Asynchronous Transfer Mode (ATM) .pptx

8/14/2019 Asynchronous Transfer Mode (ATM) .pptx

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Muhammad Khurram Khalil

Ali Raza


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ISeek refuge in Allah with accursed devil

In the name of Allah, the most Gracious, the

most Merciful


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ATMis a high-speed networking standard designed tosupport both voice and data communications.

ATMoperates at the data link layer (Layer 2 in the OSImodel) over either fiber or twisted-pair cable.

ATMdiffers from more common data link technologieslike Ethernet in several ways. For example, ATMutilizes no routing. Hardware devices known asATMswitchesestablish point-to-point connections betweenendpoints and data flows directly from source todestination

ATMcellsare 53 bytes in length, that includes 48 bytes ofdata and five (5) bytes of header information.

ATMis connection-oriented and converts all incomingdata into a 53 byte cell.


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A Private ATM Network and a Public ATM Network Both Cancarry voice, video and data traffic.


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ATM is a cell-switching and multiplexing technology thatcombines the benefits of circuit switching (guaranteed capacityand constant transmission delay) with those of packet switching(flexibility and efficiency for intermittent traffic). It providesscalable bandwidth from a few megabits per second (Mbps) tomany gigabits per second (Gbps). ATM is more efficient thansynchronous technologies, such as time-division multiplexing(TDM). With TDM, each user is assigned to a time slot, and noother station send in that time slot. If a station has much data tosend, it can send only when its time slot comes up, even if all

other time slots are empty. however, if a station has nothing totransmit when its time slot comes up, the time slot is sent emptyand is wasted. Because ATM is asynchronous, time slots areavailable on demand with information identifying the source ofthe transmission contained in the header of each ATM cell.


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ATM transfers information in fixed-size units called cells.Each cell consists of 53 octets, or bytes. The first 5 bytes containcell-header information, and the remaining 48 contain thepayload (user information). Small, fixed-length cells are well

suited to transferring voice and video traffic because such traffic isintolerant of delays that result from having to wait for a large datapacket to download, among other things. Figure illustrates thebasic format of an ATM cell.


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An ATM network is made up of an ATM switch and ATMendpoints. An ATM switch is responsible for cell transit throughan ATM network. The job of an ATM switch is well defined: Itaccepts the incoming cell from an ATM endpoint or another ATMswitch. It then reads and updates the cell header information andquickly switches the cell to an output interface toward itsdestination. An ATM endpoint (or end system) contains an ATMnetwork interface adapter. Examples of ATM endpoints areworkstations, routers, etc.


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An ATM network consists of a set of ATM switchesinterconnected by point-to-point ATM links or interfaces. ATMswitches support two primary types of interfaces: UNIandNNI. The UNI connects ATM end systems (such as hosts androuters) to an ATM switch. The NNI connects two ATM

switches. Depending on whether the switch is owned andlocated at the customer's premises or is publicly owned andoperated by the telephone company, UNI and NNI can befurther subdivided into public and private UNIs and NNIs. Aprivate UNI connects an ATM endpoint and a private ATMswitch. Its public counterpart connects an ATM endpoint or

private switch to a public switch. A private NNI connects twoATM switches within the same private organization. A publicone connects two ATM switches within the same publicorganization. An additional specification, the broadbandintercarrier interface (B-ICI), connects two public switches fromdifferent service providers.


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ATM Interface Specifications Differ for Privateand Public Networks


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An ATM cell header can be one of two formats:UNI or NNI. The UNI header is used forcommunication between ATM endpoints andATM switches in private ATM networks. The

NNI header is used for communicationbetween ATM switches. Unlike the UNI, theNNI header does not include the Generic FlowControl (GFC) field. Additionally, the NNIheader has a Virtual Path Identifier (VPI) fieldthat occupies the first 12 bits, allowing forlarger trunks between public ATM switches.


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In addition to GFC and VPI header fields, several others are usedin ATM cell header fields. The following descriptions summarizethe ATM cell header fields

Generic Flow Control (GFC)Provides local functions, such as identifying multiple stations thatshare a single ATM interface. This field is typically not used and is

set to its default value of 0 (binary 0000).

Virtual Path Identifier (VPI) In conjunction with the VCI, identifies the next destination of a

cell as it passes through a series of ATM switches on the way toits destination.


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Virtual Channel Identifier (VCI)

In conjunction with the VPI, identifies the next

destination of a cell as it passes through a series ofATM switches on the way to its destination.

Payload Type (PT)Indicates in the first bit whether the cell contains userdata or control data. If the cell contains user data, thebit is set to 0. If it contains control data, it is set to 1.

The second bit indicates congestion (0 = no congestion,1 = congestion), and the third bit indicates whether thecell is the last in a series of cells that represent a singleAAL5 frame (1 = last cell for the frame).


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Cell Loss Priority (CLP)

Indicates whether the cell should be discarded if it

encounters extreme congestion as it moves through thenetwork. If the CLP bit equals 1, the cell should bediscarded in preference to cells with the CLP bit equalto 0.

Header Error Control (HEC)

Calculates checksum only on the first 4 bytes of theheader. HEC can correct a single bit error in thesebytes, thereby preserving the cell rather thandiscarding it.


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Three types of ATM services exist:Permanent virtual circuits (PVC), Switched virtual circuits(SVC),and Connectionless service (which is similar to SMDS). PVCallows direct connectivity between sites. In this way, a PVC issimilar to a leased line. Among its advantages, PVC guaranteesavailability of a connection and does not require call setupprocedures between switches. Disadvantages of PVCs includestatic connectivity and manual setup. Each piece of equipmentbetween the source and the destination must be manuallyprovisioned for the PVC. Furthermore, no network resiliency isavailable with PVC. An SVC is created and released dynamicallyand remains in use only as long as data is being transferred. In

this sense, it is similar to a telephone call. Dynamic call controlrequires a signaling protocol between the ATM endpoint and theATM switch. The advantages of SVCs include connectionflexibility and call setup that can be handled automatically by anetworking device. Disadvantages include the extra time andoverhead required to set up the connection.


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ATM networks are fundamentally connection-oriented, whichmeans that a virtual channel (VC) must be set up across the ATMnetwork prior to any data transfer. Two types of ATM connectionsexist: virtual paths, which are identified by virtual pathidentifiers, and virtual channels, which are identified by the

combination of a VPI and a virtual channel identifier (VCI). Avirtual path is a bundle of virtual channels, all of which areswitched transparently across the ATM network based on thecommon VPI.


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The basic operation of an ATM switch isstraightforward: The cell is received across a link on aknown VCI or VPI value. The switch looks up theconnection value in a local translation table to

determine the outgoing port (or ports) of theconnection and the new VPI/VCI value of theconnection on that link. The switch then retransmitsthe cell on that outgoing link with the appropriate

connection identifiers. Because all VCIs and VPIs haveonly local significance across a particular link, thesevalues are remapped, as necessary, at each switch.


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The ATM architecture uses a logical model to describe thefunctionality that it supports. ATM functionality corresponds tothe physical layer and part of the data link layer of the OSIreference model. The ATM reference model is composed of thefollowing planes, which span all layers:

Control

This plane is responsible for generating and managing signalingrequests.

User

This plane is responsible for managing the transfer of data.


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ManagementThis plane contains two components:Layer management manages layer-specific functions, such as thedetection of failures and protocol problems. Plane management managesand coordinates functions related to the complete system. The ATMreference model is composed of the following ATM layers:

Physical layer

Analogous to the physical layer of the OSI reference model, the ATMphysical layer manages the medium-dependent transmission.

ATM layer

Combined with the ATM adaptation layer, the ATM layer is roughly

analogous to the data link layer of the OSI reference model. The ATMlayer is responsible for the simultaneous sharing of virtual circuitsover a physical link (cell multiplexing) and passing cells through theATM network (cell relay). To do this, it uses the VPI and VCIinformation in the header of each ATM cell.


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ATM adaptation layer (AAL)Combined with the ATM layer, the AAL is roughly analogous tothe data link layer of the OSI model. The AAL is responsible forisolating higher-layer protocols from the details of the ATMprocesses. The adaptation layer prepares user data for conversion

into cells and segments the data into 48-byte cell payloads. Finally,the higher layers residing above the AAL accept user data, arrangeit into packets, and hand it to the AAL

The ATM Physical Layer

The ATM physical layer has four functions: Cells are convertedinto a bit stream, the transmission and receipt of bits on thephysical medium are controlled, ATM cell boundaries are tracked,and cells are packaged into the appropriate types of frames for thephysical medium. For example, cells are packaged differently forSONET than for DS-3/E-3 media types.


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The benefits of ATM are the following:

High performance via hardware switching

Dynamic bandwidth for busty traffic

Class- of service support for media

Scalability in speed and network size

Common LAN / WAN architecture

Opportunity for simplification via VCarchitecture

International standard compliance


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ATM is a technology that can handle all types of traffic(voice, video and data) multiplexed on the samenetwork in an ATM network band width can bereassigned in real time to different traffic based on

demand. ATM is the only technology common to allenvironments from LAN to GAN. The main reason forATMs popularly is its selection as switch technologyfor future BISDN service and can handle future

unknown service. ATM combines both circuit andpacket - switching modes and thus able to handletraffic with characters ties on the same network.


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ATM

By: William Marold Kochan Wood

Asynchronous Transfer Mode (ATM) .pptx

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