ASYNCHRONOUS TRANSFER MODEASYNCHRONOUS TRANSFER MODE

BY:

Er. Amit Mahajan

ATM – definitionATM – definition

A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".

High-speed transfer technology for voice, video, and data over public networks.

ATM VS X.25ATM VS X.25As 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.

X.25 was designed to support direct connection of terminals and computers over long distances.

X.25 packets may be of varying length,whereas ATM packets are of fixed size.

X.25, with its substantial overhead, is being recognized as an inadequate tool for wide area networking.

ATM VS FRAME RELAY ATM VS FRAME RELAY

ATM is normally utilized for high bandwidths of 34 Mbps and upwards.

At speeds of 2 Mbps and below, Frame Relay is more bandwidth efficient than ATM.

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.

Both frame relay and ATM take advantage of the reliability and fidelity of modern digital facilities to provide faster packet-switching than X.25.

ATM is even more streamlined than frame relay in its functionality, and can support data rates several orders of magnitude greater than frame relay.

Frame relay was developed as part of the work of ISDN

ATM was developed as part of the work on broadband ISDN

 

ATM VS SONETATM VS SONET

The industry offers two solutions for achieving a large integrated network: SONET/SDH and ATM .

The SONET is a globally accepted, non-proprietary standard for broadband transmission through fiber-optic cables.

It handles transmissions from 51 Mbps to 10 Gbps. SONET/SDH is a physical transport medium that occupies the two

bottom layers of OSI model. ATM is a high-speed packet switching technique suitable for LAN,

wide-area network and broadband ISDN. SONET is a physical structure, while ATM is a transmission

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

carry traffic over the network

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

An ATM endpoint contains an ATM network interface adapter. 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 A Transmission path (TP) is the physical connection

(wire,cable,satellite,and so on) between an end point and a switch or between two switches

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

A Virtual Channel (VC) provides the transport of ATM cells which 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 ATM layer 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

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

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

Generic Flow Control (GFC)—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.

Virtual Path Identifier (VPI)—The VPI is an 8-bit field in a UNI cell & a 12-bit field 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.

Virtual Channel Identifier (VCI)—VCI is 16-bit field in both. Payload Type (PT)—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 control data, 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).

Cell Loss Priority (CLP)—Indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. If the CLP bit equals 1, the cell should be discarded in preference to cells with the CLP bit equal to 0.

Header Error Control (HEC)—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

Was developed to enable two ATM concepts.

This layer is responsible for providing an interface between higher-layer protocols and the ATM layer.

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.

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

Class A services are data streams with a constant bit rate, running over established connections.

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.

Class C services are those carrying data messages on established connections. These are inherently variable bit-rate, as Class B. Examples include X.25 and Frame Relay.

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 such as 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

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.

AAL1 is designated for transporting constant bit rate (CBR) data, such as real-time voice and video traffic.

First byte in the normal 48-byte cell payload is used for cell sequencing and protection of the sequence number, limiting the actual payload to 47 bytes per AAL1-generated cell.

The AAL2 was intended to support a VBR . But now used for low-bit-rate traffic & short frame traffic such as

audio,video or fax .ex mobile telephony.

AAL3 is designed to transport delay-insensitive user data, such as Frame Relay, X.25, or IP traffic.

AAL3/4 uses four additional bytes beyond the cell header. The use of those bytes makes 44 bytes in the cell available for transporting the actual payload.

In comparison, AAL5 uses all 48 bytes beyond the cell header to transport the payload, providing a minimum 10% enhanced throughput in comparison to AAL3/4.

THE ATM CONNECTION-ESTABLISHMENT THE ATM CONNECTION-ESTABLISHMENT PROCESSPROCESS

ATM signalling uses the one-pass method of connection setup that is used in all modern telecommunication networks, such as the telephone network.

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 the connection 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 EMULATION LAN 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.

LANE Components

  LAN Emulation client (LEC)- It is an entity in an end system that performs data

forwarding, address resolution, and registration of MAC addresses with the LAN Emulation Server (LES). An ATM end system that connects to multiple ELANs has one LEC per ELAN. 

LES—The LES provides a central control point for LECs to forward registration and control information.When a station receives a frame to be sent to another station using a physical address , LEC sends a special frame to the LES.

The server creates a virtual circuit between the source & the destination station.The source station can now use this virtual circuit(& the corresponding identifier) to send the frame or frames to the destination.

Broadcast and Unknown Server (BUS)—Multicasting & Broadcasting require 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 all stations,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

LAN Emulation Configuration Server (LECS)—This is used for the initial connection between the client & LANE.

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

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

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

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

Traffic contract specifies an envelope that describes the intended data flow. When an ATM end system connects to an ATM network, it enters a contract with the network, based on QoS parameters.

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.

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 any switch 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 SCR : The Sustained cell rate is the average cell rate over a long time

interval.The actual cell rate may be lower or higher than this value, but the average should be equal to or less than the SCR.

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

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.

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 that the 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:

CLR : The cell loss ratio defines the fraction of cells lost(or delivered so late 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

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.

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

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

IP over ATMIP over ATM 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.

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.

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

 

For n entry points, n(n - 1) permanent VCs are needed to directly connect n entry/exit points. Each router interface that connects to the ATM network will have two addresses. The router interface will have an IP address, as usual, and the router will have an ATM address, which is essentially a LAN address.

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—ATM interconnects these four routers just as Ethernet can be used to connect four routers.

Let us refer to the router at which the datagram enters the ATM network as the “entry router” and the router at which the datagram leaves the network as the “exit router.”

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 exit router

3. To move the datagram to the next router, the physical address of the next-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.

ATM must now move the datagram to the ATM destination address. This task has two subtasks:

Determine the VCI for the VC that leads to the ATM destination address. Segment the datagram into cells at the sending side of the VC (that is, at the

entry router), and reassemble the cells into the original datagram at the receiving side of the VC (that is, at the exit router).

ATM uses AAL5 to provide a more efficient way to segment and reassemble a datagram. Recall that IP in the entry router passes the datagram down to ATM along with the ATM address of the exit router. ATM in the entry router indexes an ATM table to determine the VCI for the VC that leads to the ATM destination address. AAL5 then creates ATM cells out of the IP datagram:

The datagram is encapsulated in a CPCS-PDU using the format in fig. The CPCS-PDU is chopped up into 48-byte chunks. Each chunk is placed in the

payload field of an ATM cell. All of the cells except for the last cell have the third bit of the PT field set to 0.

The AAL_indicate bit is used to reassemble IP datagrams from ATM cells.

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.

The ATM network then moves each cell across the network to the ATM destination 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.

When the cells arrive at the ATM destination address, they are directed to an 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: Provides scalable bandwidth from a few megabits per second (Mbps) to

many gigabits per second (Gbps). 

Fixed-length cells enables low-cost hardware to be developed to perform required cell switching based on the contents of the cell header, without requiring more complex and costly software

Fixed-size cells allow ATM to support quantifiable QoS

Because of its asynchronous nature, ATM is more efficient than synchronous technologies, such as TDM.

Simplified Network Management.

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

Overhead of cell header (5 bytes per cell) Complex mechanisms for achieveing QoS Congestion may cause cell lossesATM handles data traffic smoothly, but runs into delay

problems with voice transmissions.

CONCLUSIONCONCLUSION ATM is a high-speed packet switching technique suitable for LAN, wide-area

network and broadband ISDN (integrated services digital network) transmissions.

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 performance requirements.

ATM is ideally suited for converged voice,data and video networks because it assures quality of service.

It also provides the high amounts of bandwidth that businesses are increasingly demanding for data and other applications.

Frame relay, on the other hand, continues to be a highly economical and reliable choice, especially for medium-speed, data only applications.

It provides scalable bandwidth from a few megabits per second (Mbps) to many gigabits per second (Gbps). ATM provides no retransmissions on a link-by-link basis.

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

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8493-1805-X.-22/2/09 Ieeexplore.ieee.org/ieI4/5541/14857/00675184.pdf-5/3/09 http://www.cisco.com/en/US/docs/internetworking/technology/handbook/atm.html-15/3/09 www.pcc.qub.ac.uk/tec/courses/network/ATM/ATM-15/3/09 www.npac.syr.edu/users/mahesh/homepage/atm_tutorial/-15/3/09 www.mhhe.com/frouzen/Dcn4Sie-18/3/09 http://www.atmforum.com for ATM standards document-28/3/09 Ginsburg, David. ATM: Solutions for Enterprise Internetworking. Boston: Addison-Wesley Publishing Co, 1996-

3/4/09 Clark, Kennedy, and Kevin Hamilton. CCIE Professional Development: Cisco LAN Switching.Indianapolis: Cisco

Press, 1999.24/4/09 Behrouz A Forouzan,Fourth edition,Data communications & networking

 

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