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Network Protocols
Unit II – ATM AND BISDN
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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
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Protocol Architecture (diag)
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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
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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
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ATM Connection Relationships
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Advantages of Virtual PathsSimplified network architectureIncreased network performance and
reliabilityReduced processingShort connection setup timeEnhanced network services
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Call Establishment Using VPs
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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
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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
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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
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Control Signaling - VPCSemi-permanentCustomer controlledNetwork controlled
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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
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ATM Cell Format
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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
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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
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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
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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
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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
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Two Queue ModelTwo counters
GO_CNTR_A, GO_VALUE_A,GO_CNTR_B, GO_VALUE_B
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Header Error Control8 bit error control fieldCalculated on remaining 32 bits of headerAllows some error correction
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HEC Operation at Receiver
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Effect of Error in Cell Header
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Impact of Random Bit Errors
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Transmission of ATM Cells622.08Mbps155.52Mbps51.84Mbps25.6MbpsCell Based physical layerSDH based physical layer
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Cell Based Physical LayerNo framing imposedContinuous stream of 53 octet cellsCell delineation based on header error
control field
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Cell Delineation State Diagram
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Impact of Random Bit Errors on Cell Delineation Performance
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Acquisition Time v Bit Error Rate
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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
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STM-1 Payload for SDH-Based ATM Cell Transmission
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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)
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Real Time ServicesAmount of delayVariation of delay (jitter)
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CBRFixed data rate continuously availableTight upper bound on delayUncompressed audio and video
Video conferencing Interactive audio A/V distribution and retrieval
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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
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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
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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
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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
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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
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ATM Bit Rate Services
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Adaptation Layer ServicesHandle transmission errorsSegmentation and re-assemblyHandle lost and misinserted cellsFlow control and timing
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Supported Application typesCircuit emulationVBR voice and videoGeneral data serviceIP over ATMMultiprotocol encapsulation over ATM
(MPOA) IPX, AppleTalk, DECNET)
LAN emulation
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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
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AAL Protocols
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Segmentation and Reassembly PDU
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AAL Type 1CBR sourceSAR packs and unpacks bitsBlock accompanied by sequence number
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AAL Type 2VBRAnalog applications
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AAL Type 3/4Connectionless or connectedMessage mode or stream mode
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AAL Type 5Streamlined transport for connection
oriented higher layer protocols
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CPCS PDUs
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Example AAL 5 Transmission
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Multiplexing
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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
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FDM System
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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
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TDM System
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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
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Data Link Control on TDM
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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
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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
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TDM of Analog and Digital Sources
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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
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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
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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
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ISDN User Network InterfaceISDN allows multiplexing of devices over
single ISDN lineTwo interfaces
Basic ISDN Interface Primary ISDN Interface
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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
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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
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Frame Structure