Note that a virtual connection is defined by a pair of numbers: the VPI and the vel in Software

Maker Code 3/9 in Software Note that a virtual connection is defined by a pair of numbers: the VPI and the vel

Figure 1812 shows the VPIs and VCls for a transmission path The rationale for dividing an identifier into two parts will become clear when we discuss routing in an ATM network The lengths of the VPIs for UNIs and NNIs are different In a UNI, the VPI is 8 bits, whereas in an NNI, the VPI is 12 bits The length of the VCI is the same in both interfaces (16 bits) We therefore can say that a virtual connection is identified by 24 bits in a UNI and by 28 bits in an NNI (see Figure 1813) The whole idea behind dividing a virtual circuit identifier into two parts is to allow hierarchical routing Most of the switches in a typical ATM network are routed using VPIs The switches at the boundaries of the network, those that interact directly with the endpoint devices, use both VPIs and VCIs

The basic data unit in an ATM network is called a cell A cell is only 53 bytes long with 5 bytes allocated to the header and 48 bytes carrying the payload (user data may be less

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than 48 bytes) We will study in detail the fields of a cell, but for the moment it suffices to say that most of the header is occupied by the VPI and VCI that define the virtual connection through which a cell should travel from an endpoint to a switch or from a switch to another switch Figure 1814 shows the cell structure Figure 1814 An ATM cell

Like Frame Relay, ATM uses two types of connections: PVC and SVc PVC A permanent virtual-circuit connection is established between two endpoints by the network provider The VPls and vcrs are defined for the permanent connections, and the values are entered for the tables of each switch

SVC In a switched virtual-circuit connection, each time an endpoint wants to make a connection with another endpoint, a new virtual circuit must be established ATM cannot do the job by itself, but needs the network layer addresses and the services of another protocol (such as IP) The signaling mechanism of this other protocol makes a connection request by using the network layer addresses of the two endpoints The actual mechanism depends on the network layer protocol

ATM uses switches to route the cell from a source endpoint to the destination endpoint A switch routes the cell using both the VPls and the VCls The routing requires the whole identifier Figure 1815 shows how a VPC switch routes the cell A cell with a VPI of 153 and VCI of 67 arrives at switch interface (port) 1 The switch checks its switching table, which stores six pieces of information per row: arrival intetface number, incoming VPI, incoming VCI, corresponding outgoing interface number, the new VPI, and the new VCL The switch finds the entry with the interface 1, VPI 153, and VCI 67 and discovers that the combination corresponds to output interface 3, VPI 140, and VCI 92 It changes the VPI and VCI in the header to 140 and 92, respectively, and sends the cell out through interface 3 Figure 1815

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The switching technology has created many interesting features to increase the speed of switches to handle data We discussed switching fabrics in 8

The ATM standard defines three layers They are, from top to bottom, the application adaptation layer, the ATM layer, and the physical layer (see Figure 1816) The endpoints use all three layers while the switches use only the two bottom layers (see Figure 1817)

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