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Data Communications TDC 362 / TDC 460

Data Communications TDC 362 / TDC 460. Circuit Switching and Packet Switching. 8.1 Circuit Switching. Space-Division Switch Time-Division Switch TDM Bus Combinations. Figure 8.1 Circuit-switched network. Figure 8.2 A circuit switch. Blocking or Non-blocking. Blocking

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Data Communications TDC 362 / TDC 460

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  1. Data CommunicationsTDC 362 / TDC 460 Circuit Switching and Packet Switching

  2. 8.1 Circuit Switching Space-Division Switch Time-Division Switch TDM Bus Combinations

  3. Figure 8.1Circuit-switched network

  4. Figure 8.2A circuit switch

  5. 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

  6. Figure 8.4Crossbar switch

  7. Figure 8.5Multistage switch

  8. Figure 8.6Switching path

  9. Three Stage Switch

  10. Figure 8.7Time-division multiplexing, without and with a Time-slot interchange

  11. Figure 8.8Time-slot interchange

  12. Figure 8.9TDM bus

  13. Figure 8.10TST (Time-space-time) switch

  14. Circuit-Switched Routing • Many connections will need paths through more than one switch • Need to find a route • 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 • Uses a peer structure for nodes

  15. Alternate Routing • Possible routes between end offices predefined • Originating switch selects appropriate route • Routes listed in preference order • Different sets of routes may be used at different times

  16. Alternate Routing Diagram

  17. Control Signaling Functions • Audible communication with subscriber • Transmission of dialed number • Call can not be completed indication • Call ended indication • Signal to ring phone • Billing info • Equipment and trunk status info • Diagnostic info • Control of specialist equipment

  18. Control Signals

  19. Location of Signaling • Subscriber to network • Depends on subscriber device and switch • Within network • Management of subscriber calls and network • ore complex

  20. In Channel Signaling • Use same channel for signaling and call • Requires no additional transmission facilities • Inband • Uses same frequencies as voice signal • Can go anywhere a voice signal can • Impossible to set up a call on a faulty speech path • Out-of-band • Voice signals do not use full 4kHz bandwidth • Narrow signal band within 4kHz used for control • Can be sent whether or not voice signals are present • Need extra electronics • Slower signal rate (narrow bandwidth)

  21. Drawbacks of In Channel Signaling • Limited transfer rate • Delay between entering address (dialing) and connection • Overcome by use of common channel signaling

  22. Common Channel Signaling • Control signals carried over paths independent of voice channel • One control signal channel can carry signals for a number of subscriber channels • Common control channel for these subscriber lines • Associated Mode • Common channel closely tracks interswitch trunks • Disassociated Mode • Additional nodes (signal transfer points) • Effectively two separate networks

  23. Common vs. In Channel Signaling

  24. Signaling Modes

  25. Signaling System Number 7 • SS7 • Most widely used common channel signaling scheme • Internationally standardized and general purpose

  26. SS7 • SS7 network and protocol used for: • Basic call setup, management, tear down • Wireless services such as PCS, roaming, authentication • Toll free and toll (900) wireline services • Enhanced features such as call forwarding, caller ID, 3-way calling • Efficient and secure worldwide telecommunications

  27. SS7 • SS7 messages are exchanged between central offices and specialized databases via signal transfer points (packet switches). • Control plane • Responsible for establishing and managing connections • Information plane • Once a connection is set up, info is transferred in the information plane

  28. SS7 Signaling Network Elements • Service switching point (SSP) • SSPs enable central offices to communicate with SS7 databases (the user entry point into SS7) • Signal transfer point (STP) • A signaling point (packet switch) capable of routing control messages • Service control point (SCP) • SCPs contain databases with call routing instructions

  29. SS7 SCP SCP STP SSP Central Office STP SSP Central Office SSP Central Office

  30. SS7 Characteristics • SSPs are telephone switches that send signaling messages to other SSPs to setup, manage, and release voice circuits • An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g. a toll-free number) • Because the SS7 network is critical to call processing, SCPs and STPs are deployed in mated pair configurations in separate physical locations • Links between signaling points are also in pairs

  31. Packet Switching Principles • Circuit switching designed for voice • Resources dedicated to a particular call • Much of the time a data connection is idle • Data rate is fixed • Both ends must operate at the same rate • What if we don’t want a dedicated call, or the data rate is bursty? You want packet switching!

  32. Basic Operation • Data transmitted in small packets • Typically 1000 bytes • Longer messages split into series of packets • Each packet contains a portion of user data plus some control info (such as addressing info or packet type) • Packets are received, stored briefly (buffered) and passed on to the next node • Store and forward (only ATM does not do this)

  33. Advantages • Line efficiency • Single node to node link can be shared by many packets over time • Packets queued and transmitted as fast as possible • Data rate conversion • Each station connects to the local node at its own speed • Nodes buffer data if required to equalize rates • Packets are accepted even when network is busy • Delivery may slow down • Priorities can be used

  34. Two Basic Forms of Packet Switching • Packets handled in two ways • Datagram • Virtual circuit

  35. Datagram • Each packet treated independently • Packets can take any practical route • Packets may arrive out of order • Packets may get lost or delayed • Up to receiver to re-order packets and recover from missing packets

  36. Virtual Circuit • Preplanned route established before any packets sent • Call request and call accept packets establish connection (handshake) • Each packet contains a virtual circuit identifier instead of destination address • No routing decisions required for each packet • Clear request to drop circuit • Not a dedicated path

  37. Figure 18.2Virtual Circuit Identifier (VCI) VCI is known only between two switches. (It is not a global address.)

  38. Figure 18.4Switch and table

  39. Figure 18.5Source-to-destination data transfer

  40. S(witched)VC vs. P(ermanent)VC setup A virtual circuit can be either switched or permanent. If permanent, an outgoing VCI is given to the source, and an incoming VCI is given to the destination. The source always uses this VCI to send frames to this particular destination. The destination knows that the frame is coming from that particular source if the frame carries the corresponding incoming VCI. If a duplex connection is needed, two virtual circuits are established.

  41. S(witched)VC vs. P(ermanent)VC setup A PVC has several drawbacks: 1. Always connected, so always paying 2. Connection is between two parties only. If you need a connection to another point, you need another PVC. Don’t like these disadvantages? Use an SVC.

  42. Figure 18.6SVC setup request 1 - Setup frame sent from A to Switch I. Note how the Outgoing VCI is not yet known.

  43. Figure 18.7SVC setup acknowledgment As the acknowledgment frame goes back, the VCI number is placed into the Outgoing VCI entry in each table.

  44. Virtual Circuits vs Datagram • Virtual circuits • Network can provide sequencing and error control • Packets are forwarded more quickly • No routing decisions to make • Less reliable • Loss of a node looses all circuits through that node • Datagram • No call setup phase • Better if few packets • More flexible • Routing can be used to avoid congested parts of the network

  45. Packet Size

  46. Event Timing

  47. Routing • Complex, crucial aspect of packet switched networks • Characteristics required • Correctness • Simplicity • Robustness • Stability • Fairness • Optimality • Efficiency

  48. Performance Criteria • Used for selection of route • Minimum hop • Least cost • Dijkstra’s algorithm most common • Finds the least cost path from one starting node to all other nodes • Algorithm can be repeated for each starting node

  49. Dijkstra’s Least Cost Example

  50. Dijkstra’s Least Cost Example

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