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Asynchronous Transfer Mode

Developed as part of broadband ISDN used in private non-ISDN networks Also called Cell Relay More streamlined than Frame Relay Supports speeds at 155.52 Mbps and

622.08 Mbps Higher and lower speeds are possible

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ATM

Data is organised into fixed-size packets called cells

This simplifies the process of switching data at each node

Protocol is streamlined with minimal overhead

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ATM Protocol Structure

Higher Layerprotocols

Higher Layerprotocols

ATM Adaptation Layer

ATM Layer

Physical Layer

Management plane

Control Plane User Plane

Layer management

Plane management

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ATM layer

Logical connections are called virtual channel connections (VCC)

Full-duplex, fixed-size cell, variable rate connection between users is established on a VCC

VCC also used for user-network control signalling and network management and routing

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ATM layer

VCCs are bundled into groups that have same endpoints

These are called virtual path connections (VPC)

All cells in all VCCs in a VPC are switched together

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Advantages of VPC

Network transport functions can be split into those for individual VCCs and those for groups of VCCs in VPCs– Network deals with smaller number of entities – Increased network performance– When VPC is established new VCCs can be set

up with no transit node processing

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VCC Uses

Between end-users– VPC between users provides overall capacity– VCCs can be organised between the users up to

VPC capacity End user - network used for control

signalling Network-network used for network traffic

management and routing

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Characteristics of VPC/VCC

Switched and semi-permanent connections Packet sequence is preserved Traffic parameters can be negotiated

(e.g.peak and average rate) Quality of service is specified (cell loss

ratio, delay variation)

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Control Signalling for VCCs Not needed for semi-permanent connection Takes place on separate signalling channel Meta-signalling channel is a permanent

channel used to set up signalling channel This can be used to set up a user-to-

network signalling channel This can also be used to set up a user-to-

user signalling channel within an existing VPC which users can use to set up user-user VCC

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Control Signalling for VPCs Not needed for semi-permanent connection Customer controlled VPC

– Signalling VCC used by user to request/release VPC

Network controlled VPC– Network establishes VPC for own use. It may

be» Network-network» User-to-network» User-user

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ATM Cell Format

Generic flow control Virtual Path Identifier

Virtual Path Identifier Virtual channel ident

Virtual channel Identifier

Payload type CLPVirtual channel ident

Header error control

Information Field(48 octets)

User-network interface

8 7 6 5 4 3 2 1

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ATM Cell Format

Virtual Path Identifier

Virtual Path Identifier Virtual channel ident

Virtual channel Identifier

Payload type CLPVirtual channel ident

Header error control

Information Field(48 octets)

Network-network interface

8 7 6 5 4 3 2 1

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Header Error Control

CorrectionMode

DetectionMode

Single-bit error detected (correction)

no error detected(no action)

Multi-bit error detected(cell discarded)

no error detected(no action)

error detected(cell discarded)

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Physical Layer Two rates specified - 155.52 Mbps or

622.08 Mbps Lower Rate Can use Synchronous Digital

Hierarchy (SDH) interface or a cell-based physical layer

The lower rate can support one or more video channels

The higher rate can support multiple video channels

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Cell Based Physical Layer

No framing is imposed Some form of synchronising is needed This is performed by header error control

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SDH-based Physical Layer

Defined for 155.52 Mbps STM-1 service Advantages

– SDH can carry either ATM or STM traffic allowing mixed circuit and cell relay on common medium

– Four ATM streams can be combined to use a 622 Mbps STM-4 interface if available

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ATM Adaptation Layer

Allows higher protocols not based on ATM– PCM– LAPF

AAL handles– Transmission errors– Segmentation and re-assembly– Lost and mis-inserted cells– Flow and timing control

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Classes of Service Supported

Timing relationbetween sourceand destination

Bit rate

Connection mode

AAL protocol

Class A Class B Class C Class D

Required Not Required

Constant variable

Connection oriented Connectionless

Type 1 Type2 Type3/4,5 Type3/4

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ATM Adaptation Layer

Two logical sub-layers defined Common part convergence sub-layer

(CPCS)– Provides the functions needed to support

specific applications using AAL Segmentation and re-assembly sub-layer

(SAR)– Places data into cells for transmission and un-

packs data from received cells

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ATM Adaptation Layer

Four Protocol Types are defined at both CPCS and SAR sub-layers to support different services

Type 1 Type 2 not yet defined Type 3 and four merged to form type 3/4 Type 5

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SAR PDUs

SN SNP SAR-PDU payload

Header, 1 octet 47 octets

AAL Type 1

ST SN SAR-PDU payload

Header, 2 octets 44 octets

AAL Type 3/4

MID LI CRC

Trailer, 2 octets

SAR-PDU payload 48 octetsAAL Type 5

SN= Sequence no. (4 bits)SNP = Sequence no. protectionMID = multiplexing identification

LI= Length identificationCRC = Cyclic redundancy checkST = Segment type

Legend:

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AAL Type 5

Increasingly popular in ATM LANS Provides streamlined transport for higher-

layer connection-oriented protocols

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CPCS PDU for AALType 5

CPCS-PDU trailerPADCPCS-PDU payload

CPCS-UU = user to user indicationCPI = common-part indicatorLength = length of CPCS-PDU payloadCRC = cyclic redundancy checkPAD = pads out payload so PDU is a multiple of 48 octets

CRCLengthCPICPCS-UU

1 Octet 1 Octet 2 Octets 4 Octets

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CPCS PDU Trailer AALType 5

CPCS User to user indication (1 octet)– Used to transparently transfer user information

CRC (4 octets)– Used to detect bit errors in PDU

Common Part Indicator (1 octet) indicates interpretation of fields in trailer

Length (2 octets)– Length of payload field

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SAR PDU for AALType 5

No sequence number in PDU– Assumes all SAR PDUs received are in

sequence No MID field

– No interleaving of cells from different CPCS PDUs

– Each SAR PDU carries either part of current CPCS PDU or start of next CPCS PDU

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SAR PDU for AALType 5

AAU bit in ATM cell header is used to mark last block of a group of SAR PDUs to be reassembled into one CPCS PDU

Padding is used before trailer in CPCS PDU so that last bit of trailer coincides with last bit of last SAR PDU

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ATM Traffic and Congestion Control

Traffic control describes methods used to avoid congestion

Congestion control describes methods used to minimise intensity, spread and duration of congestion

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ATM Traffic and Congestion Control

Excessive traffic at nodes can cause buffer overflow and therefore data loss

High speed and low number of header bits creates problems not found in slower systems

Full strategy has not yet been developed

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Conventional Traffic and Congestion Control

Most Packet switched networks carry non-real-time ‘bursty’ data

Each node does not have to replicate the timing pattern of the data at the exit node

Statistical multiplexing can therefore be used

This makes efficient use of link capacity Proven techniques are available for

congestion control

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ATM Traffic and Congestion Control Voice and data traffic is not amenable to

flow control Wide range of traffic makes fair congestion

control difficult Different applications require different

services ( e.g. delay sensitive , loss sensitive)

Very high speed switching and transmission makes stable ATM congestion control more difficult

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Cell Delay Variation

Network– Minimal due to:

» Low overhead protocol, fixed frame size

» ATM switches are extremely fast acting

– Only occurs if network congested Most delay variation occurs at user-network

interface

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Cell Delay Variation at UNITime

48 octets, X Mbps

48 octets, Y Mbps

H

H

Connection A, X Mbps

Connection B, Y Mbps

OAM H

OAM

HHH H

H

ATM layer SAP

AAL layer

PHYlayer SAP

ATM layer

PHY layer

H H H H H H H H HH

PHY layer overhead

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Control of Cell Delay Variation

Cells with voice and standard video are inserted into the network at a constant rate

They suffer variable delay in the network The destination node delays the first cell by an

additional amount V equal to the estimated cell delay variation

Subsequent cells are delayed by a variable amount so that they are delivered to the user at a constant rate

Cells delayed by more than V are discarded

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Control of Cell Delay VariationLet R = required constant delivery rate

Required time T between cell delivery = 1/R Let t(0) = time of arrival of first cell

First cell is delayed by v(0)

Next cell arrives at time t(1)

It is delayed by v(1) such that:

t(1) + v(1) = t(0) +v(0) + T

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Traffic Control

Strategy Based on– Determining if new connection can be

accommodated– Agreeing a ‘contract’ with subscriber on

performance parameters that will be supported Functions are concerned with establishing

and enforcing agreed parameters

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ATM Traffic Control Functions

Network resource management Connection admission control Usage parameter control Priority control Fast resource management

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Network Resource Management

At present deals with virtual path connections

Primary parameters are cell loss ratio, cell transfer delay and cell delay variation

VCCs with similar traffic characteristics are grouped into the same VPC

Capacity can be allocated to VPC based on– aggregate peak demand or – statistical multiplexing

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Connection Admission Control

User selects the traffic characteristics required in both directions when requesting a new VPC or VCC

Four parameters are defined– Peak Cell Rate– Cell Delay Variation– Sustainable cell rate– Burst tolerance

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Connection Admission Control

These may be specified – At connection time– At subscription time– By signalling– By subscription– by default network-wide

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Usage Parameter Control

Monitors connections to ensure compliance with contract

Can be performed at VCC and VPC level Two separate functions performed

– Control of peak cell rate and cell-delay variation

– Control of sustainable cell rate and associated burst tolerance

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Usage Parameter Control

Two methods of control are used– Non-compliant cells discarded– Non-compliant cells are tagged with

CLP=1and passed

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Priority Control

Low priority cells are discarded to protect network performance for higher priority cells before congestion occurs

Cells with CLP=1 may be discarded Cells may have been tagged earlier either

by the network or by the user

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Fast Resource Management

These functions operate on same time-scale as round-trip propagation delay of ATM connection

This is for further study An example would be ability of a user to

request that agreed parameters be exceeded for a brief period

If network resources are available network should oblige

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Congestion Control

Selective cell discarding occurs when congestion is experienced

Network is free to discard all CLP=1 cells and may even discard CLP=0 cells in circuits which are not compliant with traffic contract

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Congestion Control

Explicit forward congestion notification– Nodes experiencing congestion may set

explicit forward congestion control indication in payload type field on passing cells

– The user may invoke actions in higher level protocols to lower cell rate