ASYNCHRONOUS TRANSFER MODE TRANSMISSION NETWORKS

Abstract: An ATM transmission network of variable bandwidth in which a network transmission element (12) is inserted at each end of the network (10) between said network and the ATM device (14), the transmission network element being informed of traffic parameters on each virtual path to enable it to compare the available network bandwidth with the required bandwidth and shut down one or more virtual paths (16, 16A) by inserting AIS OAM cells until the demanded bandwidth does not exceed the available bandwidth.

Description: Title Asynchronous Transfer Mode Transmission Networks Field of the Invention This invention relates generally to a telecommunications network for providing a transmission network for connecting asynchronous transfer mode (ATM) devices and more specifically to such a network having bandwidth control . Background to the Invention A transmission network provides an ATM transport service to interconnect ATM devices. The transmission network may be able to offer variable amounts of bandwidth to the interconnected ATM devices, either due to configurability in the network, fault conditions in the network or the normal operation of the network. For example, if the transport network is using inverse multiplexing the supplied bandwidth will depend on the number of configured links and on fault conditions on individual links in the inverse multiplexed group; if the transport network uses an adaptive line coding, the supplied bandwidth will depend on the current physical condition of the line. In order to offer a good quality of service the ATM devices need to know what bandwidth is available over the transmission network, so that cell loss can be avoided when the bandwidth is reduced and so that a normal good quality service can be restored when the supplied bandwidth is restored. Ideally, in a layered network model, the transport network should be transparent to ATM signalling. ATM signalling is concerned with setting up and tearing down ATM circuits through the network and so should only involve devices that are offering ATM switching and/or termination functionality. Adding ATM signalling to the transport network increases the complexity of transmission network elements and of the ATM layer of the network, which is undesirable. In addition, many network operators only offer a permanent virtual pipe service to their subscribers, and so do not support any ATM signalling in their networks . Proprietary signalling of available bandwidth is possible, but this restricts the operator to a single vendor solution, which is also undesirable. An aim of the present invention is to provide a method for controlling the bandwidth used by the ATM devices when the bandwidth available on the transmission network varies, without requiring any ATM network interface or network node interface signalling to be supported in the transmission network, but allows such signalling to be transparently transported between ATM devices over the transmission network. The invention seeks to take advantage of standard features of ATM transmission to allow the used bandwidth to be controlled in a non-proprietary fashion that will function with existing ATM devices . Possible available solutions to the problem of variable bandwidth in an ATM transmission network are : - 1. The bandwidth that is available to a port on an ATM device can be configured by management operation. The ATM device then shapes traffic that is output on that port to ensure that it does not exceed the available bandwidth. This is a static configuration and so cannot cope with variations in the available bandwidth. In addition, traffic shaping is normally only offered by ATM devices that are intended to be used at the edge of the network, so this facility is only able to control bandwidth entering the network, not the bandwidth that is used from the network to the edge device. 2. ATM network interface and network node interface signalling protocols can be used to negotiate the parameters of each ATM virtual pipe or circuit that is transported across the transport link. Traffic shaping and policing functions ensure that the ATM virtual pipes and circuits conform to these parameters . The ATM devices need statically programmed data about the available bandwidth in order to perform admission control when new paths and circuits are signalled. This static information cannot cope with variation in the bandwidth that is made available by the network. 3. The generic flow control bits in the UNI (network interface) header can be used to control the flow of cells in to the transport network. This method works well provided that the ATM device supports the use of the generic flow control bits but this is not universally the case. Generic flow control is only available at the UNI interface. Normally at least one end of the transport path will be at an NNI (network node interface) port on an ATM device and so generic flow control cannot be used at this point. This limits the bandwidth control to a single direction through the network; the NNI to UNI direction cannot be controlled and so unexpected cell loss can occur if the supplied bandwidth decreases . Even in cases where UNI is provided at both ends of the transport link, it will be in the same direction (towards the user equipment) at both ends. Bandwidth from the user equipment can be controlled but bandwidth towards the user equipment cannot be controlled. 4. The Available Bit Rate (ABR) ATM protocol is able to control the bandwidth used by a virtual circuit in the network. This protocol is intended to avoid congestion in the network. If a reduction in the bandwidth of a transmission link is considered to be a form of congestion then the ABR protocol can be used to reduce the ATM bandwidth to match. Unfortunately the ABR protocol only applies to traffic that uses this transport class. Other traffic classes (e.g. constant bit rate) cannot be controlled in this way. To implement ABR in the transmission network requires some implementation of ATM signalling in the transmission network elements, which increases their complexity and makes these elements visible in the ATM layer. 5. The simplest solution to a reduction in transmission bandwidth is randomly to discard cells when the supplied cell rate is greater than the supplied bandwidth. This has the disadvantage that it affects all traffic that is carried on the link. Also, this can cause "traffic melt down" if the cells are part of a higher layer packet structure, since whole packets will be re-transmitted by the higher layer protocol . Since there is no signalling of the loss of bandwidth from the network to the ATM devices, in this case the ATM devices are not able to take any action to cope with the loss of bandwidth. 6. In a traffic dependent cell discard solution the transmission network elements are aware of the ATM adaptation layer that is being used for each virtual circuit on the link. When the transmission bandwidth is reduced cells are discarded in groups in such a way that complete higher layer packets are lost. This helps to avoid traffic melt down and helps to localise cell loss to specific virtual circuits. However, this method still suffers the disadvantage that virtual circuits are randomly affected and that the ATM devices have no knowledge of the loss of transmission bandwidth. In addition this method requires the transmission network elements to have full knowledge of all virtual circuits on the link, which is a large processing and signalling overhead. The Invention According to one aspect of the present invention, there is provided a method of controlling the bandwidth used by Asynchronous Transfer Mode (ATM) devices interconnected by a transmission network of variable bandwidth, wherein at each end of the transmission path a transmission network element is inserted between the ATM device and the network, said transmission network element being capable of comparing the bandwidth required by the ATM device and the available bandwidth of the transmission network and of shutting down one or more virtual paths through the network until the calculated required bandwidth does not exceed the available bandwidth. According to another aspect of the invention, there is provided a transmission network of varying available bandwidth providing an ATM transport service to interconnect ATM devices, including, at each end of the network, a transport network element between the ATM device and the network, said transmission network element being capable of comparing the bandwidth required by the ATM device and the available bandwidth of the transmission network and of shutting down one or more virtual paths through the network until the calculated required bandwidth does not exceed the available bandwidth. Thus, in accordance with the invention, at each end of the transmission path a transmission network element is connected to an ATM device. Each path end point transmission network element is responsible for controlling bandwidth used by the ATM device that it is connected to. The transmission network element has imparted to it knowledge of the virtual paths (VPs) that pass through it. For each VP this may conveniently consist of the virtual path identifier (VPI) and the traffic contract parameters. This information may be either configured by management operation or is discovered automatically by the network element observing (but not taking part in) the ATM signalling that passes through it. The total required bandwidth from the ATM device to the network is calculated by the transmission network element from the traffic contract parameters for each virtual path. Preferably, each virtual path is given a unique priority. This is either configured by management operation, derived from the properties of the virtual path (e.g. from the traffic contract parameters) or is arbitrarily assigned. Under normal operation it is assumed that there is sufficient transmission bandwidth to fully support all of the virtual paths. In this case the transmission network element simply transports ATM cells from all virtual paths to the other end of the transmission link. However, if the link bandwidth is reduced such that it is less than the total required bandwidth the transmission network element shuts down the lowest priority virtual path and recalculates the total required bandwidth of the remaining active virtual paths. If the calculated value is less than or equal to the available link bandwidth no further action is taken. If the calculated bandwidth is still larger than the available link bandwidth, the next lowest priority VP is shut down and the total required bandwidth of the remaining active VPs calculated. This process is continued until the total required bandwidth calculated for the active virtual paths is less than or equal to the available link bandwidth. More preferably, moreover, if the available link bandwidth is increased and one or more virtual paths are in the shut down state, the virtual paths are re-activated in order of priority. A virtual path is re-activated if the total required bandwidth for that VP and all active VPs is calculated to be less than or equal to the current available link bandwidth. A virtual path is preferably shut down by discarding all cells for that path (with that path's VPI) that the transmission network element receives and by inserting AIS (alarm indicator signal) OAM (operation administration and maintenance) cells both towards the ATM device and towards the transmission network for that virtual path. A virtual path is then preferably re-activated by ceasing to insert AIS OAM cells and allowing the normal transmission of ATM cells on that virtual path. The ATM devices thus see a decrease in available network bandwidth as the failure of one or more virtual pipes, and restoration of bandwidth as a clearing of faults on the "failed" virtual pipes. This results in an orderly throttling of ATM traffic as cells are only lost on the VPs that have been shut down. Also, since the ATM devices conveniently receive an AIS indication for each VP that is shut down, they have an opportunity to handle the loss of bandwidth in the services that are using the affected VPs. Clearly the granularity in which the bandwidth is controlled depends on the bandwidth that is allocated to each virtual path. This is an operational matter and forms no part of the present invention. It will be appreciated that the invention can be applied at virtual circuit level on the ATM transmission network, or on defined sets of virtual circuits. The latter enables finer granularity but requires the network transmission elements to have knowledge of the individual virtual circuits. In practice, bandwidth control at virtual path level is a less expensive and more cost effective solution. Description of Embodiment The invention will be further understood from the accompanying drawings, in which: - Figure 1 is a simplified diagram of an ATM transmission network embodying the invention; Figure 2 is a diagram showing the effect of shutting down a virtual pipe (VP) or circuit; and Figure 3 shows the network as seen by ATM devices at opposite ends of a transmission network. Figure 1 shows the insertion, at the opposite ends of an ATM transmission network 10, of transmission network elements 12, 12A respectively between the network and two ATM devices 14, 14A. The transmission network 10 is of variable bandwidth, comprising a plurality of virtual pipes or circuits which can be available or unavailable for a variety of reasons, as hereinbefore explained. At each end of the network, the transmission network element 12 or 12A is able, by knowledge of the virtual path identifiers and the traffic contract parameters, to compare the bandwidth required by the ATM device 14 or 14A and the available bandwidth on the network 10 and shut down one or more virtual pipes until the available bandwidth is not less than the required bandwidth. In Figure 2, a simplified network 10 of two virtual pipes 16, 16A shows virtual pipe 16 shut down by the transmission network element 12 connected to the ATM device 14. As will be apparent from Figure 3, the transmission network elements 12, 12A are transparent to the ATM devices 14, 14A. Analogously, the transmission network elements 12, 12A are transparent to ATM signalling. In Figure 3, as far as ATM devices 14, 14A are concerned, these devices see only the open virtual pipes 16A, 16B, virtual pipe 16 having been shut down, although the transmission network elements 12, 12A will be aware from the AIS (alarm indication signals) if an increased bandwidth becomes available on the network, and are then able to re-open one or more virtual pipes. In short, the invention is able to control the bandwidth of a transmission link that is used by ATM devices in real time by the use of standard ATM OAM cells. The real time nature of this control addresses the problems of a link that has variable capacity, and is able to handle this for all traffic classes. This invention does not require any support of the ATM signalling protocols in the transmission equipment, and so keeps the transport layer distinct from the ATM layer. Virtual paths are preferably shut down based on priority levels. Priorities can be arranged such that high priority services are the last ones to be affected by a loss of bandwidth. This ensures that applications requiring high quality of service can be maintained for as long as possible, even when there is a degradation of the transmission link. By shutting down virtual paths this invention makes the end ATM devices aware that there has been a loss of bandwidth and so gives these devices the opportunity to handle the loss of bandwidth in the services that are using the affected virtual paths . It will be understood that it is possible to use the technique at the virtual circuit level, or on defined sets of virtual circuits. The latter allows finer granularity but requires the transmission network elements to have knowledge of individual virtual circuits. Bandwidth control at the virtual path level is a more cost effective solution.

Claims: Claims 1. A method of controlling the bandwidth used by Asynchronous Transfer Mode (ATM) devices interconnected by a transmission network of variable bandwidth, wherein at each end of the transmission path a transmission network element is inserted between the ATM device and the network, said transmission network element being capable of comparing the bandwidth required by the ATM device and the available bandwidth of the transmission network and of shutting down one or more virtual paths through the network until the calculated required bandwidth does not exceed the available bandwidth. 2. A method according to claim 1, wherein the transmission network element is also able to open up one or more shut down virtual paths through the network if an increased bandwidth becomes available. 3. A method according to claim 1 or claim 2, wherein each transmission network element has imparted to it knowledge of the virtual paths which pass through it . 4. A method according to claim 3, wherein said knowledge consists of the virtual path identifier and the traffic contract parameters. 5. A method according to claim 4, wherein the transmission network element observes but does not take part in the ATM signalling which passes through it. 6. A method according to any of claims 1 to 5, wherein the transmission network element calculates the required bandwidth from the traffic contract parameters for each virtual path. 7. A method according to any of claims 1 to 6, wherein each virtual path is allotted a unique priority, and virtual paths are shut down sequentially starting at the virtual path of lowest priority. 8. A method according to claim 7, wherein priorities are assigned based on the traffic contract parameters of each virtual path. 9. A method according to any of claims 1 to 8 , wherein virtual paths are shut down by discarding all cells for a virtual path to be shut down and inserting alarm indication signal/operation administration and maintenance (AIS OAM) cells for that virtual path. 10. A method according to claim 9 when appendant to claim 2, wherein a shut down virtual path is re-opened by ceasing to insert AIS OAM cells. 11. A method of controlling bandwidth in an ATM transmission network substantially as hereinbefore described. 12. A transmission network of varying available bandwidth providing an ATM transport service to interconnect ATM devices, including, at each end of the network, a transport network element between the ATM device and the network, said transmission network element being capable of comparing the bandwidth required by the ATM device and the available bandwidth of the transmission network and of shutting down one or more virtual paths through the network until the calculated required bandwidth does not exceed the available bandwidth. 13. A transmission network substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings .

Inventors: LAUDER ANDREW GEORGE, ROBERTS CLIVE ADRIAN, LAUDER, ANDREW, GEORGE, ROBERTS, CLIVE, ADRIAN
Applicants: LAUDER ANDREW GEORGE, NOKIA NETWORKS OY, ROBERTS CLIVE ADRIAN, LAUDER, ANDREW, GEORGE, ROBERTS, CLIVE, ADRIAN
Application year: 1999
Main classification: H04L12
IPC codes: H04L12