Broadband ISDN and ATM
Now a days ISDN interface standards increased end-user transmission bandwidth to several Mb/s.This new technique is known as broadband ISDN (B-ISDN) .This technique based on asynchronous transfer mode (ATM).This allows packet switching rates up to 2.4 Gbps and total switching capacities a€ high as 100 Gbps.
Asynchronous transfer mode (ATM):
- ATM is a packet switching and multiplexing technique which handle both voice users and packet data users in a single physical channel.
- ATM data rates vary from low traffic rates (64 kbps) over twisted pair to over 100 Mbps over fiber optic cables for high traffic rates between network nodes
- ATM supports bidirectional transfer of data packets of fixed length between two end points
- ATM data units, called cells, are routed based on header information in each unit (called a label) that identifies the cell as belonging to a specific ATM virtual connection
- The ATM header also includes data for congestion control, priority information for queuing of packets, and a priority which indicates which ATM packets can be dropped in case of congestion in the network.
Cell format of ATM:
Figure 8.5 Cell format of (ATM).
- Figure 8.5 shows the cell format of ATM.
- ATM cells (packets) have a fixed length of 53 bytes, consisting of 48 bytes of data and 5 bytes of header information.
- Fixed length packets result in simple implementation of fast packet switches, since packets arrive synchronously at the switch