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Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM). Rizwan Rehman , CCS. ATM – definition. "A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".

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Asynchronous Transfer Mode (ATM)

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  1. Asynchronous Transfer Mode (ATM) Rizwan Rehman , CCS

  2. ATM – definition "A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".

  3. Fast packet switching, high data rates What is it really? • Low-level network layer—above physical layer, below AAL (ATM adaptation layer) • Single transport mechanism for different types of traffic (voice, data, video, etc.) • Streamlined protocol, minimal error and flow control capabilities • Fixed packet size = ATM cell • Simplified processing, management

  4. Synchronous Transfer Mode • Pre-assigned “slots,” frame boundaries, global timing • Slots identified by position from the start of the frame • BW allocated in units of slots • Idle slots wasted • Efficient for Constant Bit Rate traffic

  5. Contrast with STM • “Bandwidth on demand”—Slots assigned on demand, users take any empty slot • Nothing pre-assigned, no global timing • Slot  Cell, fixed size of 53 bytes • Arbitrary bit rates: can support T-1 using CBR, voice/video using real-time VBR, IP-based traffic using ABR and UBR, etc. • Each cell must be self-identifying (overhead)

  6. ATM cell contents Header – 5 bytes • General Flow Control (GFC) – traffic control for different QoS, alleviates short-term overloads • VPI – routing field for network • VCI – routing to/from user • Payload type • Cell loss priority (CLP) • Header error control (HEC) – can correct single bit errors in header Information – 48 bytes

  7. Stallings, Data & Computer Communications, 6th ed., Table 11.2 ______________Interpretation____________ User data cell, congestion not experienced, SDU type=0 User data cell, congestion not experienced, SDU type=1 User data cell, congestion experienced, SDU type=0 User data cell, congestion experienced, SDU type=1 OAM segment associated cell OAM end-to-end associated cell Resource management cell Reserved for future function PT coding 000 001 010 011 100 101 110 111 SDU = Service Data Unit OAM = Operations, Administration, and Maintenance

  8. ATM Cells • Small size, may reduce queuing delay of high priority cells • Fixed size, more efficient switching UNI NNI GFC VP identifier VP identifier VP identifier 5-byte header VC identifier VC identifier PT CLP PL type CLP Header error control Header error control 53 byte cell Info field, 48 bytes Info field, 48 bytes

  9. Physical channel Virtual path Virtual channels VC’s are not always VC’s • Virtual Channel • Transmission is connection-oriented • VC set up by some signaling protocol before any cells can be sent • Virtual Path Connection (VPC), bundle of VCC’s

  10. Logical connections • VPC = bundle of VCC’s with the same endpoints  all switched together • Network management of group of connections, not many individual ones • Setup time is for a VP, adding VC’s to it involves minimal processing

  11. Request for VCC originates VPC exists? Block VCC or request more capacity Establish new VPC Reject VCC request Make connection Request granted? Request for VCC originates Yes Can QoS be satisfied? Yes No No Yes No

  12. ATM Adaptation Layer (AAL) • Layer above ATM • Service dependent • Mask ATM specifics from user; universality • SAR: segmentation and reassembly. “Translates” service data from a non-ATM format into ATM cells, then back again at destination • CS: convergence sublayer. Takes care of delay jitter, error checking, remove corrupted cells AAL user CS AAL SAR sublayer ATM layer Physical layer

  13. ATM Service Categories • Real-time services • Constant bit rate – uncompressed audio/video info • Videoconferencing, TV, pay-per-view, VOD, etc. • rt-Variable bit rate • Non-real-time services • nrt-VBR – high end system QoS, critical response time • Bank transactions, airline reservations, etc. • Unspecified bit rate – best-effort service • e.g. text/image messaging, telecommuting • Available bit rate – bursty apps requiring reliable end-to-end connection • e.g. LAN, router-to-router reliability

  14. ATM advantages • Universality • Mixed traffic types, real-time and non-real-time • Scalability • LANs, MANs, WANs, WLANs • Efficient use of network resources Bandwidth on demand concept • Simplified network infrastructure

  15. ATM challenges • In-network mux/buffering can lead to cell delay or loss  QoS guarantees • Many types of traffic • Large geographic distribution Traffic modeling, control

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