Chapter 4 Packet Switching Networks Frame Relay 1

Chapter 4 Packet Switching Networks & Frame Relay 1

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

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

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

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

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

Advantages over Circuit. Switching Greater line efficiency (many packets can go over shared link) l Data rate conversions l Non-blocking (e. g. no “busy signals”) under heavy traffic (but increased delays) l Each packet can be handled based on a priority scheme l Chapter 4 Frame Relay 9

Disadvantages relative to Circuit -Switching l Packets incur delay with every node they pass through Q * (dprop + dtrans + dqueue + dproc) Jitter: variation in end-to-end packet delay l Data overhead in every packet for routing information, etc l More processing overhead for every packet at every node traversed… circuit switching has little/no processing at each node l Chapter 4 Frame Relay 10

Switching Technique l l l Large are messages broken up into smaller “chunks, ” generically called packets Store and forward packet handling in core Two approaches to switching data: – Datagram l l l Each packet sent independently of the others No call setup More reliable (can route around failed nodes or congestion) – Virtual circuit l l Fixed route established before any packets sent No need for routing decision for each packet at each node Chapter 4 Frame Relay 11

Packet Switching: Datagram Approach Advantages: • No call setup • Flexible routes • Reliability Chapter 4 Frame Relay 12

Packet Switching: Virtual-Circuit Approach Advantages: • Network services • sequencing • error control • Performance Chapter 4 Frame Relay 13

Routing Key function of any packet-switched network: forwarding packets to a destination l Adaptive routing, routes are adjusted based on: l – Node/trunk failure – Congestion l Nodes (routers/switches) must exchange information about the state of the network Chapter 4 Frame Relay 14

The Use of Virtual Circuits Virtual end-to-end circuits Chapter 4 Frame Relay 15

X. 25 l l l First commercial packet switched network interface standard Motivates discussion of frame relay and ATM design X. 25 defines 3 levels of functionality L 1 - Physical level (X. 21, EIA-232, etc. ): physical connection of a station to the link L 2 - Link/frame level (LAPB, a subset of HDLC): logical, reliable transfer of data over the physical link L 3 - Packet level: network layer, provides virtual circuit service to support logical connections between two subscriber stations (multiplexing) Chapter 4 Frame Relay 16

User Data and X. 25 Protocol Control Information • Virtual circuit id# • Sequence #s 3 bytes 128 bytes • Flags, address, control, FCS • Link layer framing • Reliable physical transfer Chapter 4 Frame Relay 17

X. 25 Features l Call control packets – set up and tear down virtual circuits – use same channel and VC as data packets Multiplexing of VCs at layer 3 l Layers 3 (packet) and 2 (frame) both include extensive flow control and error control mechanisms l Chapter 4 Frame Relay Processing Overhead (tproc) at each node! RESULT: 64 kbps Max. data rate 18

Frame Relay Networks l l l 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 19

Frame Relay Networks Important Improvement over X. 25: l Call control signaling is on a separate logical connection from user data l Multiplexing/switching of logical connections is at layer 2 (not layer 3) l No hop-by-hop flow control and error control; responsibility of higher layers l Frames sizes can vary (up to 9000 bytes), supporting all current LAN frame sizes l Direct support for TCP/IP packets, since no network layer redundancy Chapter 4 Frame Relay 20

Comparison of X. 25 and Frame Relay Protocol Stacks Chapter 4 Frame Relay 21

Frame Relay Architecture X. 25 has 3 layers: physical, link, network l Frame Relay has 2 layers: physical and data link (or LAPF) l LAPF core: minimal data link control l – Preservation of order for frames – Small probability of frame loss Chapter 4 Frame Relay 23

LAPF Core Frame delimiting, alignment and transparency l Frame multiplexing/demultiplexing l Inspection of frame for length constraints l Detection of transmission errors l Congestion control l Chapter 4 Frame Relay 24

LAPF-core Formats 10 -bit address 23 -bit address 16 -bit address Chapter 4 Frame Relay 25

User Data Transfer Frame l No connection control fields, which are normally used for: – Identifying frame type (data or control) – Sequence numbers, used for error/flow control l 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 26

Frame Relay Call Control l Data transfer involves: – Details of call control depend on the context of its use – Assumes FR over ISDN – Generally simpler for point-to-point use – Establish logical connection and assign a unique DLCI – Exchange data frames – Release logical connection Chapter 4 Frame Relay 27

Frame Relay Call Control 4 message types needed l SETUP…request link establishment l CONNECT…reply to SETUP with connection accepted l RELEASE…request to clear (tear down) a connection l RELEASE COMPLETE… reply to SETUP with connection denied, or response to RELEASE Chapter 4 Frame Relay 28