Data and Computer Data and Computer CommunicationsCommunications

Chapter 11 – Asynchronous Transfer Chapter 11 – Asynchronous Transfer ModeMode

ATMATM

a streamlined packet transfer interfacea streamlined packet transfer interface similarities to packet switchingsimilarities to packet switching

transfers data in transfers data in discrete chunksdiscrete chunks supports multiple logical connections over a supports multiple logical connections over a

single physical interfacesingle physical interface ATM uses ATM uses fixedfixed sized packets called cells sized packets called cells with with minimalminimal error control and flow control error control and flow control data rates of 25.6Mbps to 622.08Mbpsdata rates of 25.6Mbps to 622.08Mbps

Protocol ArchitectureProtocol Architecture

Reference Model PlanesReference Model Planes user planeuser plane

provides for user information transfer, associated provides for user information transfer, associated controls (flow control, error control)controls (flow control, error control)

control planecontrol plane call and connection controlcall and connection control

management planemanagement plane plane managementplane management

• whole system functionswhole system functions layer managementlayer management

• Resources and parameters in protocol entitiesResources and parameters in protocol entities

ATM Logical ConnectionsATM Logical Connections

virtual channel connections (virtual channel connections (VCCVCC)) analogous to virtual circuit in X.25analogous to virtual circuit in X.25

basic unit of switching between two end usersbasic unit of switching between two end users full duplexfull duplex fixed size cellsfixed size cells

also for also for user-network exchange (control) user-network exchange (control) network-network exchange (network mgmt & routing)network-network exchange (network mgmt & routing)

ATM Virtual Path ConnectionATM Virtual Path Connection

virtual path connection (virtual path connection (VPCVPC)) bundle of VCC with same end pointsbundle of VCC with same end points

Advantages of Virtual PathsAdvantages of Virtual Paths

simplified network architecturesimplified network architecture increased network performance and increased network performance and

reliabilityreliability reducedreduced processing processing short connection setup time for new short connection setup time for new

channelchannel

Call Call Establishment Establishment

Using VPsUsing VPs

Virtual Channel Connection Virtual Channel Connection UsesUses

between between end usersend users end to end user dataend to end user data VPC provides overall capacityVPC provides overall capacity

• VCC organization done by users within the capacityVCC organization done by users within the capacity

between between end user and networkend user and network control signalingcontrol signaling

between between network entitiesnetwork entities network traffic managementnetwork traffic management routingrouting

VP/VC CharacteristicsVP/VC Characteristics

quality of service quality of service - cell loss ratio, cell delay - cell loss ratio, cell delay variationvariation

switched and semi-permanent channel switched and semi-permanent channel connectionsconnections

cell cell sequence integritysequence integrity traffic traffic parameter negotiation parameter negotiation and usage and usage

monitoring - average rate, peak rate, burstiness, monitoring - average rate, peak rate, burstiness, and peak durationand peak duration

VPC onlyVPC only virtual channel identifier restriction within VPCvirtual channel identifier restriction within VPC

Fixed vs. Variable Length CellFixed vs. Variable Length Cell ## Efficiency Consideration: Efficiency Consideration: Efficiency N = Information Octets / (Information Efficiency N = Information Octets / (Information

Octets + Header Octets)Octets + Header Octets) Fixed Length Packets:Fixed Length Packets: L = Data Field Size (Octets) in a PacketL = Data Field Size (Octets) in a Packet H = Header Size (Octets)H = Header Size (Octets) X = Total Message Size (Octets)X = Total Message Size (Octets) # # Expression for N = ? (Needs ceiling function), Expression for N = ? (Needs ceiling function),

Nopt = ?Nopt = ?

Fixed vs. Variable Length CellFixed vs. Variable Length Cell # # Variable Length Packets:Variable Length Packets: Needs an additional length field,Needs an additional length field, Hv = Additional overhead octetsHv = Additional overhead octets H = Header Size (Octets)H = Header Size (Octets) X = Message Size (Octets)X = Message Size (Octets) # # Expression for N = ? (Assume Single Cell)Expression for N = ? (Assume Single Cell) # # Plot of N vs. Message Size (X = 48, 96, Plot of N vs. Message Size (X = 48, 96,

144, 192, 240), for fixed and variable length 144, 192, 240), for fixed and variable length cellscells

What size of Fixed Length What size of Fixed Length Assume that the cells are completely filled Assume that the cells are completely filled

(X / L = integer) (X / L = integer) Expression for N = ? Expression for N = ? Packetization Packetization DelayDelay = buffering bits until = buffering bits until

the entire packet is filled before the entire packet is filled before retransmissionretransmission

Expression for this Expression for this delay Ddelay D = ? (function of = ? (function of L and source data rate R)L and source data rate R)

What size of Fixed Length What size of Fixed Length Plot of D vs. Data Field Size (L = 16, 32, 64, Plot of D vs. Data Field Size (L = 16, 32, 64,

128 octets) (128 octets) (RR = 64 kbps for voice coding) = 64 kbps for voice coding)

Plot of efficiency N vs. Data Field Size (16, Plot of efficiency N vs. Data Field Size (16, 32, 64, 128 octets) 32, 64, 128 octets)

=> 48 octet provides a => 48 octet provides a trade-offtrade-off between between efficiency and delayefficiency and delay

ATM CellsATM Cells

ATM Header FieldsATM Header Fields

generic flow controlgeneric flow control Virtual path identifier - routingVirtual path identifier - routing Virtual channel identifier – end to end userVirtual channel identifier – end to end user payload type payload type (3-bits, user information, (3-bits, user information,

congestion)congestion) cell cell loss priorityloss priority header error controlheader error control

ExamplesExamples ## Consider compressed video Consider compressed video

transmission in ATM network, Cells must transmission in ATM network, Cells must pass through 5 switches. The data rate is pass through 5 switches. The data rate is 43 Mbps.43 Mbps.

- Transmission time of one cell through - Transmission time of one cell through one switch?one switch?

- Assume other cells have lower priority - Assume other cells have lower priority (but (but non-premptivenon-premptive) than cell A. What is ) than cell A. What is the the maximummaximum time from arrival at the first time from arrival at the first

ExamplesExamples Switch to the completion of transmission by Switch to the completion of transmission by

the fifth for cell A? (all other delays are the fifth for cell A? (all other delays are negligible)negligible)

- if the probability that a switch is busy is 0.6 - if the probability that a switch is busy is 0.6 and the average delay to wait for current and the average delay to wait for current transmission completion is one half a cell transmission completion is one half a cell transmission time, what is the transmission time, what is the average timeaverage time from arrival at the first Switch to the from arrival at the first Switch to the completion of transmission by the fifth for completion of transmission by the fifth for cell A?cell A?

ExamplesExamples - what is the maximum and average - what is the maximum and average

variability in encountered delay (variability in encountered delay (the jitterthe jitter)?)?

## IP datagrams are segmented into ATM IP datagrams are segmented into ATM cells and sent over the ATM network. Loss cells and sent over the ATM network. Loss of a cell means loss of entire IP packet.of a cell means loss of entire IP packet.

Pc = cell loss ratePc = cell loss rate n = number of cells for a datagramn = number of cells for a datagram Expression for Pp = datagram loss rate? Expression for Pp = datagram loss rate?

SummarySummary

Asynchronous Transfer Mode (ATM)Asynchronous Transfer Mode (ATM) architecture & logical connectionsarchitecture & logical connections ATM Cell formatATM Cell format transmission of ATM cellstransmission of ATM cells