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Module 3.4: Switching

Module 3.4: Switching. Circuit Switching Packet Switching. 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.

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Module 3.4: Switching

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  1. Module 3.4: Switching • Circuit Switching • Packet Switching K. Salah

  2. 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 K. Salah

  3. 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 • Inefficient • Channel capacity dedicated for duration of connection • If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Developed for voice traffic (phone) K. Salah

  4. Circuit Switch Elements • 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 K. Salah

  5. Circuit Switching Concepts • Blocking or Non-blocking • Blocking • A network is unable to connect stations because all paths are in use • Used on voice systems since we have Short duration calls • Non-blocking • Permits all stations to connect (in pairs) at once • Used for some data connections • Two types of switches: • Space Division Switching • Time Division Switching K. Salah

  6. Space Division Switching • Developed for analog environment • Separate physical paths • Switching is instantaneous • 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. • Statistically, only 25% of the crosspoints are used at any time. • Non-blocking • Multistage Switch • Reduced number of crosspoints • More than one path through network • Increased reliability • More complex control • May be blocking K. Salah

  7. Time Division Switching • Switching Techniques: • TSI (Time Slot Interchange) • TST (Time Space Time) • TDM Bus K. Salah

  8. TSI and TST • In TSI: • No crosspoints • Processing delays • RAM Write and Read for each time slot Combining the advantages of Space Division switching and TSI with TST. K. Salah

  9. TDM Bus This is a complete switch architecture found at the receiver end. K. Salah

  10. Circuit Switching Issues • Routing • Many connections will need paths through more than one switch • Need to find a route with Efficiency and 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 • Control Signaling Functions • Subscriber to switch • Switch to switch ( commonly known as SS7) • 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 K. Salah

  11. Packet Switching • Data transmitted in small packets • Longer messages split into series of packets • Each packet contains a portion of user data plus some control info • Control info: Routing (addressing) info • Packets are received, stored briefly (buffered) and past on to the next node, I.e., Store and forward • 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 K. Salah

  12. Datagram Approach • Each packet treated independently • Packets can take any practical route • Packets may arrive out of order • Packets may go missing • Up to receiver to re-order packets and recover from missing packets K. Salah

  13. Virtual Circuit Approach • Each packet contains a virtual circuit identifier instead of destination address. No routing decisions required for each packet. • Two types: • SVC (Switched Virtual Circuit) • Connection establishment • Data transfer • Connection Release • PVC (Permanent Virtual Circuit) K. Salah

  14. 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 K. Salah

  15. Circuit-switched vs. Packet Switched Networks K. Salah

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