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Transcript
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 5.7.1 Statistical Multiplexing 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 Statistical Multiplexing Asynchronous Time Division Multiplexing Statistical Multiplexing involves the sharing of transmission channels (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) A2 A1 Shared Channel B1 MUX B2 DeMUX Z1 Statistical Multiplexing Z2 3 Multiplexer/Demultiplexer inherent in Packet Switches Multiplexer 1 1 2 2 DeMultiplexer N N Packets/frames forwarded to buffer prior to transmission from switch Multiplexing occurs in these buffers Multiplexer Modeling Input lines A Output line B Buffer C 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 Chapter 7 Packet-Switching Networks 7.3 Datagrams and Virtual Circuits 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 Switch Access Network Backbone Network Packet Switching Network Packet switching network User Transmission line Network Packet switch Transfers packets between users Transmission lines + packet switches (routers) Origin in message switching Two modes of operation: Connectionless Virtual Circuit 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 Packet 1 Packet 1 Packet 2 Packet 2 Packet 2 Routing Tables in Datagram Networks Destination address Output port 0785 7 1345 12 1566 6 2458 12 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 Example: Internet Routing Internet protocol uses datagram packet switching across networks Hosts have two-port IP address: Network address + Host address Routers do table lookup on network address Networks are treated as data links 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 Packet Switching – Virtual Circuit Packet Packet Packet Packet 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 Connection Setup Connect request Connect confirm SW 1 Connect request Connect confirm SW 2 … SW n Connect request Connect confirm 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 Connection Setup Delay t Connect request CC CR CC CR Connect confirm 1 2 3 1 2 Release 3 t t 1 2 3 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 t Virtual Circuit Forwarding Tables Input VCI Output port Output VCI 12 13 44 15 15 23 27 13 16 58 7 34 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 Cut-Through switching Source Switch 1 Switch 2 t 2 1 3 t 2 1 t 1 Destination 3 2 3 Minimum delay = 3 + T t 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 Chapter 7 Packet-Switching Networks Datagrams and Virtual Circuits Structure of a Packet Switch 1 1 2 2 N ••• ••• ••• Packet Switch: Intersection where Traffic Flows Meet 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 Generic Packet Switch “Unfolded” View of Switch Ingress Line Card (input port) Controller N Line card 1 Line card 2 Line card 3 Controller Line card N Data path Control path Output ports (a) Transfer packets between line cards Egress Line Card (output port) Input ports Routing in small switches Signalling & resource allocation Interconnection Fabric … Line card Line card … … 3 Line card Interconnection fabric 2 Line card … 1 Header processing Demultiplexing Routing in large switches Scheduling & priority Multiplexing Line Cards Framer Network processor Transceiver To physical ports Backplane transceivers Framer To switch fabric 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 To other line cards Interconnection fabric Transceiver Shared Memory Packet Switch Ingress Processing Connection Control Output Buffering 1 1 Queue Control 2 2 3 N Shared Memory … … 3 N Small switches can be built by reading/writing into shared memory Crossbar Switches (b) Output buffering (a) Input buffering Inputs Inputs 1 3 1 2 83 2 3 … … 3 N N … 1 2 3 Outputs … N 1 2 3 Outputs N 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) Self-Routing Switches 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 do not require controller Output port number determines route 101 → (1) lower port, (2) upper port, (3) lower port 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 A1 (a) Each signal transmits 1 unit every 3T seconds 0 T B1 … B2 6 T 3T 0 T 0 T 1T 2T C1 A2 3T 4T t t 6 T B2 t … C2 3T A1 B1 0T 6 T 3T C1 (b) Combined signal transmits 1 unit every T seconds … A2 C2 … Framing required Telephone digital transmission Digital transmission in backbone network t 5T 6T 24 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 Control Switch User n 1 2 3 … User n – 1 User 1 N Connection of inputs to outputs 1 2 3 … Link N 25 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 1 2 … N 1 2 … N –1 N 26 Multistage Space Switch 2(N/n)nk + k (N/n)2 crosspoints nk 1 N inputs nk N/n N/n 1 2 N/n N/n nk 2 3 1 kn 2 kn N outputs 3 kn nk N/n kn … … 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 … N/n N/n k N/n 27 謝 謝 聆 聽 謝 謝 合 作