ATM (Asynchronous Transfer Mode).

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Asynchronous T
ransfer Mode (ATM)


Everyday the world seems to be moving at a faster and faster pace
with new technological advances occurring constantly. Changes in
the structure of the telecommunications industry and market
conditions have brought new opportunities and challe
nges for
network operators and public service providers. Networks that have
been primarily focused on providing better voice services are
evolving to meet new multimedia communications challenges and
competitive pressures. In order to deliver new services
such as video
conferencing and video on demand, as well as provide more
bandwidth for the increasing volume of traditional data, the
communications industry introduced a technology that provided a
common format for services with different bandwidth require
This technology is Asynchronous Transfer Mode (ATM). Services
based on asynchronous transfer mode (ATM) architecture provide
the flexible infrastructure essential for success in that evolving

Asynchronous T
ransfer Mode (ATM)

1. Historical Background

When the computers wer
e becoming more prevalent in the office.
Networking these computers together was desirable and beneficial.
When linking these computers over a long distance, the existing
optimized WANs were used. Because computers send data
instead of voice, and dat
a has different characteristics, these WANs
did not send computer data very efficiently. Therefore, separate
WANs were sometimes built specifically to carry data traffic. Also, a
network that could carry voice, data and video had been considered,
as someth
ing needed to be done.

To address these concerns, ITU
T (formerly CCITT) and other
standards groups started work in the 1980s to establish a series of
recommendations for the transmission, switching, signaling and
control techniques required to implement
an intelligent fiber
network that could solve current limitations and would allow
networks to be able to efficiently carry services of the future. This
network was termed Broadband Integrated Services Digital Network
ISDN). By 1990, decisions had
been made to base B
SONET/SDH (Synchronous Optical Network/Synchronous Digital
Hierarchy) and ATM.

For B
ISDN, two types of switching were considered by the ITU
Synchronous and Asynchronous. An intelligent switching fabric
with the ability to sw
itch all forms of traffic at extremely high
speeds, while maximizing the use of bandwidth, was needed to
Asynchronous T
ransfer Mode (ATM)

optimize the potential of B
ISDN. Ideally, maximum bandwidth
should be accessible to all applications and users, and should be
allocated on demand. ATM

was chosen as the standard for B
that will ultimately satisfy these stringent requirements. Even
though ATM was initially considered part of the solution for WANs,
local area network (LAN) architects and equipment vendors saw
ATM as a solution to man
y of their network limitations, and cable
TV operators looked at ATM as a possible addition to their existing

By 1996 The ATM Forum presented the Anchorage Accord objective.
Fundamentally, the message is that the set of specifications needed

the development of multiservice ATM networks is available.
These specifications were complete to implement and manage an
ATM infrastructure, and ensure backward compatibility.

Entering the new millennium, ATM services are still in demand. The
global mark
et for ATM is in the billions of US dollars. Even with
emerging technologies, ATM technology is still the only technology
that can guarantee a certain and predefined quality of service. The
growth of the internet, need for broadband access and content, e
ommerce and more are spurring the need for a reliable, efficient
transport systemÖATM Technology. For voice, video, data and
images together, the next generation network depends on ATM.

Asynchronous T
ransfer Mode (ATM)

2. Definition

Asynchronous transfer mode (ATM) is a high
e, cell
oriented switching and multiplexing technology that utilizes fixed
length packets to carry different types of traffic. ATM combines the
benefits of circuit switching (guaranteed capacity and constant
transmission delay) with those of packet switchi
ng (flexibility and
efficiency for intermittent traffic). It provides scalable bandwidth
from a few megabite per second (Mb/s) to many gigabits per second
(Gb/s). Because of its asynchronous nature, ATM is more efficient
than synchronous technologies, such

as time
division multiplexing
With TDM, each user is assigned to a time slot, and no other
station can send in that time slot. If a station has much data to
send, it can send only when its time slot comes up, even if all other
time slots are empty.

However, if a station has nothing to transmit
when its time slot comes up, the time slot is sent empty and is
wasted. Because ATM is asynchronous, time slots are available on
demand with information identifying the source of the transmission
contained in
the header of each ATM cell.

3. ATM Cell Format

ATM transfers information in fixed
size units called
. Each cell
consists of 53 octets, or bytes. The first 5 bytes contain cell
information, and the remaining 48 contain the payload (user
tion). Small, fixed
length cells are well suited to transferring
voice and video traffic because such traffic is intolerant of delays
Asynchronous T
ransfer Mode (ATM)

that result from having to wait for a large data packet to download,
among other things. Figure (1) illustrates the basic
format of an
ATM cell. The cell structure was chosen for two reasons. The
relatively small cell size reduces queuing delay for high
priority cells
in the event of congestion. Fixed size cells may be switched more
efficiently since the switch need not look
for an end

3.1 ATM Cell Header Format

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
unication between ATM switches. Figure (2) depicts the basic
ATM cell format,
the ATM UNI cell
header format, and
the ATM NNI cell
header format.

Figure 1(An ATM cell consists of a header and payload data “ field length in bytes”)

Figure 2(An ATM Cell, ATM UNI Cell, and ATM NNI Cell Header Each Contain 48 Bytes of Payload)

Asynchronous T
ransfer Mode (ATM)


Unlike the UNI, the NNI header does not include the Generic Flow
Control (GFC) field. Additionally, the NN
I header has a Virtual Path
Identifier (VPI) field that occupies the first 12 bits, allowing for larger
trunks between public ATM switches.

3.2 ATM Cell Header Fields

In addition to GFC and VPI header fields, several others are used in
ATM cell header fiel
ds. The following descriptions summarize the
ATM cell header fields illustrated in Figure (2):

Generic Flow Control (GFC)

Provides local functions, such as
identifying multiple stations that share a single ATM interface. This
field is typically not used
and is set to its default value of 0
(binary 0000).

Virtual Path Identifier (VPI)

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 Ide
ntifier (VCI)

In conjunction with the VPI,
identifies the next destination of a cell as it passes through a series
of ATM switches on the way to its destination.

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),
Asynchronous T
ransfer Mode (ATM)

and the third bit indicates whether the cell is the last in a series of
cells t
hat 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 disc
arded 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.


ATM Reference Model

The ATM architecture uses a logical model to describe the
functionality that it supports. ATM functionality corresponds to the
physical layer and part of the data link layer of the OSI reference

The ATM reference model is compo
sed of the following planes,
which span all layers:


This plane is responsible for generating and managing
signaling requests.


This plane is responsible for managing the transfer of data.


This plane contains two components:

er management manages layer
specific functions, such as
the detection of failures and protocol problems.

Asynchronous T
ransfer Mode (ATM)


Plane management manages and coordinates functions related to
the complete system.

The ATM reference model is composed of the following ATM layers:

Physical layer

Analogous to the physical layer of the OSI
reference model, the ATM physical layer is concerned with the
physical transmission medium and is divided into two sublayers:


The physical medium sublayer provides the actual transmission
and recep
tion of signals over the physical transport medium. Its
main functions are Line coding, Bit rate, Generation and detection
of the transmitted signal.


The transmission convergence sublayer provides the means for
converting an arbitrary bit stream entering t
he switch into ATM

ATM layer

Combined with the ATM adaptation layer, the ATM
layer is roughly analogous to the data link layer of the OSI reference
model. The ATM layer is responsible for the simultaneous sharing of
virtual circuits over a physic
al link (cell multiplexing) and passing
cells through the ATM network (cell relay). To do this, it uses the
VPI and VCI information in the header of each ATM cell.

ATM adaptation layer (AAL)

Combined with the ATM layer, the
AAL is roughly analogous to th
e data link layer of the OSI model.
The AAL is responsible for isolating higher
layer protocols from the
details of the ATM processes. The adaptation layer prepares user
Asynchronous T
ransfer Mode (ATM)

data for conversion into cells and segments the data into 48
cell payloads.

lly, the higher layers residing above the AAL accept user data,
arrange it into packets, and hand it to the AAL. Figure (3) illustrates
the ATM reference model.

4.1 ATM Service Classes

The AAL provides four service classifications for the transport

or variable
bit rate data streams in connection or
connectionless modes and with the option of maintaining timing
synchronization between sender and receiver. These service
classifications are labeled class A through class D and are outlined i
table (1) in the next page.

Figure 3(The ATM Reference

Model Relates to the Lowest Two Layers of the OSI Reference model)

Asynchronous T
ransfer Mode (ATM)

Figure 4(An ATM Network Contains ATM Switches and Endpoints)

Class A

Class B

Class C

Class D

Bit rate













Not required

ot required

5. ATM Devices

ATM network
is made up of an
ATM switch
ATM endpoints
. An
ATM switch is responsible for cell transit through an ATM network.
The job of an ATM switch is well defined: It accepts the incoming cell
from an ATM endpoint o
r 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 (or end
system) contains an ATM network interface adapter. Examples of ATM
endpoints ar
e workstations, routers, digital service units (DSUs) and
LAN switches. Figure (4)
illustrates an ATM network made
up of ATM switches and ATM

Table 1(Service classifications provided by ATM adaptation layer)

Asynchronous T
ransfer Mode (ATM)

6. ATM Network Operation

An ATM network consists of a set of ATM switches interconnected
by point
oint ATM links or interfaces. ATM switches support
two kinds of interfaces: user
network interfaces (UNI) and network
node interfaces (NNI). UNI connect ATM end
systems (hosts,
routers, and so on) to an ATM switch, while an NNI may be
imprecisely defined a
s an interface connecting two ATM switches
together; slightly different cell formats are defined across UNI and
NNI. More precisely, however, an NNI is any physical or logical link
across which two ATM switches exchange the NNI protocol. See
figure (5).

As noted before, ATM networks are fundamentally connection
oriented. This means that a virtual circuit needs to be set up across
the ATM network prior to any data tra
nsfer. ATM circuits are of two
types: virtual paths, identified by virtual path identifiers (VPI); and
virtual channels, identified by the combination of a VPI and a
Figure 5(ATM network interface)

Asynchronous T
ransfer Mode (ATM)

virtual channel identifier (VCI). A virtual path is a bundle of virtual
channels, all of w
hich are switched transparently across the ATM
network on the basis of the common VPI. All VCI and VPI, however,
have only local significance across a particular link, and are
remapped, as appropriate, at each switch. In normal operation,
switches allocate

all UNI connections within VPI=0.

The basic operation of an ATM switch is very simple:

It receives a cell across a link on a known VCI or VPI value.

It looks up the connection value in a local translation table to
determine the outgoing port (or ports) of

the connection and
assigns the new VPI/VCI value of the connection on that link.

Then, it retransmits the cell on that outgoing link with the
appropriate connection identifiers. See figure (6).

The switch operation is so simple because external mechanisms

up the local translation tables prior to the transmittal of any data.

Figure 6(ATM switch op

Asynchronous T
ransfer Mode (ATM)

7. Quality of Service (QOS)

the ATM switch must perform its various functions while
taining an acceptable quality of service (QOS) to the customer.
This QOS includes:

A very small probability of cell blocking (so that the cell is not

Throughput capabilities reaches hundreds of megabits per
second at a negligible (10^
11 or
better) bit error rate.

A cell loss probability of 10^(
8) to 10^(
11), depending of the
type and class of service.

A cell misrouting rate of 10^(
11) to 10^(
14), depending of the
type and class of service.

A switching delay less than 1 ms.


in switching delay not to exceed 0.1ms.

The ATM performance parameters are outlined in table (2).



Cell loss ratio

Ratio of lost cells to transmitted cells

Cell misinsertion rate

Number of misinserted (misrouted) cells per connection


Cell error ratio

Ratio of errored cells to number of delivered cells

Severely errored cell
block ratio

Ratio of number of severely errored cell blocks to total
number of cell blocks

Cell transfer delay

t (switching delay of a single observed


Mean cell transfer

Arithmetic average of a specified number of cell transfer


Cell delay variation



Table 2(ATM performance parameters)

Asynchronous T
ransfer Mode (ATM)

8. Advantages of ATM

Flexible bandwidth allocation.

Simple routing due to connection oriented technology.

High bandwidth
utilization due to statistical multiplexing.

Potential QOS (Quality Of Service) guarantees.

9. Applications of ATM

Perhaps the most significant application of ATM is in broadband
ISDN (BISDN). The two technologies have developed in close
parallel. Other
applications include video
demand, video
conferencing, and switched multimegabit data service (SMDS).


Although this short paper could not cover all the ATM points, it is
clear that Asynchronous Transfer Mode (ATM) technology will play a
ral role in the evolution of current workgroup, campus and
enterprise networks. ATM delivers important advantages over
existing LAN and WAN technologies, including the promise of
scalable bandwidths at unprecedented price and performance
points and Quality

of Service (QoS) guarantees, which facilitate new
classes of applications such as multimedia.

These benefits, however, come at a price. ATM is a very complex
technology, perhaps the most complex ever developed by the
networking industry. While the struct
ure of ATM cells and cell
Asynchronous T
ransfer Mode (ATM)

switching do facilitate the development of hardware intensive, high
performance ATM switches, the deployment of ATM

requires the overlay of a highly complex, software intensive,
protocol infrastructure.

Asynchronous T
ransfer Mode (ATM)



, “Beginners' Overview Of Asynchronous
Transfer Mode (ATM)”.

, “ATM Internetworking”.

, “ ATM Fundamental”.


Modern digital and analog communication systems
Oxford: Oxford University, 1998.