ATM BASICS - Chapter 1 pps

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Chapter 1
What Is ATM
The intent of this chapter is to provide the reader with the crucial concepts
of ATM. It describes the historic background that influenced the way ATM
devices work even until today. The chapter also positions ATM as the tech-
nology that belongs to the world of telecommunication standards for more
about fourteen years already. It covers the basic idea that is actually pre-
sent behind the term ‘ATM’ without focusing on detailed and technical
descriptions.
How to describe ATM using a short definition? In very brief, ATM can be
addressed as an integrated multiplexing and packet switching technique
that supports large variety of applications and provides for incomparable
Quality of Service.
In the end of the 80’s it became obvious to anyone that a newly defined stan-
dard such as ISDN would not be able satisfy requirements from applications
envisaged for the next decade. The new wide area service called B-ISDN
(Broadband ISDN) has been proposed by the ITU-T. B-ISDN was intended
to offer video on demand, live television from many sources, CD-quality
music, LAN inter-connections, as well as high speed data transport for sci-
ence and industrial applications. As a part of B-ISDN development process,
a number of underlying technologies, capable of fulfilling these require-
ments were analyzed and tested. Finally, the ATM has been chosen as the
underlying technology that was going to make B-ISDN possible.
1.1 Why Asynchronous?
What does the term “asynchronous transmission” mean with the relation to
ATM? Most of the telecommunication systems such as fixed and mobile tele-
phony networks are based on synchronous transmission. With a synchro-
nous system the most common method for sharing network resources among
users is to grant them the access in time domain. Hence, in traditional tele-
phony and ISDN as well as in many mobile telephony systems the Time

Division Multiplexing (TDM) is the most popular method. In synchronous
systems, which are optimized for streaming applications such as uncom-
pressed voice transmission, every user is provided with a channel by means
of time slots allocated within a frame structure.
On the other hand, packet transmissions require different approach. The
data is transmitted in bursts, which appear depending on the source and the
actual network load. Using synchronous systems for packet transmission is
not optimal since some resources become unused. The asynchronous
approach is more adequate whenever data transmission is concerned. ATM
is designed to support various types of services including voice, video and
data. Therefore, to make the new technology capable of efficient delivery of
such differentiated services, the asynchronous transmission method was
chosen. Due to the requirements imposed by real-time streaming applica-
tions (voice, video), ATM transmits information in very tiny portions, which
are called cells. Their size is the compromise between efficiency for data and
other services. Unlike TDM in traditional telecommunication systems, ATM
does not assign any time slots to a given user and it is not tied to a master
clock. Instead of this it dynamically allocates ATM cells when user equip-
ment wishes to transmit information. Cells representing traffic generated
by different users are multiplexed, which is shown in the Fig. 1-1) over the
physical medium. However, ATM does not run directly over the physical
medium, so the physical layer technology must be used to provide ATM with
a set of necessary functions. In result, ATM cells are often transmitted in
the asynchronous mode but using the synchronous physical layer.
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1.2 Is ATM a packet switched or circuit switched
technology?
After some initial research had been completed, it became clear that none of
the existing technologies would fit well into the B-ISDN model. A number of

different concepts have been taken into consideration but the final agree-
ment introduced a hybrid solution. ATM represents a combination of circuit
and packet switching. On one hand, ATM transmission relies on very small
packets – cells, which benefits in large efficiency for data transmission.
Resources are used in an efficient way due to the gain resulting from multi-
plexing of cells. On the other hand, the opposite requirements from voice
and video services request for guaranteed bandwidth and quality of service.
Hence, some form of circuit switching is also necessary, even though the con-
nections (circuits) are purely logical. ATM is a technology that behaves like
a mixture of packet and circuit switching. In result some of the ATM fea-
tures are typical to packet switched networks (efficiency for data transmis-
sion) but other features are similar to those of circuit switched networks
(low and predictable delay, guaranteed bandwidth).
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Fig. 1-1, The idea of asynchronous transmission
1.3 Advantages of ATM
1.3.1 Support for variety of applications
ATM has been designed to handle practically any type of transmission. In
result, ATM is capable of simultaneous transmission of voice, video and data
representing different applications. Such a broad support for different ser-
vices highly influenced the large number of ATM mechanisms. Both voice
and video are real-time or near real-time applications that can hardly toler-
ate delays and delay variations. Therefore, they imply the need for control-
ling of some QoS parameters as long as the service should observe the qual-
ity comparable to the one present in classical networks such as PSTN,
ISDN. Data applications primarily require the opposite treatment. They can
tolerate both the delay and delay variation, but they strongly rely on reli-
able transmission. The great advantage of ATM over other technologies is
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Fig. 1-2, The origins of ATM concept.
related to its unique ability of using the consolidated network infrastruc-
ture, while independently provisioning quality of service for particular
applications. Within last decade ATM networks have become the underlying
infrastructure capable of handling voice services (Voice over ATM), video
services (e.g. Video Dial Tone) and data (IP over ATM).
1.3.2 Scalability
ATM is highly scalable. The scalability of ATM as the technological solution
can be described at two levels:
•The interface level. ATM was initially developed to operate over
optical fiber physical media. With the time passing by, it soon became clear,
that ATM should be also defined for other physical media. Today ATM can
operate not only over various types of optical fiber (single mode and multi-
mode, glass and plastic fiber) but it can be also deployed using copper twist-
ed pairs and wireless interfaces. ATM has been adapted to the transmission
systems and technologies that are widely used both in telecommunication
networks as well as in computer networks. Therefore, in many cases ATM
can be run over existing transmission networks without the need for costly
re-wiring process. The variety of interfaces allows for different transmission
speeds ranging from 1.5 Mbps (DS 1) up to 10 Gbps (the latter is available
for single-mode, multi-mode fiber as well as twisted pair copper wire)
•The network level. Scalability with regards to the network level
refers to ATM’s ability to grow in physical size and speed reflecting the num-
ber of users and volume of traffic generated. The only limiting factors are
the characteristics of the physical medium and the switching equipment
capacity. Thanks to a number of advanced traffic management mechanisms
ATM networks work and scale well in the LAN environment, campus net-
works, MANs and WANs. Therefore, a network operator can use ATM in
almost any part of its network as the uniform method of transport that can

be used on almost any physical infrastructure.
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1.3.3 Interworking with other technologies
ATM can be used to interconnect systems. Although ATM was developed to
offer the unified solution, capable of replacing existing solutions and tech-
nologies (i.e. classical telephony systems), due a number of factors it was not
able to replace them completely. Most service providers/operators actually
upgraded their backbone infrastructure to ATM yet there remain a number
of customers who demand connectivity at the level of Frame Relay or SNA.
Needless, to say ATM was defined along with various methods for inter-
working with other technologies. ATM can serve as the underlying technol-
ogy for both existing LAN and WAN network services. Furthermore, ATM
has the ability to mimic different protocols what makes possible to slowly
migrate from existing technologies to ATM. The network operator can
deploy ATM on selective basis in key parts of its network. The set of ATM
specifications defines methods for interworking with LANs such as LAN
Emulation. Other technologies present in the WAN environment can be also
a subject to interworking. They include Frame Relay, SMDS and ISDN.
Interworking between ATM and IP has been the point of the greatest inter-
est for many operators. Hence, a number of specifications describe different
scenarios for ensuring successful ATM and IP integration. They include
basic solutions such as CLIP as well as fairly advanced models including
MPOA. With the advent of MPLS it has become clear that as data trans-
mission is concerned ATM technology can be soon replaced with MPLS con-
cept. In fact most of the ATM equipment already installed in ATM networks
can be easily updated to MPLS. Unsurprisingly there is a lot of ATM relat-
ed information within MPLS specifications. It has been widely accepted that
at the current stage of MPLS development it is the ATM infrastructure that
provides the best performance. The integration from ATM based platforms

into MPLS capable solutions can be achieved in a number of ways. Not only
ATM switching devices can be converted into MPLS switching nodes and
ATM and MPLS may coexist within the same device and even at the same
physical interface. It is also foreseeable to transmit MPLS traffic across net-
work regions comprised of traditional ATM devices. To conclude, even today
it can be easily noticed that ATM is a future proof solution
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1.3.4 Maturity Stage
ATM is a proven and mature technology. It has been more than a decade
since the first commercial ATM deployments took place. From the emerging
technology ATM turned into the alternative that has been chosen by the
largest players in the telecommunication market. The current approach is
to provide services by means of common platforms. The convergence is not
just a marketing term any longer but the common practice for hundreds of
operators providing advanced services that one can hardly imagine ten
years ago. Nowadays almost any application generates a stream of bits with
variable bit rate. This may refer to the transmission of compressed voice and
video but also to the transmission of data structured in packets. However,
there exist a tremendous amount of embedded equipment and applications
that can be used even though operators upgrade some parts of the core infra-
structure. ATM applications such as Circuit Emulation Service allow ATM
to emulate existing services (e.g. T1/T3 or E1/E3) with the intention to sup-
port interconnections between end devices. Thanks to its unique capabilities
and the continuous evolution ATM is still considered as the technology of
choice for many customers, especially when reliability, scalability and QoS
are at stake.
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