Asynchronous Transfer Mode (ATM).ppt

7/27/2019 Asynchronous Transfer Mode (ATM).ppt

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Asynchronous Transfer Mode (ATM)

ATM is called Cell Relay

Designed by the ATM Forum

Adopted by ITU-T

Designed for modern propagation media eg. Fiber

Attempt to provide high data rates.

Attempt to be compatible with existing systems.

Meant to be the back bone of data networks.

ATM is connection oriented protocol.


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ATM (contd.)

Problems with existing packet networks

Variable packet sizes in different protocols.

Header takes space and header size not fixed.

A large header is wasteful.

Large data field sometimes wasteful.

Different packet sizes makes switching complicated and

inefficient at Multiplexers, Routers, etc.

A variety of packet sizes can make network trafficunpredictable. With different protocols, a packet can be as

large as 65,545 bytes or as small as 200 bytes.


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ATM (contd.)

Internetworking among different packet networks (carrying

packets of different protocols) is slow and cumbersome

With different packet sizes, priority cannot be implemented

properly at multiplexing units.

To be able to fully utilize the available bandwidth of a

transmission facility, it should be able to be shared with all

available traffic.

Real time traffic ( audio, video, etc.) are usually made of small

packets. Mixing them with normally large data packets makes

shared packet links unusable for audio and video information

and thus makes real time traffic unviable.


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Cell Networks

Cell is a small data packet of fixed size.

It can be used as a basic unit of data exchange.

All data are loaded into identical cells

As packets of different sizes and formats arrive at a cellnetwork, they are broken down to cells.

Cells are then multiplexed and routed through the cellnetwork to their destinatins.

Since all cells are of the same size multiplexing and switching

devices can operate efficiently. If cells of a traffic stream is allowed to pass through without

long gaps, it can appear as continuous traffic (at destination).


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Cell Networks (contd.)

Because of fixed cell size, switching devices can operate taking

a cell as a unit rather than a bit. This makes the switching

operations possible at hardware level, which makes the

switches fast and efficient.

Cell multiplexing done asynchronously. That is each out put

slot (which has a space for a cell) is filled with a cell taken

from any input that has cells.

If no cells are present at input lines, then empty slots flow in

the output line.


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ATM Architecture

In ATM network, the user access devices are called the User

Network Interface (UNI).

The switching points in the network (nodes) are called

Network-to-Network Interface (NNI).

Cell networks are based on Virtual Circuits. All cells belonging

to a single message follow the same VC and arrive at the

destination in the original order.

Virtual Circuit makes an end-to-end (virtual) connection.

Virtual Path is formed by a bundle of VCs (over some section

of the network) which share the transmission fiacility.


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Virtual Circuit and Virtual Path Identifiers

ATM connections are identified by their Virtual Circuit/Virtual

Path identifiers.

There is a hierarchy in these Identifiers.

VPI defines the Virtual Path that carries the virtual circuit andthe VCI defines the particular VC contained in the VP.

Dividing the identifier into two parts is done to ease routing of

ATM cells.

Thus, a virtual connection is defined by a pair of numbers: TheVPI and the VCI.


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VPs and VCs (contd.)

Lengths of VPIs are different at UNI and NNI interfaces. At aUNI a VPI is 8 bits. At a NNI a VPI is 12 bits.

The length of a VCI is the same at both UNI and NNI, and is 16bits.

Therefore, a VC is identified by a group of 24 bits at a UNI andby a group of 28 bits at a NNI.

At UNI

VPI (8 bits) VCI (16 bits)

At NNI



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PVCs and SVCs

Similar to other connection oriented protocols, ATM has

Permanent and Switched Virtual connections.

PVCs are permanently set up and their values of VPI+VCI are

entered in lookup tables at the ATM switches.

SVCs involve establishment and release of a virtual connection

every time it is used. For this ATM layer requires the services

of the Network layer.

The end to end network layer addresses are provided by the

network layer.

The actual mechanism of connection establishment/release

depends on the network layer protocol.


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ATM Switching

Types of ATM switches:

VP Switch

VPC Switch

VP switch routes the cell using the VPI. The value of the VCIstays unchanged across the switch.

VPC switch routes the cell using both VPI and VCI. Both VPI

and VCI values are changed across the switch.

Most of the switches in an ATM network are VP switches.Since they need only to lookup for VPI the switching operation

is more efficient.


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ATM Switch Fabrics

The aim of ATM is to increase the data rate

To achieve 155 Mbps, it requires 350,000 cells per sec. to pass

through a switch.

ATM switches are synchronized.

A switch has a clock and at each tick it delivers a cell (from its

input to its output).

Switching hardware:

Cross bar switchBanyan switch (Multi-stage switch)

Batcher-Banyan switch reduces blocking probability


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ATM Layers

ATM defines 3 layers:

Application Adaptation Layer

ATM Layer

Physical Layer

End points use all 3 layers.

Switches use only the 2 bottom layers.

Application Adaptation Layer (some times called the ATMAdaptation Layer) accepts packets from upper layers and map

them into fixed size ATM cells. These packets (coming fromlayers above) could be carrying traffic of any type: voice, data,audio, video, etc.


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ATM Data Types

ATM Adaptation Layer divides traffic into 4 different

categories:

Constant Bit Rate (CBR) traffic

Variable Bit Rate (VBR) traffic

Connection Oriented Packet Data traffic

Connectionless datagram traffic

There is no unique agreement about these divisions. AAL3

and AAL4 traffic above are usually considered as of same typeand called AAL3/4 traffic.


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ATM Data Types AAL1

For functional purposes, the activity in ATM Adaptation Layers

(in each category) are divided into tow sub-layers:

Convergence Sublayer (CS).

Segmentation and Reassembly (SAR) sublayer.

The convergence sublayer divides the bit stream into 47-byte

segments and passes them to the SAR sublayer below.

The SAR sublayer accepts the 47-byte payload from CS and

adds a one byte header.

The resulting 48-byte payload is passed onto the ATM layer.

ATM layer adds a 5-byte header to complete the cell.


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ATM Data Types AAL1 (cotd.)

The header byte added in the SAR sublayer has the followingfields:

Convergence Sublayer Identifier (CSI) 1 bit

Sequence Count (SC) 3 bits modulo 8 sequence

number that could be used for end-to-end flowcontrol of cells

Cyclic Redundancy Check (CRC) 3 bits provides alimited error correction capability. Employs thepolynomial x3 + x + 1.

Parity (P) 1 bit Parity check for the first 7 bits of byte.Along with 3 CRC bits it can correct one bit error.


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ATM Data Types AAL2

This is intended for Variable Bit Rate traffic.

A 45 byte payload is taken from the Convergence Sublayer.

The Segmentation and Reassembly sublayer then adds 1 byte

header and 2 byte trailer.

The header byte contains Convergence Sublayer Identifier

(CSI) (1 bit), Sequence Count (SC) (3 bits), Information Type

(IT) (4 bits)

First 6 bits of the trailer is Length Indicator (LI). This indicates

whether padding is there in the last cell.

Last 10 bits are a CRC for the entire data unit. This can correct

one bit error.


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ATM Data Types AAL2 (contd.)

AAL2 Data Unit:

Header (1 byte) Payload (45 bytes) Trailer (2 bytes)

Header:

CSI (1 bit) SC (3 bits) IT (4 bits)Trailer:

LI (6 bits) CRC (10 bits)


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ATM Data Types AAL3/4

AAL3 was meant for Connection Oriented traffic and AAL4 for

Connectionless traffic originally. But later they were put in the

same category.

Convergence Sublayer accepts packets up to 65536 bytes from

upper layers, and adds a 4 byte Header and a 4 byte trailer.

Header and the trailer indicates the beginning and the end of

the message.

Header (4 bytes) Message (


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ATM Data Types AAL3/4 (contd.)

The SAR sublayer adds a 2 byte header and a 2 byte trailer to

each 44 byte segment coming from CS.

The Header contains 4 fields:

Segment Type (ST),

Convergence Sublayer Identifier (CSI),

Sequence Count (SC),

Multiplexing ID (MID)

ST (2 bits) CSI (1 bit) SC (3 bits) MID (10 bits) ST tells whether the segment belongs to the beginning,

middle, or end of a message, or is a single segment message.


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ATM Data Types AAL3/4 (contd.)

The Trailer consists of 2 fields:

Length Indicator (LI) 6 bits

CRC 10 bits

LI is used along with ST to determine how much of the lastsegment is padding.

AAL5

AAL5 is called Simple and Efficient Adaptation Layer (SEAL). It

is meant for ATM backbones and point-to-point links. CS addsan 8 byte trailer (and padding if necessary) to the message

and passes it in 48 byte segments to SAR.


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ATM Layer

ATM layer provides routing, traffic management, switching,

and multiplexing services.

Accepts 48 byte segments from AAL and add 5 byte header to

form a 53 byte cells.

ATM has two header formats:

for UNI interface,

for NNI interface.

UNI interface has a field for Generic Flow Control (GFC). This 4bit field provides flow control at the UNI level. It is considered

as not necessary at the NNI level.


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ATM Layer (contd.)

Format at UNI interface:

GFC (4bits) VPI (4 bits)


VCI (8 bits)

VCI (4 bits) PT(3) CLP(1)

HEC (8 bits)

Payload (48 bytes)

In Payload Type (PT), the first bit defines whether it is user or

system data.

If user data, next bit gives congestion status, and the lastindicates signaling status.


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ATM Layer (contd.)

If system data, the next two bits define the type of system

data as follows:

link associated management

end-to-end management

resource management

reserved

Cell Loss Priority (CLP) provides congestion control. When

congestion takes at a switch, the cell will be dropped if this bitis set to 0.

Header Error Check (HEC) applied for first 4 bytes of header.

Asynchronous Transfer Mode (ATM).ppt

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