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Atm

VIEWS: 48 PAGES: 19

									                  Advanced topics
                                in
  Computer Networks
              Lecture 2: ATM: an overview

                   University of Tehran
          Dept. of EE and Computer Engineering
                              By:
                    Dr. Nasser Yazdani
Univ. of Tehran       Adv. topics in Computer Network   1
      Outline
    Why ATM
    ATM overview
    ATM standard




    Univ. of Tehran   Adv. topics in Computer Network   2
      Why ATM?
    Problems with IP?
         Best effort, lack of reliability
               TCP on top of IP
         Being stateless
               Hop to hop or point to point.
               Lack of QoS.
         How important is QoS? It is essential in some
          applications including
               VoIP
               Video on demand.
               Teleconferencing.

    Univ. of Tehran         Adv. topics in Computer Network   3
      Supporting QoS in IP
    QoS parameter
         Bounded delay and jitter.
         Min rate
    IP should identify flow and keep state in
     nodes.
         Contradictory to original design goal, flexibility
          and scalability.
    Can we have both? We try.
    This is the main challenge of Internet.
    Univ. of Tehran    Adv. topics in Computer Network    4
   ATM overview
ATM = Asynchronous Transfer Mode.
ATM = Data Network + Phone Network
 Combination of Internet method of

  communication (packet switching) and phone
  companies’ method (circuit switching)
 Asynchronous- The way bandwidth is allocated.

  The time slot is not allocated by temporal position
  like Synchronous transfer mode, STM, (circuit
  switch).
 Transfer mode= Multiplexing and switching.


 Univ. of Tehran   Adv. topics in Computer Network   5
      ATM overview (cont)
    Connection oriented- Need signaling like
     circuit switch.
         Establish connection.
         Tear down connection.
         Create a virtual connection or circuit, then
          order is preserved in carrying data.
         Problems?
               Overhead for connection and tearing down.
               Loosing connectivity in case of failure in nodes.


    Univ. of Tehran         Adv. topics in Computer Network         6
     ATM vs. STM
   STM or TDM
        Peak rate allocation
        Fast and simple logic.
        Not good for bursty traffic like data.
   ATM- uses statistical multiplexing.
          Giving bandwidth for label.
       Has to process cells .

       Congestion is possible.

       Variable cell delay (Queuing).

       But, network controls routing, errors, flow, copying and
           priorities.
       Overcome to congestion and delay is fundamental to
           ATM.
    Univ. of Tehran     Adv. topics in Computer Network       7
    ATM vs. Phone Network
 Current phone networks are synchronous
  (periodic). STM
  ATM = Asynchronous Transfer Mode
 Phone networks use circuit switching.

  ATM networks use “Packet” Switching
 In phone networks, all rates are multiple of 8
  kbps.
  With ATM service, you can get any rate. You can
  vary your rate with time.
 With current phone networks, all high speed
  circuits are manually setup. ATM allows dialing
  any speed.
 Univ. of Tehran  Adv. topics in Computer Network 8
      Interfaces
    User to Network Interface (UNI):
     Public UNI, Private UNI
    Network to Node Interface (NNI):
    Private NNI (P-NNI)
         Public NNI = Inter-Switching System Interface (ISSI)
         Intra-LATA ISSI (Regional Bell Operating Co)
         Inter-LATA ISSI (Inter-exchange Carriers)
          Broadband Inter-Carrier Interface (B-ICI)
    Data Exchange Interface (DXI)
     Between packet routers and ATM Digital Service
     Units (DSU)
    Univ. of Tehran      Adv. topics in Computer Network         9
     ATM standard
    Standard is different from concept. It deals with
     implementation.
    ATM Forum is a standard body of developer,
     service provider and other interested bodies.
    What standard does is to decide:
          Size of cell
       Header format

       Signaling

       Interfaces

       Traffic control

       Network management

       Testing

       Internetworking topics in Computer Network
    Univ. of Tehran     Adv.                         10
            ATM Cell format
          GFC=Generic Flow Control
               (Was used in UNI but not in NNI)
               The field is used for VPI in NNI
          VPI/VCI=0/0  Idle cell; 0/n  Signaling
          HEC: 1 + x + x 2 + x 8   0          4                                 7
                                                       GFC/VPI           VPI
                                                          VPI            VCI

• U.S ANSI proposed 64 bytes data with 5                         VCI
  byte header                                            VCI        PT         CLP
•Euro. ETSI proposed 32 bytes with 4                Header Error Checksum(HEC)
  bytes header                                                   Data
•Finally, 48 bytes with 5 bytes header.

          Univ. of Tehran      Adv. topics in Computer Network                       11
            Connection Identifiers
 VPI/VCI are local and swapped in each switch.
 VCI is allocated in connection setup time.
VPI is usually used by management.
 By VPI, a set of VCI are treated as the same.
   VPI/VCI=0/0  Idle cell; 0/n  Signaling

    •Each cell contains a 24/28-bit connection identifier
     First 8/12 bits: Virtual Path, Last 16 bits: Virtual Channel
    •VP service allows new VC's w/o orders to carriers


          Univ. of Tehran       Adv. topics in Computer Network     12
       Header Error Check (HEC)
   Protects header only
   Optional Correction mode: Correct one bit errors
    if no earlier errors
   Discard cells with bad HEC
   Used for cell delineation in SONET
   Recalculated on each hop
   CRC generator is 1 + x + x 2 + x 8
                      1. Single bit error (correction)
                      2. Multibit error (discard)
                                               Detection mode
No Error Correction mode
                                                                Error
                       No error detected
     Univ. of Tehran    Adv. topics in Computer Network                 13
  ATM protocol stack
AAL = ATM Adaptation Layer


                  Peer to peer connection
AAL                                                        AAL

ATM                            ATM                         ATM

PHY                            PHY                         PHY




Univ. of Tehran          Adv. topics in Computer Network         14
      Protocol layers
    The ATM Adaptation Layer
          How to break application messages to cells
    The ATM Layer
         Transmission/Switching/Reception
         Congestion Control/Buffer management
         Cell header generation/removal at source/destination
         Reset connection identifiers for the next hop (at switch)
         Cell address translation
         Sequential delivery



    Univ. of Tehran      Adv. topics in Computer Network        15
      ATM network
    ATM layer: Transporting cells
         Cell encapsulation and decapsulation.
         Insertion and extraction of idle cells
         Cell header processing
         Cell queuing
         Cell coping
         GFC management in UNI



    Univ. of Tehran    Adv. topics in Computer Network   16
      ATM network (cont)
    Connection Admission Ctrl
         Connection admission or rejection
         VPI/VCI assignment and removal
         Exchanging and processing signaling cells
    Traffic Ctrl: Maintain agreed QoS
         Usage parameter ctrl
         Selective cell discarding
         Implicit service priorities
         Explicitly forward
    Univ. of Tehran     Adv. topics in Computer Network   17
      ATM network (cont)
    ATM layer management
         Configuring the network and switch fabric
         Monitoring the network operation
         Detecting troubles
    ATM switch
         Doing actual cell switching.




    Univ. of Tehran    Adv. topics in Computer Network   18
      History of ATM
    1980: Narrowband ISDN adopted
    Early 80's: Research on Fast Packets
    Mid 80's: B-ISDN Study Group formed
    1986 ATM approach chosen for B-ISDN
    June 1989: 48+5 chosen (64+5 vs 32+4).
     October 1991: ATM Forum founded
    July 1992: UNI V2 released by ATM Forum
     1993: UNI V3 and DXI V1
    1994: B-ICI V1
    Univ. of Tehran   Adv. topics in Computer Network   19

								
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