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Transcript
Circuit Switching Malathi Veeraraghavan University of Virginia Outline Types of switches Add multiplexers and demultiplexers TDM circuit switch Practice: SONET switch Some figures: Courtesy of Leon-Garcia & Widjaja’s textbook web site 1 Types of switches Line card Circuit-switch (CS) (multiplexing) (position-based) Packet-switch (PS) (header-based) Controller (admission control or not) Connectionless (CL) (no admission control) Connection-oriented (CO) (admission control) 2 e.g., IP routers; Ethernet switches e.g., telephone network circuit switches, SONET switches Virtual-circuit switches • Routing: Required in controller for all three types of switches • Signaling: Admission control – hence required only for connection-oriented switches Types of switches The type of switch is determined by the type of multiplexing used on its links Circuit switch: Position-based multiplexing Time/Frequency/Wavelength Division Multiplexing TDM, FDM, WDM: Term WDM is used for optical range Packet switch: Packet-based multiplexing 3 Recall Time Division Multiplexing Example: 12 OC1 signals multiplexed on to an OC12 signal 1 2 . . 12 1 2 12 ........... 90 x 9 x 8 bits every 125s 12 x 90 x 9 x 8 bits every 125s RATE: 51.84Mbps - OC1 RATE: 622.08 - OC12 4 Example of a circuit switch (TDM): SONET Controller 1 OC12 2 .. .. .. .. . . . . OC12 OC1 OC1 OC12 Q .. .. Routing and signaling .. .. PxP space switch (also called switch fabric or interconnection fabric) OC12 1 OC12 .. .. .. .. demultiplexers Crossconnect rate: OC1 What is the relation between P and Q? Answer: ? 2 Q . . . . OC12 multiplexers 5 Example of a circuit switch: a SONET switch Endpoint A OC3 Ports or interfaces a b Endpoint B OC3 SONET switch OC3 Endpoint C c d OC3 Endpoint D Create a bidirectional OC1 circuit between host A and host C Use it for application 1 (leased-line from one customer) Create another bidirectional OC1 circuit between host A and host B Use it for application 2 (leased-line from another customer) 6 Example of what happens inside a SONET switch (crossbar assumed) Controller 125s Endpoint A Endpoint B OC1 A2 A1 a B1 b C1 Endpoint C c Endpoint D d OC3 a Input ports b c d EP B EP C A1 EP A A2 C1 B1 Output ports EP D 7 Timeslot mapping table Input Output Port Timeslot Port Timeslot a c 1 1 c a 1 1 a b 2 1 b a 1 2 We need to use timeslot 2 on port a to/from host A for the AB OC1 circuit since timeslot 1 on this port was already used for the AC OC1 circuit 8 Circuit switch: position based “Unfolded” View of Switch Input line card functions Controller Q Line card Line card 2 Line card 3 Line card Line card Line card Q Position-based demultiplexing Position-based multiplexing Output line card functions Space switch … 1 … … 3 Line card Space switch 2 Line card … 1 Controller Input ports Data path Control path Close crosspoints when a circuit is setup Transfer data bits between line cards Open crosspoints when a circuit is released Circuit setup/release phases Next lecture Output ports (a) Folded view: 1 line card has both input and output functions 9 Analogy for a circuit-switched network 10 Analogy for "position based" multiplexing Think of Metro Center as a circuit switch If trains run exactly on time, the switching action of which train (red line, orange line) to direct to which outgoing track would be time-dependent http://www.wmata.com 11