Frame Relay Chapter 04
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Frame Relay is a packet switching technology that was developed to improve on X.25 networks. It uses virtual circuits to transfer user data in frames more efficiently than X.25 by eliminating much of the overhead and removing hop-by-hop flow and error control. Frame Relay networks operate at the data link layer and use logical connections identified by a Data Link Connection Identifier to multiplex and switch user data frames, while call setup and teardown is handled on a separate control channel.
Frame Relay Chapter 04
- 1. Chapter 4 Frame Relay Chapter 4 Frame Relay 1
- 2. Introduction • Packet-Switching Networks – Switching Technique – Routing – X.25 • Frame Relay Networks – Architecture – User Data Transfer – Call Control Chapter 4 Frame Relay 2
- 3. Packet-Switching Networks • Basic technology the same as in the 1970s • One of the few effective technologies for long distance data communications • Frame relay and ATM are variants of packet- switching • Advantages: – flexibility, resource sharing, robust, responsive • Disadvantages: – Time delays in distributed network, overhead penalties – Need for routing and congestion control Chapter 4 Frame Relay 3
- 4. Circuit-Switching • Long-haul telecom network designed for voice • Network resources dedicated to one call • Shortcomings when used for data: – Inefficient (high idle time) – Constant data rate Chapter 4 Frame Relay 4
- 5. Packet-Switching • Data transmitted in short blocks, or packets • Packet length < 1000 octets • Each packet contains user data plus control info (routing) • Store and forward Chapter 4 Frame Relay 5
- 6. Figure 4.1 The Use of Packets Chapter 4 Frame Relay 6
- 7. Figure 4.2 Packet Switching: Datagram Approach Chapter 4 Frame Relay 7
- 8. Advantages over Circuit-Switching • Greater line efficiency (many packets can go over shared link) • Data rate conversions • Non-blocking under heavy traffic (but increased delays) Chapter 4 Frame Relay 8
- 9. Disadvantages relative to Circuit- Switching • Packets incur additional delay with every node they pass through • Jitter: variation in packet delay • Data overhead in every packet for routing information, etc • Processing overhead for every packet at every node traversed Chapter 4 Frame Relay 9
- 10. Figure 4.3 Simple Switching Network Chapter 4 Frame Relay 10
- 11. Switching Technique • Large messages broken up into smaller packets • 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 11
- 12. Figure 4.4 Packet Switching: Virtual- Circuit Approach Chapter 4 Frame Relay 12
- 13. Routing • Adaptive routing • Node/trunk failure • Congestion Chapter 4 Frame Relay 13
- 14. X.25 • 3 levels • Physical level (X.21) • Link level (LAPB, a subset of HDLC) • Packet level (provides virtual circuit service) Chapter 4 Frame Relay 14
- 15. Figure 4.5 The Use of Virtual Circuits Chapter 4 Frame Relay 15
- 16. Figure 4.6 User Data and X.25 Protocol Control Information Chapter 4 Frame Relay 16
- 17. Frame Relay Networks • Designed to eliminate much of the overhead in X.25 • Call control signaling on separate logical connection from user data • Multiplexing/switching of logical connections at layer 2 (not layer 3) • No hop-by-hop flow control and error control • Throughput an order of magnitude higher than X.25 Chapter 4 Frame Relay 17
- 18. Figure 4.7 Comparison of X.25 and Frame Relay Protocol Stacks Chapter 4 Frame Relay 18
- 19. Figure 4.8 Virtual Circuits and Frame Relay Virtual Connections Chapter 4 Frame Relay 19
- 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 • LAPF control: additional data link or network layer end-to-end functions Chapter 4 Frame Relay 20
- 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 21
- 22. Figure 4.9 LAPF-core Formats Chapter 4 Frame Relay 22
- 23. User Data Transfer • No control field, which is normally used for: – Identify frame type (data or control) – Sequence numbers • Implication: – Connection setup/teardown carried on separate channel – Cannot do flow and error control Chapter 4 Frame Relay 23
- 24. Frame Relay Call Control • Frame Relay Call Control • Data transfer involves: – Establish logical connection and DLCI – Exchange data frames – Release logical connection Chapter 4 Frame Relay 24
- 25. Frame Relay Call Control 4 message types needed • SETUP • CONNECT • RELEASE • RELEASE COMPLETE Chapter 4 Frame Relay 25