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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.)