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10-Circuit-Packet - Data and Computer Communications...

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Unformatted text preview: Data and Computer Communications Communications Chapter 10 – Circuit Switching and Packet Switching Packet Eighth Edition by William Stallings Lecture slides by Lawrie Brown Circuit Switching and Packet Circuit Switching Switching He got into a District Line train at Wimbledon He Park, changed on to the Victoria Line at Victoria and on to the Jubilee Line at Green Park for West Hampstead. It was a long and awkward journey but he enjoyed it. awkward —King Solomon's Carpet, Barbara Vine (Ruth Rendell) Switched Network Switched Nodes Nodes a collection of nodes and connections is a collection communications network nodes may connect to other nodes only, or to nodes stations and other nodes stations network is usually partially connected some redundant connections are desirable some have two different switching technologies circuit switching packet switching Circuit Switching Circuit uses a dedicated path between two stations has three phases establish transfer disconnect channel capacity dedicated for duration of connection if no data, capacity wasted inefficient set up (connection) takes time once connected, transfer is transparent Public Circuit Switched Network Network Circuit Establishment Circuit Circuit Switch Elements Elements Blocking or Non-blocking Blocking blocking network may be unable to connect stations because may all paths are in use all used on voice systems permits all stations to connect at once used for some data connections non-blocking network Space Division Switch Space 3 Stage Space Division Switch Switch Time Division Switching Time modern digital systems use intelligent modern control of space & time division elements control use digital time division techniques to set use up and maintain virtual circuits up partition low speed bit stream into pieces partition that share higher speed stream that individual pieces manipulated by control individual logic to flow from input to output logic Softswitch Softswitch Traditional Circuit Switching Traditional Packet Switching Packet circuit switching was designed for voice packet switching was designed for data transmitted in small packets packets contains user data and control info user data may be part of a larger message control info includes routing (addressing) info packets are received, stored briefly (buffered) packets and past on to the next node and Packet Switching Packet Advantages Advantages line efficiency single link shared by many packets over time packets queued and transmitted as fast as possible stations connects to local node at own speed nodes buffer data if required to equalize rates data rate conversion packets accepted even when network is busy priorities can be used Switching Techniques Switching station breaks long message into packets packets sent one at a time to the network packets can be handled in two ways datagram virtual circuit Datagram Datagram Diagram Virtual Virtual Circuit Diagram Virtual Circuits v Datagram Virtual virtual circuits network can provide sequencing and error network control control packets are forwarded more quickly less reliable no call setup phase more flexible more reliable datagram Packet Size Size Circuit v Packet Switching Circuit performance depends on various delays propagation delay transmission time node delay transparency amount of overhead range of other characteristics, including: Event Timing Event X.25 X.25 ITU-T standard for interface between host ITU-T and packet switched network and almost universal on packet switched almost networks and packet switching in ISDN networks defines three layers Physical Link Packet X.25 - Physical X.25 interface between station node link two ends are distinct two Data Terminal Equipment DTE (user Data equipment) equipment) Data Circuit-terminating Equipment DCE Data (node) (node) physical layer specification is X.21 can substitute alternative such as EIA-232 X.25 - Link X.25 Link Access Protocol Balanced (LAPB) Subset of HDLC see chapter 7 provides reliable transfer of data over link sending as a sequence of frames X.25 - Packet X.25 provides a logical connections (virtual provides circuit) between subscribers circuit) all data in this connection form a single all stream between the end stations stream established on demand termed external virtual circuits X.25 Use of Virtual Circuits X.25 User Data and X.25 Protocol Control Information Control Issues with X.25 Issues key features include: call control packets, in band signaling multiplexing of virtual circuits at layer 3 layers 2 and 3 include flow and error control hence have considerable overhead not appropriate for modern digital systems not with high reliability with Frame Relay Frame designed to eliminate most X.25 overhead has large installed base key differences: call control carried in separate logical connection multiplexing and switching at layer 2 no hop by hop error or flow control hence end to end flow and error control (if used) are hence done by higher layer done a single user data frame is sent from source to single destination and higher layer ACK sent back destination Advantages and Disadvantages Disadvantages lost link by link error and flow control increased reliability means less an issue streamlined communications process lower delay higher throughput frame relay can be used for access frame speeds up to and over 2Mbps speeds Protocol Architecture Protocol LAPF Functionality LAPF LAPF (Link Access Procedure for Frame Mode LAPF Bearer Services) defined in Q.922 Bearer only core functionality used: frame delimiting, alignment and transparency frame mux and demux using addressing field ensure frame is integral number of octets ensure ensure frame is neither too long nor short detection of transmission errors congestion control functions data transfer between subscribers only form sub-layer of data link layer Frame Relay Data Link Connections Connections logical connection between subscribers data transferred over them not protected by flow or error control uses separate connection for call control overall results in significantly less work in overall network network User Data Transfer User only have one frame type which carries user data no inband signaling no sequence numbers no control frames means flag and FCS function as in HDLC address field carries DLCI DLCI (Data Link Connection Identifier) has DLCI local significance only local Summary Summary circuit verses packet switching network circuit approaches approaches X.25 frame relay ...
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