atm switching(colour slides)

8/8/2019 ATM Switching(Colour Slides)

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With material from Cisco (http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm ) and Linda Crane

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ATM Devices and the Network Environment

ATM Cell Header Format ATM Services

ATM Reference Model

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History: Asynchronous Transfer Mode (ATM)

The ATM cell size was chosen by the CCITT internationalstandards committee (now called ITU)

48 bytes of data per ATM cellEuropean community wanted 32 bytes of data per ATM cell

American community wanted 64Result: compromise!

=

thus, 48 bytes of data per ATM cellBoth sides equally (un)happy

5 bytes of header

European community wanted 4 bytes of header per ATM cellAmerican community wanted 6Result: compromise!

=

thus, 5 bytes of header per ATM cell48 + 5 = 53 bytes per ATM cell

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Packet Switched: ATM

Asynchronous Transfer Mode (ATM)ATM technology is capable of transferring voice,video, and data through private and public networks.It is built on a cell-based architecture rather than ona frame-based architecture.

ATM cells are always a fixed length of 53 bytes. The ATM cell contains a 5 byte ATM header followed by

48 bytes of ATM payload.

-,voice and video traffic because this traffic is intolerantof delay.

Video and voice traffic do not have to wait for a larger

data packet to be transmitted. The 53 b te ATM cell is less efficient than the bi er

frames and packets of Frame Relay and X.25. A typical ATM line needs almost 20 percent greater

bandwidth than Frame Relay to carry the same volumeof network layer data. Why?

ATM was designed to be extremely scalable and can

-and higher.ATM offers both PVCs and SVCs,

although PVCs are more common with WANs.

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Two types of interfaces NNI

UNI

Switches can be public or private

Private UNI connects private end system to private switch

Private NNI connects two private switches

Public UNI connects end point or private switch to a public switch

Public NNI connects two switches in the same public network

Broadband Intercarrier Interface (B-ICI) connects two public switches fromdifferent providers

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Two formats UNI and NNI

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Notice NNI header has a larger VPI field allows for more VPIs


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ATM cells with UNI header used for communication

between ATM endpoints and ATM switches in privatenetworks

ATM cells with NNI header used for communication

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ATM Cell Header Fields Generic Flow Control (GFC)Provides local functions, such as identifying multiple

stations that share a single ATM interface. This field is typically not used and is set toits default value of 0 (binary 0000).

Virtual Path Identifier (VPI)In conjunction with the VCI, identifies the nextest nat on o a ce as t passes t roug a ser es o sw tc es on t e way to ts

destination. Virtual Channel Identifier (VCI)In conjunction with the VPI, identifies the next

destination of a cell as it passes through a series of ATM switches on the way to itsdestination.

Payload Type (PT)Indicates in the first bit whether the cell contains user data orcontrol data. If the cell contains user data, the bit is set to 0. If it contains control data,it is set to 1. The second bit indicates congestion (0 = no congestion, 1 = congestion),

and the third bit indicates whether the cell is the last in a series of cells that represent= .

Cell Loss Priority (CLP)Indicates whether the cell should be discarded if itencounters extreme congestion as it moves through the network. If the CLP bitequals 1, the cell should be discarded in preference to cells with the CLP bit equal to0.

ea er rror on ro a cu a es c ec sum on y on e rs y es o eheader. HEC can correct a single bit error in these bytes, thereby preserving the cellrather than discarding it.

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Three services

PVC permanent virtual circuits

SVC - switched virtual circuits and

Connectionless (similar to SMDS)

PVC

Advantages always available, no call setup

Disadvantage manual setup, no network resiliency ac p ece o equ pmen e ween source an es na on mus e

manually provisioned for PVCresults in no resiliency in network

SVC

Disadvantage call setup overhead and time

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Connection oriented

Establishes virtual channels Two types of connections

Virtual Channel identified by Virtual Channel Identifier (VCI) Virtual Path is a bundle of virtual channels

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ATM Reference model is composed of three planes Planes traverse all layers Control Plane responsible for generating and managing signaling

requests

Management Plane two components Layer management manages layer-specific functions, detection of

failures and protocol problems Pane management manages and coordinates functions related to

complete system

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Physical layerAnalogous to the physical layer of the OSI reference

, -

transmission. ATM layerCombined with the ATM adaptation layer, the ATM layer isroughly analogous to the data link layer of the OSI reference model. The

a physical link (cell multiplexing) and passing cells through the ATM network(cell relay). To do this, it uses the VPI and VCI information in the header of

each ATM cell. ATM ada tation la er AALCombined with the ATM la er the AAL is

roughly analogous to the data link layer of the OSI model. The AAL isresponsible for isolating higher-layer protocols from the details of the ATMprocesses. The adaptation layer prepares user data for conversion into cellsand segments the data into 48-byte cell payloads.

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Four functions

Convert cells to bit stream Control transmission and receipt of bits on physical medium

Track cell boundaries

Package cells appropriately for the medium

Physical layer has two parts- Transmission convergence sublayer

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AAL1 synchronous CBR traffic

Connection oriented service for constant bit rate sources

EX: Uncompressed voice and video

Uses circuit-emulation services

Circuit-emulation service also accommodates equipment usingleased-lines to attach to an ATM backbone network

AAL1 requires timing synchronization between source and destinationso de ends on a medium that su orts clockin

Prepares cells for transmission so that they the correct order

Samples data

Inserts Sequence number and sequence number protection fields

Uses one byte for AAL1 information and 47 bytes for data

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AAL2 S nchronous VBR traffic

Traffic has timing requirements but is bursty EX: Compressed voice and video

o u y e ne

Reserves 4 bytes of payload for AAL2 process support

Traffic can be characterized as either VBR-RT or VBR-NRT

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AAL3/4

Connection orient and connection-less data Asynchronous service

es gne or ne wor serv ce prov ers

Closely aligned with SMDS (Switched Multimegabit Data

Service) AAL3/4 is used to transmit SMDS over ATM

2 byte header, 44 bytes data, 2 byte trailer

Can interleave data from different sources over the one VCC

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AAL5 - Sim le and Efficient Ada tation La er (SEAL) Primary AAL for data

Supports connection-orient and connectionless data

Used for most non-SMDS data

EX: IP over ATM, LAN Emulation (LANE)

Accepts data frame from higher level

Appends trailer with frame length and 32 bit CRC

All cells except last have PT bit set to 0

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atm switching(colour slides)

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