ATM Adaptation Layers

1. ATM Adaptation Layers

2. A Description of ATM • ATM is a connection-oriented, packet-like switching and multiplexing principle • Three layers in the B-ISDN protocol reference model • Physical Layer • ATM Layer • ATM Adaptation layer

3. Three layers in the B-ISDN protocol reference model

4. ATM Cells From Packet

5. Original Classes of Traffic

6. Traffic Demands on Network

7. ATM Service Categories • Real 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)

8. Real Time Services • Amount of delay • Variation of delay (jitter)

9. CBR • Fixed data rate continuously available • Tight upper bound on delay • Uncompressed audio and video • Video conferencing • Interactive audio • A/V distribution and retrieval

10. rt-VBR • Time 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

11. nrt-VBR • May be able to characterize expected traffic flow • Improve QoS in loss and delay • End system specifies: • Peak cell rate • Sustainable or average rate • Measure of how bursty traffic is • e.g. Airline reservations, banking transactions

12. UBR • May 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 basis • Best efforts service

13. ABR • Application specifies peak cell rate (PCR) and minimum cell rate (MCR) • Resources allocated to give at least MCR • Spare capacity shared among all ABR sources • e.g. LAN interconnection

14. ATM Adaptation Layer • OUTPUT • Encapsulates outgoing data in frames • Segment frames into cell payloads and pass these to the ATM layer for transmission on a specified connection • INPUT • Reassemble cell payloads arriving on a connection into frames • Pass frames up to the higher layer

15. AAL Protocols

16. ATM Adaptation Layer • Support for information transfer protocol not based on ATM • PCM (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

17. ATM Adaptation Layer • How to break messages to cells • Segmentation and Reassembly • Convergence sublayer: Defines services AAL provides to higher layers. • CS is broken into two parts: • Service Specific Convergence Sublayer (SSCS) • Specific to video service, CBR, etc. • SSCS of AAL5 is empty. • Common Part Convergence Sublayer (CPCS)

18. Convergence Sublayer • This layer wraps the user-service data units in a header and trailer which contain information used to provide the services required. • The information in the header and trailer depends on the class of information to be transported but will usually contain error handling and data priority preservation information.

19. Segmentation and reassembly sublayer • This layer receives the convergence sublayer protocol data unit and divides it up into pieces which it can place in an ATM cell. • It adds to each piece a header which contains information used to reassemble the pieces at the destination.

20. ATM Adaptation Layer Parts

21. Adaptation Layer Services • Handle transmission errors • Segmentation and re-assembly • Handle lost and misinserted cells • Flow control and timing

22. Supported Application types • Circuit emulation • VBR voice and video • General data service • IP over ATM • Multiprotocol encapsulation over ATM (MPOA) • IPX, AppleTalk, DECNET) • LAN emulation

23. AAL Protocols • Convergence 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

24. ATM Reference Model - ATM Adaptation Layer (AAL) Two Sublayers: • Convergence Sublayer (CS) • Segmentation and Reassembly (SAR) ATM Adaptation Layer (AAL) ATM Layer Physical Layer

25. AAL ATM Adaptation Layer (AAL) S A R C S ATM Layer Physical Layer ATM Adaptation Layer—AAL AAL = CS + SAR • CS — packet <-> cell • SAR — cell tax

26. AAL1 • AAL-1 is used to transfer constant bit rate data which is time dependent. It must therefore send timing information with the data so that the time dependency maybe recovered. • AAL-1 provides error recovery and indicates errored information which could not be recovered.

27. AAL1 • SAR packs and unpacks bits • Misordering bad  Sequence number • Convergence Sublayer Indication (CSI): Two uses • CSI bits from 4 successive cells = Synchronous Residual timestamp for clock recovery • For structured data transfers:: CSI = 1  8-bit pointer to first byte of payload, 0 no pointer for partially filled cells

28. AAL1

29. AAL2 • Ideal for low bit rate voice • Designed for VBR Video/Audio • Compression and Silence suppression • Idle channel suppression • Under development. One proposal above. • CRC is used for error correction and detection

30. AAL 3/4 • Designed for Data (3 and 4 were merged) • Connectionless or Connection Oriented: • Connectionless PDUs are handled independently • Connection-oriented PDUs may be multiplexed - up to 210 logical connections per VC • Message or Streaming Mode: • Message-oriented protocols provide blocks of data • Stream-oriented protocols provide a continuous stream of data presented in fixed size blocks. Blocks may be as small as one byte. One block per cell.

31. AAL 3/4

32. AAL 3/4

33. AAL 3/4

34. AAL 5 • Streamlined transport for connection oriented higher layer protocols Designed for data traffic • Less overhead bits than AAL 3/4 • Simple and Efficient AAL (SEAL) • No per cell length field, No per cell CRC

35. AAL 5 • No per cell overhead. - AAL 3/4 uses up 4 bytes per cell for overhead • CPCS User-to-user Indication: Transparently transfer user-to-user information. • Common Part Indicator: Interpretation of the PDU. Only one interpretation is defined. • Higher layers pre-allocate buffers  BA size is not required • No sequence number  Assume ordered delivery • No MID field  no PDU multiplexing. End of PDU is marked by AAU bit in the header • No LI field  pad is large enough to make PDU a multiple of 48 bytes (rather than 32-bits as in AAL 3/4)

36. AAL 5 CPCS PDU

37. Example AAL 5 Transmission

38. AAL Type 5

39. AAL-1 Cell Tax AAL-2 Cell Tax 5 ByteHeader 5 ByteHeader 1–48Bytes 1 Byte 1–47 BytePayload 47 BytePayload AAL-3/4 Cell Tax AAL-5 Cell Tax 5 ByteHeader 5 ByteHeader no tax 48 BytePayload 44 BytePayload 4 Bytes AAL Cell Tax

40. ATM Adaptation Layers Summary • AAL5 - For data transfers. Behaves like a data link protocol AAL Type 5 --- high-speed data protocol • AAL 3/4 - Rarely used AAL Type 3/4 --- connectionless services/data protocols • AAL2 - Designed to carry compressed voice and video - AAL Type 2 --- variable bit rate services • AAL1 - Emulates conventional telecommunications circuits - AAL Type 1 --- constant bit rate services • AAL0 - Non-standard extension that enables experimental methods of packaging data to be tried. Raw data is loaded into cells without any formatting