Transfer 802
Asynchronous Mode
Another advantage of ATM is that it employs switching even in a local-area environment, unlike other LAN technologies like Ethernets, token rings, and FDDI, which use a shared medium such as a bus or a ring. This enables it to provide quality-of-service guarantees more easily than these other technologies. The xed size of the packets used in an ATM network is particularly advantageous for the development of low-cost, high-speed switches. Various lower or physical layer standards are specied for ATM. These range from 25.6 Mb/s over twisted-pair copper cable to 622.08 Mb/s over single-mode optical ber. Among the optical interfaces is a 155.52 Mb/s optical interface that operates over distances up to 2 km using LEDs over multimode ber in the 1300 nm band. Using the specied minimum transmit and receive powers, the loss budget for this interface is 9 dB. The line code used in this case is the (8, 10) line code specied by the Fibre Channel standard. These interfaces are called private usernetwork interfaces in ATM terminology, since they are meant for interconnecting ATM users and switches in networks that are owned and managed by private enterprises. A number of public usernetwork interfaces, which are meant for connecting ATM users and switches to the public or carrier network, are also dened. In these latter interfaces, ATM uses either PDH or SONET/SDH as the immediately lower layer. These interfaces are dened at many of the standard PDH and SONET/SDH rates shown in Tables 6.1 and 6.2, respectively. Among these are DS3, STS-3c, STS-12c, and STS-48c interfaces. In the terminology of the ATM standards, since the layer below ATM is called the physical layer, these interfaces to PDH and SONET/SDH are called physical layer interfaces. On the other hand, in the classical layered view of networks, which we discussed in Section 1.4, PDH and SONET/SDH must be viewed as data link layers when ATM is viewed as a network layer.
J.1
Functions of ATM
ATM data can be transmitted from an ATM user to an ATM network across a user-to-network interface (UNI) or the data can be transmitted across a network-tonetwork interface (NNI) between two ATM switches. Of the 53 bytes in an ATM cell, 48 bytes form the payload, that is, carry information sent from the higher layers, and 5 bytes constitute the header inserted by the ATM layer. The structure of the 5-byte ATM header is slightly different for the UNI and NNI. The two headers are shown in Figure J.1. The elds in the ATM header are as follows. GFC or Generic Flow Control: 4 bits on UNI, not present on NNI.
J.1 Functions of ATM
803
4 bits GFC VPI VCI HEC (a)
3 bits VPI PT
1 bit
4 bits VPI VCI
3 bits
1 bit
CLP HEC (b)
PT
CLP
Figure J.1 The header structure of ATM cells across (a) the UNI and (b) the NNI. The GFC eld is used for ow control across the UNI. The VPI and VCI elds are used for forwarding the cells within the network. PT indicates the payload type and CLP is the cell loss priority bit. The HEC eld provides error checking for the ATM header.
VPI or Virual Path Identier: 8 bits on UNI, 12 bits on NNI. VCI or Virtual Circuit Identier: 16 bits. PT or Payload Type: 3 bits. CLP or Cell Loss Priority: 1 bit. HEC or Header Error Control: 8 bits. The HEC constitutes a CRC on the 5 ATM header bytes and is used to detect corrupted ATM cells. The functions of each of these elds are described in the following sections.
J.1.1
Connections and Cell Forwarding
ATM establishes a connection between two end points the for purpose of transferring data between them. This is unlike IP (which we study in the next section), which transfers data in a connectionless manner. ATM connections are termed virtual channels and are assigned a virtual channel identier (VCI). The VCI for a connection is unique for each link that the ATM connection traverses between its end points but can vary from link to link on the path, as illustrated in Figure J.2(a). For example, the top connection has a VCI of a1, a2, and b on the three links it traverses. The VCIs for each connection on every link of the path are determined at the time of connection setup and released when the connection is torn down. Each node (switch) maintains a VCI table as illustrated in Figure J.2(b). The table species, for each incoming VCI, the outgoing link and the outgoing VCI. For example, at node 1, incoming cells with a VCI of a1 are sent on the link 12 with a VCI of a2.
804
Asynchronous Transfer Mode
VCI = a1 0 VCI = c1 1
VCI = a2 2 VCI = c2 (a)
VCI = b
3
Incoming Outgoing Outgoing VCI link VCI a1 c1 1 to 2 1 to 2 (b) a2 c2
VCI = d
4
Figure J.2 The use of ATM VCIs for cell forwarding across a path. The ATM switches use the VCI to determine the outgoing link for a cell. The switches also rewrite the VCI eld with the value assigned to the virtual channel on the outgoing link. (a) Illustration of the cell forwarding and VCI swapping. (b) The VCI table maintained at node 1 of (a).
J.1.2
Virtual Paths
There could be millions of virtual channels sharing a link. Looking up a VCI table larger than 216 = 65,536 entries for forwarding every single cell is expensive. Thus we need to have some mechanism for bundling or aggregating virtual channels for the purpose of forwarding. It is quite likely that thousands of virtual channels will have the same path, if not end to end, at least over signicant parts of the network. This property of virtual channels can be used for aggregation and is accomplished by the use of VPIs. The use of VPIs can be understood through the following example. Consider Figure J.3. Here we have four links, connecting the nodes 0, 1, 2, and 3, as shown. The two virtual circuits shown share the links 01 and 12. These virtual channels can be assigned a common VPI on each of these links (which can be, and generally is, different on the two individual links). For example, a VPI of x can be assigned on link 01 and a VPI of y on link 12. The set of two links constitutes a virtual path in the network, with node 0 constituting the beginning of the virtual path and node 2 constituting the end of the virtual path. All cells belonging to any virtual circuit assigned to this path are routed on these links based on the smaller VPI value. When the cells reach the end of the virtual path, node 2 in this example, they are again forwarded based on the VCI values. Simply put, the virtual channels treat each virtual path as a segment in their route between the source and destination: the switches within a virtual path forward cells based only on the VPI eld. Use of the two level labels, VPI and VCI, simplies the cell-forwarding process and enables the development of cost-effective ATM switches. If a single eld were used, it would be 24 bits long across the UNI and 28 bits long across the NNI. Such a large eld would make the cell-forwarding process expensive. Another advantage of virtual paths is that it enables the creation of logical links between nodes: the virtual path between two nodes is treated like a logical link by