Final Chapter Packet-Switching and Circuit Switching

1. Final Chapter Packet-Switching and Circuit Switching 7.3. Statistical Multiplexing and Packet Switching: Datagrams and Virtual Circuits 4. 4 Time Division Multiplexing and Circuit Switching

2. Multiplexing concentrates bursty traffic onto a shared line information flow should include source address and destination address Greater efficiency and lower cost Header Data payload Input lines A B Buffer Output line C 5.7.1 Statistical Multiplexing

3. A2 A1 B2 B1 DeMUX MUX Z1 Z2 Statistical Multiplexing -Asynchronous Time Division Multiplexing • Statistical Multiplexing involves the sharing of transmissionchannels (resource) by several connections A, B, ···, Z or information flows which will be transmitted (served) on demand (statistically). Thus Significant economies of scale can be achieved • Ports A, B, ···, Z in the multiplexer should provide sufficient number of buffers; information packets will be stored and forward, thus cause delay • The shared channel would be E-carrier (T-carrier) or SDH (SONET) Shared Channel Statistical Multiplexing

4. 1 1 2 2       N N Multiplexer/Demultiplexer inherent in Packet Switches Multiplexer • Packets/frames forwarded to buffer prior to transmission from switch • Multiplexing occurs in these buffers DeMultiplexer

5. Input lines A Output line B Buffer C Multiplexer Modeling • Arrivals: What is the packet interarrival pattern? • Service Time: How long are the packets? • Service Discipline: What is order of transmission? • Buffer Discipline: If buffer is full, which packet is dropped? • Performance Measures: Delay Distribution; Packet Loss Probability; Line Utilization

6. Chapter 7Packet-Switching Networks 7.3 Datagrams and Virtual Circuits

7. Switch Access Network Backbone Network The Packet Switching Function • Store and then forward packet by switching it to an appropriate output port according to a routing table. Notice that the routing table could be updated dynamically, depending on the traffic condition • Dynamic interconnection of input ports to output ports • Enables dynamic sharing of transmission resource • Two fundamental approaches: • Connectionless • Connection-Oriented: Call setup control, Connection control, Connection release

8. Packet switching network Transfers packets between users Transmission lines + packet switches (routers) Origin in message switching Two modes of operation: Connectionless Virtual Circuit User Transmission line Network Packet switch Packet Switching Network

9. Packet 1 Packet 1 Packet 2 Packet 2 Packet 2 Packet Switching - Datagram • Messages are broken into smaller units (packets) • Source & destination addresses included in the packet header • Datagram: Connectionless, where packets are routed independently, no dedicated path for the data transfer phase • Packets may arrive out of order • Pipelining of packets across network can induce out of order, but increase system throughput

10. Route determined by table lookup Routing decision involves finding the next hop in the route to the given destination Routing table has an entry for each destination specifying output port that leads to the next hop Size of table becomes impractical for very large number of destinations Destination Output address port 0785 7 1345 12 1566 6 2458 12 Routing Tables in Datagram Networks

11. Example: Internet Routing • Internet protocol uses datagram packet switching across networks • Networks are treated as data links • Hosts have two-port IP address: • Network address + Host address • Routers do table lookup on network address • This reduces size of routing table • In addition, network addresses are assigned so that they can also be aggregated • Discussed as §8.2.5 Classless Interdomain Routing (CIDR) in Chapter 8

12. Packet Packet Packet Packet Virtual circuit Packet Switching – Virtual Circuit • Call set-up phase establishes a fixed path along network before the data transfer phase • All packets for the connection follow the same path • Abbreviated header identifies connection on each link • Packets queue for transmission • Variable bit rates possible, negotiated during call set-up • Delays are still variable, but will not be less than circuit switching

13. Connect request Connect request Connect request SW 1 SW n SW 2 … Connect confirm Connect confirm Connect confirm Connection Setup • Signaling messages propagate as route is selected • Signaling messages identify connection and setup tables in switches • Typically a connection is identified by a local tag, Virtual Circuit Identifier (VCI) • Each switch only needs to know how to relate an incoming tag in one input to an outgoing tag in the corresponding output • Once tables are setup, packets can flow along the path

14. t Connect request 1 3 2 CC t Release 3 CR 1 2 CC t Connect confirm 1 2 CR 3 t Connection Setup Delay • Connection setup delay is incurred before any packet can be transferred • Delay is acceptable for sustained transfer of large number of packets • This delay may be unacceptably high if only a few packets are being transferred

15. Each input port of packet switch has a forwarding table Lookup entry for VCI of incoming packet Determine output port (next hop) and insert VCI for next link Very high speeds are possible Table can also include priority or other information about how packet should be treated Input VCI Output port Output VCI 12 44 13 15 15 23 16 27 13 58 7 34 Virtual Circuit Forwarding Tables

16. Source t 2 1 3 Switch 1 t 2 3 1 Switch 2 t 2 1 3 t Destination Minimum delay = 3 + T Cut-Through switching • Some networks perform error checking on header only, so packet can be forwarded as soon as the header is received & processed • Delays reduced further with cut-through switching

17. Chapter 7Packet-Switching Networks Datagrams and Virtual Circuits Structure of a Packet Switch

18. • • • • • • • • • Packet Switch: Intersection where Traffic Flows Meet 1 1 2 2       N N • Input ports contain multiplexed flows from access muxs & other packet switches • Flows are demultiplexed at input ports, routed and/or forwarded to output ports • Packets buffered, prioritized, and multiplexed on output ports

19. “Unfolded” View of Switch Ingress Line Card (input port) Header processing Demultiplexing Routing in large switches Controller Routing in small switches Signalling & resource allocation Interconnection Fabric Transfer packets between line cards Egress Line Card (output port) Scheduling & priority Multiplexing Controller 1 Line card 1 Line card 2 2 Line card Line card 3 3 Line card Interconnection fabric Line card … … … … N N Line card Line card Input ports Output ports Data path Control path (a) Generic Packet Switch

20. Folded View 1 circuit board is ingress/egress line card Physical layer processing Data link layer processing Network header processing Physical layer across fabric + framing Transceiver Framer Network processor Backplane transceivers Framer Transceiver To other line cards To physical ports To switch fabric Interconnection fabric Line Cards

21. Shared Memory Packet Switch Output Buffering Ingress Processing Connection Control 1 1 Queue Control 2 2 3 3 Shared Memory … … N N Small switches can be built by reading/writing into shared memory

22. Crossbar Switches (b) Output buffering (a) Input buffering Inputs Inputs 3 1 1 2 8 3 2 3 3 … … N N … … 1 2 3 N 1 2 3 N Outputs Outputs • Large switches built from crossbar & multistage space switches • Requires centralized controller/scheduler (who sends to whom when) • Can buffer at input, output, or both (performance vs complexity)

23. Inputs Outputs 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Stage 1 Stage 2 Stage 3 Self-Routing Switches • Self-routing switches do not require controller • Output port number determines route • 101 → (1) lower port, (2) upper port, (3) lower port

24. A1 A2 … t 0T 6T 3T B1 B2 … t 6T 3T 0T C1 C2 … t 0T 6T 3T C2 A2 B2 … A1 C1 B1 t 0T 1T 2T 3T 4T 5T 6T Time-Division Multiplexing • High-speed digital channel like E-carrier and SDH, where the channel is divided into fixed number of time slots and dedicated to some ports (a) Each signal transmits 1 unit every 3T seconds • Framing required • Telephone digital transmission • Digital transmission in backbone network (b) Combined signal transmits 1 unit every T seconds

25. Control 1 1 2 2 3 3 Connection of inputs to outputs … … Link Switch N User n N User n – 1 User 1 Circuit Switches • Circuits consist of dedicated resources in sequence of links & switches across network for data transfer phase • Circuit switch connects input links to output links • Network • Switch

26. 1 2 … N … N –1 2 N 1 Crossbar Space Switch • N x N array of crosspoints • Connect an input to an output by closing a crosspoint • Nonblocking (internal blocking) : Any input can connect to idle output • Complexity: N2 crosspoints

27. 2(N/n)nk + k (N/n)2 crosspoints kn nk N/n N/n 1 1 1 kn nk 2 N outputs N inputs 2 N/n N/n 2 kn nk 3 3 … … … kn nk N/n N/n N/n N/n k Multistage Space Switch • Large switch is built with multiple stages • The n inputs to a first-stage switch share k paths through intermediate crossbar switches • Larger k (more intermediate switches) means more paths to output   

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