Ch 8. Switching

Switch  Devices that interconnected with each other  Connecting all nodes (like mesh network) is not cost-effective  Some topology like bus has limitation on distance  Switched network, where end systems (e.g., A, B, … J) are connected through switches (e.g., I, II, … V)

Taxonomy of Switched Networks Telephone system The Internet (IP)

Circuit-Switched Networks  A set of switches are connected by physical links  Each link is divided into n channels using FDM or TDM  Each connection has one dedicated channel  Example (n = 3)

Circuit Switching  Resources (channels, switch buffer, switch processing time, switch ports, etc) must be reserved before communication, and released after communication  Three phases  Setup – establish a connection and reserve the resources for communication  Data transfer – do communication  Teardown – finishing communication, release the resources

Setup Teardown

Example of Circuit-Switched Network  Three switches are used to “route” 4 connections

Properties of Circuit-Switching  Require resource reservation and release  Data do not needed to be packetized  No addressing is involved during data transfer  Low efficiency  Reserved resources are unavailable to others  Low delay  Except the setup delay for the resource reservation

Datagram Networks  Messages pass through packet-switched networks, without reserving resources  Resources are allocated on demand  Data should be divided into small pieces, called packet or datagram  Each packet is treated independently of others  Network has no idea about data stream  Often called, connectionless networks, since the switch does not keep information about the connection state

Example of Datagram Network

Routing  How the switches route packets without reserving resources?  Each packet carries its destination address  Each switch keep routing table, which is dynamic and updated periodically  Routing table  Specifies the output port of the switch for each destination address

Properties of Packet-Switching  Resources are allocated on demand  Data should be packetized, and each packet should include its destination address  High efficiency – more multiplexing  High delay

Recall the Taxonomy Telephone system The Internet (IP)

Virtual-Circuit Networks  Can be regarded as a blend of both a circuit-switched network and a datagram network  Three phases: setup, transfer, and teardown  Resources can be reserved at setup, or allocated on demand  Data are packetized and each packet carries an address  Normally, switches are implemented at  physical layer - Circuit-switched networks  network layer - Packet-switched networks  data link layer - Virtual-circuit networks

Addressing  Two-level addressing  Global addressing – address is unique over networks  Virtual-circuit identifier (VCI, local addressing) – used by a frame between two switches

Routing Table  Routing using (port, VCI)

End-to-end Data Transfer

Connection Setup (1)  Request (source  destination) How does switch 1 know it should go to port 3?  This will be covered later

Connection Setup (2)  Acknowledgement (destination  source)

Properties of VC Switching  Three phases  Setup  Data transfer – all packets belonging to the same source and destination travel the same path  Teardown – the similar method as setup (i.e., request and confirm)  Efficiency and delay  Depends on whether resources are either reserved during the setup, or allocated on demand  Advantage  The source can check availability of the resources, without actually reserving it.

Structure of a Switch  Switches are used in both circuit-switched and packet- switched networks  Circuit switch  Space-division switch: paths in the circuit are separated from one another spatially  Time-division switch: internally uses time-division multiplexing (TDM)  Packet Switch

Space-Division Switch (1)  Crossbar Switch  Connect n inputs to m outputs in a grid  Switch with too many crosspoints is impractical and inefficient

Space-Division Switch (2)  Multi-stage Switch  Combine crossbar switches in several stages (usually three)  Ex: number of crosspoints?  N/n (n x k) + k (N/n x N/n) + N/n (k x n) = 2kN + k(N/n) 2  This is much smaller than single-stage crossbar: N 2

Space-Division Switch (3)  Blocking – problem of multistage switch  Under heavy traffic, resources (i.e., crosspoints) are limited, if many users want a connection at the same time  Blocking refers to times when one input cannot be connected to an output due to no available path  Can we avoid blocking?  Clos criterion: n = (N/2) 1/2, k > 2n-1  Number of crosspoitns ≥ 4N ((2N) 1/2 – 1)  This is still huge, though less than N 2

Time-Division Switch  Time-Slot Interchange (TSI)  TDM muxer, demuxer  TSI with Random Access Memory (RAM)  To support inputs continuously, TSI should operate at a faster rate – speed-up

Time- and Space-Division Switch  Space-division requires many cross-points  Time-division requires speed-up (or delay if store in the switch)  Time-space-time (TST) switch

Packet Switches  Components:  Input port, routing processor, switching fabric, output port

Packet Switch Structure (1)  Input port performs the physical and data link functions: decapsulates packet from the frame, detects/corrects errors, and store packets at its queue  Output port performs the same function of the input port, but in the reverse order

Packet Switch Structure (2)  Routing processor performs the functions of the network layer: finds the output port number by looking up the routing table (table lookup)  Switching fabrics move packets from the input queue to the output queue  Crossbar  Banyan  Batch-Banyan

Banyan Switch  Multistage switch with many 2x2 micro-switches  log 2 n stages, n/2 micro-switches at each stage  Packets are automatically routed to the destination using the binary expression of the destination address Banyan tree from dailycognition.com

Banyan Switch  Micro-switch (2x2)  A packet has a control bit, 0 or 1  The packet goes up if the control bit is 0  The packet goes down if the control bit is or 1

Banyan Switch  Two examples  Left figure: packet to output 6 (= 110)  Right figure: packet to output 2 (= 010) Control bit at 1 st stage Control bit at 2 nd stage Control bit at 3 rd stage

Banyan Switch  First bit determines the block of the next stage  Two blocks are separated  First bit indicates which block the packet should go  Procedure repeats at the next stage with the next bit

Batcher-Banyan Switch  Collision of packets even for a different dest.  At port 0, to dest. 4 (100)  At port 6, to dest. 5 (101)  Pre-sorting can solve the problem  E.g., the second packet arrives at port 1  Batcher switch does the sorting Collision

Homework  Exercise in Chap. 8  13  18  22  23