ATM was originally conceived as a solution for completely integrating the entire set of communication technologies, starting from the Local Area Network to the IP based systems and the broad band voice and data communication systems. ATM was proposed as the final and ultimate protocol to take care of all the requirements of all communication systems in the world. However, it was found that ATM could satisfy only some of these like ADSL could make full use of it. ATM was not used in its original form on the other devices. The best features and proposals of ATM were incorporated in a number of other standards that came about, typically, MPLS kind of systems. However, ATM has taken a rebirth in terms of the standards that are currently being developed IEC 62365 which provides for both audio and video over IP using ATM technologies.
The aim of this paper is to present the asynchronous transfer mode as a standard protocol, its description and development over a period of time. The paper will also look at the places were this is being employed and to what extent these were successful in implementing the systems that are needed for the users.
Asynchronous Transfer mode systems integrate Voice, Video and Data. Since it is connected oriented technology, one packet with the same source and destination will travel through the same route. This employs best effort delivery system which would enable performance and bandwidth on demand.
Figure 1: Aimed at providing best service (IEC, 2005)
ATM as per the standards defined would employ connection oriented, packet-like switching and multiplexing model. It starts by defining the fixed sized cell that comprises of 48 bytes of data and 5 bytes of header together making up the 53 bytes of cell information. ATM is fundamentally connection oriented. Therefore, it does multiplexing and reserves a route for a specific user on getting a request from the user. Multiplexing such lines over a network either from a user to a destination or from one switch to another would optimize the usage of the network to a great extent. This would save the cost of switching when requests are made one after the other and switch keeps changing the route. ATM also specifies a number of standards on how an user would get connected to an ATM network depending upon the speed of the network. ATM to User interface standards are governed by the ATM UNI standards.
ATM has the following three layers:
1. The physical layer
2. ATM Layer
3. ATM adaptation layer
The physical layer consists of various media that make up the transmission system, which would eventually transmit from kilobits per second to gigabits per second. The ATM layer defines the cells that make up the data transfer along with the other components; multiplexer that would switch the cell to the specific route that it should take, switches that would transmit data over a high speed switching hardware so that the transmission speed is achieved and finally have necessary software or hardware to take care of the issues that concern the traffic and do traffic management. In the ATM adaptation layer, four different types of data transfer is provided for the ATM system.
AAL Type 1
Constant bit rate services
AAL Type 2
Variable bit rate services
AAL Type 3 / 4
Connectionless services and data protocols
AAL Type 5