Asynchronous Transfer Mode

        ATM belongs to the fast packet switching family. It also uses Time Division Multiplexing (TDM) technique in transmission of data and so called traffics which also carry data or information. It sends asynchronously to the network. Traffics are sent in a form of cells or slots. This carries data to one network to the other. When there is no traffic to send the cell contains idle or empty cells because the network is synchronous.  ATM is the combination of the TDM technique and the transportation of data or traffic in a form of cells. This is an advantage for both end users of the network. ATM uses Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC).

        There are two choices of connections types, the PVC and SVC. The protocol is connection oriented. The circuits here are not the same. It depends on where the traffic is present on the network. When a user want’s to use it, a service provider will provide a committed bandwidth for the user to access the network. This is the basis of an ATM network.  ATM networks are on demand and has high speed communication network.

Here is a figure for an end to end connection through the network:

               In the figure the network uses VPI switching. It is handled by the network switched. It means that the switches use the virtual path for mapping through the network and will remap from one virtual path to the other while the virtual channel number is held consistent.

                Comparing the OSI model and the ATM. The ATM would probably belong to the two lower layers. The Data link layer and the Physical layer of the architecture. Such as SONET, ATM has been designed to run on a physical medium.  When comparing OSI and ATM, there is no exactly the same mapping on both architectures.

              In the ATM adaption layer. There are several sub layers present. The upper most portion of the layer is the Service Specific Convergence Sub layer (SSCS).  It is used on mapping Frame Relay and on Switched Multimegabit Data Service (SMDS). Under this layer is the Common Part Convergence Sub layer (CPCS).The combination of the SSCS and the CPCS make up the Convergence Sub layer (CS). This layer is the changing and melding of the data into a common interface for the ATM networks.  The next layer is the Segmentation And Reassembly (SAR). In this layer the data is prepared into a 48 byte payload prior to being submitted to the ATM layer for the header. 

Here are some benefits of ATM:

·   ATM uses dynamic bandwidth for burst of traffic meeting applications needs and delivering high utilization of networking resources.

·         High-speed communication

·         Fast, hardware-based switching

·         A single network connection that can reliably mix voice, video and data

·         Flexible and efficient allocation of network bandwidth

·         High speed communication

·         Opportunities for simplification via switched VC architecture; this is particularly for LAN based traffic that today is connectionless in nature.

·         High performance via hardware switching with terabit switches on the horizon 

Sources:

Broadband Telecommunications Handbook by Regis J. Bates

http://www.telecomspace.com/vop-atm.html

http://technet.microsoft.com/en-us/library/cc740081(WS.10).aspx