Asynchronous Transfer Mode (ATM)

1. Asynchronous Transfer Mode (ATM) • Introduction • Background • Physical and ATM Layers • AALs • Applications

2. Introduction Asynchronous Transfer Mode (ATM) technology delivers important advantages over existing LAN and WAN technologies, including the promise of scalable bandwidths at unprecedented price and performance points and Quality of Service (QoS) guarantees, which facilitate new classes of applications such as multimedia. However, ATM is the most complex technology ever developed by the networking industry. In fact, the deployment of ATM networks requires the overlay of a highly complex, software intensive, protocol infrastructure. This infrastructure is required to both allow individual ATM switches to be linked into a network, and for such networks to internetwork with the vast installed base of existing local and wide area networks.

3. ATM Quick Highlights • The Telecom Industry’s thrust into multi-media data networking •Fixed-sized “cell” (53 bytes) • Built to provide Quality-of-service • Connection-oriented • Designed to run over SONET/SDH

4. B-ISDN (contd.) Plane Management Layer Management Management Plane Control Plane User Plane Data Transfer Upper Layers Upper Layers User and Control Convergence Sub layer Segmentation and Reassembly Sub layer Transmission Convergence Sub layer (Framing) Physical Medium Dependent Sub layer Packets CS ATM adaptation layer(AAL) SAR Cells, VC’s ATM Layer TC Physical layer Physical Medium PMD

5. Why? • Why a small cell instead of a large packet? • Queue delays tend to grow as packet size grows. A small cell helps maintain streamlined flows • No/little performance loss due to padding large fields • Small cells better for voice • No need for in-route fragmentation

6. Why? •Why a fixed cell size instead of variable-size packets? • Switch architecture can be optimized to the fixed size, so switching can be done in hardware • Scalable parallel switch designs

7. Why? • Why 53 bytes? – US wanted 64 payload bytes, Europe wanted 32 – Compromised on 48 – +5 header = 53

8. ATM + and – + • QoS • Multimedia Support • Hardware Switching -> High Speed • Connection-Oriented (-?) - • IP Support • LAN arena dominated by huge installed Ethernet base • Ethernet growing toward MAN, WAN • Connection-Oriented (+?)

9. ATM Architecture End Station End Station User Application User Application ATM Adaptation Layer (AAL) ATM Adaptation Layer (AAL) Switch ATM Layer ATM Layer ATM Layer 53-Byte Cells 53-Byte Cells Physical Layer Physical Layer Physical Layer Physical Layer

10. ATM • Physical and ATM Layers • Cells, Formats, and Addressing • Virtual Circuits • Switches and Media • Interfaces

11. (48 bytes PDU) 5 bytes ATM Cell Header User-Network Interface (UNI) (48 bytes PDU) PTI 3 bits GFC 4 bits HEC 8 bits VCI 16 bits VPI 8 bits CLP (1) 5 bytes ATM Header Network-Network Interface (NNI) (48 bytes PDU) PTI 3 bits HEC 8 bits VCI 16 bits VPI 12 bits CLP (1) 5 bytes ATM Header

12. Header Error Control (HEC) HEC covers the header only, not the payload -- the goal is to ensure correct delivery First Four Cell Header Bytes bbbbbbbbb bbbbbbbbb bbbbbbbbb bbbbbbbbb x8 + x2 + x + 1 Remainder + 01010101 HEC If P(bit error) = 10-8, then P(Undetected header error) = ~10-20: HEC also assists in synchronizing: - Look at 53-byte sequences until you find one where the HEC field works correctly - If this holds up for D sequences in a row, assume you’ve synched - p(bad synch) = ~2-8D

13. UNI Header Fields (48 bytes PDU) PTI 3 bits GFC 4 bits HEC 8 bits VCI 16 bits VPI 8 bits CLP (1) 5 bytes ATM Header GFC - General Flow Control Only used between host and network. Overwritten by first switch. VPI - Virtual Path ID VCI - Virtual Circuit ID PTI - Payload type ID CLP - Cell Priority (ID’s cells for deletion when congestion experienced) HEC -Header Checksum (all 1-bit errors corrected, 90% of multi-bit errors detected)

14. NNI Header Fields (48 bytes PDU) PTI 3 bits HEC 8 bits VCI 16 bits VPI 12 bits CLP (1) 5 bytes ATM Header VPI - Virtual Path ID VCI - Virtual Circuit ID PTI - Payload type ID CLP - Cell Priority (used to ID cells for deletion when congestion experienced) HEC -Header Checksum (all 1-bit errors corrected, 90% of multi-bit errors

15. PTI Field Codes PTI Meaning Used by AAL5 to denote end of message 000 User Data Cell Type 0 No congestion experienced 001 User Data Cell Type 1 No congestion experienced 010 User Data Cell Type 0 Congestion experienced 011 User Data Cell Type 1 Congestion experienced 100 Maintenance info between adjacent switches 101 Maintenance info between source and destination switches 110 Resource management cell (for ABR congestion control) 111 Reserved 0 b b Explicit Forward Congestion Indicator (EFCI) Set by Congested switch

16. ATM services • Three types of ATM services exist: permanent virtual circuits (PVC), switched virtual circuits (SVC), and connectionless service. • PVC allows direct connectivity between sites. • PVC guarantees availability of a connection and does not require call setup procedure between switches. • Disadvantages of PVCs include static connectivity and manual setup.

17. An SVC is created and released dynamically and remains in use only as long as data is being transferred. • In this sense, it is similar to a telephone call. Dynamic call control requires a signaling protocol between the ATM endpoint and the ATM switch. • The advantages of SVCs include connection flexibility and call setup that can be handled automatically by a networking device. Disadvantages include the extra time and overhead required to set up the connection.

18. Virtual circuit • ATM networks are fundamentally connection-oriented, which means that a virtual channel (VC) must be set up across the ATM network prior to any data transfer. • Two types of ATM connections exist: virtual paths, which are identified by virtual path identifiers, and virtual channels, which are identified by the combination of a VPI and a virtual channel identifier (VCI).

19. Virtual path and virtual channel

20. VC connection msg

22. ATM Broadcasting

23. ATM switching • The basic operation of an ATM switch is straightforward: The cell is received across a link on a known VCI or VPI value. The switch looks up the connection value in a local translation table to determine the outgoing port (or ports) of the connection and the new VPI/VCI value of the connection on that link. The switch then retransmits the cell on that outgoing link with the appropriate connection identifiers.

24. a VP3 VP5 a b ATM Sw 1 ATM Sw 2 b ATM DCC c ATM Sw 3 c d e VP2 VP1 d ATM Sw 4 e Sw = switch Digital Cross Connect Only switches virtual paths

26. ATM over SONET • ATM designed to run over SONET OC-3c • Basic: 155.52 Mbps gross rate • New generation runs at OC-12 (622 Mbps), • OC-48 (2.4 Gbps)

27. Public Private Public NNI Private UNI Public UNI Private NNI B-ICI Private UNI Private UNI ATM interfaces

28. Broadband Inter-Carrier Interface (B-ICI) Public Network-to-Network Interface • NNI Switch-to-switch interface protocol Two versions: Public and private (similar,more flexibility in private version) NNI Includes: Routing protocol (Link-sate/OSPF) Signaling protocol for link setup/teardown

29. UNI Protocol for interfacing with user equipment Two versions: Public and private

30. ATM ADAPTATION LAYER (AAL) • Specifically, the AAL receives packets from upper-level protocols and breaks them into the 48-byte segments that form the payload field of an ATM cell. • AALprotocol model consists of a Segmentation and Reassembly (SAR) sublayer and Service-Specific Convergence Sublayers (CPCS and SSCS). • The ATM Adaptation Layer (AAL) provides support for higher-layer services such as signaling, circuit emulation, voice, and video. AALs also support packet-based services, such as IP, LANs, and Frame Relay

31. AAL

32. AAL Types

33. AAL 1 • AAL1, a connection-oriented service, is suitable for handling constant bit rate sources (CBR), such as voice and videoconferencing. • The sequence number field (SN) and sequence number protection (SNP) fields provide the information that the receiving AAL1 needs to verify that it has received the cells in the correct order. The rest of the payload field is filled with enough single bytes to equal 48 bytes. • AAL1 requires timing synchronization between the source and destination and, for that reason, depends on a media that supports clocking, such as SONET. The standards for supporting clock recovery are currently being defined.

34. AAL 1

35. AAL 1 SAR PDU (non-pointer type) Adds sequence # with protection (checksum)Adds parity bit (even) over header AAL1 SAR PDU(Pointer Type) • Pointer field gives offset to start of next message (0-92 bytes)

36. Structured mode AAL1 SAR and CS

37. Unstructured mode AAL1 SAR and CS

38. AAL 2 • Designed to support Variable Bit Rate (“Bandwidth on Demand”) • Provides for partial payloads to support low rate data • Error protection over full PDU • Simple flag to indicate position in message • Also AAL 2 was designed to multiplex a number of such low variable bit rate data streams on to a single ATM connection.

39. AAL2 Operation CPS packet

40. CPS packet • Channel identifier (CID): CPS can multiplex several streams onto a single ATM connection. The CID identifies each channel. CID values are allocated as follows: the 0 value is used as padding, and the 8 to 255 values are valid CID values used to identify channels. • Length indicator (LI): Its value is one less than the number of bytes in the CPSpacket payload. The default maximum length of the CPS-packet payload is 45 bytes. • Header error control (HEC): It use the pattern x5 + x2 + 1. The receiver uses the contents of the HEC to detect errors in the header. • User-to-user-indication (UUI): used for transferring information between the peer CPS users. The CPS transports this information transparently.

41. AAL2 Operation • (CPS) PDU format

42. CPS-PDU • Parity (P): A 1-bit field used to detect errors in the STF. • Sequence numbers (SN): A 1-bit field used to number modulo 2 the successive CPSPDUs. • Offset field (OSF): The CPS-PDU payload can carry CPS packets in a variety of different arrangements. To extract the CPS-packets from the CPS-PDU payload, a 6-bit offset field (OSF) is used to indicate the start of a new CPS-packet in the CPS-PDU payload. Specifically, OSF gives the number of bytes between the end of the STF and the start of the first CPS-packet in the CPS-PDU payload.

43. AAL2 Operation

44. AAL 3/4 • AAL3/4 supports both connection-oriented and connectionless data. It was designed for network service providers and is closely aligned with Switched Multimegabit Data Service (SMDS). AAL3/4 is used to transmit SMDS packets over an ATM network. • Originally 2 separate AALs: – AAL3: Connection-oriented packet svcs (X.25) – AAL4: Connectionless svcs (IP) • Eventually combined into a single type for all data service

45. AAL3/4 CS PDU

46. AAL3/4 SAR PDU

47. AAL3/4 Operation

48. AAL5 • AAL 5 is used for the transfer of data. Due to its simplicity, it is the most popular adaptation layer. • AAL5 is a Simple Efficient Adaptation Layer (SEAL). The Common Part (CP) AAL5 supports Variable Bit Rate (VBR) traffic, both connection-oriented and connectionless. • It is used to transfer most non-SMDS data, such as classical IP over ATM and LAN Emulation (LANE). • Efficiency: • AAL3/4: 4 bytes per message + 4 bytes per cell => 44 User Data Bytes / Cell • AAL5: 8 bytes per message => 48 User Data Bytes / Cell, 8% improvement

49. AAL5 CS PDU

50. AAL5 CS PDU • Padding (Pad): It can be between 0 and 47 bytes long, and is added so that the entire CPS-PDU including the padding and the remaining fields in the trailer becomes an integer multiple of 48 bytes. • CPS user-to-user indication (CPS-UU): A 1-byte field used to transfer transparently CPS user-to-user information. • Common part indicator (CPI): A 1-byte field to support future AAL 5 functions. • Length: A 2-byte field used to indicate the length in bytes of the CPS-PDU payload . • CRC-32: This 4-byte field contains the FCS calculated by the transmitting CPS over the entire contents of the CPS-PDU The pattern used is: x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1.