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Chapter 4

Chapter 4. Packet Switching Networks & Frame Relay. Introduction. Packet-Switching Switching Technique Routing X.25 Frame Relay Networks Architecture User Data Transfer Call Control. Introduction - Taxonomy. Communication Networks. Circuit -Switched. Packet -Switched.

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Chapter 4

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  1. Chapter 4 Packet Switching Networks & Frame Relay

  2. Introduction • Packet-Switching • Switching Technique • Routing • X.25 • Frame Relay Networks • Architecture • User Data Transfer • Call Control Chapter 4 Frame Relay

  3. Introduction - Taxonomy Communication Networks Circuit -Switched Packet -Switched Virtual Circuit FDM Datagram TDM Frame Relay ATM The Internet (TCP/IP) Chapter 4 Frame Relay

  4. Circuit-Switching • Historically – long-haul telecom networks designed for voice and/or constant bit rate applications • Network resources dedicated to one “call” after circuit setup • Shortcomings when used for data: • Inefficient (high idle time) for “bursty” sources • Constant data rate not appropriate for varied endpoint capabilities Chapter 4 Frame Relay

  5. Packet-Switching • Historically – network technology designed for general data communications • Basic technology is the same as in the 1970s • One of the few effective technologies for long distance data communications in use today • Frame relay and ATM are variants of packet-switching (using virtual circuits) • Advantages: • flexible, resource sharing, robust, responsive • Disadvantages: • Time delays in distributed network, overhead penalties • Need for routing and congestion control Chapter 4 Frame Relay

  6. Packet-Switching • Data transmitted in short blocks, or packets • Packet length typically < 1000 octets • Each packet contains user data plus control info (routing) • Store and forward Chapter 4 Frame Relay

  7. Advantages over Circuit-Switching • Greater line efficiency (many packets can go over shared link) • Data rate conversions • Non-blocking (e.g. no “busy signals”) under heavy traffic (but increased delays) • Each packet can be handled based on a priority scheme Chapter 4 Frame Relay

  8. Disadvantages relative to Circuit-Switching • Packets incur delay with every node they pass through Q * (dprop + dtrans + dqueue + dproc) • Jitter: variation in end-to-end packet delay • Data overhead in every packet for routing information, etc • More processing overhead for every packet at every node traversed… circuit switching has little/no processing at each node Chapter 4 Frame Relay

  9. Switching Technique • Large are messages broken up into smaller “chunks,” generically called packets • Store and forward packet handling in core • Two approaches to switching data: • Datagram • Each packet sent independently of the others • No call setup • More reliable (can route around failed nodes or congestion) • Virtual circuit • Fixed route established before any packets sent • No need for routing decision for each packet at each node Chapter 4 Frame Relay

  10. Packet Switching: Datagram Approach • Advantages: • No call setup • Flexible routes • Reliability Chapter 4 Frame Relay

  11. Packet Switching: Virtual-Circuit Approach • Advantages: • Network services • sequencing • error control • Performance Chapter 4 Frame Relay

  12. Routing • Key function of any packet-switched network: forwarding packets to a destination • Adaptive routing, routes are adjusted based on: • Node/trunk failure • Congestion • Nodes (routers/switches) must exchange information about the state of the network Chapter 4 Frame Relay

  13. The Use of Virtual Circuits Virtual end-to-end circuits Chapter 4 Frame Relay

  14. X.25 • First commercial packet switched network interface standard • Motivates discussion of frame relay and ATM design • X.25 defines 3 levels of functionality L1 - Physical level (X.21, EIA-232, etc.): physical connection of a station to the link L2 - Link/frame level (LAPB, a subset of HDLC): logical, reliable transfer of data over the physical link L3 - Packet level: network layer, provides virtual circuit service to support logical connections between two subscriber stations (multiplexing) Chapter 4 Frame Relay

  15. User Data and X.25 Protocol Control Information • Virtual circuit id# • Sequence #s  128 bytes 3 bytes • Flags, address, control, FCS • Link layer framing • Reliable physical transfer Chapter 4 Frame Relay

  16. X.25 Features • Call control packets • set up and tear down virtual circuits • use same channel and VC as data packets • Multiplexing of VCs at layer 3 • Layers 3 (packet) and 2 (frame) both include extensive flow control and error control mechanisms Processing Overhead (tproc) at each node! RESULT: 64kbps Max. data rate Chapter 4 Frame Relay

  17. Frame Relay Networks • Most widely deployed WAN link-layer protocol in use today • Designed to eliminate much of the processing overhead in X.25 • Designed to support “bandwidth on demand” for modern, bursty applications • Throughput is an order of magnitude higher than X.25 • ITU-T Recommendation I.233 indicates effective rates of frame relay of up to 2 Mbps, but current practice is much higher (up to T-3 equivalent, or 44.376 Mbps) Chapter 4 Frame Relay

  18. Frame Relay Networks Important Improvement over X.25: • Call control signaling is on a separate logical connection from user data • Multiplexing/switching of logical connections is at layer 2 (not layer 3) • No hop-by-hop flow control and error control; responsibility of higher layers • Frames sizes can vary (up to 9000 bytes), supporting all current LAN frame sizes • Direct support for TCP/IP packets, since no network layer redundancy Chapter 4 Frame Relay

  19. Comparison of X.25 and Frame Relay Protocol Stacks Chapter 4 Frame Relay

  20. Frame Relay Architecture • X.25 has 3 layers: physical, link, network • Frame Relay has 2 layers: physical and data link (or LAPF) • LAPF core: minimal data link control • Preservation of order for frames • Small probability of frame loss Chapter 4 Frame Relay

  21. LAPF Core • Frame delimiting, alignment and transparency • Frame multiplexing/demultiplexing • Inspection of frame for length constraints • Detection of transmission errors • Congestion control Chapter 4 Frame Relay

  22. LAPF-core Formats 10-bit address 23-bit address 16-bit address Chapter 4 Frame Relay

  23. User Data Transfer Frame • No connection control fields, which are normally used for: • Identifying frame type (data or control) • Sequence numbers, used for error/flow control • Implication: • Connection setup/teardown carried on separate channel • No flow and error control, must be handled by higher layer in protocol stack Chapter 4 Frame Relay

  24. Frame Relay Call Control • Frame Relay Call Control • Details of call control depend on the context of its use • Assumes FR over ISDN • Generally simpler for point-to-point use • Data transfer involves: • Establish logical connection and assign a unique DLCI • Exchange data frames • Release logical connection Chapter 4 Frame Relay

  25. Frame Relay Call Control 4 message types needed • SETUP…request link establishment • CONNECT…reply to SETUP with connection accepted • RELEASE…request to clear (tear down) a connection • RELEASE COMPLETE… reply to SETUP with connection denied, or response to RELEASE Chapter 4 Frame Relay

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