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WAN – Packet and Cell-switched Networks

WAN – Packet and Cell-switched Networks. X.25, Frame Relay ATM. Introduction.

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WAN – Packet and Cell-switched Networks

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  1. WAN – Packet and Cell-switched Networks X.25, Frame Relay ATM Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  2. Introduction • Starting from the 1980’s, the emphasis in wide area networking has been shifting from dedicated point-to-point connections (circuit-switched and leased lines) to switched alternatives (packet and cell-switched networks) because: • A full mesh network requires a large number of point-to-point connections while switched networks provide any-to-any connectivity. • Point-to-Point links are engineered to meet peak traffics. During non-peak hours, transmission capacity is wasted. Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  3. Agenda • Packet-switched Network • Connectionless and Connection-oriented Services • Datagram and Virtual Circuit • Permanent and Switched Virtual Circuits (PVC and SVC) • X.25 and Frame Relay Packet-switched Networks • Cell-switched Network – ATM Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  4. Packet-Switched Network • In packet-switched network, messages are broken up into small packets before they are sent. • Packets from multiple sources travel through links and routers in an interleaved (STDM) fashion (i.e. the physical links are shared). Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  5. Packets from multiple sources traveling through a packet-switch network Source: Forouzan, Data Comm. and Networking, McGraw-Hill Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  6. Advantages of Packet-Switched Network • Use network bandwidth more efficiently – as the communication links are shared by many users (instead of being dedicated to only one user as in the case of circuit-switched network) • Ideal for organizations that need to be on line all the time – because the service is usually paid by a fixed monthly connect fee plus usage charges Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  7. Connectionless Transmission Service • Also known as Datagram service • Packets are forwarded on-the-fly by routers, based on the current best path to a destination. • Packets may travel through different routes and hence arrive the destination out of orders. • Normally work without error recovery procedures  unreliable • used to transfer short messages • e.g. UDP protocol Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  8. Connectionless (i.e. Datagram) Service Source: Forouzan, Data Comm. and Networking, McGraw-Hill Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  9. Connection-oriented Transmission Services • A path (virtual circuit) must be established between source and destination across the network before packets can start to flow. • All packets go through the same path so they always arrive the destination in the right order. • Normally work with error recovery procedures  reliable • used to transfer long messages • e.g. X.25, Frame Relay networks, TCP protocol Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  10. Virtual Circuits: PVC and SVC A. Permanent Virtual Circuit (PVC) • similar to leased lines • established by user subscription and is always available to the DTE as a permanent resource • the major type of circuits available from packet-switched and cell-switched network providers B. Switched Virtual Circuit (SVC) • like telephone calls • a connection is established first, then data are transferred and finally the connection is released. Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  11. Permanent Virtual Circuit (PVC) Source: Forouzan, Data Comm. and Networking, McGraw-Hill Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  12. Switched Virtual Circuit (SVC) Source: Forouzan, Data Comm. and Networking, McGraw-Hill Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  13. X.25 Packet-switched Network • Has been around since the mid 1970's when most WAN circuits were still analog and thus susceptible to errors. • Provides very robust error checking to guarantees data integrity at the cost of large network delays • Speed ≤ 64 kbps Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  14. Frame Relay Packet-switched Network • Became popular since the mid 1980’s when modern digital and optical networks have advanced to the extent that node-to-node error correction throughout the network is no longer necessary. • Leaves it up to the end stations (i.e. transport layer) to handle any errors. • Fast packet switching with minimal network delay. • Speed: up to T1 or even T3 Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  15. Frame Relay Packet-switched Network • Committed Information Rate (CIR) • defined as the rate at which the network agrees to accept data from a client over a specific virtual circuit • chosen at the time the connection is ordered from the communications provider • data sent in excess of the CIR may be discarded and not delivered when the frame relay network is congested • some inexpensive Frame Relay services are based on a CIR of zero Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  16. Asynchronous Transfer Mode (ATM) • International standard for high-speed (up to 2.5 Gbps, usually run on optical fibers), connection-oriented, cell-switched technology. • Foundation of B-ISDN (Broadband ISDN) which supports both constant and variable bit rates. • No error recovery – just like Frame Relay network. • Unlike packet-switched networks where the packet size varies, ATM transmits fixed-length (53 bytes) data units called “cells. • because the cell size is so small and the throughput of the circuit is high, delay-sensitive traffic can be carried along with bursty types of data successfully. Voice and video work without glitches and data customers get bandwidth-on-demand. Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  17. Asynchronous Transfer Mode (ATM) • Makes optimal use of circuit capacity – by statistically multiplexing cells onto the network according to the Quality-of-Service information contained in the cell header (i.e. like STDM) • QoS Classes: • Class A: constant bit rate (CBR), for uncompressed voice and video transmission • Class B: real-time variable bit rate (RT-VBR), for applications that can tolerate slight delays, e.g. compressed voice and video transmission • Class C: non-real-time variable bit rate (NRT-VBR), for bursty, non-critical applications, e.g. LAN-to-LAN data traffic. • Class D: two types - available bit rate (ABR) and unspecified bit rate (UBR),for file transfer and datagram transmission Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  18. Source: WestNet Learning Technologies Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

  19. Where to Get More Information • WestNet Learning Technologies • http://www.webclasses.net/3comu/intro/units/unit07/ • Data Communications and Networking, by Behrouz A. Forouzan, Publisher: McGraw-Hill • Chapter 17, 18, 19 Data Communication (E&T2760): WAN - Packet and Cell-Switched Networks

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