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ECS5365 Lecture 5 ATM Protocols and Networks Philip Branch Centre for Telecommunications and Information Engineering (CTIE) Monash University 1 Copyright © Monash University ATM Protocols and Networks Overview • • • • • • ATM Virtual Circuits ATM Cell Services ATM Adaptation Layers Examples 2 Copyright © Monash University 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 3 Copyright © Monash University 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 Copyright © Monash University 4 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 5 Copyright © Monash University Virtual Circuits in ATM • Non Broadcast Multiple Access (NBMA) • Virtual circuits across same medium • Order preserving – simplifies switch design 6 Copyright © Monash University Types of Virtual Circuits • Permanent Virtual Circuits – Established once manually • Semi-permanent Virtual Circuits – Time based • Switched Virtual Circuits – On demand 7 Copyright © Monash University The ATM connection hierarchy • Virtual Channel • Virtual Path • Physical Medium 8 Copyright © Monash University 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 9 Copyright © Monash University Tunnelling SVCs through PVPs • Carriers may offer only Permanent Virtual Paths • Can ‘tunnel’ Switched Virtual Channels through them 10 Copyright © Monash University ATM Layer Functions • Cell multiplexing and demultiplexing • Virtual Path and Virtual Circuit Identifier translation • Cell header generation and extraction 11 Copyright © Monash University 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 12 Copyright © Monash University 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 13 Copyright © Monash University Generic Flow Control • • • • Present in UNI but not in NNI Use not fully specified Intended for priority scheme Rarely used 14 Copyright © Monash University 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 15 Copyright © Monash University 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 16 Copyright © Monash University 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 17 Copyright © Monash University 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 18 Copyright © Monash University Problems with ATM UNI Header • Generic Flow Control – Better done at higher layer • GFC limits number of VCI values • User-network interface and networknetwork interface distinction artificial 19 Copyright © Monash University 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 Copyright © Monash University 20 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 21 Copyright © Monash University ATM Services • Constant Bit Rate (CBR) • Variable Bit Rate (VBR) – real time – non-realtime • Available Bit Rate (ABR) • Unspecified Bit Rate (UBR) 22 Copyright © Monash University Constant Bit Rate • Circuit emulation – voice, H.320 Videoconferencing • Parameters – Peak Cell Rate – Cell Delay Variation Tolerance – Quality of service parameters 23 Copyright © Monash University 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 24 Copyright © Monash University 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 25 Copyright © Monash University Unspecified Bit Rate • No parameters specified • No QoS guarantees • ATM Forum only (ITU-T not defined) – VBR with SCR 0 and CLP 1 26 Copyright © Monash University 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 27 Copyright © Monash University 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 28 Copyright © Monash University 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 29 Copyright © Monash University 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 30 Copyright © Monash University AAL3/4 • Variable bit rate services • 2 bytes per cell header – type – sequence number – Multiplexing ID • 2 bytes per cell trailer – Length – CRC 31 Copyright © Monash University 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 32 Copyright © Monash University 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 33 Copyright © Monash University 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. 34 Copyright © Monash University 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 35 Copyright © Monash University 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 36 Copyright © Monash University 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 37 Copyright © Monash University Summary • • • • • • ATM Virtual Circuits ATM Cell Services ATM Adaptation Layers Examples 38 Copyright © Monash University 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? 39 Copyright © Monash University