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Asynchronous Transfer Mode

(ATM)

EE290F22 April 2004

Shanna Crankshaw

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"A transfer mode in which information isorganized into cells; it is asynchronous inthe sense that the recurrence of cellscontaining information from an individual

user is not necessarily periodic".

ATM definition

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What is it really?

Low-level network layerabove physicallayer, below AAL (ATM adaptation layer)

Single transport mechanism for differenttypes of traffic (voice, data, video, etc.)

Streamlined protocol, minimal error and

flow control capabilities Fixed packet size = ATM cell

Simplified processing, management

Fast

packetswitching,high data

rates

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Contrast with STMBandwidth on demandSlots assigned

on demand, users take any empty slot

Nothing pre-assigned, no global timing Slot Cell, fixed size of 53 bytes

Arbitrary bit rates: can support T-1 usingCBR, voice/video using real-time VBR, IP-based traffic using ABR and UBR, etc.

Each cell must be self-identifying(overhead)

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ATM cell contentsHeader 5 bytes

General Flow Control (GFC) traffic control for

different QoS, alleviates short-term overloads VPI routing field for network

VCI routing to/from user

Payload type

Cell loss priority (CLP) Header error control (HEC) can correct single

bit errors in header

Information 48 bytes

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PT coding

000

001

010

011

100

101

110

111

______________Interpretation____________

User data cell, congestion not experienced, SDU type=0

User data cell, congestion not experienced, SDU type=1

User data cell, congestion experienced, SDU type=0

User data cell, congestion experienced, SDU type=1

OAM segment associated cell

OAM end-to-end associated cell

Resource management cell

Reserved for future function

SDU = Service Data Unit

OAM = Operations, Administration, and Maintenance

Stallings, Data & ComputerCommunications, 6th ed., Table 11.2

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ATM Cells Small size, may reduce queuing delay of high priority cells

Fixed size, more efficient switching

Info field,48 bytes

Header error control

PT CLP

VC identifier

VP identifier

GFC VP identifier

5-byteheader

Info field,48 bytes

Header error control

PL type CLP

VC identifier

VP identifierNNIUNI

53 bytecell

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VCs are not always VCsVirtual Channel

Transmission is

connection-orientedVC set up by some

signaling protocol beforeany cells can be sent

Virtual Path Connection(VPC), bundle of VCCs Virtual channels

Virtual path

Physicalchannel

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Logical connections

VPC = bundle of VCCs with the same

endpoints all switched together Network management of group of

connections, not many individual ones

Setup time is for a VP, adding VCs to itinvolves minimal processing

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Request for

VCC originates

VPC exists?

Request for

VCC originates

Establishnew VPC

Can QoS besatisfied?

Block VCC orrequest more

capacity

Reject VCC

request

Makeconnection

Requestgranted?

Yes

YesYes

NoNo

No

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ATM Adaptation Layer (AAL) Layer above ATM

Service dependent

Mask ATM specifics from user;universality

SAR: segmentation and reassembly.Translates service data from a

non-ATM format into ATM cells,then back again at destination

CS: convergence sublayer. Takescare of delay jitter, error checking,remove corrupted cells

Physical layer

ATM layer

SAR sublayer

CS

AAL user

AAL

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ATM Service Categories Real-time services

Constant bit rate uncompressed audio/video info

Videoconferencing, TV, pay-per-view, VOD, etc. rt-Variable bit rate

Non-real-time services

nrt-VBR high end system QoS, critical response time

Bank transactions, airline reservations, etc.

Unspecified bit rate best-effort service

e.g. text/image messaging, telecommuting

Available bit rate bursty apps requiring reliable end-to-end connection

e.g. LAN, router-to-router reliability

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ATM challenges

In-network

mux/buffering canlead to cell delay orloss QoSguarantees

Many types of traffic

Large geographicdistribution

Traffic modeling,control