1、Asynchronous Transfer Mode (ATM),EE290F 22 April 2004 Shanna Crankshaw,“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“.,ATM definition,What is it re
2、ally?,Low-level network layerabove 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,Sy
3、nchronous 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
4、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 Flo
5、w 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,PT coding 000 001 010 011 100 101 11
6、0 111,_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 Resour
7、ce management cell Reserved for future function,SDU = Service Data Unit OAM = Operations, Administration, and Maintenance,Stallings, Data & Computer Communications, 6th ed., Table 11.2,ATM Cells,Small size, may reduce queuing delay of high priority cells Fixed size, more efficient switching,Info fie
8、ld, 48 bytes,Header error control,PT CLP,VC identifier,VP identifier,GFC,VP identifier,5-byte header,Info field, 48 bytes,Header error control,PL type CLP,VC identifier,VP identifier,NNI,UNI,53 byte cell,VCs are not always VCs,Virtual Channel Transmission is connection-oriented VC set up by some sig
9、naling protocol before any cells can be sent Virtual Path Connection (VPC), bundle of VCCs,Logical connections,VPC = bundle of VCCs with the same endpoints all switched together Network management of group of connections, not many individual ones Setup time is for a VP, adding VCs to it involves min
10、imal processing,Request for VCC originates,Can QoS be satisfied?,Yes,Yes,Yes,No,No,No,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 ag
11、ain at destination CS: convergence sublayer. Takes care of delay jitter, error checking, remove corrupted cells,Physical layer,ATM layer,SAR sublayer,CS,AAL user,AAL,ATM Service Categories,Real-time services Constant bit rate uncompressed audio/video info Videoconferencing, TV, pay-per-view, VOD, et
12、c. 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
13、.g. LAN, router-to-router reliability,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,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,
