Broadband ATM networks 10.1 Introduction Broadband ISDN (B-ISDN) was designed to support multimedia communication applications. The source information associated with each call is first converted into small fixed-sized packets known as cells. Instead of allocating a fixed portion of transmission bandwidth per call, the cell streams relating to different calls are multiplexed together on a statistical basis. The switching units then operate using a form of packet switching called cell switching. Small fixed-sized cell switches can operate at a much higher rate than variable-length packet switches and hence it is known as fast packet switching. In order to keep the header of a cell small, the connection-oriented mode instead of the connectionless mode is used. A unique identifier called as virtual circuit identifier (VCI) is assigned to the call on each line making up the virtual circuit as a virtual circuit is set up. In order to make a compromise between packet delay and relative overheads, a cell comprises a 48-byte payload (information) field and a 5-byte header for the VCI and other fields. No error control is performed and hence no sequence numbers are required for retransmission purposes. The header contains error check bits for error detection. An optical fiber access network is used to connect subscriber premises to the core network as the bit rate is expected to be high. The cells relating to different calls have varying time intervals between them during both transmission and switching and hence this mode of operation is known as the asynchronous transfer mode (ATM). Networks operate in this way is called ATM networks. CYH/MMT/ATM/p.1 CYH/MMT/ATM/p.2
10.2 Cell format and switching principles In an ATM network, the virtual circuit identifier used on each link is known as the protocol connection identifier (PCI). Associated with each incoming link/port is a routing table that contains, for each incoming PCI, the corresponding outgoing link route and the new PCI to be used. The routing of cells is very fast as it involves a simple look-up operation. The PCI is made up of 2 field: a virtual path identifier (VPI) and a virtual channel identifier (VCI). Routing can be performed using either one or a combination of the two. Example of the use of virtual path switching: multiple calls originating at the same network entry point are all intended for the same destination. Example of the use of virtual channel switching: each call at the network entry point is intended for a different destination. CYH/MMT/ATM/p.3 CYH/MMT/ATM/p.4
Format of a cell: Generic flow control (GFC): enables a local switch to flow control the entry of cells by a user into the network. Virtual path identifier (VPI): used for identification/routing purposes within the network. Virtual channel identifier (VCI): used for identification/routing purposes within the network. Payload type identifier (PTI) indicates the type of information carried in the cell. Cell loss priority (CLP): enable a user to specify a preference as to which cells should be discarded during heavy load conditions. Header error checksum (HEC): error detection for the header CYH/MMT/ATM/p.5 CYH/MMT/ATM/p.6
10.3 Switch architectures This involves a simple look-up and mapping operation of the VPI/VCI in the header of the incoming cell into the corresponding output VPI/VCI. Buffering is provided by either the input controllers or the output controller to handle the case that more than 1 cells arrive at the different input ports for the same output port. Output controllers simply forward received cells at the appropriate bit rate. The main role of the control processor is to download routing information into routing tables in each input controller. The control processor may generate network management messages itself. The switch fabrics used in ATM switches can be classified as either time division or space division. In a time-division switch, a time-division backplane bus that is capable of transferring N cells, where N is the number of input ports, in a single cell arrival time. Each input link is terminated by an input controller (IC) which performs the routing of cells arriving at each link (port) to their required output link. In a space-division switch the switch fabric comprises a matrix of interconnected switching elements that collectively provide a number of alternative paths through the network. CYH/MMT/ATM/p.7 CYH/MMT/ATM/p.8
The matrix is known as a fully-connected switch matrix. To avoid internal queuing with this type of switch the cell transfer operation must be performed N times faster than the cell arrival rate, where is the number of input ports. No additional buffering is required within the switching matrix and the switch is said to be internally nonblocking. Ways of overcoming blocking: Discard cells Perform the switching operation several times faster than the cell arrival rate Introduce buffering Others Most practical matrix switch designs use multiple switching stages, each made up of a number of smaller switching elements interconnected in a regular matrix. This type of switching fabric is blocking. CYH/MMT/ATM/p.9 CYH/MMT/ATM/p.10