atm seminar

8/3/2019 ATM Seminar

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1. Introduction

Asynchronous Transfer Mode (ATM) is a standard switching

technique designed to unify telecommunication and computer

networks. It uses asynchronous time-division multiplexing, and

it encodes data into small, fixed-sized cells.

This technology differs from approaches such as the Internet

Protocol or Ethernet that use variable sized packets or frames.

ATM provides data link layer that run over a wide range of OSI

physical layer links. ATM has functional similarity with both

circuit switched before the actual data exchange begins.

Networking and small packet switched networking. It

represents a relatively recently developed communications

technology designed to overcome the constraints associated

with traditional and for the most part separate, voice and data

networks. ATM uses a connection-oriented model in which a

virtual circuit must be established between two endpoints

ATM has its roots in the work of a CCITT (now known as ITU-T)

study group formed to develop broadband ISDN standards

during the mid-1980s. In 1988, a cell switching technology was

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chosen as the foundation for broadband ISDN, and in 1991, the

ATM Forum was founded.

2. The Rationale and Underlying Technology

ATM can be considered to represent a unifying technology

because it was designed to transport voice, data, and video

(including graphics images) on both local and wide area

networks. Until the development of ATM, networks were

normally developed based on the type of data to be

transported. Thus, circuit-switched networks, which included

the public switched telephone network and high-speed digital

transmission facilities, were primarily used to transport delay-

sensitive information, such as voice and video. In comparison,

on packet-based networks, such as X.25 and Frame Relay,

information can tolerate a degree of delay. Network users can

select a networking technology to satisfy a specific

communications application, but most organizations support a

mixture of applications. Thus, most organizations are forced to

operate multiple networks, resulting in a degree of inefficiency

and escalating communications costs. By combining the

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features from both technologies, ATM enables a single network

to support voice, data, and video.

ATM is designed to be scalable, enabling its 53-byte cell to be

transported from LAN to LAN via WAN, as well as for use on

public and private wide area networks at a range of operating

rates. On LANs, ATM support is currently offered at 25 and

155Mbps, whereas access to WAN-based ATM carrier networks

can occur at T1 (1.544Mbps), at T3 (45Mbps), or via different

SONET facilities at data rates up to 622Gbps, all based on the

transportation of 53-byte cells. A key to ATM's ubiquitous

transmission capability is its fixed 53-byte cell length, which

remains static regardless of changes in media, operating rates,

or framing.

The use of a fixed-length cell 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. Thus, ATM can be considered to represent

a unifying technology that will eventually become very

economical to implement when its development expenses are

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amortized over the growing production cycle of ATM

communications equipment.

Although many organizations merged voice and data through

the use of multiplexers onto a common circuit, this type of

merger is typically not end-to-end. For example, traffic from a

router connected to a LAN might be fed into a port on a high-

speed multiplexer with another connection to the multiplexer

from the company PBX. Although this type of multiplexing

enables a common WAN circuit to be used for voice and data, it

represents an interim and partial solution to the expense

associated with operating separate voice and data networks. In

addition, the emergence of multimedia applications requiring

the transmission of video can wreak havoc with existing LANs

and WANs due to their requirement for high bandwidth for

short periods. ATM represents an emerging technology

designed to provide support for bandwidth-on-demand

applications, such as video, as well as voice and data. A

comparison of the key features associated with each

technology can give you an appreciation for ATM technology in

comparison to conventional data communications- and

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telecommunications-based technology. Table 2.1 compares

nine features of data communications and telecommunications

networks with those of an ATM network.

In a data communications environment, the network can range

in scope from a token-ring LAN to an X.25 or Frame Relay

WAN. Thus, although some features are common to both LAN

and WAN environments, there is also some variability. In

general, a data communications network transports data by

using variable-length packets. Although many WAN protocols

are connection-oriented, some are connectionless. Similarly,

many LAN protocols are connectionless, whereas others are

connection-oriented. Because data communications networks

were designed to transport files, records, and screens of data,

transmission delay or latency, if small, does not adversely

affect users. In comparison, in a telecommunications network,

a similar amount of latency that is acceptable on a data

network could wreak havoc with a telephone conversation.

Recognizing the differences among voice, video, and data

transportation, ATM was designed to adapt to the time

sensitivity of different applications. It includes different classes

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of service that enable the technology to match delivery to the

time sensitivity of the information it transports.

Table 2.1 Comparing Network Features

Feature

Data

Communications Telecommunications ATM

Traffic support Data Voice Data, voice,

video

Transmission

unit

Packet Frame Cell

Transmission

length

Variable Fixed Fixed

Switching type Packet Circuit Cell

Connection

type

Connectionless or

Connection-oriented

Connection-oriented Connection-

oriented

Time sensitivity None to some All Adaptive

Delivery Best effort Guaranteed Defined class

or guaranteed

Media and

operating rate

Defined by protocol Defined by class Scalable

Media access Shared or dedicated Dedicated Dedicated

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Thus, ATM provides a mechanism for merging voice, data, and

video onto LANs and WANs. You can gain an appreciation for

how ATM accomplishes this by learning about its architecture.

3. Architecture

ATM is based on the switching of 53-byte cells, in which each

cell consists of a 5-byte header and a payload of 48 bytes of

information. Figure 3.1 illustrates the format of the ATM cell,

including the explosion of its 5-byte header to indicate the

fields carried in the header.

Figure 3.1: The 53-byte ATM cell.

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The 4-bit Generic Flow Control (GFC) field is used as a

mechanism to regulate the flow of traffic in an ATM network

between the network and the user. The use of this field is

currently under development. As we will shortly note, ATM

supports two major types of interfaces: Network-to-User (UNI)

and Network-to-Network (NNI). When a cell flows from the

user to the network or from the network to the user, it will

carry a GFC bit value. However, when it flows within a network

or between networks, the GFC field is not used. Instead of

being wasted, its space can be used to expand the length of

the Virtual Path Identifier field.

The 8-bit Virtual Path Identifier (VPI) field represents one half

of a two-part connection identifier used by ATM. This field

identifies a virtual path that can represent a group of virtual

circuits transported along the same route. Although the VPI is

eight bits long in a UNI cell, the field expands to 12-bit

positions to fill the Generic Flow Control field in an NNI cell. It

is described in more detail later in this chapter.

The Virtual Channel Identifier (VCI) is the second half of the

two-part connection identifier carried in the ATM header. The

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16-bit VCI field identifies a connection between two ATM

stations communicating with one another for a specific type of

application. Multiple virtual channels (VCs) can be transported

within one virtual path. For example, one VC could be used to

transport a disk backup operation, while a second VC is used to

transport a TCP/IP-based application. The virtual channel

represents a one-way cell transport facility. Thus, for each of

the previously described operations, another series of VCIs is

established from the opposite direction. You can view a virtual

channel as an individual one-way end-to-end circuit, whereas a

virtual path that can represent a collection of virtual channels

can be viewed as a network trunk line. After data is within an

ATM network, the VPI is used to route a common group of

virtual channels between switches by enabling ATM switches to

simply examine the value of the VPI. Later in this chapter, you

will examine the use of the VCI.

The Payload Type Identifier (PTI) field indicates the type of

information carried in the 48-byte data portion of the ATM cell.

Currently, this 3-bit field indicates whether payload data

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represents management information or user data. Additional

PTI field designators have been reserved for future use.

The 1-bit Cell Loss Priority (CLP) field indicates the relative

importance of the cell. If this field bit is set to 1, the cell can be

discarded by a switch experiencing congestion. If the cell

cannot be discarded, the CLP field bit is set to 0.

The last field in the ATM cell header is the 8-bit Header Error

Control field. This field represents the result of an 8-bit Cyclic

Redundancy Check (CRC) code, computed only over the ATM

cell header. This field provides the capability for detecting all

single-bit errors and certain multiple-bit errors that occur in the

40-bit ATM cell header.

4. Advantages of the Technology

(a) Integration of various services such as voice, image, video, data

and multimedia.

(b) Standardization of network structures and components. This

results in cost savings for network providers.

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(c) Transmission that is independent of the medium used PDH, SDH,

SONET and other media can be used to transport ATM cells.

(d) ATM is scalable, i.e. the bandwidth can be adapted extremely

flexible to meet user requirements.

(e) Guaranteed transmission quality to match the service required by

the user (quality of service, QoS).

5. Cell Routing

The actual routing of ATM cells depends on whether a

connection was pre-established or set up as needed on a

demand basis. The pre-established type of connection is

referred to as a Permanent Virtual Connection (PVC), and the

other type is referred to as a Switched Virtual Connection

(SVC). Examine the 5-byte ATM cell header shown in Figure

3.1 and note the VCI and VPI fields. The VPI is 8 bits in length,

whereas the VCI is 16 bits in length, enabling 256 virtual paths

of which each path is capable of accommodating up to 65,536

(216) virtual connections.

By using VPs and VCs, ATM employs a two-level connection

identifier that is used in its routing hierarchy. A VCI value is

unique only in a particular VPI value, whereas VPI values are

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unique only in particular physical links. The VPI/VCI value

assignment has only local significance, and those values are

translated at every switch a cell traverses between endpoints in

an ATM network. The actual establishment of a virtual path is

based on ATM's network management and signaling

operations. During the establishment of a virtual path routing

table, entries in each switch located between endpoints map an

incoming physical port and a Virtual Path Identifier pair to an

outgoing pair. This initial mapping process is known as network

provisioning, and the change of routing table entries is referred

to as network reprovisioning.

Figure 5.1 illustrates an example of a few possible table entries

for a switch, where a virtual path was established such that

VPI=6 on port 1 and VPI=10 on port 8, representing two

physical links in the established connection.

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Fig 5.1: Switch operations based on routing table

entries.

Next, examine the entries in the routing table shown in Figure

5.1, and note that the table does not include values for VCIs.

This is by design because a VP in an ATM network can support

up to 65,536 VC connections. Thus, only one table entry is

required to switch up to 65,536 individual connections if those

connections all follow the same set of physical links in the same

sequence. This method of switching, which is based on the VPI

and port number, simplifies the construction and use of routing

tables and facilitates the establishment of a connection through

a series of switches. Although VCIs are not used in routing

tables, they are translated at each switch. To help you

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understand the rationale for this technique, you must focus on

their use. As previously noted, a VCI is unique within a VP and

is used at an endpoint to denote a different connection within a

virtual path. Thus, the VPI/VCI pair used between an endpoint

and a switch has a local meaning and is translated at every

switch; however, the VCI is not used for routing between

switches.

The establishment of a connection between two end stations is

known as a Virtual Channel Connection (VCC). To illustrate the

routing of cells in an ATM network based on a VCC, consider

Figure 5.2, which represents a small two-switchbased ATM

network. The VCC represents a series of virtual channel links

between two ATM endpoints. In Figure 5.2, one VCC could be

represented by VCI=1, VCI=3, and VCI=5, which collectively

form a connection between workstations at the two endpoints

shown in the network. A second VCC could be represented by

VCI=2, VCI=4, and VCI=6. The second VCC could represent

the transportation of a second application between the same

pair of endpoints or a new application between different

endpoints served by the same pair of ATM switches.

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Fig 5.2: Connections in an ATM network.

As indicated by the previous examples, each VC link consists of

one or more physical links between the location where a VCI is

assigned and the location where it is either translated or

removed. The assignment of VCs is the responsibility of

switches during the call setup process.

6. The ATM Protocol Reference Model

Three layers in the ATM architecture form the basis for the ATM

Protocol Reference model, illustrated in Figure 6.1 Those layers

are the Physical layer, the ATM layer, and the ATM Adaptation

layer.

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Fig 6.1: The ATM protocol suite.

The Physical Layer

As indicated in Figure 6.1, the lowest layer in the ATM protocol

is the Physical layer. This layer describes the physical

transmission of information through an ATM network. It is not

actually defined with respect to this new technology. The

absence of a Physical layer definition results from the design

goal of ATM to operate on various physical interfaces or media

types. Thus, instead of defining a specific Physical layer, ATM

depends on the Physical layers defined in other networking

protocols. 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

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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.

The actual manner by which the ATM layer performs its

relaying function depends on the location of the layer at a

switch or at an endpoint. If the ATM layer is located in an

endpoint, it receives a stream of cells from the Physical layer

and transmits either cells with new data or empty cells if there

is no data to send to the AAL. When located in a switch, the

ATM layer is responsible for determining where incoming cells

are routed and for multiplexing cells by placing cells from

individual connections into a single-cell stream.

The ATM Adaptation Layer

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The ATM Adaptation layer (AAL) represents the top layer in the

ATM Protocol model. This layer is responsible for providing an

interface between higher-layer protocols and the ATM layer.

Because this interface normally occurs based on a voice, data,

or video application accessing an ATM network, the operations

performed by the AAL occur at endpoints and not at ATM

switches. Thus, the AAL is shown in Figure 6.1 to reside at ATM

endpoints. The primary function of the ATM Adaptation layer is

format conversion. That is, the 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.

The capability to support connection-oriented or connectionless

applications enables ATM to support various existing higher-

layer protocols. For example, Frame Relay is a connection-

oriented protocol, whereas IP is a connectionless protocol.

Through the use of different AALs, both can be transported by

ATM.

Based on the four application classes, four different types of

AALs were defined: AAL1, 2, 3/4, and 5. At one time, AAL3 and

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AAL4 were separate types; however, they had a sufficient

degree of commonality to be merged. Figure 6.2 illustrates the

relationship between application classes and ATM Adaptation

layers with respect to the different parameters used to classify

the application classes.

Table 6.1: The ATM Application Classes

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

Figure 6.2: Application classification and associated

AALs.

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AAL1 is designated for transporting continuous bit rate (CRR)

data, such as real-time voice and video traffic. The AAL1

specification defines the manner by which a continuous signal

is transported in a sequence of individual ATM cells. As

indicated in Figure 6.3, the 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 cell will eventually be used to

transport packet video services and should be defined in the

near future.

Figure 6.3: AAL 1 cell payload format.

AAL3 is designed to transport delay-insensitive user data, such

as Frame Relay, X.25, or IP traffic. There is a high degree of

probability that such data will have to be fragmented because

the maximum payload of an ATM cell is 48 bytes. AAL3/4 uses

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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.

Although several aspects of different AAL operations remain to

be specified, the use of different AALs provides the mechanism

for the cell-based switching technology on which ATM is based

to transport different types of information using a common cell

structure.

7. Service Definitions

Perhaps the major benefit of ATM is that it enables users to

obtain a Quality of Service (QoS) for each class of service. The

QoS represents a guaranteed level of service that can be based

upon such parameters as peak cell rate (PCR), sustained cell

rate (SCR), cell delay variation tolerance (CDVT), minimum cell

rate (MCR), and burst tolerance (BT). Each of these parameters

is used with other parameters to define one of the five classes

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of service for which a carrier may offer cell loss, cell delay, and

bandwidth guarantees. Those classes of service include

Continuous Bit Rate (CBR), Variable Bit RateReal Time (VBR

RT), Variable Bit RateNon-Real Time (VBR-NRT), Unspecified

Bit Rate (UBR), and Available Bit Rate (ABR).

Continuous Bit Rate and Variable Bit RateReal Time services

generally correspond to Class A and Class B services,

respectively. Variable Bit RateNon-Real Time is a less time-

stringent version of VBRRT.

Both UBR and ABR services are for transporting delay-

insensitive traffic, corresponding to Classes C and D. UBR

represents a best-effort delivery mechanism for which cells can

be discarded during periods of network congestion. In

comparison, an ABR service is allocated all the bandwidth

required by the application that is available on a connection,

with a feedback mechanism employed to control the rate the

originator transmits cells to minimize cell loss when available

bandwidth contracts. Table 7.1 provides a summary of the five

types of ATM services.

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Table 7.1 ATM Services

Guarantees

ATM

Service

Feedback

Metric

s Loss Delay Bandwidth

Constant Bit

Rate (CBR)

PCR, CDVT Yes Yes Yes No

Variable Bit

RateReal

Time (VBR

RT)

PCR, CDVT,

SCR, BT

Yes Yes Yes No

Variable Bit

RateNon-

Real Time

(VBR-NRT)

PCR, CDVT,

SCR, BT

Yes Yes Yes No

Unspecified

Bit Rate

(UBR)

Unspecified No No No No

Available Bit

Rate (ABR)

PCR, CDVT,

MCR

Yes No Yes Yes

Legend:

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PCR = Peak Cell Rate

CDVT = Cell Delay Variation Tolerance

SCR = Sustained Cell Rate

BT = Burst Tolerance

MCR = Minimum Cell Rate

8. CONCLUSION

This seminar is all about the combination of the collective

knowledge which I gained through existence reading and

research work in the field networking, communication and

computer engineering. This research work has actually shown

the face of advancement in the areas of communication.

ATM represents cell-switching technology designed to transport

voice, data, and video by using a common cell format on both

local and wide area networks.ATM represents a scalable

technology for which 53-byte cells can be transported at a

range of operating rates from 25Mbps on LANs to 622Gbps on

SONET.

Recognizing the differences between voice and data

transportation requirements, ATM is designed to adapt to the

time sensitivity of different applications. A two-part identifier

consisting of a Virtual Path Identifier and Virtual Channel

Identifier enables multiple connections to be carried on the

same path.

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The ATM Protocol Reference model has three layers: the

Physical layer, the ATM layer, and the ATM Adaptation layer.

The routing of ATM cells between switches is based on Virtual

Path Identifiers and port number routing table entries in the

two switches. A Virtual Path Connection (VPC) represents a

concatenation of virtual paths between switches; a Virtual

Channel Connection (VCC) represents a connection between

two end stations via a VPC. ATM users can obtain a guaranteed

level of service referred to as a Quality of Service (QoS).

REFERENCES

. Black, Uyless D. (1998). ATMVolume III: Internetworking with ATM.

Toronto: Prentice Hall.

. De Prycker, Martin (1993). Asynchronous Transfer Mode. Solutions for

Broadband ISDN. Prentice Hall.

. Joel, Amos E., Jr. (1993).Asynchronous Transfer Mode. IEEE Press.

. Golway, Tom (1997). Planning and Managing ATM Networks. New :

Manning.

. McDysan, David E.; Darren L. Spohn (1999). ATM Theory and

Applications. Montreal: McGraw-Hill.

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. Neelakanta, P. S. (2000). A Textbook on ATM Telecommunications,

Principles and implementation. CRC Press.

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