Module 3 4 Switching Circuit Switching Packet Switching

Module 3. 4: Switching • • Circuit Switching Packet Switching K. Salah 1

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 2

Circuit Switching • • • Dedicated communication path between two stations • • Must have intelligence to work out routing • • • Three phases: Establish, Transfer, & Disconnect Must have switching capacity and channel capacity to establish connection 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 3

Circuit Switch Elements • • • Digital Switch – Provide transparent signal path between devices Network Interface Control Unit – Establish connections Ø Generally on demand Ø Handle and acknowledge requests Ø Determine if destination is free Ø construct path – Maintain connection – Disconnect K. Salah 4

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

Space Division Switching • • Developed for analog environment • Multistage Switch – Reduced number of crosspoints – More than one path through network Ø Increased reliability – More complex control – May be blocking 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 Ø All stations connected, only a few crosspoints in use. Ø Statistically, only 25% of the crosspoints are used at any time. – Non-blocking K. Salah 6

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

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

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

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 Ø Static routing uses the same approach all the time – Dynamic routing allows for changes in routing depending on traffic Ø Uses a peer structure for nodes Control Signaling Functions – Subscriber to switch – Switch to switch ( commonly known as SS 7) – 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 10

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 Ø Single node to node link can be shared by many packets over time Ø Packets queued and transmitted as fast as possible – Data rate conversion Ø Each station connects to the local node at its own speed Ø Nodes buffer data if required to equalize rates – Packets are accepted even when network is busy K. Salah 11

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 12

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 13

Virtual Circuits vs. Datagram • • Virtual Circuits – Network can provide sequencing and error control – Packets are forwarded more quickly Ø No routing decisions to make – Less reliable Ø Loss of a node looses all circuits through that node Datagram – No call setup phase Ø Better if few packets – More flexible Ø Routing can be used to avoid congested parts of the network K. Salah 14

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