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Chapter 2 – X.25, Frame Relay & ATM

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

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Chapter 2 – X.25, Frame Relay & ATM

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  1. Chapter 2 – X.25, Frame Relay & ATM

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

  3. Three Types of Switched Communication Network • Circuit-switched • Message-switched • Packet-switched

  4. 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

  5. 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

  6. 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

  7. 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

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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

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

  15. 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

  16. 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

  17. 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

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