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Atm network

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Atm network


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?

In the late 1800's 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 recognized 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, separated by silence. The constant connection provided by circuit switching therefore does not provide optimum usage of the network resources. The PSTNs were ordinally 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.

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 realized, 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 susceptibility 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 realization 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.

The ATM design has been influenced by the performance 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.

ATM has grown out of the need for a worldwide standard to allow inter-operability of information, regardless of the "end_system" or type of information. With ATM, the goal is one international standard. There is an unprecedented level of acceptance throughout the industry of both the technology and the standardization process. With ATM, we are seeing an "emerging technology" being driven by international consensus, not by a single vendor' s view or strategy.

Historically, there have been separate methods used for the transmission of information among users on a Local Area Network (LAN), versus "users" on the Wide Area Network (WAN). This situation has added to the complexity of networking as user' s needs for connectivity expand from the LAN to metropolitan, national, and finally world wide connectivity. ATM is a method of communication which can be used as the basis for both LAN and WAN technologies. Over time, as ATM continues to be deployed, the line between local and wide networks will blur to form a seamless network based on one standard_ATM.



Today, in most instances, separate networks are used to carry voice, data and video information_mostly because these traffic types have different characteristics. For instance, data traffic tends to be "bursty"_not needing to communicate for an extended period of time and then needing to communicate large quantities of information as fast as possible. Voice and video, on the other hand, tend to be more even in the amount of information required_but are very sensitive to when and in what order the information arrives. With ATM, separate networks will not be required. ATM is the only standards based technology which has been designed from the beginning to accommodate the simultaneous transmission of data, voice and video. ATM is the emerging standard for communications. This is possible because ATM is available at various speeds from Megabits to Gigabit speeds. When information needs to be communicated, the sender negotiates a "requested path" with the network for a connection to the destination. When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end_to_end quality of service. An analogy for this negotiation of qualities would be ...

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