ECS5365 Lecture 5 ATM Protocols and Networks

1. ECS5365 Lecture 5ATM Protocols and Networks Philip Branch Centre for Telecommunications and Information Engineering (CTIE) Monash University

2. ATM Protocols and Networks Overview • ATM • Virtual Circuits • ATM Cell • Services • ATM Adaptation Layers • Examples

3. 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 Circuit Switching vs Packet Switching

4. 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

5. 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

6. Virtual Circuits in ATM • Non Broadcast Multiple Access (NBMA) • Virtual circuits across same medium • Order preserving • simplifies switch design

7. Types of Virtual Circuits • Permanent Virtual Circuits • Established once manually • Semi-permanent Virtual Circuits • Time based • Switched Virtual Circuits • On demand

8. The ATM connection hierarchy • Virtual Channel • Virtual Path • Physical Medium

9. 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

10. Tunnelling SVCs through PVPs • Carriers may offer only Permanent Virtual Paths • Can ‘tunnel’ Switched Virtual Channels through them

11. ATM Layer Functions • Cell multiplexing and demultiplexing • Virtual Path and Virtual Circuit Identifier translation • Cell header generation and extraction

12. 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

13. 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

14. Generic Flow Control • Present in UNI but not in NNI • Use not fully specified • Intended for priority scheme • Rarely used

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. ATM Services • Constant Bit Rate (CBR) • Variable Bit Rate (VBR) • real time • non-realtime • Available Bit Rate (ABR) • Unspecified Bit Rate (UBR)

23. Constant Bit Rate • Circuit emulation • voice, H.320 Videoconferencing • Parameters • Peak Cell Rate • Cell Delay Variation Tolerance • Quality of service parameters

24. 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

25. 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

26. Unspecified Bit Rate • No parameters specified • No QoS guarantees • ATM Forum only (ITU-T not defined) • VBR with SCR 0 and CLP 1

27. 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

28. 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

29. 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

30. 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

31. AAL3/4 • Variable bit rate services • 2 bytes per cell header • type • sequence number • Multiplexing ID • 2 bytes per cell trailer • Length • CRC

32. 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

33. 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

34. 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.

35. 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

36. 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

37. 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

38. Summary • ATM • Virtual Circuits • ATM Cell • Services • ATM Adaptation Layers • Examples

39. 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?