Switch Design - Local Area Network - Lecture Slides, Slides of Network security

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Switch Design

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Switching

• The process by which a network element

forwards data arriving at one of its inputs to

one of its outputs.

• Three kinds of switching elements:
  • Telephone switches
  • Packet switches
  • ATM switches

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Classification

• Before sending a packet, a connection needs to be set up to configure (enter VCID) all the switches on the path.
• Each packet is fixed-sized and contains a VC ID that switches can use to index into a switching table. 9- 4

Classification

• Before sending voice data, a connection needs to be set up first to configure all the switches (e.g., switching schedule for TDM) on the path.
• Switched data thus need not contain meta-data (i.e., the control information carried in a header).9- 5

A Switch’s Jobs

• Switching data between input and output ports.
• (Layer 3) Participating in routing algorithm to build routing tables - A connectionless switch consults the routing table for every incoming packet. - A connection-oriented switch consults the routing table during connection setup.
• (Layer 2) Or participating in spanning tree algorithm
• Scheduling access to an output link
  • Buffer management and packet scheduling (e.g., FIFO, WRR)
• Admission control
  • Connection-oriented switches can perform admission control during connection setup. 9- 7

Requirement for Switches

• High forwarding capacity
  • A switch’s capacity is the maximum rate at which it can move information, assuming all data paths are simultaneously active. (e.g., CISCO 12400’s packet forwarding engine has a 25Mpps forwarding rate.)
• Low call blocking or packet loss rate
  • A connection-oriented switch must reject a call if it does not have a path from an input port to an output port to carry the call. (call blocking)
  • A connectionless switch need not reject a call because packets can be buffered. However, packet may be lost due to buffer overflow.
• No packet reordering
  • Otherwise receivers must buffer out-of-order packets, increasing memory cost and packet end-to-end delay. (^) Docsity.com9- 8

Two Types of Blocking

• Internal blocking
  • Two samples coming from two different input ports and going to two different output ports collide in the switch fabric and one of them must be dropped.
  • Bad design. Should be avoided.
• Output blocking
  • Two samples coming from different input ports want to go to the same output port at the same time.
  • Only one of them can go through the output port.
  • Should be avoided. 9- 10

Circuit Switching Techniques

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Speed Conversions

• Multiplexor and Demultiplexor can work with switches so that a switch can switch traffic between input and output lines with various speeds. (^) 9- 13

Time-Division Switching

• A Time-Slot Interchanger (TSI) writes incoming

samples in an N-byte buffer, one byte per incoming circuit, and read them out in a different order. 9- 14

Space-Division Switching

• In SDS, each sample takes a different path through

the switch, depending on its destination.

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Space-Division Switching

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Multistage Crossbar

• Try to reduce the number of crosspoints.
• However, it is internally blocking.
• Example 1: at E1 if more than 3 input ports has packets to send at the same time, at most 3 of them can be sent to the second stage.
• Example 2: if more than 3 input ports have packets to send to E2 at the same time.

E1 E

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Time-Space Switching

• By rearranging

the order of

incoming

samples, call

blocking or

packet loss can

be avoided.

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