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.

Computer interconnections

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 communications evolution.



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:

Service Bit rate (bit/s)
Telemetry 101
Low Rate Data 103
Voice 105
High Rate Data 107
Hi Fi Audio 107
Video 109

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 bit rate.

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.



Concluding remarks

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.


Bibliography

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"


Abbreviations

ATM
asynchronous transfer mode
B-ISDN
broadband integrated services digital network
FCS
fast circuit switching
FDM
frequency division multiplexing
ISDN
integrated services digital network
ITU-T
International Telecommunications Union Telecom Standards Sector
PCM
pulsed code modulation
PSTN
public switched telephone network
STM
synchronous transfer mode
TDM
time division multiplexing


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