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ATM: What it is, and what it isn't

ATM: What it is, and what it isn't. Carey Williamson. University of Calgary. ATM: What it is. Asynchronous Transfer Mode A low-layer networking technology based on fast packet-switching of small fixed size packets called cells

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ATM: What it is, and what it isn't

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  1. ATM: What it is, and what it isn't Carey Williamson University of Calgary

  2. ATM: What it is • Asynchronous Transfer Mode • A low-layer networking technology based on fast packet-switching of small fixed size packets called cells • ATM provides a single transport mechanism for integrated services traffic: data, voice, video, image, graphics... • All statistically multiplexed at ATM layer

  3. ATM: What it isn’t • Synchronous Transfer Mode (STM) • STM relies on pre-assigned “slots” for each user within a frame, and global timing information to mark frame boundaries • Example: T1 transmission (1.544 Mbps)

  4. T1 Transmission Framing bit Room for 24 calls, with 8 bits from each 8 bit sample for call i 24 x 8 bits = 192 bits + 1 framing bit = 193 bits 125 microseconds (8000 cycles/sec) An example of one frame from T1 digital transmission scheme 193 bits/frame X 8000 frames/sec = 1.544 Mbps

  5. T1 Transmission (Cont’d) Frame 1 Frame 2 Frame 3 Call i uses slot i in each frame 8 bits/slot X 1 slot/frame X 8000 frames/sec = 64000 bits/sec = 64 kbps Calls can be allocated k slots per frame to give allocated bandwidth that is k x 64 kbps Idle slots are wasted

  6. Synchronous Transfer Mode • STM relies on positional association: slots are identified by their relative position from the start of the frame (global timing info) • Each user knows which slot(s) to use • All slots are the same size (e.g., 8 bits) • Bandwidth allocated in multiples of slots • Efficient for Constant Bit Rate traffic • Inefficient for Variable Bit Rate traffic

  7. Asynchronous Transfer Mode • ATM does not use a priori assignment of slots to users • Slots are assigned “on demand” on an as needed basis • Users can use whichever slots are empty

  8. Slotted transmission scheme All slots are the same size (53 bytes = 1 ATM cell) Any user can use any empty slot No notion of specific slots assigned to specific users Bandwidth allocation in ATM may reserve a certain percentage of the total slots for a given call, but which slots you get is determined at time of transmission ATM Transmission Can support arbitrary bit rates

  9. Asynchronous Transfer Mode • No global timing relationship between slots (i.e., cells) of different users (asynchronous) • Efficient for Variable Bit Rate traffic • Implication: the cell in each slot has to be completely self-identifying (i.e., overhead)

  10. Advantages of ATM • Better for bursty traffic (i.e., VBR) • Statistical multiplexing gain • Better network utilization • Same mechanism works for all traffic types • Simple and fast hardware switching

  11. Characteristics of ATM • Point to point technology • Connection-oriented: an end-to-end connection (called a virtual channel) must be set up using a signalling protocol before any data cells can be sent on that VC • “Bandwidth on demand” • Statistical multiplexing • Integrated services

  12. ATM Cell • 53 bytes • 5 byte header • 48 byte payload (data) • Virtual Path Identifier (VPI) • Virtual Channel Identifier (VCI) • Simple control fields

  13. ATM Cell Format GFC VPI VPI VCI VCI VCI PT RES CLP HEC Payload (48 bytes) ATM UNI Cell Specification

  14. Why ATM will win • ATM is a scalable technology • scalable in bandwidth • scalable in distance • scalable in deployment

  15. ATM: Scalable Bandwidth • ATM is not tied to any particular bit rate or physical layer network technology • ATM is simply the abstract concept of fast packet switching with small fixed size cells • Can do low speed ATM (e.g., 1.5 Mbps) • Can do high speed ATM (e.g., 155 Mbps) • Primary interest: high speed ATM networks

  16. ATM: Scalable Distance • ATM can be used for LANs • ATM can be used for MANs • ATM can be used for WANs • Initial market: ATM LANs, enterprise area networks, LAN backbones • Future: wide area network backbone, ATM to the desktop, wireless ATM

  17. ATM: Scalable Deployment • Emphasis on interoperability, compatibility • Incremental evolutionary path to ATM • Ethernet => switched Ethernet => ATM hub • Start with one switch, N ports, plus NICs • Add more ports as needed • Add more switches as needed • Hierarchical cascading structure

  18. Why ATM will win • ATM is a scalable technology • scalable in bandwidth • scalable in distance • scalable in deployment • Global standards (ATM Forum, ITU) • Billions of dollars invested

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