Resource Management with Virtual Paths in ATM Networks

10 n asynchronous transfer mode (ATM) networks, multiplexing and switching are performed on 53-byte cells which are transported across the network on virtual channel connections (VCCs) [1]. A virtual path connection (VPC) is a labeled path which can be used to transport, process, and manage a bundle of VCCs. Two levels of cell forwarding are defined: virtual path (VP) and virtual channel (VC), which use the virtual path identifier (VPI) and virtual channel identifier (VCI) fields in the cell header, respectively [2]. A VPC can be characterized by its two VPC terminators, a physical route between these terminators, and a possible assigned capacity. A VCC, on the other hand, traverses a set of concatenated VPCs. An example of this relationship between the VPC and VCC is shown in Fig. 1. At the originating VPC terminator (node A), a cell belonging to a given VCC is identified by a VCI (i.e., VCI 6). This VCI, as well as the identifier of the VPC used to carry this VCC (i.e., VPI 4), are then written in the header of the cell. At the other end of the VPC (node D), VCI and VPI translation takes place for transmission onto the next VPC. At transit nodes, only the VPI label needs to be recognized, and the VCI field remains unchanged. Transit nodes are therefore freed from performing VCI translation when routing cells.1 In general, a VCC is constructed from one or more VPCs (or VC links). The number of VPCs traversed by a VCC will be referred to as its VPC hop count. Likewise, a VPC is constructed from one or more physical links (or VP links). The number of physical links traversed by a VPC (or a VCC) will be referred to as its physical hop count. In Fig. 1, the VCC has a VPC hop count of two and a physical hop count of five,, while VPC 1 and VPC 2 have physical hop counts of three and two, respectively. Using the virtual path concept, the network is organized as a collection of VPCs which form a VPC, or logical, overlay network (Fig. 2). In this logical network, links correspond to VPCs, while nodes correspond to VPC terminators. The logiResource Management with Virtual Paths in ATM Networks