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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 (Cont’d)

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 VCI’s 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 (Cont’d)

Virtual Path (VP)
– a group of virtual channels (VC’s) 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 (Cont’d)

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 VPI’s 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 (Cont’d)
VP switch: an ATM switch that deals only
with the VPI’s 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 VCI’s in cell headers (e.g., access
switch at the edge of an ATM network)
 VP/VC switch: an ATM switch that deals
with both VPI’s and VCI’s in cell switching

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Definitions (Cont’d)

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 (Cont’d)

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 (Cont’d)

ATM Adaptation Layer (AAL)
– a protocol for converting between higher layer
protocol data units (PDU’s), 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 (Cont’d)

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 (Cont’d)

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 (Cont’d)

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 < 50 msec, CLR < 0.001
» e.g., data: delay < 1 sec, CLR < 0.000001
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Definitions (Cont’d)

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 VCI’s
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Definitions (Cont’d)

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 (Cont’d)

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 (Cont’d)

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 (Cont’d)

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 (Cont’d)

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 (Cont’d)

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 (Cont’d)

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 TLA’s (three
letter acronyms)
 Get used to it!
 Soon you will be speaking it too!!!

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