1 All about ATM: ATM Terminology Explained Carey Williamson Department of Computer Science University of Calgary.

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All about ATM:All about ATM: ATM Terminology Explained ATM Terminology Explained

Carey WilliamsonCarey Williamson

Department of Computer ScienceUniversity of Calgary

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Introduction Introduction

ATM networking is filled with a lingo of its ATM networking is filled with a lingo of its very own, many of which are acronyms, and very own, many of which are acronyms, and many of which are quite fundamental to an many of which are quite fundamental to an understanding of what is going on in an understanding of what is going on in an ATM networkATM network

Examples: VCI, VPI, PVC, SVC, AAL, Examples: VCI, VPI, PVC, SVC, AAL, CBR, VBR, ABR, PCR, SCR, QOS, CDVCBR, VBR, ABR, PCR, SCR, QOS, CDV

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ReviewReview

ATM: Asynchronous Transfer ModeATM: Asynchronous Transfer Mode ATM is a statistical multiplexing technique ATM is a statistical multiplexing technique

for high speed integrated services networks, for high speed integrated services networks, based on the fast packet switching of small based on the fast packet switching of small fixed size (53 byte) packets called fixed size (53 byte) packets called cellscells

ATM is a ATM is a connection-orientedconnection-oriented low-layer low-layer networking conceptnetworking concept

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ATM is Connection-OrientedATM is Connection-Oriented

An end-to-end path called a An end-to-end path called a virtual channel virtual channel must must be set up in advance, using an ATM signalling be set up in advance, using an ATM signalling (control) protocol, before any data cells can be (control) protocol, before any data cells can be sentsent

All cells of a virtual channel travel on the same All cells of a virtual channel travel on the same pathpath

Cells arrive in the order that they were sentCells arrive in the order that they were sent Switches must maintain state about the virtual Switches must maintain state about the virtual

channels passing through themchannels passing through them

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DefinitionsDefinitions

Virtual Channel (VC)Virtual Channel (VC)– a connection between two communicating a connection between two communicating

ATM entities (e.g., host-switch, switch-switch)ATM entities (e.g., host-switch, switch-switch)– set up at time of call arrivalset up at time of call arrival– provides a certain grade of service provides a certain grade of service

(negotiated at time of call arrival)(negotiated at time of call arrival)– cell sequence is preservedcell sequence is preserved

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Definitions (Cont’d)Definitions (Cont’d)

Virtual Channel Identifier (VCI)Virtual Channel Identifier (VCI)– the label associated with a VCthe label associated with a VC– 16-bit integer in UNI ATM cell format16-bit integer in UNI ATM cell format– carried in ATM cell header for identificationcarried in ATM cell header for identification– note that VCI’s are locally significant only note that VCI’s are locally significant only

(i.e., assigned on a per link basis by the ATM (i.e., assigned on a per link basis by the ATM devices at either end of that link)devices at either end of that link)

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Definitions (Cont’d)Definitions (Cont’d)

Virtual Path (VP)Virtual Path (VP)– a group of virtual channels (VC’s) all travelling a group of virtual channels (VC’s) all travelling

between the same two points in ATM networkbetween the same two points in ATM network– used by the network to simplify provisioning, used by the network to simplify provisioning,

resource management, providing different grades resource management, providing different grades of service, etc.of service, etc.

– ““bundles up” traffic heading to same destinationbundles up” traffic heading to same destination

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Definitions (Cont’d)Definitions (Cont’d)

Virtual Path Identifier (VPI)Virtual Path Identifier (VPI)– the label associated with a VPthe label associated with a VP– 8-bit integer in UNI ATM cell format8-bit integer in UNI ATM cell format– carried in ATM cell header for identificationcarried in ATM cell header for identification– part of two level addressing scheme in ATMpart of two level addressing scheme in ATM– note that VPI’s are locally significant only note that VPI’s are locally significant only

(i.e., assigned on a per link basis by the ATM (i.e., assigned on a per link basis by the ATM devices at either end of that link)devices at either end of that link)

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Definitions (Cont’d)Definitions (Cont’d)

VP switch: an ATM switch that deals only VP switch: an ATM switch that deals only with the VPI’s in cell headers (e.g., a core with the VPI’s in cell headers (e.g., a core switch in middle of a large ATM network)switch in middle of a large ATM network)

VC switch: an ATM switch that deals only VC switch: an ATM switch that deals only with the VCI’s in cell headers (e.g., access with the VCI’s in cell headers (e.g., access switch at the edge of an ATM network)switch at the edge of an ATM network)

VP/VC switch: an ATM switch that deals VP/VC switch: an ATM switch that deals with both VPI’s and VCI’s in cell switchingwith both VPI’s and VCI’s in cell switching

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Definitions (Cont’d)Definitions (Cont’d)

Permanent Virtual Channel (PVC)Permanent Virtual Channel (PVC)– a virtual channel connection (virtual channel) a virtual channel connection (virtual channel)

that is set up on a long term basis (e.g., hours, that is set up on a long term basis (e.g., hours, days, months, years) by a human operatordays, months, years) by a human operator

– involves statically configuring the “routing involves statically configuring the “routing table” in ATM equipmenttable” in ATM equipment

– done as part of network provisioning in current done as part of network provisioning in current ATM network testbedsATM network testbeds

– supported by all ATM switch vendorssupported by all ATM switch vendors

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Definitions (Cont’d)Definitions (Cont’d) Switched Virtual Channel (SVC)Switched Virtual Channel (SVC)

– a virtual channel connection (virtual channel) that is set a virtual channel connection (virtual channel) that is set up by the ATM signalling protocol between two up by the ATM signalling protocol between two communicating ATM entitiescommunicating ATM entities

– set up on an as needed basis, and torn down when set up on an as needed basis, and torn down when completecomplete

– short term basis (e.g., seconds, minutes)short term basis (e.g., seconds, minutes)– involves dynamically configuring the “routing table” involves dynamically configuring the “routing table”

in ATM equipmentin ATM equipment– supported by very few ATM switch vendorssupported by very few ATM switch vendors

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Definitions (Cont’d)Definitions (Cont’d) ATM Adaptation Layer (AAL)ATM Adaptation Layer (AAL)

– a protocol for converting between higher layer a protocol for converting between higher layer protocol data units (PDU’s), such as TCP protocol data units (PDU’s), such as TCP packets, IP packets, or JPEG images, packets, IP packets, or JPEG images, and ATM cells for actual transmissionand ATM cells for actual transmission

– defines procedures for segmentation and defines procedures for segmentation and reassembly (SAR)reassembly (SAR)

– segmentation: packets to cells (done by sender)segmentation: packets to cells (done by sender)– reassembly: cells to packets (done by receiver)reassembly: cells to packets (done by receiver)

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Definitions (Cont’d)Definitions (Cont’d) Traffic DescriptorsTraffic Descriptors

– a numerical specification of the statistical a numerical specification of the statistical characteristics of an ATM traffic flowcharacteristics of an ATM traffic flow

– used by ATM switches at time of call setupused by ATM switches at time of call setup– specifies Peak Cell Rate (PCR), Sustained Cell Rate specifies Peak Cell Rate (PCR), Sustained Cell Rate

(SCR), Maximum Burst Size (MBS), etc.(SCR), Maximum Burst Size (MBS), etc.– different calls can specify different values for their different calls can specify different values for their

traffic descriptortraffic descriptor» e.g., voice: PCR = SCR = 1000 cells/sece.g., voice: PCR = SCR = 1000 cells/sec

» e.g., data: PCR = 10,000 cells/sec, SCR = 1000e.g., data: PCR = 10,000 cells/sec, SCR = 1000

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Definitions (Cont’d)Definitions (Cont’d) Quality of Service (QOS)Quality of Service (QOS)

– a specification of the desired (or acceptable) grade a specification of the desired (or acceptable) grade of service required for a traffic flowof service required for a traffic flow

– some traffic is delay-sensitive (e.g., voice)some traffic is delay-sensitive (e.g., voice)– some traffic is loss-sensitive (e.g., data)some traffic is loss-sensitive (e.g., data)– some traffic is both (e.g., compressed video)some traffic is both (e.g., compressed video)– some traffic is neither (e.g., LAN emulation)some traffic is neither (e.g., LAN emulation)– QOS requested at time of call setupQOS requested at time of call setup– ATM network tries to provide requested QOSATM network tries to provide requested QOS

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Definitions (Cont’d)Definitions (Cont’d) QOS ParametersQOS Parameters

– the parameters that can be specified as part of the the parameters that can be specified as part of the QOS request at time of call setupQOS request at time of call setup

– examples: cell loss ratio (CLR), mean cell delay, examples: cell loss ratio (CLR), mean cell delay, maximum tolerable cell delay, cell delay maximum tolerable cell delay, cell delay variation (CDV)variation (CDV)

– different calls can specify different values for their different calls can specify different values for their QOS parametersQOS parameters

» e.g., voice: delay < 50 msec, CLR < 0.001e.g., voice: delay < 50 msec, CLR < 0.001» e.g., data: delay < 1 sec, CLR < 0.000001e.g., data: delay < 1 sec, CLR < 0.000001

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Definitions (Cont’d)Definitions (Cont’d)

QOS ClassesQOS Classes– generic service classes for ATM trafficgeneric service classes for ATM traffic– used to help simplify the management and used to help simplify the management and

support of QOS requirements in ATM networkssupport of QOS requirements in ATM networks– currently there are five proposed classes: currently there are five proposed classes:

CBR, rt-VBR, nrt-VBR, ABR, and UBRCBR, rt-VBR, nrt-VBR, ABR, and UBR– class specification determines the order of class specification determines the order of

service for cells of different VCI’sservice for cells of different VCI’s

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Definitions (Cont’d)Definitions (Cont’d) Constant Bit Rate (CBR)Constant Bit Rate (CBR)

– simplest type of traffic: constant bit rate simplest type of traffic: constant bit rate (e.g., voice traffic, T1 circuit emulation)(e.g., voice traffic, T1 circuit emulation)

– this is the highest priority class because of the this is the highest priority class because of the delay-sensitive (i.e., time-dependent) nature of the delay-sensitive (i.e., time-dependent) nature of the traffic carriedtraffic carried

– suitable for periodic (isochronous) trafficsuitable for periodic (isochronous) traffic– need to specify only PCR (which equals SCR)need to specify only PCR (which equals SCR)– supported by some ATM switch vendorssupported by some ATM switch vendors

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Definitions (Cont’d)Definitions (Cont’d) Variable Bit Rate (VBR)Variable Bit Rate (VBR)

– more complicated type of traffic: the bit rate varies more complicated type of traffic: the bit rate varies with time (e.g., compressed video)with time (e.g., compressed video)

– peak bit rate (i.e., short term) may be much higher peak bit rate (i.e., short term) may be much higher than the mean bit rate (i.e., long term)than the mean bit rate (i.e., long term)

– must specify PCR, SCR, and burstinessmust specify PCR, SCR, and burstiness– next highest priority class(es)next highest priority class(es)– versions: real-time (rt) and non-real-time (nrt)versions: real-time (rt) and non-real-time (nrt)– supported by some ATM switch vendorssupported by some ATM switch vendors

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Definitions (Cont’d)Definitions (Cont’d) Available Bit Rate (ABR)Available Bit Rate (ABR)

– unpredictable type of traffic: traffic is willing to use unpredictable type of traffic: traffic is willing to use as much or as little bandwidth as is available (e.g., as much or as little bandwidth as is available (e.g., Internet traffic, LAN emulation)Internet traffic, LAN emulation)

– usually variable bit rate, delay-insensitiveusually variable bit rate, delay-insensitive– referred to as “elastic traffic” (e.g., ftp)referred to as “elastic traffic” (e.g., ftp)– next lowest priority classnext lowest priority class– supported by few ATM switch vendorssupported by few ATM switch vendors– still under discussion by ATM Forumstill under discussion by ATM Forum

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Definitions (Cont’d)Definitions (Cont’d)

Unspecified Bit Rate (UBR)Unspecified Bit Rate (UBR)– the “bottom feeder” in the ATM food chainthe “bottom feeder” in the ATM food chain– Minimum Cell Rate (MCR) is zero; guaranteed Minimum Cell Rate (MCR) is zero; guaranteed

nothing, but may get more than this if there nothing, but may get more than this if there happens to be extra bandwidth availablehappens to be extra bandwidth available

– lowest priority classlowest priority class– much discussion at ATM Forummuch discussion at ATM Forum– may become Guaranteed Frame Rate (GFR)?may become Guaranteed Frame Rate (GFR)?

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Definitions (Cont’d)Definitions (Cont’d) Call Admission Control (CAC)Call Admission Control (CAC)

– a control function in ATM switches that makes a control function in ATM switches that makes the decision on whether or not to accept a newly the decision on whether or not to accept a newly incoming callincoming call

– considers traffic descriptor (TD) and quality of considers traffic descriptor (TD) and quality of service (QOS) parameters, as well as impact on service (QOS) parameters, as well as impact on the QOS of existing calls in the networkthe QOS of existing calls in the network

– can be statistical or deterministiccan be statistical or deterministic– still an active research topicstill an active research topic

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Definitions (Cont’d)Definitions (Cont’d)

Usage Parameter Control (UPC)Usage Parameter Control (UPC)– a control function performed in switches to a control function performed in switches to

“police” ATM traffic flows“police” ATM traffic flows– monitors traffic on a VCI basis, to make sure monitors traffic on a VCI basis, to make sure

that it conforms to the declared traffic that it conforms to the declared traffic descriptor (TD)descriptor (TD)

– in case of violation, can tag cells with a in case of violation, can tag cells with a violation tag (CLP = 1), discard cells, or abort violation tag (CLP = 1), discard cells, or abort the connectionthe connection

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Definitions (Cont’d)Definitions (Cont’d)

Cell Loss Priority (CLP)Cell Loss Priority (CLP)– a single bit in ATM cell headers for denoting a single bit in ATM cell headers for denoting

the cell loss priority (e.g., violation cells)the cell loss priority (e.g., violation cells)– in the event of congestion, drop tagged cells in the event of congestion, drop tagged cells

before dropping any untagged cells before dropping any untagged cells – not to be confused with “delay priority” not to be confused with “delay priority”

(i.e., the service classes)(i.e., the service classes)

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SummarySummary

ATM has much confusing terminology, and ATM has much confusing terminology, and an endless (growing) list of TLA’s (three an endless (growing) list of TLA’s (three letter acronyms)letter acronyms)

Get used to it!Get used to it! Soon you will be speaking it too!!!Soon you will be speaking it too!!!