Asynchronous Transfer Mode (ATM)

SLIDES

1. Asynchronous Transfer Mode
(ATM)

2. ATM
•Bythemid1980s,threetypesofcommunication
networkshadevolved.
•Thetelephonenetworkcarriesvoicecalls,television
networkcarriesvideotransmissions,andnewly
emergingcomputernetworkcarriesdata.
•Telephonecompaniesrealizedthatvoice
communicationwasbecomingacommodityservice
andthattheprofitmarginwoulddecreaseovertime.
•Theyrealizedthatdatacommunicationwas
increasing.
•Thetelecommunicationindustrydecidedtoexpand
itsbusinessbydevelopingnetworkstocarrytraffic
otherthanvoice.

3. Goal of ATM
•Universal Service
•Support for all users
•Single, unified infrastructure
•Service guarantees
•Support for low-cost Devices

4. ATM
•ThephonecompaniescreatedIntegratedService
DigitalNetwork(ISDN)andAsynchronousTransfer
Mode(ATM).
•ATMisintendedasauniversalnetworking
technologythathandlesvoice,video,anddata
transmission.
•ATMusesaconnection-orientedparadigminwhich
anapplicationfirstcreatesavirtualchannel(VC),
usesthechannelforcommunication,andthen
terminatesit.
•Thecommunicationisimplementedbyoneormore
ATMswitches,eachplacesanentryfortheVCinits
forwardingtable.

5. ATM
•APVCiscreatedmanuallyandsurvivepower
failures,andanSVCiscreatedondemand.
•WhencreatingaVC,acomputermustspecify
qualityofservice(QoS)requirements.
•TheATMhardwareeitherreservesthe
requestedresourcesordeniestherequest.

6. Development of ATM
•ATMdesignersfacedadifficultchallengebecause
thethreeintendeduses(voice,video,anddata)have
differentsetsofrequirements.
•Forexample,bothvoiceandvideorequirelowdelay
andlowjitter(i.e.lowvarianceindelay)thatmakeit
possibletodeliveraudioandvideosmoothlywith
gapsordelaysintheoutput.
•Videorequiresasubstantiallyhigherdataratethan
audio.
•Mostdatanetworksintroducejitterastheyhandle
packets.

7. Development of ATM
•Toallowpacketswitchestooperateathigh
speedsandtoachievelowdelay,lowjitter,and
echocancellation,ATMtechnologydividesall
dataintosmall,fixed-sizepacketscalledcells.
•EachATMcellcontainsexactly53octets.
•5octetsforheader
•48octetsfordata

8. ATM Protocol Layer
•PhysicalLayer:ThelowestlayerintheATMprotocol.It
describesthephysicaltransmissionmedia.Wecanuse
shieldedandunshieldedtwistedpair,coaxialcable,andfiber-
opticcable.
•ATMLayer:Itperformsallfunctionsrelatingtotherouting
andmultiplexingofcellsoverVCs.Itgeneratesaheadertothe
segmentstreamsgeneratedbytheAAL.Similarly,onreceipt
ofacellstreams,itremovestheheaderfromthecellandpass
thecellcontentstotheAALprotocol.Toperformallthese
functions,theATMlayermaintainsatablewhichcontainsa
listofVCIs.

9. ATM Protocol Layer
•ATMAdaptationLayer:
ToplayerintheATMprotocolModel.Itconvertsthe
submittedinformationintostreamsof48-octetsegments
andtransportstheseinthepayloadfieldofmultiple
ATMcells.Similarly,onreceiptofthestreamofcells
relatingtothesamecall,itconvertsthe48-octet
informationfieldintorequiredformfordeliverytothe
particularhigherprotocollayer.

10. Disadvantages
•ATM has not been widely accepted. Although
some phone companies still use it in their
backbone networks.
•The expense, complexity and lack of
interoperability with other technologies have
prevented ATM from becoming more
prevalent.

11. Disadvantages
•Expense: ATM technology provides a comprehensive lists of
services, even a moderate ATM switch costs much more than
inexpensive LAN hardware. In addition, the network interface
card needed to connect a computer to an ATM network is
significantly more expensive than a corresponding Ethernet
NIC.
•Connection Setup Latency: ATM’s connection-oriented
paradigm introduces significant delay for distant
communication. The time required to set up and tear down the
ATM VC for distant communication is significantly larger
than the time required to use it.

12. Disadvantages
•Cell Tax: ATM cell headers impose a 10% tax on all data
transfer. In case of Ethernet, cell tax is 1%.
•Lack of Efficient Broadcast: Connection-oriented networks
like ATM are sometimes called Non Broadcast Multiple
Access (NBMA) networks because the hardware does not
support broadcast or multicast. On an ATM network,
broadcast to a set of computers is ‘simulated’ by arranging for
an application program to pass a copy of the data to each
computer in the set. As a result, broadcast is inefficient.

13. Disadvantages
•ComplexityofQoS:Thecomplexityofthe
specificationmakes implementation
cumbersome and difficult.Many
implementationsdonotsupportthefull
standard.
•AssumptionofHomogeneity:ATMis
designedtobeasingle,universalnetworking
system.Thereisminimalprovisionfor
interoperatingwithothertechnologies