Switching Overview

8/7/2019 Switching Overview

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ATM Switching: An Overview

Carey Williamson

Department of Computer Science

University of Calgary


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Introduction

ATM switches are central to the

operation of ATM networks

Lots of research on ATM switch design

in the last 10 years

This presentation: an overview of the

basic components and functionalitycommon to all ATM switching systems


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ATM Switch Components Input ports

Output ports

Switch fabric

Buffering

VPI

/ VCI

table Control unit


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Generic ATM Switch

OutputPortsSwitch

Fabric

InputPorts

Control

UnitVPI

VCI


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Switch Ports Provide the physical connection to other

ATM entities

Dedicated point-to-point links

Each port (or card) is for a particular

type of interface and particular data

rate (e.g., OC-3, DS-3, JPEG,Ethernet,T1,...)

N inputs, N outputs


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Switch Fabric Defines the interconnection between the

input ports and the output ports

Distinguishing characteristic of different

types of ATM switches (e.g., shared

memory, banyan, Benes, hypercube, ...)

Affects performance (cell delay, cellloss)


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Buffering Some switches are bufferless

Some switches are buffered

Buffers provide physical storage (i.e.,

memory) to hold cells that cannot yet be

sent on their desired output ports

Uses RAM, DRAM, or FIFO...


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Buffering (Contd) Buffering can be located at the input

ports, at the output ports, internal to the

switch fabric, or any combination

Buffers are usually limited in size:

e.g., 100 cells per port

e.g., 4096 cells per port

Buffer size and location affects

performance


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VPI/ VC

ITable

Physical memory that is used to keep

track of how to map cells of different

VCs from input ports to output ports

Translation table:

1 4 15 1 5 12

3 27 104 2 2 33

Port PortVPI VPIVCI VCI

INPUT OUTPUT


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VPI/ VC

ITable (Contd)

Entries initialized during call setup

Lookup done for each data cell

Determines routing tag to be used for

switch fabric


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Control Unit Controls entire operation of switch

Cell switching

Signalling (UNI and NNI)

Call Admission Control (CAC)

Usage Parameter Control (UPC)Accounting statistics


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ATM Switch Functionality Cell switching

also known as label switching

or label multiplexing

cell with VPI = a and VCI = b on input port

c is sent out with VPI = d and VCI = e on

output port f uses VPI / VCI table

simple fast harware switching (for data

cells)


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ATM Switch Functionality Signalling

participate in UNI signalling protocol

(edge switches)

participate in NNI signalling protocol

(core switches)

connection setup and teardown update VPI / VCI table


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ATM Switch Functionality Call Admission Control (CAC)

makes decision to accept / reject new call

considers traffic descriptor (TD) and QOS

requested by the new call

(e.g., PCR, SCR, CLR, CDV)

considers available resources, and(potential) impact on the QOS of existing

calls

many different approaches proposed


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ATM Switch Functionality Usage Parameter Control (UPC)

mechanism to monitor incoming traffic

flows on a VPI / VCI basis

check for conformance to traffic descriptor

e.g., leaky bucket, dual leaky bucket

can provide traffic shaping tags or discards non-conforming cells


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ATM Switch FunctionalityAccounting

statistics accumulation

simple counters (e.g., per port)

statistics are reported to network

management station (e.g., Newbridge

4602)


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SummaryATM switches form the very heart

of ATM networks

Basic components and functionality are

common to all switches

Switches from different vendors differ in

the details (e.g., switch fabric, buffering,control software, etc.)

Documents

Switching Overview