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Data and Computer Communications

Data and Computer Communications. Chapter 10 – Circuit Switching and Packet Switching (Wide Area Networks). Switched Network. Nodes. a collection of switching nodes and connections is a communications network nodes may connect to other nodes only, or to stations and other nodes

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Data and Computer Communications

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  1. Data and Computer Communications Chapter 10 – Circuit Switching and Packet Switching (Wide Area Networks)

  2. Switched Network

  3. Nodes • a collection of switching nodes and connections is a communications network • nodes may connect to other nodes only, or to stations and other nodes • network is usually partially connected • some redundant connections are desirable • have two different switching technologies • circuit switching • packet switching

  4. Circuit Switching • uses a dedicated path between two stations • has three phases • Establish - an end-to-end (station-to-station) circuit • Transfer – Data transmitted through the network • Disconnect - the connection is terminated, usually by of one of the two stations • inefficient • channel capacity is dedicated for duration of connection • if no data, capacity wasted (Client / server) • set up (connection) takes time • once connected, transfer is transparent (no delay)

  5. Public Circuit Switched Network

  6. Circuit Establishment

  7. Circuit Switch Elements

  8. Blocking or Non-blocking • blocking network • may be unable to connect stations because all paths are in use • used on voice systems • non-blocking network • permits all stations to connect (in pair) at once • used for some data connections

  9. Space Division Switch

  10. 3 Stage Space Division Switch

  11. Time Division Switching • modern digital systems use intelligent control of space & time division elements • use digital time division techniques to set up and maintain virtual circuits • partition low speed bit stream into pieces that share higher speed stream • individual pieces manipulated by control logic to flow from input to output

  12. Time Slot Interchange • TSI receives input in sequence, but sends them to the output out-of sequence (as needed by the devices) • Stored in memory, and scanned out • Delivery of data is delayed • # How many cross points are needed for a crossbar switch to connect 1000 telephones?

  13. Traditional Circuit Switching

  14. Softswitch

  15. Packet Switching • circuit switching was designed for voice • packet switching was designed for data • transmitted in small packets (1000 octets) • packets contain 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) and passed on to the next node

  16. Packet Switching

  17. Advantages • line efficiency • single link shared by many packets over time • packets queued and transmitted as fast as possible • data rate conversion • stations connects to local node at own speed • nodes buffer data if required to equalize rates • packets accepted even when network is busy • priorities can be used

  18. Packet Switching Techniques • 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

  19. DatagramDiagram

  20. VirtualCircuitDiagram

  21. Virtual Circuits v Datagram • virtual circuits • network can provide sequencing and error control • packets are forwarded more quickly • less reliable • datagram • No setup phase • more flexible • more reliable

  22. Packet Size

  23. Circuit v Packet Switching • performance depends on various delays • propagation delay - negligible • transmission time • node delay – processing time • range of other characteristics, including: • Network size & topology • Load pattern

  24. Event Timing

  25. # N = hops = 4, L = message length = 3200 bits, B = data rate = 9600, P = packet (data + overhead) size = 1024 bits, H = overhead = 16 bits, S = call set up time = 0.2 s, D = propagation delay / hop = 0.001 s. (ignore the time for ACK in circuit switching and virtual circuit packet switching) • Find end to end delay for circuit switching, datagram and virtual circuit packet switching.

  26. Circuit: • T = C1 + C2 • C1 = S, C2 = Tr. + Prop. Time • Datagram: • T = D1 + D2 + D3 + D4 • D1 = (Tr. + Prop. Time) of all packets over 1st hop, Packet data = 1024 -16 =1008 • D2 = D3 = D4 = (Tr. + Prop. Time) of the last frame over 2nd, 3rd, and 4th hops.

  27. Virtual: • T = V1 + V2 • V1 = S, V2 = Datagram T

  28. Summary • circuit verses packet switching network approaches

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