Why has communications evolved towards the ATM concept?
The Asynchronous Transfer Mode has been chosen as the standard
system concept for integrated broadband communication networks
by the ITU-T. The system is predicted to grow rapidly as soon as it
becomes widely accepted by network operators and users. Why has
communications evolved in history towards the ATM concept and
why has it been chosen as the broadband solution?
Arran Derbyshire, 3rd June 1996
Points addressed in this article:
Evolution Towards Integrated Broadband Communications
Early telephone networks
In the late 1800s public telephone networks capable of transmitting
analogue voice signals were established. The users were connected
together via switches across the network to form a circuit. This was
the first transfer mode used in telephone networks, and it is known
as circuit switching.
The invention of the vacuum tube led to the introduction of frequency
division multiplexing (FDM) in 1925, and therefore the ability to make
multiple connections on a single line. The public switched telephone
networks (PSTNs) expanded, but with the circuit switched transfer
mode remaining in use. This is because it provided an obvious way
of keeping the constant connection necessary for voice traffic.
The invention of the transistor and the concept of pulse code
modulation (PCM) allowed digital communications to be developed
in the late 1960's. The interconnection of computer systems over
telecommunication networks soon became a requirement. Modems
were used at first to generate analogue signals compatible with the
PSTNs from the digital computer data to allow such interconnections.
The already wide spread use of the PSTNs was an advantage of this
scheme, however it was soon recognised as not being an optimum
solution for data transfer.
The analogue PSTNs were unsuitable in terms of switching, capacity
(bandwidth) and channel noise. In data communication applications,
data tends to be transferred in bursts, seperated by silence. The
constant connection provided by circuit switching therefore does
not provide optimum usage of the network resources. The PSTNs
were orginally dimensioned to provide capacity to transport voice
signals, which provides very limited bandwidth and therefore a low
maximum rate of data transfer. The noise present in the PSTNs due
to poor channel quality lowers information transfer rates, since
redundancy in the data is required to perform error checking.
Solutions for data communications were developed, including the
packet switching transfer mode. With this system, the transmitter
splits the data to be transferred into discrete units and sends them
individually across a network, where at the other end, the data is
reconstructed by the receiver. Packets need only be sent when
data is available which therefore provides a more optimum use
of network resources.
Technological advances yielding high speed data
Computer technology advances created the ability for faster
information processing and therefore the need for faster
communication systems. Specific data networks were introduced
such as packet switched data networks (PSDNs), to meet the new
service requirements. In the early 1970's, high bit rate digital time
division multiplexing (TDM) systems were realised, allowing
multiple high speed digital connections on a single line. The
requirement for the integration of voice and data signals on a
single network emerged.
In the early 1980's, the design for an integrated systems digital
network (ISDN) was proposed. The design described a digital
packet switched network capable of providing telephone services
and other data services. Optical fibre technology emerged providing
a high speed transmission media with a low susceptability to noise.
These advancements coupled with the ISDN concept have led to
the current information age: the notion of wideband networks
capable of supporting high speed data communications, video,
multi-media, etc. is emerging.
The broadband-ISDN (B-ISDN) concept is the proposed realisation
of an integrated broadband communications network. The transfer
mode for implementing the B-ISDN defined and accepted by the
ITU-T is the asynchronous transfer mode (ATM). Is has been widely
accepted that a single network capable of supporting all required
communications services is at the core of the current movement of
A Comparison of Network Transfer Modes
The ATM design has been influenced by the peformance
requirements of the B-ISDN, new wideband teleservice
requirements and networking technology advancements.
By considering all these factors, it can be shown why the
ITU-T accepts ATM as the ultimate solution for B-ISDN.
B-ISDN performance requirements
The B-ISDN network is required to be able to transport many
different types of services over the same channels. However,
the bandwidth of the signals of the different expected services
varies considerably. A few examples are as follows:
||Bit rate (bit/s)
|Low Rate Data
|High Rate Data
|Hi Fi Audio
The quoted bit rates give an idea of the order of magnitude of
the bandwidths of different signals and they are stated in terms of the
rate at which data is generated by a service. Clearly, the B-ISDN
transfer mode could not be designed to support just a single type
of service. For instance, a transfer mode specific to high speed
video data would be extremely inefficient if the bandwidth of
video was assigned to the transfer of low speed data.
For the definition of a general transfer mode, the services can be
considered in terms of their natural information rates. This can be
described as the rate at which a source generates non-redundant
data which cannot be predicted in advance. Data generated from
a source could be transferred at a fixed rate and within a fixed
bandwidth, but this leads to inefficiency and quality loss problems.
Inefficiency and quality loss from fixed rate transfer
The natural information rate of a source fluctuates with time as a
random process. A source has a peak natural information rate, and
an average information rate. Consider a real time video image where
only some of the picture is changing: the static part of the image
contains redundant data and can be predicted from one frame to
the next. The natural information rate of this scenario will be less
than the natural information rate of a video image which is changing
entirely from one frame to the next.
To ensure that information is not lost, a signal could be transferred
with a bit rate equal to that of the peak natural information rate.
However, at the times when the natural information rate is lower
than the peak rate (like the partially redundant video image),
network resources would not be used effiently, and bandwidth
would be wasted.
To try and reduce overall bandwidth usage, the signal could be
transmitted with a bit rate lower than the peak natural information
rate. This would require however, that some of the signal information
is not transferred when the natural information rate exceeds the fixed
transfer rate. The would force an occasional quality loss during
the transfer of the service.
Natural information rates can be realised by the use of compression
systems. These systems only generate data from the non-redundant
information of the source data. By transferring this compressed data,
network resources would be used efficiently. However, this requires
a transfer mode flexible enough to handle a signal with a fluctuating
Evolution away from circuit switching
As described previously, circuit switching was first used in public
telephone networks. It establishes a constant connection for the
duration of a communications session. Communicating users can
be described as at the 'edges' of the network, with the circuit
connected through a series of network nodes, or switches. It is
this circuit path which is allocated at the start of a session and then
held constant during the session.
The links of such a network may be multiplexed between different
digital signals by the use of TDM. The bits from multiple signals are
transmitted alternatively at regular intervals to provide more
effective use of the network resources. Once a time slot has been
allocated to a connection, it remains for the duration of the session
and is repeated with a fixed period. Therefore, circuit switching
will only provide a fixed bit rate for transmission which leads to
problems in handling services of vastly different bit rates and
problems with fluctuating information rates (as outlined above).
This transfer mode is also referred to as synchronous transfer mode
(STM) and it is unable to dynamically assign resources with the
flexibility required to satisfy the B-ISDN concept. Multirate circuit
switching is the next evolution away from circuit switching. This is
similar to the original STM system, but now a connection can be
allocated to multiple channels, each of a basic bit rate. This allows
for services of different rates to be accomodated, where the
number of channels allocated is proportional to the service
bandwidth. However, this system also cannot cope efficiently
with signals with fluctuating rates and it suffers from inflexibility
in terms of accomodating different data rates effectively.
The next evolutionary step from pure circuit switching leads to
the concept of fast circuit switching (FCS). This transfer mode
attempts to address the problem of handling sources with a
fluctuating natural information rate. FCS only allocates resources
and establishes a circuit when data is needed to be sent. However
the rapid allocation and deallocation of resources required to
efficiently transfer data is complex and unfeasible for the required
high data rate services.
The packet switching transfer mode is a significant step away from
circuit switching. As described previously, packet switching only
uses resources as required by the natural information rate by only
transferring packets as data is available. This attribute of the mode
makes it flexible and efficient in terms of bandwidth allocation.
A packet switching type of transfer mode has been accepted as
the most suitable solution for B-ISDN.
The effect of technology on packet switching concepts
Two important functions that are offered by networks are
semantic transparency and time transparency. They are the ability
to guarantee the correct transfer of data and the ability to guarantee
the timely transfer of data through the network, respectively. The
requirements of the B-ISDN define these functions for the packet
switching transfer mode specifically, which has led to an evolution
of the packet switching transfer mode towards ATM.
Packet switching meets the general conceptual requirements of
B-ISDN, but the original systems designed in the 1960's reflected
the state of networking technology as it was then. The X.25 packet
switching protocol was standardised at this time and it was designed
to operate on low quality channels which were highly susceptable to
bit errors due to channel noise. The semantic transparency functions
of X.25 were therefore designed to cope with many errors, which as
a result made them complex.
The X.25 system performs all its semantic transparency functions at
every node between the edges of the network which introduces
time delays in the transfer. So, in order to provide semantic
transparency, time transparency is reduced. The delays in such a
system prevent it from being able to transfer the high speed data
to be supported by B-ISDN, since the typical operating speed of the
system was about 64 kbit/s.
In order to provide higher speed transfer, the semantic transparency
functions need to be reduced. This has led to two further evolutionary
steps. The first was the frame relay transfer mode, which reduces
complexity and delays by moving some of the semantic transparency
functions to the edge of the network. This is offered by network
operators as an upgrade on X.25, and it provides an enhanced
throughput of 140 Mbit/s.
The asynchronous transfer mode solution
The ultimate step towards high speed flexible networking is taken by
the ATM concept. The system concept is to move all the semantic
transparency functions to the edge of the network, so that minimal
semantic functionality is provided by the intermediate switching nodes.
This keeps delays to a minimum providing a platform for high speed
transfer and delay sensitive real time applications.
The current proposed top transfer rate of ATM is 600 Mbit/s, which if
used efficiently should provide a realisation of the B-ISDN system.
It is important to note that the above stated system concepts have
been possible by the advances in networking technology. The most
significant area of development is optical transmission systems, which
provide high speed and low noise links. The reduction of noise offers
a reduction in required system complexity and therefore delays, yielding
an overall faster transfer rate. This coupled with the high bandwidth
potential of optical systems provides an overall feasible solution to
future wideband communications.
The early evolution of communications systems through history was
first influenced by the simple desire to communicate messages. With
increasing influence from computer technology, communications
started to take a new direction towards mass data communication.
The development of multimedia computer technology is starting to
have a profound effect now on communications, with the possiblity
for mass multimedia communication systems coming into our lives.
The networking technologies for these new applications must be
powerful and flexible to provide a large scale and long term solution.
ATM has been developed to provide these features by evolving
the system concept to give high speed data transfer. With the
support of fibre technologies, ATM is the optimum transfer mode
and most widely accepted transfer mode for future communications.
ASYNCHRONOUS TRANSFER MODE Solution for Broadband ISDN
2nd Edition, Martin de Prycker, Ellis Horwood Publishing.
"ISDN" by Thomas A. Fine from "Recent advances in networking"
"ATM Asynchronous Transfer Mode" by Brendan McKeon
from "The OSI Reference Model"
- asynchronous transfer mode
- broadband integrated services digital network
- fast circuit switching
- frequency division multiplexing
- integrated services digital network
- International Telecommunications Union Telecom Standards Sector
- pulsed code modulation
- public switched telephone network
- synchronous transfer mode
- time division multiplexing
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