Method and apparatus for caching switched virtual circuits in an ATM network

METHOD AND APPARATUS FOR CACHING
SWITCHED VIRTUAL CIRCUITS IN AN ATM
NETWORK

TECHNICAL FIELD 5

This invention relates to the routing of real-time service requests through Asynchronous Transfer Mode (ATM) transport networks and, in particular, to the rapid establishment of a communications connection through an ATM network using cached switched virtual connections (SVCs) 10 and ATM switching using cached cross-connections.

BACKGROUND OF THE INVENTION

In telecommunications systems, the current trend is to use an ATM protocol in the backbone transport network. The ATM protocol is based on a data structure called a "cell". Cells are data packets having a fixed length of 53 octets. Each cell includes a header of 5 octets and a payload of 48 octets. The header is used to direct the cell through the

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network and the payload is the useful information portion of each packet.

ATM cells are routed through the network using routing information in the cell header. The routing information consists of a Virtual Path Identifier (VPI) and a Virtual 2J Channel Identifier (VCI). The VPI and VCI pair have only local significance on the link between ATM nodes. ATM nodes, which include ATM switches and cross-connect apparatus, use routing tables to map VCI and VPI values received in an incoming cell to outgoing values used to select an outgoing link as a way of routing the associated cell through the ATM node. A Virtual Circuit Link (VCL) is a logical link between two nodes in the ATM network and is identified by a VCI value. A Virtual Path Link (VPL) is a logical link between two nodes in the network and is identified by a VPI value. A Virtual Circuit Connection (VCC) is identified by a Virtual Circuit Connection Identifier (VCCI). The VCC is an end-to-end connection between two nodes in the network and is formed by the concatenation of VCLs.

When real-time connection-oriented traffic is transferred to an ATM transport backbone, a path must be set up through the backbone network to provide a connection over which cells are routed. Each communication request may also be associated with a service characteristic which may guarantee a specific bandwidth to a communication connection. One way to set up a path through the network is to define a VPC between each pair of ingress/egress points in the network. A VPC generally reserves adequate bandwidth to accommodate an anticipated traffic load for a specific communications application. The VPC eliminates most connection setup signaling and processing but tends to waste network resources because the VPC lacks the granularity required for efficient use of the network capacity.

Network capacity can be utilized more efficiently if a Switched Virtual Circuit (SVC) is used for each communication connection. However, it is well known that the signaling involved in setting up an SVC consumes a certain overhead in the network and may cause connection processing delays. For example, when voice traffic is transported over an ATM network, the setup of SVCs is known to sometimes cause unacceptable call setup delays. This problem is particularly acute during peak busy hours when ATM switches may not be able to sustain a peak time call setup rate.

Solutions have been invented to overcome this problem. For example, U.S. Pat. No. 5,719,863 which issued Feb. 17,

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1998 to Hummel describes a method and arrangement for fast through-connect of virtual connections in ATM communication systems. Hummel proposes that virtual connections be set up using cells containing call-associated signaling information. The signaling information is communicated using cells defined as administration and maintenance cells. A permanent virtual connection is set up from an originating to a destination ATM communication terminal equipment. Cells containing the signaling information are labelled as administration and maintenance cells. They are transmitted via the permanent virtual connection. Using the permanent virtual connection for setting up SVCs facilitates and accelerates SVC setup. A drawback of this solution is that a certain portion of the network bandwidth is reserved exclusively for signaling and that portion of the network cannot be used for other purposes even when signaling requirements are minimal. A further disadvantage is that SVC setup still takes time and uses resources in every node involved in the connection.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method and apparatus for rapidly establishing communications connections through an ATM network which ensures minimal delay in the setup of communications connections while ensuring efficient usage of network capacity.

It is another object of the invention to provide a method and apparatus for rapidly establishing cross-connections through an ATM switch which ensures minimal delay in the setup of cross-connections while ensuring efficient usage of switch capacity.

It is a further object of the invention to provide a method and apparatus for establishing a communications connection through an ATM network which permits the caching of SVCs.

It is another object of the invention to provide a method and apparatus for establishing a cross-connection through an ATM switch which permits the caching of crossconnections.

It is yet a further object of the invention to provide a method and apparatus for rapidly establishing a communications connection through an ATM network which permits the intelligent caching of SVCs using application-specific parameters.

It is yet another object of the invention to provide a method and apparatus for rapidly establishing a crossconnection through an ATM switch which permits an intelligent caching of cross-connections using applicationspecific parameters.

It is yet a further object of the invention to provide a method and apparatus for rapidly establishing a communications connection through an ATM network which proactive ly responds to the level of use of an ATM network by any given real-time communications application to ensure that network resources are efficiently utilized.

It is yet another object of the invention to provide a method and apparatus for rapidly establishing a crossconnection through an ATM switch that proactively responds to the level of usage of the switch fabric by any communications application to ensure that switch resources are efficiently utilized.

These and other objects of the invention are realized in a method of rapidly establishing a communications connection through an asynchronous transfer mode (ATM) network, comprising the steps of:

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asynchronous transfer mode (ATM) network 12. Many other functionally equivalent arrangements for ATM network ingress/egress points are also known. The ingress/egress point includes at least one application interface 14. The application interface may be any one of a number of devices, 5 including: ATM line cards in the trunk side of a Service Switching Point (SSP) in a synchronous transfer mode (STM) switched telephone network (STN); an STM tandem peripheral which serves as a gateway between the STN and the ATM network; an IP router or an IP host with an ATM Network Interface Card (NIC). Those skilled in the art will understand that this list is not an exhaustive list of application interfaces. The application interface 14 is connected to an ATM switch fabric 16 which is well known in the art. Connected to the ATM network side of the ATM switch fabric 16 are a plurality of auxiliary cross-connect line cards 18, also well known in the art. The ATM switch fabric 16 is controlled by a switch control element 20, which handles ATM signaling, routing and switch fabric resource management, as is well known in the art. The ATM network 2Q 12 is typically used as a backbone network for a number of different communications services. The ATM network 12 typically serves as a transport for a variety of real-time communications services which are delay sensitive, as well as a variety of data communications services which are 2J delay tolerant. When real-time communications are transported through the ATM network, it is important that setup delays be minimized in order to ensure that excessive call setup delays are avoided and peak-hour call setup rates can be sustained. 30

FIG. 2 is a block diagram of the functional components for switched virtual circuit (SVC) caching in accordance with the invention. A communications application 22 associated with the communications application interface 14 communicates with the functional components of the SVC 35 caching system in accordance with the invention using an appropriate communications protocol. The SVC caching system in accordance with the invention includes an SVC selector 24, an SVC control agent 26, a caching manager 28 and a plurality of cache tables 30 maintained by the caching 40 manager 28. The SVC selector 24, the SVC control agent 26 and the caching manager 28 may be functional components of a single computer program or may be substantially independent software modules or software objects that execute on one or more computer controlled machines. 45

The SVC selector 24 serves as an interface between the application 22 and the SVC control agent 26. The SVC selector 24 is responsible for filtering SVC operation requests for the setup and release of SVCs from the application 22. The SVC selector 24 determines if an SVC is to 50 be taken from a cache table 30 on receipt of an SVC setup request or is to be set up on demand, as will be explained below with relation to FIGS. 3-5.

The SVC control agent 26 is responsible for SVC setup and release. Each of the SVC selector 24 and the caching 55 manager 28 may request the SVC control agent 26 to perform an SVC operation such as the setup or release of an SVC. The SVC control agent 26 also communicates with the communications application 22 to inform the communications application 22 when an SVC is released if the com- 60 munications application 22 requires such information, as will also be explained below with reference to FIGS. 3-5.

The caching manager 28 is responsible for maintaining the SVC cache tables 30 to ensure that an appropriate number of cached SVCs are available for the rapid estab- 65 lishment of communications connections. The caching manager 28 is advised by the SVC selector 24 when a release

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request is received from the communications application 22, and the caching manager executes a management-defined algorithm to determine whether that SVC should be kept in the cache tables or deleted from the cache tables and released back through the ATM network 12. When an SVC is released from another ingress/egress point 10 in the ATM network 12, the SVC control agent 26 will inform the communications application 22 that an SVC belonging to the application has been released. The communications application may directly inform the caching manager 28 of the SVC release. On receipt of the SVC release notice, the caching manager 28 deletes the SVC entry from its cache tables. The function of the caching manager 28 is to ensure that just enough cached SVCs are available to handle delay-sensitive traffic without the over-reservation of bandwidth in the ATM network 12. Overview of System Operations

Preferably the SVC caching system in accordance with the invention may be configured by network administration to operate in accordance with the requirements of a specific communications application. In general, however, the SVC system in accordance with the preferred embodiment of the invention operates as follows. When an SVC is requested by the communications application 22, the SVC selector examines an SVC cache policy (rule base) associated with the application to determine whether the communications application 22 uses SVC caching. If SVC caching is used by the application 22, the SVC selector examines a cache table 30 associated with the application 22 for an appropriate, unoccupied SVC, as will be explained below with reference to FIGS. 3-5. If the selection is successful, the SVC is acquired by the SVC selector 24 which changes the status of the SVC to indicate that it is occupied and an identifier (VCCI) for the SVC is returned to the requesting communications application 22. If all of the SVCs available in the cache are occupied, the SVC selector generates an SVC setup request to the SVC control agent 26 to request that a new SVC be set up through the ATM network 12. After the SVC control agent 26 sets up the SVC, the SVC is passed by the SVC selector 24 to the communications application 22 and to the caching manager 28. The caching manager 28 executes its caching algorithm to determine whether the SVC should be added to the cache table.

When an SVC release request is received from the communications application 22, the SVC selector 24 checks the communications applications caching policy (rule base) to determine if the SVC release request should be passed to the caching manager 28, or passed directly to the SVC control agent 26. If the caching policy indicates that caching is used by the communications application 22, the SVC release request is passed to the caching manager 28. The caching manager 28, on receipt of the release request, executes the caching algorithm for the communications application 22 to determine whether the SVC should be cached or released through the network. The caching algorithm may likewise indicate that only a portion of the SVC is to be released. As will be explained below, it is often advantageous to release only the cross-connect at the ATM switch fabric 16 (FIG. 1) while maintaining the SVC between auxiliary line cards 18. This enables more flexibility for use of the SVC since the SVC is no longer linked to a particular port in the application interface 14. In other instances, there may be sufficient traffic to warrant that the cross-connect is maintained at the application interface 14. If the communications application 22 does not use SVC caching, the SVC selector 24 passes the release request directly to the SVC control agent 26 which releases the SVC through the ATM network 12 in a manner