ATM (Asynchronous Transfer Mode)

7/17/2019 ATM (Asynchronous Transfer Mode)

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ATM – Asynchronous Transfer Mode

Tuan



General

• The ITU-T definition of ATM is:

"A transfer mode in which information is organised into cells; it is

asynchronous in the sense that the recurrence of cells containing

information from an individual user is not necessarily eriodic"!

• i#ed-si$ed %loc& or ac&et' referred to as cell' of () %ytes' 5 of which are for switching and control !

• *ells assigned on demand' deending on source activity and

availa%le resources!

Cell information Field

(Cell Payload)

Cell

Header

48 bytes information5 byte header

ATM cell



ATM rocess: rom ac&etisation to cell server

• *onsider a single switch and focus on a single outut ort :

finite buffer fixed rate server

multiplexor ATMcell segmentation

variable bit rate

data bursts

high bandwidth

constant bit rate

constant bit rate

lowbandwidth

services

: ac!etiser

: ATMcell

: "eader



ATM rocess: receiving station end

ATMcell de#ittering

variable bit rate

data bursts

high bandwidth

constant bit rate

constant bit ratelowbandwidthasynchronous arrival of cells

arriving ATMcells signal reconstruction

and reassebmly



ATM rocess: receiving station end +cont,d

• .e-/ittering and reassem%ly $ .uring transortation of ATM cells /ittering is introduced due to differences in local

cloc&,s fre0uency and hase! Also due to varia%le 0ueuing delays cells arriving at ATM

terminal will %e out of hase from those transmitted

$ 1ence' re0uired to de-/itter and reassem%le arriving cell stream' so that when layed

%ac& its timing relationshi will reflect the original one!

$ *onsider a stream of arriving cells +from a voice connection

dijittering and eassembly

Analog to digital

time



2ayers for ATM +cont,d

%%%

output by an

example application

output by

convergence

sublayer &'()

output by

(A* sublayer

message

'( '(

'( '( ( A *

( A *

( A *

( A *

( A *

( A *

( A *

( A *

unused

%%%'( '( ( A *

( A *

( A *

( A *

( A *

( A *

( A *

( A *

output by

ATM layer

A T M

A T M

A T M

A T M

48 bytes5+ bytes

header

header

trailer

trailer



2ayers for ATM

ATM-3.U

ATM-4.U

ATM!.ATA!

Indication

ATM!.ATA!

5e0uest

Indicate

5e0uest

ATM Adatation 2ayer

+AA2

ATM 2ayer

3hysical layer +316

ATM-4.U: ATM 4ervice .ata

Unit +78 %ytes of user dataincluding A22 1eader

1igher layers

Alication

information

9ideo .ata 9oice

Alications

*onvert to correct

electrical or

otical format

orward +or receive

cell through networ&

*onversion to +from

ATM data tyes

ATM-3.U: ATM 3rocess .ata

Unit +() %yte cells

To +from ATM switch



3hysical 2ayer: 3hysical Medium +3M su%-layer

• Tas& of hysical layer +316 is to transort ATM cells!

• 3hysical layer standards define fundamental schemes for %asic digital transmission to

suort ATM' including signal definitions' coding' %it timing' formats' scram%ling and

oeration and maintenance caa%ilities!

• ne asect of these standards is uni0ue to ATM: They all define a mechanism to delineate

+cell synchroni$ed ATM cells!

• At resent' wide range of hysical-layer transort otions:

$ ver 4<T=4.1 +agreed %y <T4I and ITU and T>4> - e!g! 4.1 4TM-> at >(( M%s u to

4TM->? at @!7 Gg%it=s! *ells transmitted as tri%utaries within 4.1 frame!

$ ver 3.1 +5ecs! G!B)' G!8B7 +.4> >!( M%s and <> @ M%s' .4) +7( M%s' <) +)7

M%s ' <7 +>)C M%s

$ <tc!



ATM rocess for transmission



ATM etwor& Interfaces

3u%lic

4witch

3u%lic

4witch

2ocal 4witch

*omuter

5outer.ata 4ervice

Unit +.4U

*omuter

I 3u%lic

4witch

I

UI

UI

UI

UI

UIUI

I - etwor& ode Interface

UI - User networ& Interface



ATM *ell ormat



ATM 4witching

• ATM networ&s' which are connection-oriented devices' need a virtual circuit +9* to %e set u

across the networ& rior to any data transfer!

• Two tyes of circuits include the virtual ath +93' which is identified %y a virtual ath

identifier +93I' and the channel ath' which is identified %y the virtual channel identifier

+9*I!

• Decause 9*I=93I values are locali$ed' each segment of the connection has a uni0ue 93I=9*I

com%ination! Ehen a cell travels through the networ& from the user networ& interface +UI

through the switch to the networ& node interface +I' the 93I=9*I value is changed to the

value the ne#t segment of connection uses through a rocess called a 93I=9*I translation'shown in igure %elow:



ATM 4witching : Translation

• igure %elow shows the function of an ATM switch where 93I=9*I values are translated!

93I=9*I values are uni0ue er interface; however' the values can %e re-used along thenetwor&! or e#amle' in igure %elow' the 93I=9*I value of @C is used in two different

interfaces!



ATM 4witching

• Virtual Channel=Virtual Path +VC=VP

$ Virtual Channel +VC connection

• connection %etween two communicating ATM end entities!

• may consist of several ATM 9* lin&s!

• reserves cell se0uence and rovides a certain 0uality of service!

• A Virtual Channel Identifier +VCI in the ATM cell header is assigned er networ&

entity-to-entity lin&' i!e! it may change across the networ& within the same 9*

connections!

$ Virtual Path +VP connection:

• Grous 9*s carried %etween two ATM entities!

• may involve many ATM 93 lin&s!

• allows for efficient and simle management of resources %y alying controls on a

small num%er of grous of connections +the 93s rather than a large num%er ofindividual connections +the 9*s!

• 9*s associated with 93s are glo%ally switched without un%undling or rocessing of

the individual 9*s!

• reduction of si$e of routing ta%les in ATM 93 switches

• allows carriers to offer Virtual Private Networks (VPNs) to cororate customers



9irtual 3ath and 9irtual *ircuit *oncet

VCI =1 (text)

VCI =2 (voice)

VCI =3 (video)

TRA!"I!!I# PATH

Text

Voice

Video

AT" et$or%

Interface

Virt&al Pat'

VP

VC



ATM switching +cont,d

• *ell multile#ing and switching

$ for more than one ATM connection' ATM layer is resonsi%le for the multile#ing

function $ 9* %asic routing entity for switched services! 1andled in 9* multile#ers=demultile#ers

and switches!

$ 9*s are aggregated in 93s which may %e routed through multile#ers=demultile#ers

and 93 switches=cross connects

, , , ,

,' ,' ,'

,

,'' ,''

,' ,'

, ATM

switch

,-,' ATM

switch

transmission pathvirtual path

virtual

channels



4EIT*1IG 9*s and 93s

• 5outing functions for 93s are erformed at a 93 switch!

• This routing involves translation of the 93I values of the incoming 93 lin&s to the 93I values

of the outgoing 93 lin&s! 9*I values remain unchanged!

• 9* switches terminate %oth 9* lin&s and necessarily 93 lin&s!

• 93I and 9*I translation is erformed!

VP Switch/Cross Connect

VPI1

VPI2

VPI3

VPI4

VPI5

VPI6

VCI 21

VCI 22

VCI 23

VCI 24

VCI 25

VCI 24

VCI 23

VCI 24

VCI 25

VCI 24

VCI 21

VCI 22

VP Switching



93 and 9* 4EIT*1IG

VCI 23

VCI 24

VPI 2

VCI 25

VCI 21

VPI 4

VPI 5

VCI 23

VCI 24

VCI 25

VCI 21

VC !$itc'Cro Connect



1eader 3rocessing

• 1eader translation and interretation

$ Ehen cells enter the switch' their headers are analysed!

• for ATM cross-connect system' only 93I field of the header is analysed and used for

routing!

• for ATM VCI switch' %oth 93I and 9*I field are analysed and used for routing!

$ Idle cells are discarded from the cell stream and all other cells are send to the headerinterpreter !

$ In general' the header is interreted using a ta%le!

• or each valid 9* lin&' an entry e#ists in this ta%le! This ta%le contains at least @

entries er inde#:

$ >! The destination ort num%er

$ @! The new 9*I=93I value



1eader 3rocessing +cont,d

• <ach incoming ort has its own ta%le!

$ The 93I=9*I art of header of incoming cell is relaced with new 9*I=93I header valuecoming from ta%le and routed to actual switching %loc&!

$ There %ased on header cell is routed to correct outlet!

xxy

xys

n!!

ggm

nil

.

cells

/0

/n

10

12

13

header and lin!

translation unit

incoming

headers outgoingheaders

translation table

incominglin!

outgoing

in!header header

/0x

.

y10

!m

l

13

12

%%% note routing information at a switch has only local meaning

updated at connection establishment for each connection

switching fabric



ATM 4witching : *AM

• The time re0uired to comute 93I=9*I translations is critical in order to determine the

erformance of ATM networ&s!

• *AM can act as an address translator for loo&-u ta%les +2UTs in ATM switches and erform

93I=9*I translation 0uic&ly!

• 93I=9*I fields from the ATM controller are comared against a list of current connections

stored in the *AM array!

• *AM generates an address that is used to access an em%edded 5AM' where 93I=9*I maing

data and other connection information are stored!

• 93I=9*I data from 5AM is added on to the cell and sent to the switch' as shown in igure

%elow :



.iagram of the >?#>? 4witching ode

Structure of the Control Chip



An ATM ode



Generating 93-AI4=5.I and 9*-AI4=5.I tye AM cells



ormat of AM cells



Than& 6ou F

ATM (Asynchronous Transfer Mode)

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