Wave Division Multiplexing. Circuit Switching (1) Switching Networks. Nodes. Last Lecture Multiplexing (2) Source: chapter8

Transcription

1 Circuit Switching (1) Last Lecture Multiplexing (2) Source: chapter8 This Lecture Circuit switching (1) Source: chapter9 Next Lecture Circuit switching (2) Source: chapter 9 Reference books A. Tanenbaum, Computer Networks, TK/5105.5/T447/1996 B. Forouzan, Introduction to Data Communications and Networking, TK/5105/FQ19 W. Shay, Understanding Data Communications and Networks, TK/5105/SH55/1999 Wave Division Multiplexing Switching Networks Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data End devices are stations Computer, terminal, phone, etc. A collection of nodes and connections is a communications network Data routed by being switched from node to node Nodes Nodes may connect to other nodes only, or to stations and other nodes Node to node links usually multiplexed Network is usually partially connected Some redundant connections are desirable for reliability Three different switching technologies Circuit switching Packet switching Message switching (store and forward) similar to packet switching Simple Switched Network Circuit Switching Dedicated path between two stations Three phases: Establish - Transfer - Disconnect Must have capacity to establish connection and intelligence to work out routing But inefficient: Channel capacity dedicated for duration of connection If no data, capacity wasted Set up (connection) takes time, but once connected, transfer is transparent Developed for voice traffic (phone)

2 Circuit-switched network Switch Public Circuit Switched Network Telecomms Components Subscriber Devices attached to network Local Loop Subscriber loop Connection to network Exchange Switching centers End office - supports subscribers Trunks Branches between exchanges Multiplexed Public Switched Telephone Network (PSTN) Touch-Tone Dialing to the end office

3 Folded switch Circuit Switch Elements Circuit Switching Concepts Digital Switch Provide transparent signal path between devices Network Interface Control Unit Establish connections Generally on demand Handle and acknowledge requests Determine if destination is free construct path Maintain connection Disconnect Blocking or Non-blocking Blocking A network is unable to connect stations because all paths are in use A blocking network allows this Used on voice systems Short duration calls Non-blocking Permits all stations to connect (in pairs) at once Used for some data connections Space Division Switching Crossbar Matrix Developed for analog environment Separate physical paths Crossbar switch

4 Space Division Switching (crossbar switch) Number of crosspoints grows as square of number of stations Loss of crosspoint prevents connection Inefficient use of crosspoints All stations connected, only a few crosspoints in use Multistage Switch Reduced number of crosspoints A path requires more than one gate More than one path through network Increased reliability Non-blocking Three Stage Switch Multistage Switch Usually the middle stages have fewer switches than the first and last stages More complex control May be blocking when traffic is high Three Stage Switch Time Division Switching Involves partitioning a low speed bit stream into pieces that share a higher speed stream. e.g. TDM bus switching Based on synchronous time division multiplexing. Source and destination of data in each time slot are known no need for address bits in slot. Each station connects through controlled gates to high speed bus Time slot allows small amount of data onto bus Another line s gate is enabled for output the same time Time slot must equal the transmission time of the input + propagation delay across the bus. Input data are buffered at the gate.

5 TDM with Time-Slot Interchange Time-Slot Interchange TSI How it works Control of TDM Bus Switch !3 2!5 3!1 5!2 6!4 Control logic Space and Time Switching Combinations TST (Time-Space-Time) Switch Space-division switching Advantage: instantaneous (no delays) Disadvantage: too many crosspoints Time-division swiching Advantage: no crosspoints Disadvantage: TSI processing takes time (delays) We can combine these two to take advantage of the best of both.