Asynchronous Transfer Mode (ATM) EE290F 22 April 2004 Shanna Crankshaw

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".

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 Fast packet switching, high data rates

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

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)

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

______________Interpretation____________ Stallings, Data & Computer Communications, 6th ed., Table 11.2 PT coding 000 001 010 011 100 101 110 111 ______________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 SDU = Service Data Unit OAM = Operations, Administration, and Maintenance

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

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 Virtual channels Virtual path Physical channel

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

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

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

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

ATM advantages Universality Scalability 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

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