Packet Switching Networks & Frame Relay Chapter 4 Packet Switching Networks & Frame Relay

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

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

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

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

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

Use of Packets Chapter 4 Frame Relay

A Simple Switching Network Chapter 4 Frame Relay

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

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

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

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

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

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

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

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

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

X.25 Features Call control packets Multiplexing of VCs at layer 3 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

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

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

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

Virtual Circuits and Frame Relay Virtual Connections (a) X.25 Packet Switching X.25 Packet-Switching network (b) Frame Relay Frame Relay network Chapter 4 Frame Relay

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

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

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

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

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

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