Method and system to transport narrowband calls across an ATM network

Abstract: The use of multiple AAL2 virtual circuits to support narrow band calls with multiple types of narrow band signalling systems leads to inefficient use of bandwidths over an ATM network. The present invention provides a method of transporting narrow bend calls of multiple narrow band signalling type between narrow band networks across a single virtual circuit in an ATM network, where for each call connection within a circuit the call connection signalling information includes the narrow band signalling type.

Description: FIELD OF THE INVENTION This invention relates to telecommunications networks and in particular a system and method for asynchronous transfer mode (ATM) transmission of traffic. BACKGROUND OF THE INVENTION ATM technology is a flexible from of transmission which allows various types of service traffic, eg. Voice, video or data, to be multiplexed together onto a common means of transmission, the traffic being carried in cells each having a header indicating its destination. Service traffic is adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of the ATM network. This form of adaption is performed in the ATM adaption layer (AAL). The technique allows large volumes of traffic to be handled reliably and efficiently. A limiting factor in the introduction of ATM is the difficulty of interfacing new broadband ATM networks with existing narrowband networks. The presently proposed AAL2 ATM trunking for narrowband services encapsulates and transports short user packages inside an ATM cell stream. AAL2 allows multiple narrowband users to share a single ATM virtual circuit- in this manner the length of each user package can be kept short to minimise packetisation delay, whilst the overall bandwidth efficiency of the VCC is maintained. However, as presently proposed, a single AAL2 virtual circuit can only support AAL2 connections which are controlled by the same type and variant of narrowband signalling system. In general, transmission facilities terminating on a single inter-working function (IWF) may each carry a different narrowband signalling system, or a single transmission facility may carry multiple narrowband signalling systems. In either case, this requires the IWF to terminate and process multiple different narrowband signalling systems. This means that as presently proposed, in order to transport user data across the ATM network, the IWF would need to initiate multiple AAL2 virtual circuits, one for each narrowband signalling system terminated. At present it is not otherwise possible for the narrowband signalling handling function at the receiving IWF to determine which signalling system to associate with an individual narrowband call. This information is required so that the receiving IWF can successfully process the signalling information in order to provide onward routing of the call. The use of multiple AAL2 virtual circuits to overcome this problem leads to an inefficient use of bandwidth, especially when an AAL2 virtual circuit has only a few calls to support. GB-A-2305812 discloses a system for interconnecting narrowband networks via a broadband network in which signalling messages are transmitted in the broadband network to indicate that a number of different connections are required to access a multi-media service. Lui S S: "Impacts of Signalling-Intensive Local Services (SILS) on B-ISDN Switching Systems: A Simulation Study" Computer Networks and ISDN Systems, NL, North Holland Publishing, Amsterdam, vol. 25, no. 2, 17 th August 1992, pages 121-143 discloses that SILS CPEs mark their signalling cells by the standard VPI and use the VCIs to identify different types of SILS to signalling processors. SUMMARY OF THE INVENTION It is an object of the present invention to minimise or to overcome these disadvantages. It is a further object of the present invention to provide an improved apparatus and method for the transport of narrowband communications traffic over ATM networks. In a first aspect, the present invention provides a method of transporting a multiplicity of narrowband calls between first and second narrowband networks connected by a broadband network, each of the multiplicity of narrowband calls having associated therewith a narrowband call connection signalling type, the method comprising the steps of: providing a virtual circuit (VC) across said broadband network; providing a multiplicity of call channels within said VC, multiplexing said narrowband calls into respective ones of said multiplicity of call channels; associating with each of said multiplicity of call channels respective narrowband call connection signalling type data; the method being characterised in that the step of multiplexing said narrowband calls into respective ones of said call channels of said VC comprises multiplexing a multiplicity of narrowband calls having more than one narrowband call connection signalling type associated therewith into the same VC. Preferably, the step of associating with each of said multiplicity of call channels respective narrowband call connection signalling type data comprises providing in the VC a narrowband call connection signalling type channel for each of said more than one narrowband call connection signalling types associated with said multiplicity of narrowband calls and mapping each of said call channels to a respective one of said narrowband call connection signalling type channels. In a second aspect, the present invention provides an apparatus for transporting a multiplicity of narrowband calls between first and second narrowband networks connected by a broadband network, each of the multiplicity of narrowband calls having associated therewith a narrowband call connection signalling type, the apparatus comprising: means for providing a virtual circuit (VC) across said broadband network; means for providing a multiplicity of call channels within said VC, means for multiplexing said narrowband calls into respective ones of said multiplicity of call channels; means for associating with each of said multiplicity of call channels respective narrowband call connection signalling type data; the apparatus being characterised in that the means for multiplexing said narrowband calls into respective ones of said call channels of said VC is arranged to multiplex a multiplicity of narrowband calls having more than one narrowband call connection signalling type associated therewith into the same VC. BRIEF DESCRIPTION OF THE DRAWINGS In order that a greater understanding of the invention can be obtained, an embodiment of the invention will now be described with reference to the accompanying drawings by way of example only and without intending to be limiting, in which: Figure 1 is a schematic diagram of a telecommunications system comprising narrowband services connected over an ATM network; Figure 2 Is a schematic diagram showing virtual circuit connections between the ATM network IWF's; Figure 3 is a schematic diagram showing an AAL2 virtual circuit data stream; and Figure 4 is a schematic diagram illustrating an inventive AAL2 connection signalling packet within the AAL2 virtual circuit data stream. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 shows a telecommunications system comprising a plurality of narrowband networks 2 such as public switch telephone networks or wireless networks, connected by an ATM network 3. The narrowband networks 2 are connected to the ATM network 3 by narrowband transmission facilities 4 such as E1 or DS1, which terminate at the ATM network 3 in an interworking function (IWF) 5. In the ATM network structure 3, narrowband channels are transported across the network 3 between pairs of IWF's 5, each providing an interface between the respective narrowband network transmission facility 4 and the ATM network 3, as shown in figure 2. The IWF 5 will process any incoming narrowband calls to determine a suitable outgoing route (to another IWF 5) as well as performing other functions such as compression and in the case of voice or voice band data calls suppressing silent intervals. The IWF 5 also performs an ATM adaption layer function using AAL type 2 to adapt user data and signalling information into packets or minicells - said packets are multiplexed into the AAL2 virtual circuit (VC) for transport across the ATM network 3 to a receiving IWF5. Each user is allocated a unique channel within the AAL2 VC. The receiving IWF retrieves the packets and reassembles the narrowband call. In this way packets of user information from different users may be multiplexed onto a single AAL2 VC. The narrowband signalling information, associated with each individual call being transported between IWF's5 is similarly transported either in a given AAL2 channel within the AAL2 VC, or in a separate AAL5 VC, as shown schematically in Figure 2. This information is similarly retrieved by the receiving IWF's which provides information on the forward routing of a particular narrowband call for example. However the different types of variants of narrowband signalling systems used require the receiving IWF to be able to associate a particular narrowband call with the type of signalling system used to retrieve for example the required forward routing information to complete the call. Upon set up of a VC, a channel is allocated for the narrowband signalling information associated with the calls to be transported by the newly set up VC, either within the VC itself (as a unique channel within the AAL2 VCC), or within a separate AAL5 VC. In prior art arrangements as shown in phantom outline in figure 2, the association of narrowband calls with the type of signalling system in which the call was terminated at the sending IWF is achieved by the use of VCs which are dedicated to a particular signalling type. The receiving IWF's thus know that all calls received on a particular VC will be of a predetermined signalling type, this signalling information then being retrievable from the dedicated AAL2 VC or AAL5 VC channel allocated to it. However, as described above, the use of separate VC's for each narrowband call signalling type leads to inefficient use of the VC's and hence inefficient use of bandwidth within the ATM network, especially when an AAL2 VC has only a few calls of a given signalling type to support. The invention achieves much greater multiplexing gain by combining all narrowband calls, irrespective of their associated narrowband signalling system, onto the same AAL2 VC, as shown in figure 2 by the solid line circuits. Figure 3 shows an AAC2 VC data stream which, for the purposes of illustration only, comprises a channel 1 dedicated to AAL2 connection control signalling messages for setup and termination of calls within the VC; a channel 2 dedicated to carrying (for example) a Common Channel Signalling (CCS) channel to provide the narrow band signalling for the narrowband calls terminating at the sending IWF which support this type of signalling The other channels shown comprise user data from the various narrowband calls transported by the VC. AAL2 connection control signalling concerned with the control of individual AAL2 connections within a AAL2 VC is also used to forward information about a connection to set up the receiving IWF5. This connection control signalling takes the form of a set up message sent on a dedicated channel within the AAL2 VC or a separate AAL 5 VC. AAL2 connection control signalling is concerned with conveying the identities of the channels occupied by the individual AAL2 connections, as well as with other AAL2 parameters necessary for correct operation of the sending and receiving IWFs, for example the identification of the voice coding algorithm to be used on the call. These connection control signalling messages are used during the set up and release phases of individual AAL2 connections. Figure 4 shows the field and format arrangement for AAL2 connection control signalling messages of the invention and includes; information on the coding algorithm to be used during the call; the SSCS type of a call; the call timeslot to channel identification value mapping information; the narrowband call signalling type information. The mapping information allows the destination IWF to reassemble timeslot by timeslot the narrowband call for forwarding via a narrowband transmission facility 4 (eg E1 or DS1) to the connected narrowband network 2. The narrowband call signalling type information allows the IWF to associate the correct narrowband signalling information with the new call, by being directed to the channel previously set up for transport of the signalling information. Some narrowband signalling systems include the information inband (ie as part of the user data), and use of this signalling system can also be conveyed to the receiving IWF Use of narrowband signalling type information in the AAL2 signalling allows the carrying on a single AAL2 VC of narrowband calls with different narrowband signalling systems. This extra information field conveys the type and variant of narrowband signalling associated with a given AAL2 connection. This information is conveyed by the AAL2 connection control signalling at AAL2 connection set up time to the receiving IWF. The AAL2 connection control signalling function at the receiving IWF then transfers this narrowband signalling information to the narrowband signalling handling function, which is then provided with the per-call information that it requires to correctly determine which narrowband call is associated with which type and variant of narrowband signalling system, regardless of how many different signalling systems are in use for a given AAL2 VC. A single AAL2 VC is thus enabled to carry narrowband calls associated with multiple different narrowband signalling systems. In use, VCs are set up within the ATM network 3 between IWFs 5 which are in turn connected to a narrowband network 2 by narrowband transmission facilities 4. Upon set up of a VC, a channel is allocated for AAL2 signalling for individual call set up within the VC, a channel is also allocated for each type of narrowband signalling system call envisaged to be provided for within the VC. Once the VC is set up, narrowband calls routed along it are first set up using the AAL2 connection message protocol as described above with reference to figure 4, detailing the mapping of the call timeslot on the narrowband transmission facility 4 to the channel value to be used for this timeslot as well as the narrowband call signalling system type. Once the call connection is set up, the call user data is placed on to the AAL2 VC and transported across the ATM network 3 to the receiving IWF, which is enabled to retrieve the call user data as well as the narrowband signalling associated with the call using the AAL2 connection message information as described above The foregoing describes the invention including a preferred form thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof as defined by the appended claims.

Claims: A method of transporting a multiplicity of narrowband calls between first and second narrowband networks (2) connected by a broadband network (3), each of the multiplicity of narrowband calls having associated therewith a narrowband call connection signalling type, the method comprising the steps of: providing a virtual circuit (VC) across said broadband network; providing a multiplicity of call channels within said VC, multiplexing said narrowband calls into respective ones of said multiplicity of call channels; associating with each of said multiplicity of call channels respective narrowband call connection signalling type data; the method being characterised in that the step of multiplexing said narrowband calls into respective ones of said call channels of said VC comprises multiplexing a multiplicity of narrowband calls having more than one narrowband call connection signalling type associated therewith into the same VC. A method as claimed in claim 1, characterised in that the step of associating with each of said multiplicity of call channels respective narrowband call connection signalling type data comprises providing in the VC a narrowband call connection signalling type channel for each of said more than one narrowband call connection signalling types associated with said multiplicity of narrowband calls and mapping each of said call channels to a respective one of said narrowband call connection signalling type channels. A method as claimed in claim 1 or claim 2, characterised in that it includes the step of conveying connection control signalling data across the broadband network from the first narrowband network to the second narrowband network to set up a narrowband call connection between said first and second narrowband networks. A method as claimed in claim 3, characterised in that the connection control signalling data comprises any combination of the following for a call channel: a coding algorithm for the call; call timeslot to channel identification value mapping information; and narrowband call connection signalling type information. A method as claimed in claim 3 or claim 4, characterised in that the connection control signalling data is conveyed in a connection control signalling channel in the VC or in another separate VC. A method according to any preceding claim, characterised in that it includes the step of conveying connection control signalling data across the broadband network from the first narrowband network to the second narrowband network, said connection control signalling data identifying the multiplicity of call channels within said VC into which said narrowband calls are respectively multiplexed and identifying the association of each of said multiplicity of call channels with its respective narrowband call connection signalling type data. A method as claimed in claim 6, characterised in that use of the connection control signalling data to identify the association between each of said multiplicity of call channels with respective narrowband call connection signalling type data comprises providing a narrowband call connection signalling type channel in the VC for each of said more than one narrowband call connection signalling types associated with said multiplicity of narrowband calls and mapping each of said call channels to a respective one of said narrowband call connection signalling type channels. A method as claimed in claim 6 or claim 7, characterised in that the connection control signalling data is conveyed in a connection control signalling channel in the VC or in another separate VC. A method as claimed in any of claims 1 to 8, characterised in that the VC comprises an ATM AAL2 VC. An apparatus for transporting a multiplicity of narrowband calls between first and second narrowband networks (2) connected by a broadband network (3), each of the multiplicity of narrowband calls having associated therewith a narrowband call connection signalling type, the apparatus comprising: means (5) for providing a virtual circuit (VC) across said broadband network; means (5) for providing a multiplicity of call channels within said VC, means (5) for multiplexing said narrowband calls into respective ones of said multiplicity of call channels; means (5) for associating with each of said multiplicity of call channels respective narrowband call connection signalling type data; the apparatus being characterised in that the means for multiplexing said narrowband calls into respective ones of said call channels of said VC is arranged to multiplex a multiplicity of narrowband calls having more than one narrowband call connection signalling type associated therewith into the same VC. An apparatus as claimed in claim 10, characterised in that means for associating each of said multiplicity of call channels with respective narrowband call connection signalling type data is arranged to provide in the VC a narrowband call connection signalling type channel for each of said more than one narrowband call connection signalling types associated with said multiplicity of narrowband calls and to map each of said call channels to a respective one of said narrowband call connection signalling type channels. An apparatus as claimed in claim 10 or claim 11, characterised in that it includes means (5) for conveying connection control signalling data across the broadband network from the first narrowband network to the second narrowband network to set up a narrowband call connection between said first and second narrowband networks. An apparatus as claimed in claim 12, characterised in that the connection control signalling data comprises any combination of the following for a call channel: a coding algorithm for the call; call timeslot to channel identification value mapping information; and narrowband call connection signalling type information. An apparatus as claimed in claim 12 or claim 13, characterised in that the connection control signallingdata is conveyed in a connection control signalling channel in the VC or in another separate VC. An apparatus as claimed in any of claims 10 to 14, characterised in that said apparatus comprises an inter-working function (5). An apparatus as claimed in any one of claims 10 to 15, characterised in that said VC or another separate VC is an AAL2 VC.

Inventors: CAVES KEITH, CAVES, KEITH
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1999
Main classification: H04Q11
IPC codes: H04Q11, H04L12