ECS5365 Lecture 5 ATM Protocols and Networks
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Transcript of ECS5365 Lecture 5 ATM Protocols and Networks
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ECS5365 Lecture 5ATM Protocols and Networks
Philip BranchCentre for Telecommunications and Information
Engineering (CTIE)
Monash University
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ATM Protocols and Networks Overview
• ATM
• Virtual Circuits
• ATM Cell
• Services
• ATM Adaptation Layers
• Examples
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Circuit Switching vs Packet Switching
• Circuit switching– fixed delay
– low latency
– bandwidth allocation difficult
– signalling slow
– excellent for voice
– not adaptable for bursty traffic
• Packet switching– delay is variable
– latency is high
– bandwidth allocation flexible
– signalling simple
– difficult for voice
– very flexible for bursty traffic
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Asynchronous and Synchronous Time Division Multiplexing
• Synchronous Time Division Multiplexing– eg narrow band ISDN– fixed assignment of slots to channels in frames– position specifies channel
• Asynchronous Time Division Multiplexing– ATM– no fixed assignment of slots to channels in frames– tag needed to specify channel
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ATM
• Asynchronous Transfer Mode– Also: Asynchronous Time Division Multiplexing
• Connection Oriented Fast Packet Switching• Small header
– Mostly next hop information
• Fixed size information field• Bandwidth flexibility, rather then efficiency
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Virtual Circuits in ATM
• Non Broadcast Multiple Access (NBMA)
• Virtual circuits across same medium
• Order preserving– simplifies switch design
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Types of Virtual Circuits
• Permanent Virtual Circuits– Established once manually
• Semi-permanent Virtual Circuits– Time based
• Switched Virtual Circuits– On demand
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The ATM connection hierarchy
• Virtual Channel
• Virtual Path
• Physical Medium
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Switching on Virtual Paths and Channels
• Multiple Virtual Channels can be bundled into a single Virtual Path
• Switching can be on path or circuit
• Useful for interconnection via second party
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Tunnelling SVCs through PVPs
• Carriers may offer only Permanent Virtual Paths
• Can ‘tunnel’ Switched Virtual Channels through them
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ATM Layer Functions
• Cell multiplexing and demultiplexing
• Virtual Path and Virtual Circuit Identifier translation
• Cell header generation and extraction
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ATM Cells
• 5 Byte header– mostly next hop information
• 48 Byte Payload– Compromise between 32 and 64 bytes
• Different format for UNI and for NNI– UNI has GFC field
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Fields in an ATM Cell
• GFC - Generic Flow Control
• VPI - Virtual Path Indicator
• VCI - Virtual Connection Indicator
• PTI - Payload Type Indicator
• CLP- Cell Loss Priority
• HEC- Header Error Check
• Payload
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Generic Flow Control
• Present in UNI but not in NNI
• Use not fully specified
• Intended for priority scheme
• Rarely used
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VPI / VCI
• Virtual Path Identifier / Virtual Channel Identifier
• Local only to the switch– Will change as cell passes through switch
• Index into lookup tables setup at connection time
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Payload Type
• 3 bits
• bit 1 – 0 = user cell– 1 = management cell
• bit 3 in user cells– signalling bit– used to signal end of datagram in AAL5
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Cell Loss Priority
• 1 bit
• Switch must drop CLP=1 cells before CLP=0 cells
• Can be set by network– non-conforming cells
• Can be set by application– lower priority cells
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Header CRC
• Cyclic Redundancy Check
• Calculated over 5 byte cell header
• Can correct single bit and detect large class of multiple bit errors
• Recalculated at each hop in the ATM network
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Problems with ATM UNI Header
• Generic Flow Control– Better done at higher layer
• GFC limits number of VCI values
• User-network interface and network-network interface distinction artificial
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Quality of Service
– Traffic parameters• Peak Cell Rate
• Sustainable Cell Rate
• Maximum Burst Size
• Minimum Cell Rate (ABR only)
• Cell Delay Variation Tolerance
– Negotiated Quality of Service Parameters• Cell Loss Ratio
• Cell Delay
• cell errors, cell misinsertions, block errors
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Classes of Service Defined in UNI 3.1
• Class A - constant bit rate
• Class B - variable bit rate, real time
• Class C - connection oriented data
• Class D - connectionless data
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ATM Services
• Constant Bit Rate (CBR)
• Variable Bit Rate (VBR)– real time – non-realtime
• Available Bit Rate (ABR)
• Unspecified Bit Rate (UBR)
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Constant Bit Rate
• Circuit emulation– voice, H.320 Videoconferencing
• Parameters– Peak Cell Rate– Cell Delay Variation Tolerance– Quality of service parameters
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Variable Bit Rate
• Variable encoded video and voice (rt)
• Bursty data (nrt)
• Parameters– Peak Cell Rate– Cell Delay Variation Tolerance– Sustained Cell Rate (rt only)– Maximum Burst Size (rt only)– Quality of service parameters
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Available Bit Rate
• Uses feedback for congestion control– Resource management cells
• Used mostly for TCP/IP data
• Parameters– Peak Cell Rate– Minimum Cell Rate– Cell Loss Ratio
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Unspecified Bit Rate
• No parameters specified
• No QoS guarantees
• ATM Forum only (ITU-T not defined)– VBR with SCR 0 and CLP 1
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ATM Adaptation Layer
• Adapts service to ATM cell transport
• Maps AAL Service Data Units to Cells
• Originally one AAL per class of service
• Now AAL independent of class of service
• One AAL can support more than one class of service
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ATM Adaptation Layer Functions (AAL)
• Two sublayers– Convergence sublayer (CS)– Segmentation and reassembly sublayer (SAR)
• CS handles flow of data to and from SAR– deals with cell delay variation– not really necessary for a separate layer
• SAR breaks data into cells at sender and reassembles them at receiver
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AALs
• AAL1 - constant bit rate service
• AAL3/4 - connectionless data based on DQDB protocol
• AAL5 - simple adaptation for connection oriented traffic
• AAL1 and AAL5 widely used
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AAL1
• Constant bit rate services
• Uses 1 byte per cell from the payload for AAL Service Data Unit information– convergence sublayer indicator– sequence count– CRC– parity
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AAL3/4
• Variable bit rate services
• 2 bytes per cell header– type– sequence number– Multiplexing ID
• 2 bytes per cell trailer– Length– CRC
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Problems with AAL3/4
• Wasteful– 44 bytes data / 53 byte cell (17% overhead)
• Process data a cell at a time– examine type to identify end of packet
• Complex to implement
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AAL5
• Most commonly used AAL– video– data
• Uses full 48 bytes per cell for data– efficient use of cell space
• End of PDU indicated in cell header– PTI indicator
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Early Packet Discard
• Available in AAL5
• Uses PTI indicator
• If switch drops part of packet– overflow, error
• Then switch drops rest of packet
• Prevents transmission of cells that will be retransmitted.
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Information Transmission in an ATM Network
• Connection set up (Signalling)– routing done at connection setup time– resources allocated within switches– VPI/VCI translation tables set up
• Information flow– adaptation of higher layer to cells– switching of cells based on VPI/VCI
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Example: Circuit Emulation
• Traffic class A
• AAL1
• Constant bit rate connection
• CS layer packs frames into SAR-PDU
• SAR layer prefixes header– sequence number and check sum
• ATM layer generates cells from SAR-PDU
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Example: IP Packets over ATM using AAL5
• Traffic class D
• UBR or ABR
• CS layer segments IP packet into 48 byte SAR-PDU payloads
• SAR layer presents SAR-PDU payloads to ATM layer
• ATM layer generates cells
• Sets PTI indicator for end of PDU
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Summary
• ATM
• Virtual Circuits
• ATM Cell
• Services
• ATM Adaptation Layers
• Examples
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Review Questions
– ATM has been attacked as inefficient, since it has a large cell header. What is the transmission efficiency of ATM?
– Write pseudo-code describing an algorithm to implement early packet discard.
– Why does the header CRC need to be recalculated at each hop in the ATM network?