Download - Atm Dictionary
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MADE BY
AMIT BHATI
ECE
SEMINAR
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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
xamples: VCI, VPI, PVC, SVC, AAL, C R, V R,
ABR, PCR, SCR, QOS, C V
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ATM: Asynchronous Transfer Mode
ATM is a statistical multiplexing technique
for high speed integrated services networks,
based on the fast packet switching of smallfixed size (53 byte) packets called cells
ATM is a connection-oriented low-layer
networking concept
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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
sentAll 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 virtualchannels passing through them
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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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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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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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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 onlywith 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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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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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 whencomplete
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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ATMAdaptation Layer (AAL)
a protocol for converting between higher layer
protocol data units (P Us), such as TCP packets, IP
packets, or JP G 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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Traffic escriptors
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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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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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 delayvariation (C V)
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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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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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 trafficcarried
suitable for periodic (isochronous) traffic
need to specify only PCR (which equals SCR)
supported by some ATM switch vendors
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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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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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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 therehappens to be extra bandwidth available
lowest priority class
much discussion at ATM Forum
may become Guaranteed Frame Rate (GFR)?
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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 ofservice (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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Usage Parameter Control (UPC)
a control function performed in switches to
police ATM traffic flows
monitors traffic on a VCI basis, to make sure thatit 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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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 cellsbefore dropping any untagged cells
not to be confused with delay priority
(i.e., the service classes)
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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!!!