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Data and Computer Communications

Data and Computer Communications. Chapter 11 – Asynchronous Transfer Mode. ATM. a streamlined packet transfer interface similarities to packet switching transfers data in discrete chunks supports multiple logical connections over a single physical interface

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Data and Computer Communications

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  1. Data and Computer Communications Chapter 11 – Asynchronous Transfer Mode

  2. ATM • a streamlined packet transfer interface • similarities to packet switching • transfers data in discrete chunks • supports multiple logical connections over a single physical interface • ATM uses fixed sized packets called cells • with minimal error control and flow control • data rates of 25.6Mbps to 622.08Mbps

  3. Protocol Architecture

  4. Reference Model Planes • user plane • provides for user information transfer, associated controls (flow control, error control) • control plane • call and connection control • management plane • plane management • whole system functions • layer management • Resources and parameters in protocol entities

  5. ATM Logical Connections • virtual channel connections (VCC) • analogous to virtual circuit in X.25 • basic unit of switching between two end users • full duplex • fixed size cells • also for • user-network exchange (control) • network-network exchange (network mgmt & routing)

  6. ATM Virtual Path Connection • virtual path connection (VPC) • bundle of VCC with same end points

  7. Advantages of Virtual Paths • simplified network architecture • increased network performance and reliability • reduced processing • short connection setup time for new channel

  8. Call Establishment Using VPs

  9. Virtual Channel Connection Uses • between end users • end to end user data • VPC provides overall capacity • VCC organization done by users within the capacity • between end user and network • control signaling • between network entities • network traffic management • routing

  10. VP/VC Characteristics • quality of service - cell loss ratio, cell delay variation • switched and semi-permanent channel connections • cell sequence integrity • traffic parameter negotiation and usage monitoring - average rate, peak rate, burstiness, and peak duration • VPC only • virtual channel identifier restriction within VPC

  11. Fixed vs. Variable Length Cell • # Efficiency Consideration: • Efficiency N = Information Octets / (Information Octets + Header Octets) • Fixed Length Packets: • L = Data Field Size (Octets) in a Packet • H = Header Size (Octets) • X = Total Message Size (Octets) • # Expression for N = ? (Needs ceiling function), Nopt = ?

  12. Fixed vs. Variable Length Cell • # Variable Length Packets: • Needs an additional length field, • Hv = Additional overhead octets • H = Header Size (Octets) • X = Message Size (Octets) • # Expression for N = ? (Assume Single Cell) • # Plot of N vs. Message Size (X = 48, 96, 144, 192, 240), for fixed and variable length cells

  13. What size of Fixed Length • Assume that the cells are completely filled (X / L = integer) • Expression for N = ? • PacketizationDelay = buffering bits until the entire packet is filled before retransmission • Expression for this delay D = ? (function of L and source data rate R)

  14. What size of Fixed Length • Plot of D vs. Data Field Size (L = 16, 32, 64, 128 octets) (R = 64 kbps for voice coding) • Plot of efficiency N vs. Data Field Size (16, 32, 64, 128 octets) • => 48 octet provides a trade-off between efficiency and delay

  15. ATM Cells

  16. ATM Header Fields • generic flow control • Virtual path identifier - routing • Virtual channel identifier – end to end user • payload type (3-bits, user information, congestion) • cell loss priority • header error control

  17. Examples • # Consider compressed video transmission in ATM network, Cells must pass through 5 switches. The data rate is 43 Mbps. • - Transmission time of one cell through one switch? • - Assume other cells have lower priority (but non-premptive) than cell A. What is the maximum time from arrival at the first

  18. Examples • Switch to the completion of transmission by the fifth for cell A? (all other delays are negligible) • - if the probability that a switch is busy is 0.6 and the average delay to wait for current transmission completion is one half a cell transmission time, what is the average time from arrival at the first Switch to the completion of transmission by the fifth for cell A?

  19. Examples • - what is the maximum and average variability in encountered delay (the jitter)? • # IP datagrams are segmented into ATM cells and sent over the ATM network. Loss of a cell means loss of entire IP packet. • Pc = cell loss rate • n = number of cells for a datagram • Expression for Pp = datagram loss rate?

  20. Summary • Asynchronous Transfer Mode (ATM) • architecture & logical connections • ATM Cell format • transmission of ATM cells

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