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All%20about%20ATM:%20ATM%20Terminology%20Explained

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Title: All%20about%20ATM:%20ATM%20Terminology%20Explained


1
All about ATM ATM Terminology Explained
  • Carey Williamson

Department of Computer Science University of
Calgary
2
Introduction
  • ATM networking is filled with a lingo of its very
    own, many of which are acronyms, and many of
    which are quite fundamental to an understanding
    of what is going on in an ATM network
  • Examples VCI, VPI, PVC, SVC, AAL, CBR, VBR, ABR,
    PCR, SCR, QOS, CDV

3
Review
  • ATM Asynchronous Transfer Mode
  • ATM is a statistical multiplexing technique for
    high speed integrated services networks, based on
    the fast packet switching of small fixed size (53
    byte) packets called cells
  • ATM is a connection-oriented low-layer networking
    concept

4
ATM is Connection-Oriented
  • An end-to-end path called a virtual channel must
    be set up in advance, using an ATM signalling
    (control) protocol, before any data cells can be
    sent
  • All cells of a virtual channel travel on the same
    path
  • Cells arrive in the order that they were sent
  • Switches must maintain state about the virtual
    channels passing through them

5
Definitions
  • Virtual Channel (VC)
  • a connection between two communicating ATM
    entities (e.g., host-switch, switch-switch)
  • set up at time of call arrival
  • provides a certain grade of service
    (negotiated at time of call arrival)
  • cell sequence is preserved

6
Definitions (Contd)
  • Virtual Channel Identifier (VCI)
  • the label associated with a VC
  • 16-bit integer in UNI ATM cell format
  • carried in ATM cell header for identification
  • note that VCIs are locally significant only
    (i.e., assigned on a per link basis by the ATM
    devices at either end of that link)

7
Definitions (Contd)
  • Virtual Path (VP)
  • a group of virtual channels (VCs) all travelling
    between the same two points in ATM network
  • used by the network to simplify provisioning,
    resource management, providing different grades
    of service, etc.
  • bundles up traffic heading to same destination

8
Definitions (Contd)
  • Virtual Path Identifier (VPI)
  • the label associated with a VP
  • 8-bit integer in UNI ATM cell format
  • carried in ATM cell header for identification
  • part of two level addressing scheme in ATM
  • note that VPIs are locally significant only
    (i.e., assigned on a per link basis by the ATM
    devices at either end of that link)

9
Definitions (Contd)
  • VP switch an ATM switch that deals only with the
    VPIs in cell headers (e.g., a core switch in
    middle of a large ATM network)
  • VC switch an ATM switch that deals only with the
    VCIs in cell headers (e.g., access switch at
    the edge of an ATM network)
  • VP/VC switch an ATM switch that deals with both
    VPIs and VCIs in cell switching

10
Definitions (Contd)
  • Permanent Virtual Channel (PVC)
  • a virtual channel connection (virtual channel)
    that is set up on a long term basis (e.g., hours,
    days, months, years) by a human operator
  • involves statically configuring the
    routing table in ATM equipment
  • done as part of network provisioning in current
    ATM network testbeds
  • supported by all ATM switch vendors

11
Definitions (Contd)
  • Switched Virtual Channel (SVC)
  • a virtual channel connection (virtual channel)
    that is set up by the ATM signalling protocol
    between two communicating ATM entities
  • set up on an as needed basis, and torn down when
    complete
  • short term basis (e.g., seconds, minutes)
  • involves dynamically configuring the routing
    table in ATM equipment
  • supported by very few ATM switch vendors

12
Definitions (Contd)
  • ATM Adaptation Layer (AAL)
  • a protocol for converting between higher layer
    protocol data units (PDUs), such as TCP packets,
    IP packets, or JPEG images, and ATM
    cells for actual transmission
  • defines procedures for segmentation and
    reassembly (SAR)
  • segmentation packets to cells (done by sender)
  • reassembly cells to packets (done by receiver)

13
Definitions (Contd)
  • Traffic Descriptors
  • a numerical specification of the statistical
    characteristics of an ATM traffic flow
  • used by ATM switches at time of call setup
  • specifies Peak Cell Rate (PCR), Sustained Cell
    Rate (SCR), Maximum Burst Size (MBS), etc.
  • different calls can specify different values for
    their traffic descriptor
  • e.g., voice PCR SCR 1000 cells/sec
  • e.g., data PCR 10,000 cells/sec, SCR 1000

14
Definitions (Contd)
  • Quality of Service (QOS)
  • a specification of the desired (or acceptable)
    grade of service required for a traffic flow
  • some traffic is delay-sensitive (e.g., voice)
  • some traffic is loss-sensitive (e.g., data)
  • some traffic is both (e.g., compressed video)
  • some traffic is neither (e.g., LAN emulation)
  • QOS requested at time of call setup
  • ATM network tries to provide requested QOS

15
Definitions (Contd)
  • QOS Parameters
  • the parameters that can be specified as part of
    the QOS request at time of call setup
  • examples cell loss ratio (CLR), mean cell delay,
    maximum tolerable cell delay, cell
    delay variation (CDV)
  • different calls can specify different values for
    their QOS parameters
  • e.g., voice delay lt 50 msec, CLR lt 0.001
  • e.g., data delay lt 1 sec, CLR lt 0.000001

16
Definitions (Contd)
  • QOS Classes
  • generic service classes for ATM traffic
  • used to help simplify the management and support
    of QOS requirements in ATM networks
  • currently there are five proposed classes CBR,
    rt-VBR, nrt-VBR, ABR, and UBR
  • class specification determines the order of
    service for cells of different VCIs

17
Definitions (Contd)
  • Constant Bit Rate (CBR)
  • simplest type of traffic constant bit rate
    (e.g., voice traffic, T1 circuit emulation)
  • this is the highest priority class because of the
    delay-sensitive (i.e., time-dependent) nature of
    the traffic carried
  • suitable for periodic (isochronous) traffic
  • need to specify only PCR (which equals SCR)
  • supported by some ATM switch vendors

18
Definitions (Contd)
  • Variable Bit Rate (VBR)
  • more complicated type of traffic the bit rate
    varies with time (e.g., compressed video)
  • peak bit rate (i.e., short term) may be much
    higher than the mean bit rate (i.e., long term)
  • must specify PCR, SCR, and burstiness
  • next highest priority class(es)
  • versions real-time (rt) and non-real-time (nrt)
  • supported by some ATM switch vendors

19
Definitions (Contd)
  • Available Bit Rate (ABR)
  • unpredictable type of traffic traffic is willing
    to use as much or as little bandwidth as is
    available (e.g., Internet traffic, LAN emulation)
  • usually variable bit rate, delay-insensitive
  • referred to as elastic traffic (e.g., ftp)
  • next lowest priority class
  • supported by few ATM switch vendors
  • still under discussion by ATM Forum

20
Definitions (Contd)
  • Unspecified Bit Rate (UBR)
  • the bottom feeder in the ATM food chain
  • Minimum Cell Rate (MCR) is zero guaranteed
    nothing, but may get more than this if there
    happens to be extra bandwidth available
  • lowest priority class
  • much discussion at ATM Forum
  • may become Guaranteed Frame Rate (GFR)?

21
Definitions (Contd)
  • Call Admission Control (CAC)
  • a control function in ATM switches that makes the
    decision on whether or not to accept a newly
    incoming call
  • considers traffic descriptor (TD) and quality of
    service (QOS) parameters, as well as impact on
    the QOS of existing calls in the network
  • can be statistical or deterministic
  • still an active research topic

22
Definitions (Contd)
  • Usage Parameter Control (UPC)
  • a control function performed in switches to
    police ATM traffic flows
  • monitors traffic on a VCI basis, to make sure
    that it conforms to the declared traffic
    descriptor (TD)
  • in case of violation, can tag cells with a
    violation tag (CLP 1), discard cells, or abort
    the connection

23
Definitions (Contd)
  • Cell Loss Priority (CLP)
  • a single bit in ATM cell headers for denoting the
    cell loss priority (e.g., violation cells)
  • in the event of congestion, drop tagged cells
    before dropping any untagged cells
  • not to be confused with delay priority
    (i.e., the service classes)

24
Summary
  • ATM has much confusing terminology, and an
    endless (growing) list of TLAs (three letter
    acronyms)
  • Get used to it!
  • Soon you will be speaking it too!!!
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