular channel to a called party.

The widespread use of ATM technology will require the establishment of an ATM core network including ATM switches adapted to transfer the ATM cells from the calling party to the called party through the telecommunications network. Currently, the absence of such an ATM core network prevents the widespread use of ATM technology to users. There therefore exists a need to enable the transfer of data within currently existing telecommunications networks, whilst taking advantage of ATM technology benefit.

With this in mind, one aspect of the present invention provides a method of transmitting data within a telecommunication network operating in synchronous transfer mode, the method comprising the steps of:

formatting the data into one or more ATM adaptation layer packets,

mapping each packet into one or more consecutive STM time slots, transmitting an STM data stream containing each mapped packet, and transmitting packet identifier information identifying the one or more time slots containing each said packet.

Accordingly, ATM adaptation layer packets are simply transmitted down existing STM channels, or more particularly, each packet is mapped into consecutive time slots corresponding to one or more of these channels. The channels are identified in the network by packet identifier information which enable the packet to be identified and switched in the telecommunication network to a called party.

Preferably, the data to be transmitted is formatted as one or more ATM adaptation layer 2, or AAL2, packets.

The packet identifier information may be transmitted on a signalling link, whilst the STM data stream may be transmitted through the telecommunication network on a separate data bearing link.

At least part of the signalling link and data bearing link may form part of a narrow band ISDN network.

Moreover, at least part of the signalling link and data bearing link may form part of an intelligent network using SS No. 7 signalling.

In one embodiment, the packet identifier information may form part of the ISDN user part of the SS No. 7 protocol.

The packet identifier information may include one or more identification codes identifying the time slots in the STM data stream containing each said packet. The following description refers in more detail to the various features and advantages of the present invention. To facilitate an understanding, reference is made in the description to the accompanying drawings where the method of transmitting data in an STM network is illustrated in a preferred embodiment. It is to be understood, however, that the present invention is not limited to the preferred embodiment as illustrated in the drawings.

In the drawings:

Figure 1 is a schematic diagram showing the structure of an ATM cell, and of AAL2 packets within that ATM cell;

Figure 2 is a schematic diagram of a telecommunication network operating in synchronous transfer mode; and

Figure 3 is a schematic diagram showing the mapping of a AAL2 data packet into an STM data stream for transmission in the telecommunication network of Figure 2.

Referring now to Figure 1, there is generally shown an ATM cell 1. The cell 1 has a fixed length of 53 bytes or octets, 48 for the information field 2 and 5 for the cell header 3. The ATM cell 1 is intended to carry different types of information across an ATM telecommunication network. Some cells will carry control information, whilst others will be used to carry user information of a different nature, such as continuous bit rate (CBR) or variable bit rate (VBR) data.

ATM uses a layered protocol model, having three layers: the physical layer,

ATM layer, and adaptation layer. The ATM adaptation layer (AAL) is a separate layer above the ATM layer and is necessary to distinguish the different sorts of information conveyed by the ATM cells, to make the ATM layer independent of the different types of information transferred. AAL type 2 (AAL2) is designed for VBR traffic, requiring a time relation between sender and receiver. AAL2 has been adopted to handle the problem of packetization delay for low bit rate services, and has been designed as packets that can be multiplexed on an ATM connection. Two such AAL2 packets are represented in Figure 1. The AAL2 packets 4 and 5 have a similar design to the ATM cell 1, in that they each contain a header, respectively referenced 6 and 7, and a payload, respectively referenced 8 and 9. However, unlike the ATM cell 1, the payload is not fixed in size, but can vary from 4 to 64 bytes or octets. Each three octet AAL2 packet header comprises an 8 bit connection identifier 10, a 6 bit length indicator 11 indicating the length of the payload, a 5 bit field containing user to user information 12 and a 5 bit header error control field 13. The connection identifier (CID) enables up to 248 AAL2 channels to be multiplexed on a AAL2 link or ATM connection. The user to user info (UUI) is a field that is conveyed transparently between end points with the exception of the specific binary co-point values 30 and 31 that are reserved for operation and maintenance function. The header error control (HEC) is used to detect errors in the header.

The ATM cell shown in Figure 1 has been designed for, and is currently used in, telecommunication networks operating in a synchronous transfer mode and including ATM core network elements such as switching exchanges.

ATM is a layered architecture allowing multiple services like voice, data and video, to be mixed over the network. Three lower level layers have been defined to implement the features of ATM. The adaptation layer assures the appropriate service characteristics and divides all types of data into the 48 byte payload that will make up the ATM cell. The ATM layer takes the data to be sent and adds the five byte header information that assures the cell is sent on the right connection. The physical layer defines the electrical characteristics and network interfaces. ATM is not tied to a specific type of physical transport. Turning now to Figure 2, there is shown a conventional telecommunication network 20 adapted for operation in synchronous transfer mode. The network 20 includes various integrated services digital network (ISDN) architectural elements and reference points. ISDN terminals 21 to 24 are connected to an ISDN primary rate interface (PRI) link 25 via an ISDN multiplexer 26. The PRI link provides twenty-three data bearing channels of 64Kbps each, plus a 64Kbps signalling channel.

Non-ISDN equipped devices 27 and 28 are connected to an ISDN basic rate interface (BRI) link 29 via a terminal adaptor 30 and associated interface 31. The BRI link provides two data bearing channels of 64Kbps each plus a 16Kbps signalling channel.

In both the BRI and PRI links, the separate signalling channel is used to access the control functions of various digital switches and exchanges within an N-ISDN network 32. The signalling channel is used to provide message exchange between each of the user terminals 21 to 24 and 27 to 28 and the network 32 to set up, modify, and clear the data bearing channels conveying voice, data, image, video or other data to and from the terminals.

Data is conveyed on the PRI and BRI links by time division multiplexing. Accordingly, data is transmitted in frames, commencing with a synchronisation word which enables each switching element within the network 32 to synchronise itself with the data. The remaining portion of each frame is divided into time slots corresponding to data channels. For example, it is known to divide a frame into 32 time slots of 8 bits each, each time slot corresponding to a separate data channel. The frames are transmitted at a rate of 8000 per second, so that 32 channels may be transmitted, each at a rate of 64Kbps, on a single physical data link. STM switching exchanges, such as those referenced 33 to 37 enable the transmission of each channel from a particular source terminal to an intended destination terminal. According to the present invention, a telecommunications network, such as the network referenced 20 in Figure 2, may be used to support the transmission of ATM adaptation layer packets, such as the AAL2 packets shown in Figure 1.

Referring now to Figure 3, there is shown an STM data stream 40 transmitted on a data bearing link within the telecommunication network 20. The

STM data stream 40 is composed of a series of frames, such as that referenced 41.

The frame transmission rate is 8Kbps. Each frame includes a synchronisation word 42 and a number of time slots 43, which are normally used in an STM telecommunications network to identify data channels transmitted on an ISDN data link. According to the invention, data to be transmitted is firstly formatted into one or more ATM adaptation layer packets, such as the AAL2 packet 4. Each

AAL2 packet is then mapped into one or more consecutive STM time slots. In the example shown in Figure 3, the AAL2 packet 4 is mapped into a group of consecutive STM time slots 44. The number of consecutive time slots occupied by the AAL2 packet will depend upon the length of the payload 8 attached to the header 6.

The AAL2 packet is identified in the N-ISDN network 32 by signalling information transmitted on a signalling link. This signalling information includes packet identifier information identifying the time slots into which the AAL2 packet has been mapped.

Conveniently, the application transport mechanism (APM) currently available and used in N-ISDN may be used to transmit signalling information identifying the time slots into which the AAL2 packets are mapped throughout the network 20. Accordingly, the ISDN user part currently available in narrow band ISDN networks, known as N-ISUP, can be used to identify the time slots 44 throughout the network 20. This ISDN user part is common to both the digital subscriber signalling system number 1 (DSS1) which must be used by subscribers connected to ISDN exchanges, such as the exchange referenced 34, and switching exchanges forming part of an intelligent network communicating by the signalling system number 7 protocol, such as those exchanges referenced 35 and 36. Each of the time slots in the group of time slots 44 may be identified in the control plane by a circuit identification code (CIC), rather than the connection element identifier (CEI) used in ATM. These CICs can be thought of as constant bit rate ATM connections. When the bandwidth in the connection is fully utilised, additional connections (additional time slots) may be established. Advantageously, no additional framing is required in the ATM user plane as each CIC identifies a "tunnel" in the N-ISDN (the CIC maps into time slots in the STM transmission system). As shown in Figure 3, this allows for connections of up to 31 X 64Kbps in conventional STM networks, which will meet the bandwidth requirements of most variable bit rate systems that will use AAL2 protocols.

It is to be understood that various modifications and/or additions may be made to the method of transmitting data within an STM network without departing from the ambit of the present invention. For example, whilst the invention has been described in relation to the use of AAL2 packets, other ATM adaptation layer data packets may also be transmitted according to this method.

Claims

CLAIMS :

1. A method of transmitting data within a telecommunication network operating in synchronous transfer mode, the method comprising the steps of: formatting the data into one or more ATM adaptation layer packets; mapping each packet into one or more consecutive STM time slots; transmitting an STM data stream containing each mapped packet; and transmitting packet identifier information identifying the one or more time slots containing each said packet.

2. A method of transmitting data according to claim 1, wherein the data to be transmitted is formatted as one or more ATM Adaptation Layer 2 packets.

3. A method of transmitting data according to either of claims 1 or 2, wherein the packet identifier information is transmitted on a signalling link, the

STM data stream being transmitted through the telecommunication network on a separate data bearing link.

4. A method of transmitting data according to claim 3, wherein at least part of the signalling link and the data bearing link form part of a narrow band ISDN network.

5. A method of transmitting data according to either of claims 3 or 4, wherein at least part of the signalling link and data bearing link form part of an intelligent network using SS7 signalling.

6. A method of transmitting data according to any one of the preceding claims, wherein the packet identifier information forms part of the ISDN user part of the SS7 protocol.

7. A method of transmitting data according to claim 6, wherein the packet identifier information includes one or more identification codes identifying time slots in the STM data stream containing each said packet.