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

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All about ATM ATM Terminology Explained

• Carey Williamson

Department of Computer Science University of
Calgary
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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

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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

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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

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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

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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)

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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

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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)

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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)?

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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

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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

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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)

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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!!!