Presenters: Shahid Iqbal &

Haris Tariq

ASYNCHRONOUS TRANSFER MODE

WHAT IS ATM? ATM is a cell-switching and multiplexing

technology that combines the benefits of circuit

switching (guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency for intermittent traffic)

Major AdvantagesBandwidth EfficiencyScalabilityTransparencyNetwork Flexibility

The B-ISDN Reference Model

THE ATM PROTOCOL STACK

THE ATM PROTOCOL STACKA 3 stage processo Segmentation of user data into cellso Transfer of cells through the networko Reconstruction of the original information at the receiving end

The ATM Adaptation layer (AAL)Divides higher level data streams into

segmentsThese segments are more compatible with

the requirements of the ATM LayerThe AAL process depends on the type of

traffic being transmitted

Types of traffic 5 Classes of Traffic

Class A: Constant Bit Rate (CBR)

Class B: Variable Bit Rate (VBR)

Connection-orientedRequired timing

relationship between source and destination [constant bit-rate video]

Connection-orientedRequired timing

relationship between source and destination [packet-video, voice]

Types of trafficClass C: Variable Bit Rate (VBR)

Class D: Variable Bit Rate (VBR)

Connection-oriented No timing relationship

required between source and destination [Bursty data traffic]

ConnectionlessNo timing relationship

required between source and destination [Bursty datagram traffic]

Types of traffic Class X: User defined

The AAL ProcessDivided into two sub layers:

o The convergence sub layer (CS) o The segmentation and reassembly sub layer

(SAR)

The convergence Sub Layer (CS)Divides the application data stream into

rational, fixed length protocol data units (PDUs)

Length of PDU depends on the nature of the traffic being passed (such as 64 Kbytes for Class D traffic)

Appends header and trailer to the CS payload

The SAR Sub LayerDivides CS-PDUs into small segments

ranging from 44 to 48 bytes in length depending on the type of traffic being transmitted

To form SAR-PDU header and trailer information is

added to the basic payload segment

AAL Process

The ATM LayerThe heart of the ATM communication processPackages data, received from the AAL into

cellsThe ATM layer operates between adjacent

network nodes

Physical LayerDefines the various transmission medium

used for the transmission of the ATM cellDivided into two parts:

o The Physical Medium Dependent (PMD) Sub layer

o The Transmission Convergence(TC) Sub layer

The PMD Sub LayerAssociated with a given media type and

transmission speedDeals with the actual transfer of information

bits between adjacent nodes of the networkConcerned with specific characteristics of a

particular transmission technology

The TC Sub LayerIndependent of the underlying physical mediaProvides necessary framing and convergence

operations required to fit the ATM cells into the physical transport media

Calculates and inserts the header error control information into the HEC field of the ATM cell header; and, before passing a recovered cell to the ATM layer at the receiving end, checks the received HEC field and discards the cell if an error is indicated

The ATM Protocol Data Unit : The CellThe basic unit of ATM information transfer53-byte in lengthA 5-byte header and a 48-byte payload field

The ATM Cell Structure

The ATM Cell FieldsGFC(Generic Flow

control) The last four bits of

byte-1 form the generic flow control

Used to control the flow of traffic across the User-Network Interface (UNI)

VPI/VCIThis three-byte field

makes the ATM address

First byte contains virtual path identifier (VPI)

Next two bytes contain the virtual channel identifier (VCI)

The ATM Cell FieldsPT(Payload Type)Differentiates

between user data and control messages

Indicates if a cell has experienced congestion

CLP(Cell Loss Priority)

The first bit of byte-4

Cells with a CLP set to “1” are subject to discard prior to cells for which the CLP is “0” during periods of network congestion

The ATM Cell FieldsHEC(Header Error

Control)The final byte of the

ATM cell headerDetects errors in the

cell header but not the Payload

provides protection against miss-routing of ATM cells due to addressing errors

PAYLOAD FieldThe remaining 48

bytes of the ATM Cell are reserved for the data inserted by the AAL

ATM TOPOLOGYDefines how the various network components

and user terminal devices are interconnectedInterfaces are divided into two groups

o User-to-network interface (UNI) o Network-to-network or node interface (NNI)

The UNIThree variations in UNI

o Public UNIo Private UNIo Residential UNI

The NNIThree versions of the NNI

o Public NNIo Private NNI(PNNI)o Broadband Inter-Carrier Interface (B-ICI)

ATM TOPOLOGY

THE ATM COMMUNICATIONS PROCESSBased on a simple and fast transport relay

mechanismCall connection, termination, addressing,

sequencing, and payload error control are handled at the trans-mitting and receiving ends

Association between User and Network Functions

ATM NetworkingConnection-oriented serviceEstablishes connections between adjacent

Layer 2 ATM entitiesUses virtual channels and virtual paths to

establish end-to-end virtual circuit connections

ATM Transmission PathVirtual Channel (VC)Virtual Path (VP)Virtual Circuit Connection (VCC)

ATM Transmission Path

ATM SwitchingATM switches operate very differently from

analog or digital switchesATM switches use self-routing proceduresA self-routing switch relies on information

contained in the VPI/VCI fields of the cellThe switch adds a temporary routing header

or “tag” to the cell

Routing in a Switch

ATM SignalingATM end-users require the capability to

establish connections across the networkTwo Types of End-to-End Connections

o Permanent Virtual Circuit (PVC)o Switched Virtual Circuit(SVC)

The signaling mechanism used for ATM is based on ISDN Signaling System Number 7 (SS7)

ATM SignalingThe Q.2931 signaling protocol with Signaling ATM Adaptation Layer (SAAL) is adapted for ATM Signaling

Signaling ATMAdaptation Layer (SAAL)The SAAL provides reliable transport of

signaling messages between two ATM systems

Composed of two sub layerso Common part o Service-specific part

Service-Specific PartSubdivided into Service-Specific Coordination Function

(SSCF) Service-Specific Connection Oriented

Protocol (SSCOP)

Virtual Circuit Connection (VCC)Types

4 ClassesPoint-to-point (pt-pt)Point-to-multipoint (pt-mpt)Multipoint-to-point (mpt-pt)Multipoint-to-multipoint (mpt-mpt)

ATM Addressing FormatATM-connected devices use an ATM end-system address (AES)

Initial Domain Part (IDP)This part contains Address Format Identifier (AFI)Initial Domain Identifier (IDI)The IDI

o Data Country Code (DCC)o International Code Designator (ICD)

Domain Specific Part (DSP)ContainsHigh-Order Part (HO-DSP) Lower-Order Part (LO-DSP)

THANK YOU

High-Order Part (HO-DSP) ContainsDomain Format Identifier (DFI)Address Authority (AA)Routing Domain (RD) Area

Lower-Order Part (LO-DSP)

ContainsEnd-System Identifier (ESI) Selector (SEL)

THANK YOU