atm dictionary

8/8/2019 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!!!

atm dictionary

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