1. Chapter 3 Asynchronous Transfer Mode (ATM)

2. Synchronous vs. Asynchronous  ATM- cell switching technology (asynchronous)  TDM – circuit switching technology (synchronous)  ATM is more efficient than TDM. (why?)  TDM: dedicated time slots.  ATM: time slots are available on demand with information identifying the destination of the transmission contained in the header of each ATM cell.

3. ATM  Multi-speed network environment that provides a variety of complex network services  Can carry voice, data, video separately or simultaneously  Can be used in LANs, MANs, or WANs  Fixed-length packets (cells)  Allows multiple logical connections to be multiplexed  Minimal error and flow control capabilities  Connection-oriented virtual channel

4. Cell Switched ATM  Similar to frame relay  Difference?  Frame relay switches variable length frames within frame relay cloud from source to destination  ATM switches fixed-length cells (48 byte information field, 5 byte header)  Based on packet switching (connection-oriented)  Cell sequence integrity preserved via virtual channel  VCC – virtual channel connection – is set up between end users, variable rate, full duplex  VCC also used for control  Information field is carried transparently through the network, with minimal error control

5. ATM Network (integrated voice, video, and data services)

6. Protocol Architecture

7. ATM Protocol Layers ATM Applications ATM Adaptive Layer (AAL) ATM Layer Physical Layer ATM Layer Physical Layer ATM Applications ATM Adaptive Layer (AAL) ATM Layer Physical Layer ATM End Station ATM SwitchATM End Station

8. ATM Physical Layer  Transports cells via a communications channel (either optical or electrical)  LAN support: 25-155 Mbps copper or fiber  WAN support: SONET rates over fiber  Physical Medium Sublayer: bit transfer, bit alignment, and copper/fiber conversions  Transmission Convergence Sublayer: bit/cell conversion at sending and receiving nodes

9. ATM Layer  Handles functions of the network layer  Connection-oriented without acknowledgements  Two possible interfaces:  UNI – User-Network Interface: Boundary between an ATM network and host  NNI – Network-Network Interface: Between two ATM switches

10. UNI/NNI Interface

11. ATM Adaptation Layer (AAL)  Maps higher-layer information into ATM cells to be transported over an ATM network  Collects information from ATM cells for delivery to higher layers

12. ATM Adaptation Layer  The AAL relays ATM cells between ATM Layer and higher layer (Segmentation and Reassembly (SAR) process)  When relaying information received from the higher layers, it segments the data into ATM cells.  When relaying information received from the ATM Layer, it must reassemble the payloads into a format the higher layers can understand.

13. ATM Adaptation Layer  Different AALs were defined in supporting different traffic or service expected to be used. The service classes and the corresponding types of AALs were as follows:  Class A - Constant Bit Rate (CBR) service:  AAL1 supports a connection-oriented service in which the bit rate is constant.  Examples of this service include 64 Kbit/sec voice, fixed-rate uncompressed video and leased lines for private data networks  Class B - Variable Bit Rate (VBR) service:  AAL2 supports a connection-oriented service in which the bit rate is variable but requires a bounded delay for delivery.  Examples of this service include compressed packetized voice or video. The requirement on bounded delay for delivery is necessary for the receiver to reconstruct the original uncompressed voice or video.

14. ATM Adaptation Layer  Class C - Connection-oriented data service:  For connection-oriented file transfer and in general, data network applications where a connection is set up before data is transferred, this type of service has variable bit rate and does not require bounded delay for delivery.  Two AAL protocols were defined to support this service class, and have been merged into a single type, called AAL3/4. But with its high complexity, the AAL5 protocol is often used to support this class of service.

15. ATM Adaptation Layer  Class D - Connectionless data service:  Examples of this service include datagram traffic and in general, data network applications where no connection is set up before data is transferred.  Either AAL3/4 or AAL5 can be used to support this class of service.  Operation Administration and Maintenance (OA&M)  Defined for supervision, testing, and performance monitoring. It uses loop-back for maintenance and ITU TS standard CMIP, with organization into 5 hierarchical levels:  Virtual Channel (F5 - Between VC endpoints)  Virtual Path (F4- Between VP endpoints)  Transmission Path (F3- Between elements that perform assembling, disassembling of payload, header, or control)  Digital Section (F2 Between section end-points, performs frame synchronization)  Regenerator Section (F1- Between regeneration sections)

16. ATM Adaptation Layer AAL Types Connection Mode Bit Rate Timing relationship between source & destination AAL1 Connection oriented ConstantYes AAL2 VariableYes AAL3ConnectionlessVariableNo AAL4ConnectionlessVariableNo AAL5ConnectionlessVariableNo

17. Virtual Connections  Virtual Channel Connection (VCC) – Full duplex virtual circuit with logical connection between source and destination – can be PVC or SVC  Virtual Path Connection (VPC) – Semi- permanent (or customer controlled or network controlled) connection that provides a logical collection of virtual channels that have the same endpoint  A single virtual path supports multiple virtual channels (analogy – highway = VPC, lane = VCC)

18. VCI vs VPI  VPI – Virtual Path Identifier – identified in cell’s header. Cannot establish a virtual channel before virtual path  VCI – Virtual Channel Identifier – only have local significance – different virtual paths reuse VCIs (but VCIs on same path must be unique)

19. What is so special about a virtual path?  ATM is connection-oriented, so circuit must be established before transmission  As route established, VPIs and VCIs are assigned  VPI and VCI info suffices for addressing info  Simplified network architecture (based on VC or VP)  Increased network performance and reliability (fewer, aggregated entities because of simplified network architecture)  Reduces processing and short connection setup time  User may define closed user group or closed networks of virtual channel bundles

20. ATM PVC and SVC  PVC allows direct connectivity between sites, and is similar to a leased line.  Advantages:  guarantees availability of a connection  does not require call setup procedures between switches.  Disadvantages:  requires manual setup between the source and the destination.  no network resiliency is available with PVC.  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.  Advantages:  connection flexibility  call setup that can be handled automatically by a networking device.  Disadvantages:  the extra time and overhead required to set up the connection.  network management and troubleshooting

21. ATM Connection Relationships

22. ATM Quality of Service (QoS)  CBR: Constant Bit Rate  Guaranteed transmission rate  VBR: Variable Bit Rate  rtVBR and nrtVBR  Peak Cell Rate (PCR), Sustained Cell Rate (SCR), and Max Burst Size (MBR)  UBR: Unspecified Bit Rate  No guarantee, best effort CBR VBR UBR PCR SCR Service Bit Rate

23. Call Establishment Using VPs

24. VP/VC Characteristics  Quality of service based on VCC  Switched and semi-permanent channel connections  Call sequence integrity – packets arrive in order  Traffic parameter negotiation and usage monitoring  VPC only  Virtual channel identifier restriction within VPC – some VCCs reserved for network management

25. ATM Cells  Fixed size – 53 bytes  5 octet header  48 octet information field  Small cells reduce queuing delay for high priority cells  Small cells can be switched more efficiently  Easier to implement switching of small cells in hardware

26. ATM Cell Format

27. Header Format  Generic flow control (GFC)  Only at user to network interface (UNI)  Controls flow between user and local ATM switch  2 sets of procedures: Uncontrolled and Controlled transmission  Virtual path identifier  8 bits in UNI cell, 12 bits in NNI cell  Addressing info for cell routing  Virtual channel identifier  16 bit  Additional addressing info for cell routing

28. Header Format  Payload type – 3 bits (user info or network management)  first bit: user data (0) or control data (1)  second bit: 0 = no congestion, 1 = congestion),  third bit: whether the cell is the last cell (1) in a series of AAL5 frame  Cell loss priority (CLP)  Indicates whether the cell should be discarded (1) if it encounters extreme congestion as it moves through the network  Header error control (HEC)  Checksum on the first 4 bytes of the header  Correct an isolated bit error and detect multiple bit errors

29. ATM Service Categories  ATM is designed to transfer many different types of traffic simultaneously, including real- time voice, video, and bursty TCP traffic  Way in which data flow is handled depends on the characteristics of the traffic flow and requirements of the application (ex. Real-time video must be delivered within minimum variation in delay)  Primary service categories – real time service, non-real time service

30. 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)  Guaranteed frame rate (GFR)

31. Real Time Services  If want to avoid or decrease variation of delay (jitter), use CBR or rt- VBR  CBR  Fixed data rate continuously available  Commonly used for uncompressed audio and video  Video conferencing  Interactive audio  A/V distribution and retrieval  rt-VBR  Best for time sensitive applications  Tightly constrained delay and delay variation  Applications transmit at a rate that varies with time (e.g. compressed video)  Produces varying sized image frames  Original (uncompressed) frame rate constant (isochronous)  So compressed data rate varies  Can statistically multiplex connections

32. Non-Real Time  Intended for applications with bursty traffic and limited constraints on delay and delay variation  Greater flexibility, greater use of statistical multiplexing

33. 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

34. UBR  Unused capacity of CBR and VBR traffic made available to UBR  For application that can tolerate some cell loss or variable delays  e.g. TCP based traffic  Cells forwarded on FIFO basis  Best efforts service

35. ABR  Application specifies peak cell rate (PCR) it will use and minimum cell rate (MCR) it requires  Resources allocated to give at least MCR  Spare capacity shared among ABR and UBR sources  e.g. LAN interconnection

36. Guaranteed Frame Rate (GFR)  Designed to support IP backbone subnetworks  Purpose: optimize handling of frame based traffic passing from LAN through router to ATM backbone  Used by enterprise, carrier and ISP networks  Consolidation and extension of IP over WAN  UNI establishes handshaking between NIC and switch

37. ATM vs Frame Relay  Frame relay uses variable length frames  ATM fixed length cells  ATM has higher overhead, but faster speed and traffic management (better suited for video and voice)

38. ATM vs SONET  SONET is a transport mechanism, transporting data over fiber.  Can act as a transport carrier for ATM (or FDDI, or ISDN, etc.)  ATM is a technology and protocol designed to use SONET as its carrier service

39. Why is ATM so Efficient?  Minimal error and flow control  Reduces overhead of processing ATM cells  Reduces number of required overhead bits  Fixed size simplified processing at each ATM node (can be switched more efficiently – more efficient use of router)  Small cells reduce queuing delay  Minimal addressing info on each cell  Efficient traffic management