Introduction to Asynchronous Transfer Mode | ATM Overview | InformIT

Bill Stallings discusses the technology behind Asynchronous Transfer Mode (ATM), the widely used wide area network technology.

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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. In the
early days of wide area networking, X.25 was designed to support direct
connection of terminals and computers over long distances. At speeds up to 64
Kbps or so, X.25 copes well with these demands. As LANs have come to play an
increasing role in the local environment, however, X.25, with its substantial
overhead, is being recognized as an inadequate tool for wide area networking.
This has led to increasing interest in frame relay, which is designed to support
access speeds up to 2 Mbps. But as we look to the not-too-distant future, even
the streamlined design of frame relay will falter in the face of a requirement
for wide area access speeds in the tens and hundreds of megabits per second. To
accommodate these gargantuan requirements, the latest technology has emerged:
asynchronous transfer mode (ATM), also known as cell
relay.

ATM 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 nonISDN
applications, particularly in bridges and routers. ATM was developed as part of
the work on broadband ISDN, but now finds applications in nonISDN
environments, where very high data rates are required.

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ATM Overview

ATM is a packet-oriented transfer mode. It allows multiple logical
connections to be multiplexed over a single physical interface. The information
flow on each logical connection is organized into fixed-size packets, called
cells. As with frame relay, there is no link-by-link error control or
flow control.

Figure 1
shows the overall hierarchy of function in an ATM-based network. This hierarchy
is seen from the point of view of the internal network functions needed to support
ATM as well as the user-network functions. The ATM layer consists of virtual
channel and virtual path levels; these are discussed later in this article.

Figure 1
ATM transport hierarchy.

The physical layer can be divided into three functional levels:

Transmission path level. Extends between network elements that
assemble and disassemble the payload of a transmission system. For end-to-end
communication, the payload is end-user information. For user-to-network
communication, the payload may be call-control signaling information (call setup
and call termination). Cell delineation and header error-control functions are
required at the end points of each transmission path.
Digital section level. Extends between network elements that
assemble and disassemble a continuous bit or byte stream. This refers to the
exchanges or signal transfer points in a network that are involved in switching
data streams.
Regenerator section level. A portion of a digital section. An
example of this level is a repeater that’s used to simply regenerate the
digital signal along a transmission path that’s too long to be used without
such regeneration; no switching is involved.

Virtual Channels and Virtual Paths

Logical connections in ATM are referred to as virtual channels. A
virtual channel is analogous to a virtual circuit in X.25 or a frame-relay
logical connection. It’s the basic unit of switching in ATM. A virtual
channel is set up between two end users through the network; a variable-rate,
full-duplex flow of fixed-size cells is exchanged over the connection. Virtual
channels 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 paths (see Figure
2). A virtual path is a bundle of virtual channels that have the same endpoints.
Thus, all the cells flowing over all the virtual channels in a single virtual
path are switched together.

Figure 2
ATM connection relationships.

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

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).
Increased network performance and reliability. The network deals
with aggregated (and therefore fewer) entities.
Reduced processing and short connection setup time. Much of the
work is done when the virtual path is set up. The addition of new virtual
channels to an existing virtual path involves minimal processing.
Enhanced network services. The virtual path is used internal to
the network but is also visible to the end user. Thus, the user may define
closed user groups or closed networks of virtual-channel bundles.

Virtual Path/Virtual Channel Characteristics

The following are characteristics of virtual channel connections:

Quality of service. A user of a virtual channel 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.
Switched and semi-permanent virtual channel connections. Both
switched connectionswhich require call-control signalingand
dedicated channels can be provided.
Cell-sequence integrity. The sequence of transmitted cells within
a virtual channel is preserved.
Traffic parameter negotiation and usage monitoring. Traffic
parameters can be negotiated between a user and the network for each virtual
channel. The network monitors the input of cells to the virtual channel 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 deal with congestion and to manage existing and requested virtual
channels. At the crudest level, the network may simply deny new requests for
virtual channels 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.

Quality of service, switched and semi-permanent virtual paths, cell sequence
integrity, and traffic parameter negotiation and usage monitoring are all also
characteristics of a virtual path. There are a number of reasons for this
duplication:

This provides some flexibility in how the network manages the
requirements placed on it.
The network must be concerned with the overall requirements for a virtual
path, and within a virtual path may negotiate the establishment of virtual
circuits with given characteristics.
Once a virtual path is set up, it’s possible for the end users to
negotiate the creation of new virtual channels. The virtual path characteristics
impose a discipline on the choices that the end users can make.

In addition, a fifth characteristic is listed for virtual paths:

Virtual channel identifier restriction within a virtual path. One
or more virtual channel identifiers, or numbers, may not be available to the
user of the virtual path, but may be reserved for network use. Examples include
virtual channels used for network management.

Control Signaling

For virtual channels, there are four methods for providing an
establishment/release facility. One method or a combination of these methods can
be used in any particular network:

Semi-permanent virtual channels may be used for user-to-user
exchange. In this case, no control signaling is required.
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 a virtual channel
to be used for call control. Such a channel is called a meta-signaling
channel, since the channel is used to set up signaling channels.
The meta-signaling channel can be used to set up a virtual channel
between the user and the network for call-control signaling. This
user-to-network signaling virtual channel can than be used to set up
virtual channels to carry user data.
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 virtual path. It can then be used to allow the two end users,
without network intervention, to establish and release user-to-user virtual
channels to carry user data.

For virtual paths, three methods are defined:

A virtual path can be established on a semi-permanent basis by
prior agreement. In this case, no control signaling is required.
Virtual path establishment/release may be customer controlled. In
this case, the customer uses a signaling virtual channel to request the virtual
path from the network.
Virtual path establishment/release may be network controlled. In
this case, the network establishes a virtual path for its own convenience. The
path may be network-to-network, user-to-network, or user-to-user.