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CS 4594 CS 4594 Broadband CommunicationsBroadband Communications

ATMATM

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ATMATM

• ATM = Asynchronous Transfer ModeATM = Asynchronous Transfer Mode

Asynchronous - in contrast to SONET/SDH in which Asynchronous - in contrast to SONET/SDH in which large frames of data are sent on a completely regular basis, large frames of data are sent on a completely regular basis, completely synchronized.completely synchronized.

• Purpose: Convey voice, video, and data service over the Purpose: Convey voice, video, and data service over the same channel. Each service has different timing same channel. Each service has different timing requirements. requirements.

• ATM was developed to meet the new telephone network ATM was developed to meet the new telephone network model called Broadband ISDN = Integrated Services model called Broadband ISDN = Integrated Services Digital NetworkDigital Network

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ATM LayersATM Layers

• ATM has three major layers: AAL, ATM, ATM has three major layers: AAL, ATM, Physical.Physical.

AAL

ATM

Physical

•AAL provides network services. Converts user data into fixed length cells (53 bytes)•ATM handles switching, traffic management, etc.•Physical connects to a variety of different media

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ATM CellsATM Cells

• ATM transports data in small packets called ATM transports data in small packets called cells.cells.

• Each cell is 53 bytes long with a 5 byte header Each cell is 53 bytes long with a 5 byte header and a 48 byte payload.and a 48 byte payload.

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ATM Cell HeaderATM Cell Header

• The ATM cell header has two different forms:The ATM cell header has two different forms:• UNI = User-Network Interface - as the cell UNI = User-Network Interface - as the cell

enters the networkenters the network• NNI = Network-Network Interface - as it NNI = Network-Network Interface - as it

moves through the network.moves through the network.

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ATM Cell Header – FieldsATM Cell Header – Fields

• Labeling – Cell IDLabeling – Cell ID• GFC = Generic Flow Control – 4 bits for UNI, not present in NNI, GFC = Generic Flow Control – 4 bits for UNI, not present in NNI,

misnamed, now used for multiplexing user channelsmisnamed, now used for multiplexing user channels

• VPI = Virtual Path Identifier – 8 bits for UNI, 12 bits for NNIVPI = Virtual Path Identifier – 8 bits for UNI, 12 bits for NNI

• VCI = Virtual Channel Identifier – 16 bitsVCI = Virtual Channel Identifier – 16 bits

• Cell ManagementCell Management• PTI = Payload Type – 3 bitsPTI = Payload Type – 3 bits

• CLP = Cell Loss PriorityCLP = Cell Loss Priority

• Error Control and FramingError Control and Framing• HEC = Header Error Control – 8 bits, CRC check on headerHEC = Header Error Control – 8 bits, CRC check on header

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ATM – Virtual CircuitsATM – Virtual Circuits

• VPI/VCI form a label for the cell that VPI/VCI form a label for the cell that determines how the cell will be handled in the determines how the cell will be handled in the next switch.next switch.

• Each switch has a number of inputs and Each switch has a number of inputs and outputs, each with a identifying number.outputs, each with a identifying number.

• When a cell enters a switch, its VPI/VCI is When a cell enters a switch, its VPI/VCI is input into a table look-up that returns the input into a table look-up that returns the output identifier and a new VPI/VCI.output identifier and a new VPI/VCI.

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ATM – Virtual Circuit TypesATM – Virtual Circuit Types

• PVC = Permanent Virtual Circuit – set up PVC = Permanent Virtual Circuit – set up manually by programming each switch along manually by programming each switch along the routethe route

• SVC = Switched Virtual Circuit – set up SVC = Switched Virtual Circuit – set up dynamically by signaling procedures.dynamically by signaling procedures.

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AAL = ATM Adaptation LayerAAL = ATM Adaptation Layer

• ALL is the top layer among ATM layersALL is the top layer among ATM layers

AAL

ATM

Physical

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Role of AALRole of AAL

• Provides interface between application layers Provides interface between application layers and ATM network itselfand ATM network itself

• Implemented in endpoint devices (at edge of Implemented in endpoint devices (at edge of network)network)

• Supports a number of different service classesSupports a number of different service classes• Shields ATM layer from knowing anything Shields ATM layer from knowing anything

about user dataabout user data

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Classical AAL ClassesClassical AAL Classes

ClassClass Bit rateBit rate timingtiming Connection Connection modemode

Class AClass A constantconstant Timing criticalTiming critical Connection Connection orientedoriented

Class BClass B variablevariable Timing criticalTiming critical Connection Connection orientedoriented

Class CClass C variablevariable Timing tolerantTiming tolerant Connection Connection orientedoriented

Class DClass D variablevariable Timing tolerantTiming tolerant Connection lessConnection less

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Modern Service CategoriesModern Service Categories

• CBR = Constant Bit RateCBR = Constant Bit Rate• rt-VBR = Real Time Variable Bit Ratert-VBR = Real Time Variable Bit Rate• nrt-VBR = Non Real Time Variable Bit Ratenrt-VBR = Non Real Time Variable Bit Rate• ABR = Available Bit RateABR = Available Bit Rate• UBR = Unspecified Bit RateUBR = Unspecified Bit Rate• GFR = Guaranteed Frame RateGFR = Guaranteed Frame Rate

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Traffic ContractTraffic Contract

• Agreement between user and networkAgreement between user and network• ContainsContains

• Traffic descriptors – specifying ATM cell flowTraffic descriptors – specifying ATM cell flow

• QoS parameters – specifing timing and error qualitiesQoS parameters – specifing timing and error qualities

• Conformance checking rulesConformance checking rules

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Traffic ParametersTraffic Parameters

• PCR = Peak Cell RatePCR = Peak Cell Rate• SCR = Sustainable Cell RateSCR = Sustainable Cell Rate• MBS = Maximum Burst SizeMBS = Maximum Burst Size• MCR = Minimum Cell RateMCR = Minimum Cell Rate

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QoS ParametersQoS Parameters

• TimingTiming• Cell Transfer Delay (CTD)Cell Transfer Delay (CTD)

• Cell Delay Variation (CDV)Cell Delay Variation (CDV)

• ErrorsErrors• NonegotiableNonegotiable

• Cell Error Ratio (CER)Cell Error Ratio (CER)

• Cell Misinsertion Rate (CMR)Cell Misinsertion Rate (CMR)

• Severely-Errored Cell Block Ratio (SECBR)Severely-Errored Cell Block Ratio (SECBR)

• NegotiableNegotiable• Cell Loss Ratio (CLR)Cell Loss Ratio (CLR)

• Cell Misinsertion Rate (CMR)Cell Misinsertion Rate (CMR)

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AAL SublayersAAL Sublayers

Service specific convergence sublayer (SSCS)

Common part convergence sublayer (SSCS)

Segmentation and reassembly sublayer (SAR)

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Sublayers of AALSublayers of AAL

• CS = Convergence Sublayer (CS)CS = Convergence Sublayer (CS)• Performs functions such as end-to-end synchronization, Performs functions such as end-to-end synchronization,

traffic control and monitoring, cell loss controltraffic control and monitoring, cell loss control

• SublayersSublayers• Service Specific (SSCP) – application dependentService Specific (SSCP) – application dependent

• Common Part (CPCS) – common to all users of a serviceCommon Part (CPCS) – common to all users of a service

• Segmentation and Reassembly (SAR)Segmentation and Reassembly (SAR)• Performs repackaging between convergence layer Performs repackaging between convergence layer

packets and ATM cellspackets and ATM cells

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AAL TypesAAL Types

• Designed to handle the different service typesDesigned to handle the different service types• Specify architectural details, traffic parameters, Specify architectural details, traffic parameters,

packet formats and procedurespacket formats and procedures

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AAL-0AAL-0

• UnofficialUnofficial• Designed to send ATM cells directly from Designed to send ATM cells directly from

ATM-cell based application to ATM cellATM-cell based application to ATM cell• Has empty CS and SARHas empty CS and SAR

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AAL-0 OperationsAAL-0 Operations

Higher Layers ATM Cell ATM Cell ATM Cell ATM Cell

AAL Layers

ATM Layer ATM Cell ATM Cell ATM Cell ATM Cell

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AAL-1AAL-1

• Designed for Class A= CBRDesigned for Class A= CBR• Transports a constant stream of bits with timingTransports a constant stream of bits with timing• Applications: circuit emulation, uncompressed voice and Applications: circuit emulation, uncompressed voice and

video.video.• May handle:May handle:

  segmentation and reassembly (SAR)segmentation and reassembly (SAR)  handling cell delay variation (buffering)handling cell delay variation (buffering)  handling cell delayhandling cell delay  handling lost and mininserted cellshandling lost and mininserted cells  clock managementclock management  recovery of source data framingrecovery of source data framing  monitoring and correction of error in user datamonitoring and correction of error in user data 

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AAL-1 PDUAAL-1 PDU

• SAR headerSAR header• SN (4 bits): sequence numberSN (4 bits): sequence number

• CSI (1 bit): Convergence layer indicator (can be used for clock)CSI (1 bit): Convergence layer indicator (can be used for clock)

• Sequence number (3 bits)Sequence number (3 bits)

• SNP (4 bits): sequence number protectionSNP (4 bits): sequence number protection• 3-bit CRC3-bit CRC

• 1 bit parity1 bit parity

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AAL-1 OperationsAAL-1 Operations

Higher Layers Bit Stream

AAL Layers

ATM Layer

47 47 47 47(bytes)

H1 47

H1 47

H1 47

H1 47

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

SAR

CS

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AAL-2AAL-2

• Designed to handle Class B, rt-VBRDesigned to handle Class B, rt-VBR• Transfers multiplexed streams of bits and Transfers multiplexed streams of bits and

timingtiming• Handles errors or indicates when errors have Handles errors or indicates when errors have

occurredoccurred• Two layers: packet and PDU (cell)Two layers: packet and PDU (cell)

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AAL-2 OperationsAAL-2 Operations

Higher Layers User 1

AAL 2 Layers

ATM Layer ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

User 2 User 3 User 1

STF

1 47

STF

1 47

STF

1 47

STF

1 47

H3

H3

H3

H3

packet

PDU

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AAL-2 CPS PacketAAL-2 CPS Packet

• CPS-PH (3 octets) common part sublayer CPS-PH (3 octets) common part sublayer packet headerpacket header• CID (8 bits): channel id for multiplexingCID (8 bits): channel id for multiplexing

• LI (6 bits): length indicator for number of octets in LI (6 bits): length indicator for number of octets in variable payload structuresvariable payload structures

• UUI (5 bits): user-to-user informationUUI (5 bits): user-to-user information

• HEC (5 bits): error control for packet headerHEC (5 bits): error control for packet header

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AAL-2 CS-PDUAAL-2 CS-PDU

• STF (8 bits): start fieldSTF (8 bits): start field• OSF (6 bits): offsetOSF (6 bits): offset

• SN/P (2 bits): error correction and detection for start SN/P (2 bits): error correction and detection for start fieldfield

• Payload data (47 bytes)Payload data (47 bytes)• PAD (padding as needed for real-time)PAD (padding as needed for real-time)

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AAL-3/4AAL-3/4

• Designed for Class C and Class D (nrt-VBR) connection and Designed for Class C and Class D (nrt-VBR) connection and connectionlessconnectionless

• Can do multicasting (network-based distribution)Can do multicasting (network-based distribution)• Can do multiplexing (multiple streams through same service)Can do multiplexing (multiple streams through same service)• Designed to be highly reliableDesigned to be highly reliable

• CRC check on cellsCRC check on cells• SAR supportSAR support

• SequencingSequencing• Mux id Mux id

• Beginning and end tagsBeginning and end tags• HECHEC

• Multiple modes of operation (assured and non assured delivery)Multiple modes of operation (assured and non assured delivery)

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AAL-3/4 OperationsAAL-3/4 Operations

Higher Layers packet

AAL Layers

ATM Layer

75 75 75(bytes)

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

CPCS

SSCS

packet packet

SAR

H4 75

P5

T4

H4 75

P5

T4

H4 75

P5

T4

H

48

22 44

T H

48

22 44

T H

48

22 44

TH

48

22 44

T

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AAL-3 structuresAAL-3 structures

CPI BT BA size User Data PAD AL ET Length

Header (1+1+2) Trailer (1+1+2)

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AAL-3/4 CPCS-PDUAAL-3/4 CPCS-PDU

• Header (4 octets)Header (4 octets)• CPI (1 octets): Common Part Indicator – defines how the other CPI (1 octets): Common Part Indicator – defines how the other

parts are encodedparts are encoded• BT (1 octet): Beginning TagBT (1 octet): Beginning Tag• BA size (1 octet): Buffer Allocation sizeBA size (1 octet): Buffer Allocation size

• User Data (1-65,535 octets)User Data (1-65,535 octets)• PAD: padding to keep PDU length a multiple of 4 PAD: padding to keep PDU length a multiple of 4

octetsoctets• Trailer (4 octets)Trailer (4 octets)

• AL (1 octet): Alignment to make CPS trailer 4 octetsAL (1 octet): Alignment to make CPS trailer 4 octets• ET (1 octet): Ending TagET (1 octet): Ending Tag• Length (2 octets) of entire PDULength (2 octets) of entire PDU

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AAL-3/4 SAR-PDUAAL-3/4 SAR-PDU

• Header (2 octets)Header (2 octets)• ST (2 bits): Segment Type: BOM 10, COM 00, EOM 01ST (2 bits): Segment Type: BOM 10, COM 00, EOM 01

• SN (4 bits): Sequence NumberSN (4 bits): Sequence Number

• MUX-ID (10 bits): multiplexing idMUX-ID (10 bits): multiplexing id

• SAR-PDU Payload (44 octets) segment of CPCS-SAR-PDU Payload (44 octets) segment of CPCS-PDUPDU

• Trailer (2 octets)Trailer (2 octets)• LI (6 bits): length indicatorLI (6 bits): length indicator

• CRC (10 bits): CRC check on segmentCRC (10 bits): CRC check on segment

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AAL-5AAL-5

• Designed for Class C and Class D (nrt-VBR) Designed for Class C and Class D (nrt-VBR) connection and connectionlessconnection and connectionless

• Simplified (lightweight) version of AAL-3/4Simplified (lightweight) version of AAL-3/4• No overhead in cellsNo overhead in cells

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AAL-5 OperationsAAL-5 Operations

Higher Layers packet

AAL Layers

ATM Layer

75 75 75(bytes)

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

ATM CellH

5 48

CPCS

SSCS

packet packet

SAR

75

P13

T8

48

48

75

P13

T8 75

P13

T8

48

48

48

48

48

48

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AAL-5 SDUsAAL-5 SDUs

• CPCS-PDUCPCS-PDU• User Data: 1-65,535 octetsUser Data: 1-65,535 octets

• PAD: 0-47 octetsPAD: 0-47 octets

• CPCS-PDU trailerCPCS-PDU trailer• CPCS UU: 1 octetCPCS UU: 1 octet

• CPI: 1 octetCPI: 1 octet

• CPCS-PDU length: 2 octetsCPCS-PDU length: 2 octets

• CRC: 4 octetsCRC: 4 octets