24139085 Asynchronous Transfer Mode

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ASYNCHRONOUS TRANSFER MODEASYNCHRONOUS TRANSFER MODE

BY:

Er. Amit Mahajan



ATM VS X.25ATM VS X.25y As the speed and number of local area networks (LANs)

continue their relentless growth, increasing demand is

place on wide area packet-switching networks to support

the tremendous throughput generated by these LANs.

y X.25 was designed to support direct connection of

terminals and computers over long distances.

y X.25 packets may be of varying length,whereas ATM

packets are of fixed size.

y X.25, with its substantial overhead, is being recognized

as an inadequate tool for wide area networking.



ATM VS FRAME RELAYATM VS FRAME RELAY

y ATM is normally utilized for high bandwidths of 34 Mbps

and upwards.

y At speeds of 2 Mbps and below, Frame Relay is more

bandwidth efficient than ATM.

y ATM transmits only fixed-size frames, called cells, not

variable-sized frames as frame relay do.

There are two main drivers that caused businesses migrate

from frame relay service to ATM :

1. The need for greater capacity than frame relay can handle.

2. The need to support mixed-media traffic, especially voice

and video.



ATM VS SONETATM VS SONET

y The industry offers two solutions for achieving a large integrated

network: SONET/SDH and ATM .

y The SONET is a globally accepted, non-proprietary standard for

broadband transmission through fiber-optic cables.

y It handles transmissions from 51 Mbps to 10 Gbps.

y SONET/SDH is a physical transport medium that occupies the two

bottom layers of OSI model.

y ATM is a high-speed packet switching technique suitable for LAN,

wide-area network and broadband ISDN.

y SONET is a physical structure, while ATM is a transmission

protocol.

y If the ATM protocol is used, a transport medium is still needed to

carry traffic over the network



ARCHITECTURE:ATM DevicesARCHITECTURE:ATM Devicesy An ATM network is made up of an ATM switch and ATM endpoints.

y ATM switch accepts the incoming cell from an ATM endpoint or another

ATM switch. It then reads and updates the cell header information and

quickly switches the cell to an output interface toward its destination.

y An ATM endpoint contains an ATM network interface adapter.

y Examples of ATM endpoints are workstations, routers, LAN switches, and

video coder-decoders (CODECs).

ATM Network Comprises ATM Switches and Endpoints



ATM virtual connectionsATM virtual connections

y A Transmission path (TP) is the physical connection(wire,cable,satellite,and so on) between an end point and aswitch or between two switches

y

A Virtual Path (VP) transports ATM cells belonging to virtualchannels which share a common identifier, called the VirtualPath Identifier VPI. Connects two switches.

y A Virtual Channel (VC) provides the transport of ATM cellswhich have the same unique identifier, called the Virtual

Channel Identifier (VCI).

ATM virtual connections



ATM protocol reference modelATM protocol reference model

The Physical Layer

This layer describes the physical transmission of information through an

ATM network.

The original design of ATM was based on SONET because high data rate of

SONET¶s carrier , the boundaries of cells can be clearly defined .

SONET specifies the use of pointer to define the beginning of a payload.

Types of physical media specified for ATM include shielded and unshielded

twisted-pair, coaxial cable, and fiber-optic cable, which provide cell transport

capabilities ranging from a T1 rate of 1.544Mbps to a SONET range of

622Mbps.



The ATM Layer

The ATM layer represents the physical interface between

the ATM Adaptation layer (AAL) and the Physical layer.

Thus, the ATM layer is responsible for relaying cells

from the AAL to the Physical layer for transmission, and

in the opposite direction from the Physical layer to the

AAL for use in an endpoint.

When transporting cells to the Physical layer, the ATMlayer is responsible for generating the five-byte cell

header for each cell. When receiving cells from the

Physical layer, the ATM layer performs a reverse

operation, extracting the five-byte header from each cell.



ATM cell header format

y ATM transfer information in fixed-size units called cells.

y An ATM cell header can be one of two formats: UNI or NNI. The UNI

header is used for communication between ATM endpoints and ATM

switches in private ATM networks. The NNI header is used for

communication between ATM switches.

y Each cell consists of 53 octets, or bytes. The first 5 bytes contain cell-

header information, and the remaining 48 contain the payload (user

information).



ATM Cell Header Fields

y Generic Flow Control (GFC) ² The 4-bit GFC field provides flow control at the

UNI level.The ITU-T has determined that this level of flow control is not necessary

at the NNI.

y Virtual Path Identifier (VPI) ² The VPI is an 8-bit field in a UNI cell & a 12-bitfield in an NNI cell.In conjunction with the VCI, identifies the next destination of a

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

y Virtual Channel Identifier (VCI) ² VCI is 16-bit field in both.

y Payload Type (PT) ² Indicates in the first bit whether the cell contains user data or

control data. If the cell contains user data, the bit is set to 0. If it contains controldata, it is set to 1. The second bit indicates congestion (0 = no congestion, 1 =

congestion), and the third bit indicates whether the cell is the last in a series of cells

that represent a single AAL5 frame (1 = last cell for the frame).

y Cell Loss Priority (CLP) ² Indicates whether the cell should be discarded if it

encounters extreme congestion as it moves through the network. If the CLP bitequals 1, the cell should be discarded in preference to cells with the CLP bit equal to

0.

y Header Error Control (HEC) ² Calculates checksum only on the first 4 bytes of

the header. HEC can correct a single bit error in these bytes, thereby preserving the

cell rather than discarding it.



The ATM Adaptation Layer

y Was developed to enable two ATM concepts.

y This layer is responsible for providing an interface between higher-layer

protocols and the ATM layer.

y AAL maps the data stream originated by the higher-layer protocol into the

48-byte payload of ATM cells, with the header placement being assigned by

the ATM layer.

y In the reverse direction, the AAL receives the payload of ATM cells in 48-

byte increments from the ATM layer and maps those increments into the

format recognized by the higher-layer protocol



y Class A services are data streams with a constant bit rate, running over

established connections.y Class B services are similar, but instead of being locked to a regular

data rate they send 'peaks' of data at some times, and little or none at

others. Examples include compressed video.

y

Class C services are those carrying data messages on establishedconnections. These are inherently variable bit-rate, as Class B.

Examples include X.25 and Frame Relay.

y Class D services are the so called connectionless datagrams, where a

packet of data is sent into the network and contains its own destination

address. Examples include many traditional local-area networks suchas Ethernet, wide area networks and the new switched multimegabit

Class Timing Relationship Bit Rate Type of Connection

A Yes Constant Connection-oriented

B Yes Variable Connection-oriented

C No Variable Connection-oriented

D No Variable Connectionless

The ATM Application Classes



y In AAL1 cell payload the Sequence Number Protection (SNP) field protects

the Sequence Number (SN) field from the effect of bit errors occurring

during transmission, in effect providing a forward error detection and

correction capability.

y AAL1 is designated for transporting constant bit rate (CBR) data, such

as real-time voice and video traffic.

y First byte in the normal 48-byte cell payload is used for cell sequencingand protection of the sequence number, limiting the actual payload to

47 bytes per AAL1-generated cell.



THE ATM CONNECTIONTHE ATM CONNECTION--ESTABLISHMENTESTABLISHMENT

PROCESSPROCESS

y ATM signalling uses the one-pass method of connection setup that is used

in all modern telecommunication networks, such as the telephone network.

y First, a source end system sends a setup message, which is forwarded to the first

ATM switch (ingress switch) in the network. This switch sends a call

proceeding message and invokes an ATM routing protocol. The signaling

request is propagated across the network. The exit switch (called the egress

switch) that is attached to the destination end system receives the setup

message. The egress switch forwards the setup message to the end system

across its UNI, and the ATM end system sends a connect message if theconnection is accepted. The connect message traverses back through the

network along the same path to the source end system, which sends a connect

acknowledge message back to the destination to acknowledge the connection.

Data transfer can then begin.



ATM LAN EMULATIONATM LAN EMULATIONLAN Emulation (LANE) is a standard defined by the ATM Forum that gives

to stations attached via ATM the same capabilities that they normally obtain

from legacy LANs.

The LANE protocol defines mechanisms for emulating either an IEEE 802.3

Ethernet or an 802.5 Token Ring LAN.

The LANE protocols make an ATM network look and behave like an Ethernet or

Token Ring

LANE requires no modifications to higher-layer protocols to enable their

operation over an ATM network.



y

Broadcast and Unknown Server (BUS) ² Multicasting & Broadcastingrequire the use of another server called the broadcast/unknown

server(BUS).If a station needs to send a frame to a group of stations or to

every station,the frame first goes to the bus;this server has permanent

virtual connnection to every station.The server creates copies of the

received frame & sends a copy to a group of stations or to allstations,simulating a multicasting or broadcasting process.The server can

also deliver a unicast frame by sending the frame to every station.In this

case the destination address is unknown.This is sometimes more efficient

then getting the connection identifier from the LES

y LAN Emulation Configuration Server (LECS) ² This is used for the

initial connection between the client & LANE.

y This server is always waiting to receive the initial contact.It has wellknown ATM address that is known to every client in the system.

y The LECS maintains a database of LECs and the ELANs to which they

belong.



ATM QUALITY OF SERVICEATM QUALITY OF SERVICEQuality-of-service (QoS) that guarantees traffic contract, traffic shaping,

and traffic policing is based on the service class , user related attributes,&

network-related attributes.

y

Traffic contract specifies an envelope that describes the intended dataflow. When an ATM end system connects to an ATM network, it enters a

contract with the network, based on QoS parameters.

y Traffic shaping is the use of queues to constrain data bursts, limit peak

data rate, and smooth jitters so that traffic will fit within the promised

envelope.y ATM switches can use traffic policing to enforce the contract. The switch

can measure the actual traffic flow and compare it against the agreed-upon

traffic envelope. If the switch finds that traffic is outside of the agreed-upon

parameters, it can set the cell-loss priority (CLP) bit of the offending cells.

Setting the CLP bit makes the cell discard eligible, which means that anyswitch handling the cell is allowed to drop the cell during periods of

congestion for the multimedia applications and provide overall

optimization of network resources



Service Class Quality of Service Parameter

constant bit rate (CBR)

It is designed for customers who need real time audio or video services. The cell

rate is constant with time. CBR applications are quite sensitive to cell-delay

variation. Examples of applications that can use CBR are telephone traffic ,

videoconferencing, and television.

variable bit rate±non-real time

(VBR±NRT)

This class allows users to send traffic at a rate that varies with time depending on

the availability of user information.It is designed for those users who do not need

real time services but use compression techniques to create a variable bit rate.Multimedia e-mail is an example of VBR±NRT.

variable bit rate±real time

(VBR±RT)

This class is similar to VBR±NRT but is designed for applications that are

sensitive to cell-delay variation. It is designed for those users who need real time

services & use compression techniques to create a variable bit rate. Examples for

real-time VBR is interactive compressed video.

available bit rate (ABR)

This class of ATM services provides rate-based flow control and is aimed at data

traffic such as file transfer and e-mail. Although the standard does not require the

cell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable

for switches to minimize delay and loss as much as possible. Depending upon the

state of congestion in the network, the source is required to control its rate. The

users are allowed to declare a minimum cell rate.If more network capacity is

available, this minimum rate can be exceeded.ABR is particularly suitable for

applications that are bursty.

unspecified bit rate (UBR)This class is a best effort delivery service that does not guarantee anything and is

widely used today for TCP/IP.



User related attributes

y SCR : The Sustained cell rate is the average cell rate over a long timeinterval.The actual cell rate may be lower or higher than this value, but the

average should be equal to or less than the SCR.

y

PCR : The peak cell rate defines the sender¶s maximum cell rate. Theuser¶s cell rate can sometimes reach this peak,as long as the SCR is

maintained.

y MCR : The minimum cell rate defines the minimum cell rate acceptable to

the sender.For example,if the MCR is 50,000, the network must guarantee

that the sender can send atleast 50,000 cells per second.

y CVDT : The cell variation delay tolerance is a measure of the variation in

cell transmission times.For example,if the CVDT is 5 ns ,this means thatthe difference between the minimum & the maximum delays in delivering

the cells should not exceed 5 ns.



Network related attributes

The network related attributes are those that define characteristics of the

network.The following are some network related attributes:

y CLR : The cell loss ratio defines the fraction of cells lost(or delivered solate that they are considered lost) during transmission.For example, if the

sender sends 100 cells & one of them is lost,the CLR is

CLR = 1/100 = 10-2

y CTD : The cell transfer delay is the average time needed for a cell to travel

from source to destination. The maximum CTD & the minimum CTD also

considered attributes.

y CDV : The cell delay variation is the difference between CTD maximum &

the CTD minimum.

y CER : The cell error ratio defines the fraction of cells delivered in error.



IP over ATMIP over ATMy When IP works with ATM , the IP packets are segmented into fixed length cells

of ATM, transmitted through the ATM network, & then reassembled into IP packets at the receiving end.

y Each entry/exit point is a router. An ATM backbone can span an entire continent

and may have tens or even hundreds of ATM switches.

y Most ATM backbones have a permanent virtual channel (VC) between each pair of entry/exit points.



y For n entry points, n(n - 1) permanent VCs are needed to directlyconnect n entry/exit points. Each router interface that connects to the

ATM network will have two addresses. The router interface will havean IP address, as usual, and the router will have an ATM address, whichis essentially a LAN address.

y Consider now an IP datagram that is to be moved across the ATM backbone To four IP routers, the backbone appears as a single logical

link ² ATM interconnects these four routers just as Ethernet can be usedto connect four routers.

y Let us refer to the router at which the datagram enters the ATM network as the ³entry router´ and the router at which the datagram leaves thenetwork as the ³exit router.´

y The entry router does the following:

1. Examines the destination address of the datagram.

2. Indexes its routing table and determines the IP address of the exitrouter

3. To move the datagram to the next router, the physical address of thenext-hop router must be determined.

4. IP in the entry router then passes down to the link layer (that is, ATM)the datagram along with the ATM address of the exit router.



y The last cell has the bit set to 1. AAL5 then passes the cells to the ATM

layer. ATM sets the VCI and CLP fields and passes each cell to the TC

sublayer. For each cell, the TC sublayer calculates the HEC and inserts it in

the HEC field. The TC sublayer then inserts the bits of the cells into the

PMD sublayer.

y

The ATM network then moves each cell across the network to the ATMdestination address. At each ATM switch between the ATM source and the

ATM destination, the ATM cell is processed by the ATM physical and ATM

layers, but not by the AAL layer. At each switch the VCI is typically

translated and the HEC is recalculated.

y When the cells arrive at the ATM destination address, they are directed toan AAL buffer that has been put aside for the particular VC. The CPCS-

PDU is reconstructed using the AAL_indicate bit to determine which cell is

the last cell of the CPCS-PDU. Finally, the IP datagram is extracted out of

the CPCSPDU and is passed up the protocol stack to the IP layer



ATM Advantages:ATM Advantages:y Provides scalable bandwidth from a few megabits per second (Mbps) to

many gigabits per second (Gbps).

y

Fixed-length cells enables low-cost hardware to be developed to performrequired cell switching based on the contents of the cell header, without

requiring more complex and costly software

y Fixed-size cells allow ATM to support quantifiable QoS

y Because of its asynchronous nature, ATM is more efficient than

synchronous technologies, such as TDM.

y Simplified Network Management.

y 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 ).



ATM disadvantagesATM disadvantages

y Overhead of cell header (5 bytes per cell)

y Complex mechanisms for achieveing QoS

y Congestion may cause cell losses

y ATM handles data traffic smoothly, but runs into delay

problems with voice transmissions.



CONCLUSIONCONCLUSIONy

ATM is a high-speed packet switching technique suitable for LAN, wide-area network and broadband ISDN (integrated services digital network)

transmissions.

y The decision of when to use ATM and when to use frame relay largely

depends on the applications businesses want to run over their enterprise

networks, the amount of bandwidth they need and their performancerequirements.

y ATM is ideally suited for converged voice,data and video networks because

it assures quality of service.

y It also provides the high amounts of bandwidth that businesses are

increasingly demanding for data and other applications.

y Frame relay, on the other hand, continues to be a highly economical and

reliable choice, especially for medium-speed, data only applications.

y It provides scalable bandwidth from a few megabits per second (Mbps) to

many gigabits per second (Gbps). ATM provides no retransmissions on alink-by-link basis.

y Combining the ATM & SONET offers scalability and flexibility. While

Asynchronous Transfer Mode (ATM) and Synchronous Optical Network

(SONET) technologies are still emerging technologies, the combination of

the two will drastically alter future corporate LAN design



REFERENCESREFERENCES

y Martin De Prycker , Asynchronous Transfer Mode. Solutions for Broadband ISDN

(Prentice-Hall, 1993) -19/2/09

y William Stallings,ISDN & Broadband ISDN with frame relay & ATM(Prentice-

Hall,1999).

y P.S. Neelakanta A Textbook on ATM Telecommunications, Principles and

implementation. CRC Press. 2000 ISB N 0-8493-1805-X.-22/2/09

y Ieeexplore.ieee.org/ieI4/5541/14857/00675184.pdf-5/3/09

y http://www.cisco.com/en/US/docs/internetworking/technology/handbook/atm.html-

15/3/09

y www.pcc.qub.ac.uk/tec/courses/network/ATM/ATM-15/3/09

y www.npac.syr.edu/users/mahesh/homepage/atm_tutorial/-15/3/09

y www.mhhe.com/frouzen/Dcn4Sie-18/3/09

y http://www.atmforum.com for ATM standards document-28/3/09

y Ginsburg, David. ATM: Solutions for Enterprise Internetworking. Boston: Addison-Wesley Publishing Co, 1996-3/4/09

y Clark, Kennedy, and Kevin Hamilton. CCIE Professional Development: Cisco LAN

Switching.Indianapolis: Cisco Press, 1999.24/4/09

y Behrouz A Forouzan,Fourth edition,Data communications & networking



THANK YOU

24139085 Asynchronous Transfer Mode

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