Chapter 2 – X.25, Frame Relay & ATM

Switched Network Stations are not connected together necessarily by a single link Stations are typically far apart Messages are not broadcast to every station

Three Types of Switched Communication Network Circuit-switched Message-switched Packet-switched

Circuit-Switched Network Before any data can be sent, an end-to-end circuit must be established This circuit is maintained for the duration of the transfer of all the data The data can be digital or analog and the signal can be either type as well Connection is usually full-duplex Is inefficient – channel capacity is dedicated for the duration of the connection Example – Pubic telephone system

Circuit-Switched Network – Cont. Bits are transmitted as fast as they are received – no storage of data at the intermediate nodes Disadvantages –Both stations must be available at the same time for data exchange –Resources in the network are dedicated for the duration of the transmission

Message-Switched Network It is not necessary to establish a dedicated path between the two stations The sending station appends a destination address to the message The message is passed through the network from node to node At each node the entire message is received, stored briefly, and then transmitted to the next node

Advantages of Message- Switched Network Line efficiency is greater Sender and receiver do not have to be available at the same time Duplicate copies of message can be sent to different destinations Message priorities can be established Error control and recovery can be built into the network

Disadvantage of Message- Switched Network Not suited to real-time traffic Delay through network is relatively long and varies considerably

Packet-Switched Network Very much like message switching Principal external difference is that the length of the message found internally has a maximum length A typical maximum length is several thousand bits Messages above the maximum length are divided up into smaller units and sent out one at a time These smaller units are called packets Packets, unlike messages, are typically not filed at the intermediate nodes

Packet-Switched Network The simple rule of limiting the maximum size of a data unit has a dramatic effect on performance There are two different ways the network can handle the stream of packets that make up the message: –Datagram –Virtual circuit

Datagram Approach to Packet- Switched Network Each packet is treated independently The packets may take different paths to the destination The packets might arrive in a different sequence from the order in which they were sent The packets may have to be reordered at the destination

Virtual Circuit Approach to Packet-Switched Network A logical connection is established before any packets are sent All packets follow the same path through the network This does not mean that there is a dedicated path, as in circuit switching

Advantages of the Datagram Approach Call setup phase is avoided This is important if a station wished to send only one or a few packets More flexible – incoming packets can be routed away from congestion when it develops Datagram delivery is more reliable – if a node fails, packets can be sent on an alternate route

Three Examples of Packet- Switched Protocols X.25 – Virtual Circuit Frame Relay – Virtual Circuit ATM – Virtual Circuit

X.25 Based upon existing analog copper lines that experience a high number of errors Uses the virtual circuit approach A set of international protocols approved in 1976 Provides a way to send packets across a packet- switched public data network The redundant error checking is done at each node See Figure 2-8 for X.25 encapsulation for IP datagrams

Frame Relay No longer need the overhead associated with X.25 and analog copper wires Similar to X.25, but does not have the added framing and processing overhead to provide guaranteed data transfer Link-to-link reliability is not provided – if a frame is corrupted, it is silently discarded Upper-level protocols such as TCP must detect and recover discarded frames See Figure 2-9 for Frame Relay encapsulation of IP datagrams

ATM Destined to replace most existing WAN technologies Improves on performance of Frame Relay Based upon 53-byte cells of fixed size 48 bytes of application information together with a 5-byte ATM header The standard-sized cells allow switching mechanisms to achieve faster switching rates Rates of 155 – 622 Mbps are achieved with theoretical rates up to 1.2 Gbps Compatible with twisted-pair, coax, and fiber