ASYNCHRONOUS TRANSFER MODE(ATM)

ASYNCHRONOUS TRANSFER MODE(ATM)

Asynchronous transfer mode (ATM), also known as cell relay, is similar in

concept to frame relay. 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.

In addition to their technical similarities, ATM and frame relay have similar

histories. Frame relay was developed as part of the work of ISDN, but is now finding

wide application in private networks and other non-ISDN applications, particularly

in bridges and routers. ATM was developed as part of the work on broadband

ISDN, but is beginning to find application in non-ISDN environments where very

high data rates are required.

We begin with a discussion of the details of the ATM scheme. Then, the

important concept of the ATM Adaptation Layer (AAL) is examined. Finally, the

key issue of congestion control in ATM networks is discussed.

PROTOCOL ARCHITECTURE

Asynchronous transfer mode (ATM), also known as cell relay, is in some ways similar

to packet switching using X.25 and frame relay. Like packet switching and

frame relay, ATM involves the transfer of data in discrete chunks. Also, like packet

switching and frame relay, ATM allows multiple logical connections to be multiplexed

over a single physical interface. In the case of ATM, the information flow on

each logical connection is organized into fixed-size packets, called cells.

ATM is a streamlined protocol with minimal error and flow control capabilities;

this reduces the overhead of processing ATM cells and reduces the number of

overhead bits required with each cell, thus enabling ATM to operate at high data

rates. Further, the use of fixed-size cells simplifies the processing required at each

ATM node, again supporting the use of ATM at high data rates.

The standards issued for ATM by ITU-T are based on the protocol architecture

shown in Figure 11.1, which illustrates the basic architecture for an interface

between user and network. The physical layer involves the specification of a transmission

medium and a signal encoding scheme. The data rates specified at the physical

layer include 155.52 Mbps and 622.08 Mbps. Other data rates, both higher and

lower, are possible.

Two layers of the protocol architecture relate to ATM functions. There is an

ATM layer common to all services that provides packet transfer capabilities, and an

ATM adaptation layer (AAL) that is service dependent. The ATM layer defines

the transmission of data in fixed-size cells and also defines the use of logical connections.

The use of ATM creates the need for an adaptation layer to support information

transfer protocols not based on ATM. The AAL maps higher-layer information

into ATM cells to be transported over an ATM network, then collects

information from ATM cells for delivery to higher layers.

The protocol reference model makes reference to three separate planes:

@ User Plane. Provides for user information transfer, along with associated controls

(e.g., flow control, error control).

Control Plane. Performs call control and connection control functions.

@ Management Plane. Includes plane management, which performs management

functions related to a system as a whole and provides coordination

between all the planes, and layer management, which performs management

functions relating to resources and parameters residing in its protocol entities.

11.2 ATM LOGICAL CONNECTIONS

Logical connections in ATM are referred to as virtual channel connections (VCC).

A VCC is analogous to a virtual circuit in X.25 or a data link connection in frame

relay; it is the basic unit of switching in an ATM network. A VCC is set up between

two end users through the network and a variable-rate, full-duplex flow of fixed-size

cells is exchanged over the connection. VCCs are also used for user-network

exchange (control signaling) and network-network exchange (network management

and routing).

For ATM, a second sublayer of processing has been introduced that deals with

the concept of virtual path (Figure 11.2). A virtual path connection (VPC) is a bundle

of VCCs that have the same endpoints. Thus, all of the cells flowing over all of

the VCCs in a single VPC are switched together.

The virtual path concept was developed in response to a trend in high-speed

networking in which the control cost of the network is becoming an increasingly

higher proportion of the overall network cost. The virtual-path technique helps contain

the control cost by grouping connections that share common paths through the

network into a single unit. Network management actions can then be applied to a

small number of groups of connections instead of to a large number of individual

connections.

Several advantages can be listed for the use of virtual paths:

a Simplified network architecture. Network transport functions can be separated

into those related to an individual logical connection (virtual channel)

and those related to a group of logical connections (virtual path).

a Increased network performance and reliability. The network deals with fewer,

aggregated entities.

Reduced processing and short connection setup time. Much of the work is

done when the virtual path is set up. By reserving capacity on a virtual path

connection in anticipation of later call arrivals, new virtual channel connections

can be established by executing simple control functions at the endpoints

of the virtual path connection; no call processing is required at transit

nodes. Thus, the addition of new virtual channels to an existing virtual path

involves minimal processing.

e Enhanced network services. The virtual path is used internal to the network

but is also visible to the end user. As a result, the user may define closed user

groups or closed networks of virtual-channel bundles.

Figure 11.3 suggests in a general way the call-establishment process using virtual

channels and virtual paths. The process of setting up a virtual path connection

is decoupled from the process of setting up an individual virtual channel connection:

e The virtual path control mechanisms include calculating routes, allocating

capacity, and storing connection state information.

To set up a virtual channel, there must first be a virtual path connection to the

required destination node with sufficient available capacity to support the virtual

channel, with the appropriate quality of service. A virtual channel is set

up by storing the required state information (virtual channel/virtual path

mapping).

The terminology of virtual paths and virtual channels used in the standard is

a bit confusing, and is summarized in Table 11.1. Whereas most of the networklayer

protocols that we survey in this lesson relate only to the user-network interface,

the concepts of virtual path and virtual channel are defined in the ITU-T Recom

Virtual Channel Connection Uses

The endpoints of a VCC may be end users, network entities, or an end user and a

network entity. In all cases, cell sequence integrity is preserved within a VCC; that

is, cells are delivered in the same order in which they are sent. Let us consider examples

of the three uses of a VCC.

Between end users. Can be used to carry end-to-end user data; can also be

used to carry control signaling between end users, as explained below. A VPC

between end users provides them with an overall capacity; the VCC organization

of the VPC is up to the two end users, provided the set of VCCs does

not exceed the VPC capacity.

* Between an end user and a network entity. Used for user-to-network control

signaling, as discussed below. A user-to-network VPC can be used to aggregate

traffic from an end user to a network exchange or network server.

* Between two network entities. Used for network traffic management and

routing functions. A network-to-network VPC can be used to define a common

route for the exchange of network management information.

Virtual Path/Virtual Channel Characteristics

ITU-T Recommendation 1.150 lists the following as characteristics of virtual channel

connections:

Quality of service. A user of a VCC is provided with a Quality of Service specified

by parameters such as cell loss ratio (ratio of cells lost to cells transmitted)

and cell delay variation.

a Switched and semi-pemanent virtual channel connections. Both are switched

connections, which require call-control signaling, and dedicated channels can

be provided.

a Cell sequence integrity. The sequence of transmitted cells within a VCC is

preserved.

a Traffic parameter negotiation and usage monitoring. Traffic parameters can

be negotiated between a user and the network for each VCC. The input of

cells to the VCC is monitored by the network to ensure that the negotiated

parameters are not violated.

The types of traffic parameters that can be negotiated include average rate,

peak rate, burstiness, and peak duration. The network may need a number of

strategies to handle congestion and to manage existing and requested VCCs. At the

crudest level, the network may simply deny new requests for VCCs to prevent congestion.

Additionally, cells may be discarded if negotiated parameters are violated

or if congestion becomes severe. In an extreme situation, existing connections might

be terminated.

1.150 also lists characteristics of VPCs. The first four characteristics listed are

identical to those for VCCs. That is, quality of service, switched and semipermanent

VPCs, cell sequence integrity, and traffic parameter negotiation and

usage monitoring are all also characteristics of a VPC. There are a number of reasons

for this duplication. First, redundancy provides some flexibility in how the network

service manages its requirements. Second, the network must be concerned

with the overall requirements for a VPC, and, within a VPC, it may negotiate the

establishment of virtual channels with given characteristics. Finally, once a WC is

set up, it is possible for the end users to negotiate the creation of new VCCs. The

WC characteristics impose a discipline on the choices that the end users may make.

In addition, a fifth characteristic is listed for VPCs:

" Virtual channel identifier restriction within a VPC. One or more virtual channel

identifiers, or numbers, may not be available to the user of the WC, but

may be reserved for network use. Examples include VCCs used for network

management.

In ATM, a mechanism is needed for the establishment and release of VPCs and

VCCs. The exchange of information involved in this process is referred to as control

signaling, and takes place on separate connections from those that are being

managed.

For VCCs, 1.150 specifies four methods for providing an establishment/release

facility. One or a combination of these methods will be used in any particular

network:

1. Semi-permanent VCCs may be used for user-to-user exchange. In this case, no

control signaling is required.

2. If there is no pre-established call control signaling channel, then one must be

set up. For that purpose, a control signaling exchange must take place

between the user and the network on some channel. Hence, we need a permanent

channel, probably of low data rate, that can be used to set up VCCs

that can be used for call control. Such a channel is called a meta-signaling

channel, as the channel is used to set up signaling channels.

3. The meta-signaling channel can be used to set up a VCC between the user and

the network for call control signaling. This user-to-network signaling virtual

channel can then be used to set up VCCs to carry user data.

4. The meta-signaling channel can also be used to set up a user-to-user signaling

virtual channel. Such a channel must be set up within a pre-established WC.

It can then be used to allow the two end users, without network intervention,

to establish and release user-to-user VCCs to carry user data.

For VPCs, three methods are defined in 1.150:

1. A VPC can be established on a semi-permanent basis by prior agreement. In

this case, no control signaling is required.

2. VPC establishmentlrelease may be customer controlled. In this case, the customer

uses a signaling VCC to request the VPC from the network.

3. VPC establishmentlrelease may be network controlled. In this case, the network

establishes a VPC for its own convenience. The path may be networkto-

network, user-to-network, or user-to-user.