Network Protocols

Unit II – ATM AND BISDN

Protocol ArchitectureSimilarities between ATM and packet

switching Transfer of data in discrete chunks Multiple logical connections over single physical

interface

In ATM flow on each logical connection is in fixed sized packets called cells

Minimal error and flow control Reduced overhead

Data rates (physical layer) 25.6Mbps to 622.08Mbps

Protocol Architecture (diag)

Reference Model PlanesUser plane

Provides for user information transfer

Control plane Call and connection control

Management plane Plane management

whole system functions

Layer managementResources and parameters in protocol entities

ATM Logical ConnectionsVirtual channel connections (VCC)Analogous to virtual circuit in X.25Basic unit of switchingBetween two end usersFull duplexFixed size cellsData, user-network exchange (control) and

network-network exchange (network management and routing)

Virtual path connection (VPC) Bundle of VCC with same end points

ATM Connection Relationships

Advantages of Virtual PathsSimplified network architectureIncreased network performance and

reliabilityReduced processingShort connection setup timeEnhanced network services

Call Establishment Using VPs

Virtual Channel Connection UsesBetween end users

End to end user data Control signals VPC provides overall capacity

VCC organization done by users

Between end user and network Control signaling

Between network entities Network traffic management Routing

VP/VC CharacteristicsQuality of serviceSwitched and semi-permanent channel

connectionsCall sequence integrityTraffic parameter negotiation and usage

monitoring

VPC only Virtual channel identifier restriction within VPC

Control Signaling - VCCDone on separate connectionSemi-permanent VCCMeta-signaling channel

Used as permanent control signal channel

User to network signaling virtual channel For control signaling Used to set up VCCs to carry user data

User to user signaling virtual channel Within pre-established VPC Used by two end users without network intervention

to establish and release user to user VCC

Control Signaling - VPCSemi-permanentCustomer controlledNetwork controlled

ATM CellsFixed size5 octet header48 octet information fieldSmall cells reduce queuing delay for high

priority cellsSmall cells can be switched more

efficientlyEasier to implement switching of small

cells in hardware

ATM Cell Format

Header FormatGeneric flow control

Only at user to network interface Controls flow only at this point

Virtual path identifierVirtual channel identifierPayload type

e.g. user info or network management

Cell loss priorityHeader error control

Generic Flow Control (GFC)Control traffic flow at user to network interface

(UNI) to alleviate short term overloadTwo sets of procedures

Uncontrolled transmission Controlled transmission

Every connection either subject to flow control or not

Subject to flow control May be one group (A) default May be two groups (A and B)

Flow control is from subscriber to network Controlled by network side

Single Group of Connections (1)Terminal equipment (TE) initializes two

variables TRANSMIT flag to 1 GO_CNTR (credit counter) to 0

If TRANSMIT=1 cells on uncontrolled connection may be sent any time

If TRANSMIT=0 no cells may be sent (on controlled or uncontrolled connections)

If HALT received, TRANSMIT set to 0 and remains until NO_HALT

Single Group of Connections (2)If TRANSMIT=1 and no cell to transmit on

any uncontrolled connection: If GO_CNTR>0, TE may send cell on controlled

connectionCell marked as being on controlled connectionGO_CNTR decremented

If GO_CNTR=0, TE may not send on controlled connection

TE sets GO_CNTR to GO_VALUE upon receiving SET signal Null signal has no effect

Use of HALTTo limit effective data rate on ATMShould be cyclicTo reduce data rate by half, HALT issued

to be in effect 50% of timeDone on regular pattern over lifetime of

connection

Two Queue ModelTwo counters

GO_CNTR_A, GO_VALUE_A,GO_CNTR_B, GO_VALUE_B

Header Error Control8 bit error control fieldCalculated on remaining 32 bits of headerAllows some error correction

HEC Operation at Receiver

Effect of Error in Cell Header

Impact of Random Bit Errors

Transmission of ATM Cells622.08Mbps155.52Mbps51.84Mbps25.6MbpsCell Based physical layerSDH based physical layer

Cell Based Physical LayerNo framing imposedContinuous stream of 53 octet cellsCell delineation based on header error

control field

Cell Delineation State Diagram

Impact of Random Bit Errors on Cell Delineation Performance

Acquisition Time v Bit Error Rate

SDH Based Physical LayerImposes structure on ATM streame.g. for 155.52MbpsUse STM-1 (STS-3) frameCan carry ATM and STM payloadsSpecific connections can be circuit

switched using SDH channelSDH multiplexing techniques can combine

several ATM streams

STM-1 Payload for SDH-Based ATM Cell Transmission

ATM Service CategoriesReal time

Constant bit rate (CBR) Real time variable bit rate (rt-VBR)

Non-real time Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR)

Real Time ServicesAmount of delayVariation of delay (jitter)

CBRFixed data rate continuously availableTight upper bound on delayUncompressed audio and video

Video conferencing Interactive audio A/V distribution and retrieval

rt-VBRTime sensitive application

Tightly constrained delay and delay variation

rt-VBR applications transmit at a rate that varies with time

e.g. compressed video Produces varying sized image frames Original (uncompressed) frame rate constant So compressed data rate varies

Can statistically multiplex connections

nrt-VBRMay be able to characterize expected

traffic flowImprove QoS in loss and delayEnd system specifies:

Peak cell rate Sustainable or average rate Measure of how bursty traffic is

e.g. Airline reservations, banking transactions

UBRMay be additional capacity over and

above that used by CBR and VBR traffic Not all resources dedicated Bursty nature of VBR

For application that can tolerate some cell loss or variable delays e.g. TCP based traffic

Cells forwarded on FIFO basisBest efforts service

ABRApplication specifies peak cell rate (PCR)

and minimum cell rate (MCR)Resources allocated to give at least MCRSpare capacity shared among all ARB

sourcese.g. LAN interconnection

ATM Adaptation LayerSupport for information transfer protocol

not based on ATMPCM (voice)

Assemble bits into cells Re-assemble into constant flow

IP Map IP packets onto ATM cells Fragment IP packets Use LAPF over ATM to retain all IP

infrastructure

ATM Bit Rate Services

Adaptation Layer ServicesHandle transmission errorsSegmentation and re-assemblyHandle lost and misinserted cellsFlow control and timing

Supported Application typesCircuit emulationVBR voice and videoGeneral data serviceIP over ATMMultiprotocol encapsulation over ATM

(MPOA) IPX, AppleTalk, DECNET)

LAN emulation

AAL ProtocolsConvergence sublayer (CS)

Support for specific applications AAL user attaches at SAP

Segmentation and re-assembly sublayer (SAR) Packages and unpacks info received from CS into

cells

Four types Type 1 Type 2 Type 3/4 Type 5

AAL Protocols

Segmentation and Reassembly PDU

AAL Type 1CBR sourceSAR packs and unpacks bitsBlock accompanied by sequence number

AAL Type 2VBRAnalog applications

AAL Type 3/4Connectionless or connectedMessage mode or stream mode

AAL Type 5Streamlined transport for connection

oriented higher layer protocols

CPCS PDUs

Example AAL 5 Transmission

Multiplexing

Frequency Division MultiplexingFDMUseful bandwidth of medium exceeds

required bandwidth of channelEach signal is modulated to a different

carrier frequencyCarrier frequencies separated so signals

do not overlap (guard bands)e.g. broadcast radioChannel allocated even if no data

FDM System

Synchronous Time Division MultiplexingData rate of medium exceeds data rate of

digital signal to be transmittedMultiple digital signals interleaved in timeMay be at bit level of blocksTime slots preassigned to sources and

fixedTime slots allocated even if no dataTime slots do not have to be evenly

distributed amongst sources

TDM System

TDM Link ControlNo headers and tailersData link control protocols not neededFlow control

Data rate of multiplexed line is fixed If one channel receiver can not receive data, the

others must carry on The corresponding source must be quenched This leaves empty slots

Error control Errors are detected and handled by individual

channel systems

Data Link Control on TDM

FramingNo flag or SYNC characters bracketing

TDM framesMust provide synchronizing mechanismAdded digit framing

One control bit added to each TDM frameLooks like another channel - “control channel”

Identifiable bit pattern used on control channel e.g. alternating 01010101…unlikely on a data

channel Can compare incoming bit patterns on each

channel with sync pattern

Pulse StuffingProblem - Synchronizing data sourcesClocks in different sources driftingData rates from different sources not

related by simple rational numberSolution - Pulse Stuffing

Outgoing data rate (excluding framing bits) higher than sum of incoming rates

Stuff extra dummy bits or pulses into each incoming signal until it matches local clock

Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

TDM of Analog and Digital Sources

Digital Carrier SystemsHierarchy of TDMUSA/Canada/Japan use one systemITU-T use a similar (but different) systemUS system based on DS-1 formatMultiplexes 24 channelsEach frame has 8 bits per channel plus

one framing bit193 bits per frame

Digital Carrier Systems (2)For voice each channel contains one word of

digitized data (PCM, 8000 samples per sec) Data rate 8000x193 = 1.544Mbps Five out of six frames have 8 bit PCM samples Sixth frame is 7 bit PCM word plus signaling bit Signaling bits form stream for each channel

containing control and routing info

Same format for digital data 23 channels of data

7 bits per frame plus indicator bit for data or systems control

24th channel is sync

Mixed DataDS-1 can carry mixed voice and data

signals24 channels usedNo sync byteCan also interleave DS-1 channels

Ds-2 is four DS-1 giving 6.312Mbps

ISDN User Network InterfaceISDN allows multiplexing of devices over

single ISDN lineTwo interfaces

Basic ISDN Interface Primary ISDN Interface

Basic ISDN Interface (1)Digital data exchanged between subscriber

and NTE - Full DuplexSeparate physical line for each directionPseudoternary coding scheme

1=no voltage, 0=positive or negative 750mV +/-10%

Data rate 192kbpsBasic access is two 64kbps B channels and

one 16kbps D channelThis gives 144kbps multiplexed over 192kbpsRemaining capacity used for framing and sync

Basic ISDN Interface (2)B channel is basic iser channelDataPCM voiceSeparate logical 64kbps connections o

different destinationsD channel used for control or data

LAPD frames

Each frame 48 bits longOne frame every 250s

Frame Structure