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Data and Computer Communications Part 3 - Wide area Networks - Concepts

Data and Computer Communications Part 3 - Wide area Networks - Concepts. Chapter 9 Circuit Switching. Switching Networks. Long distance transmission is typically done over a network of switched nodes Nodes are not concerned with content of data End devices are referred to as stations

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Data and Computer Communications Part 3 - Wide area Networks - Concepts

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  1. Data and Computer CommunicationsPart 3 - Wide area Networks - Concepts Chapter 9 Circuit Switching

  2. Switching Networks • Long distance transmission is typically done over a network of switched nodes • Nodes are not concerned with content of data • End devices are referred to as stations • Computer, terminal, phone, etc. • A collection of nodes and connections is a communications network • Data is routed by being switched from node to node

  3. Switching Networks • Communication Network • Switching Network (used in WAN) • Broadcast Network (used in LAN) • Switching Networks • Circuit-switching networks • Packet-switching networks

  4. Nodes • Nodes may connect to other nodes only, or to stations and other nodes • Node to node links are usually multiplexed • A network is usually partially connected • But some redundant connections are desirable for reliability • Two different switching technologies exist: • Circuit switching • Packet switching

  5. Simple Switched Network

  6. Simple switching network CommunicationNetworkNode • providesswitchingfacility (routing) NetworkStation • endnode(source & destination) Communication is achieved by transmitting data from source to destination through a network of switching nodes Communication Network

  7. Switching technology • Circuit switching • need a connection established between end nodes • connection is maintained until one of end nodes terminates • Connection is dedicated to the communication between two nodes • Example : Public Switch Telephone Network (PSTN) • Packet switching • data are transmitted in short messages called packets • a connection between the two end-nodes is not maintained • a node-to-node link can be dynamically shared by many packets • Example : Public Data Network (PDN) like X.25, Frame Relay

  8. Key differences Keys Circuit switching Packet switching data dedicated single route different routes node status both must be ready sender ready to send is enough connection dedicated shared utilization poor good data rate fixedvaries Prioritization not supported supported

  9. Public Switching Telephone Network endoffice subscriberloop Long-distanceoffice subscriber loop (local loop) linkbetweensubscriberandnetwork Intercitytrunk connecting trunk (exchanges) switchingcenter; localized support for subscriber Intercity trunk (trunks) branches between exchanges; carry multiple voice circuits connectingtrunk

  10. PSDN • PSDN is designed specifically for the transmission of data rather than voice • Communication is shared LAN PSDN= Public Switched Data Network LAN PSDN LAN

  11. Packets data header data header data header data packet packet packet • Messages are broken up into a series of packets • Header is used to route the packet through the network

  12. PSDN switching approach • Virtual Circuit • network protocols establish a logical route called virtual circuit • packets use the same route; data arrive in order • similar to circuit switching ! but the route is not dedicated • need call setup • Datagram • each packet is transmitted independently • network protocols route each packet as though it were a separate message • packet may not arrive in order; need protocol to ensure ordering • call setup is not required

  13. 1.1 1.2 1.3 1.1 1.2 1.3 2.1 2.2 2.3 2.3 2.2 2.1 4 2 1 3 5 Virtual circuit B • A logical connection known as • virtual circuit (VC) is set up • between two stations. • Packets are labeled with • a virtual circuit number and • a sequence number A C virtual circuit #1 B A C virtualcircuit #2

  14. B.1 B.2 B.3 B.1 B.2 B.3 C.1 C.2 C.3 C.3 C.2 C.1 B.2 B.3 4 2 C.1 1 B.1 3 5 C.2 C.3 Datagram B • Each packet is transmitted • independently. • Packets are labeled with • a destination address and • may arrive out of sequence A C B A C

  15. Routing methods in packet switching • Fixed routing • each packet is transmitted independently • Flooding • network protocol establishes a logical route called virtual circuit • packets use the same route; data arrive in order • Random routing • the route taken is random • Adaptive routing • the route reacts to changing conditions within the network

  16. Circuit-Switching Networks • During communication, a dedicated communication path exists between sender and receiver; e.g., telephone system. • Communication involves 3 phases: • Circuit establishment: “Call request” propagates, hop by hop through the network, to establish a dedicated link (channels in each component link in some path, from sender to receiver) • Data transfer: continuous transfer; either analog or digital signal • Circuit disconnect: “circuit disconnect” signal to deallocate the links

  17. Circuit-Switching • Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes • Dominant in both voice and data communications today • e.g. PSTN is a circuit-switched network • Relatively inefficient (100% dedication even without 100% utilization)

  18. Circuit Switching • Dedicated communication path between two stations • Three phases • Establish • Transfer • Disconnect • Must have switching capacity and channel capacity to establish connection • Must have intelligence to work out routing

  19. Circuit-Switching Stages • Circuit establishment • Transfer of information • point-to-point from endpoints to node • internal switching/multiplexing among nodes • Circuit disconnect

  20. Circuit-Switching Networks • Disadvantages: • Both stations need to be simultaneously ready • Inefficient use of link capacities, especially with bursty intermittent traffic • Advantages • Low delay once circuit is established

  21. Circuit-Switching Networks • Public telephone network • Best-known example of a circuit-switching network • Four generic architectural components • Subscribers - Devices that attach to the network • Local loop - The link between the subscriber and the network (end office). Also called subscriber loop • Exchanges - Switching centers in the network. End offices, long-distance offices, … • Trunks - Links between exchanges. Carry multiple voice channels by using FDM or synchronous TDM

  22. Circuit Switching - Applications • Inefficient • Channel capacity dedicated for duration of connection • If there is no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Developed for voice traffic (phone)

  23. Public Circuit Switched Network Connecting Trunk

  24. Circuit-Switching Networks

  25. AT&T telephone hierarchy

  26. Switching Concepts • Single circuit-switched node network • A collection of stations attached to a central switching unit • The central switch establishes a dedicated path between any two devices that wish to communicate

  27. Switching Concepts • Digital switch • Establishes a dedicated path between any two devices • Control unit • Establishes, maintains, and tears down the connection • Network interface • (circuit) switch • Blocking • Nonblocking

  28. Telecommunication 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

  29. Circuit Switching Node's Elements

  30. Circuit Switching Node's Elements • Digital Switch • Provides transparent signal path between devices • Network Interface (functions & hardware needed to connect devices to the network) • Control Unit; its function is to: • Establish connections • Generally on demand • Handle and acknowledge requests • Determine if destination is free • construct path • Maintain connection • Disconnect

  31. Circuit Switching Node:Digital Switch • Provides transparent signal path between any pair of attached devices • Typically full-duplex

  32. Circuit-Switching Node:Network Interface • Provides hardware and functions to connect digital devices to switch • Analog devices can be connected if interface includes CODEC functions • Typically full-duplex

  33. Circuit-Switching Node:Control Unit • Establishes on-demand connections • Maintains connection while needed • Breaks down connection on completion

  34. Blocking or Non-blocking • An important characteristic of a circuit-switching device is whether it is blocking or non-blocking. • Blocking • A network is unable to connect stations because all paths are in use • Used on voice systems • example: Short duration calls • Non-blocking • Permits all stations to connect (in pairs) at once • Used for some data connections

  35. Blocking/Nonblocking Networks • Blocking: network is unable to connect two stations because all possible paths are already in use • Nonblocking: permits all possible connection requests because any two stations can be connected

  36. There are 2 Types of Switching Techniques Internal to a Single Circuit-Switching Node: 1. Space-Division Switching 2. Time-Division Switching

  37. Space-Division Switching Developed for analog environment, but has been carried over into digital communication Requires separate physical paths for each signal connection Uses metallic or semiconductor “gates” Time-Division Switching Used in digital transmission Utilizes multiplexing to place all signals onto a common transmission path Bus must have higher data rate than individual I/O lines Switching Techniques

  38. Space Division Switch • Developed for the analog environment (but is now used for both analog & digital signals) • Separate physical paths for every channel (signal path) • Its basic device is the 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 • Non-blocking

  39. Crossbar Matrix (switch)

  40. Multistage Switch • Reduced number of crosspoints • More than one path through network • Results in increased reliability • More complex control • May be blocking

  41. Three Stage Switch

  42. Time Division Switching • Partition low speed bit stream into pieces that share higher speed stream • e.g. TDM bus switching • based on synchronous time division multiplexing • 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 at the same time

  43. Routing in Circuit-Switched Networks • Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks • Newer circuit-switched networks are dynamically routed: all nodes are peer-to-peer, making routing more complex

  44. Routing in Circuit-Switching Networks • Many connections will need paths through more than one switch • Therefore, the network needs to find an appropriate route. Important factors are: • Efficiency • Resilience • Public telephone switches are a tree structure • Static routing uses the same approach all the time • Dynamic routing allows for changes in routing depending on traffic

  45. Alternate Routing • Possible routes between two end offices are predefined • Originating switch selects the best route for each call • Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)

  46. Alternate Routing Diagram

  47. Control Signaling • Control unit manages the establishment, maintenance, and termination of signal paths • Includes signaling from subscriber to network, and signals within network • In-channel signaling uses the same channel for control signals and calls • Common-channel signaling uses independent channels for controls (e.g. SS7)

  48. Control Signaling Functions • The means by which the network is managed, calls are established, maintained, and terminated • Audible communication with subscriber • Transmission of dialed number • Call cannot be completed indication • Call ended indication • Signal to ring phone • Billing info • Equipment and trunk status info • Diagnostic info • Control of special equipment

  49. Control Signal Sequence • Both phones on hook • Subscriber lifts receiver (off hook) • End office switch signaled • Switch responds with dial tone • Caller dials number • If target is not busy, ringer signal is sent to target subscriber • Feedback to caller • Ringing tone, engaged tone, unobtainable (disconnected line, etc.) • Target accepts call by lifting receiver • Switch terminates ringing signal and ringing tone • Switch establishes connection • Connection is released when Source subscriber hangs up

  50. Switch to Switch Signaling • Subscribers connected to different switches • Originating switch seizes inter-switch trunk • Off hook signal is sent on trunk, requesting a digit register at target switch (so that address may be communicated) • Terminating switch sends off hook followed by on hook (known as wink) to show register-ready status • Originating switch sends address

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