Asynchronous transfer mode (ATM) is one of many network transmission protocols included in Windows Server 2003. The most commonly used transmission protocol included in Windows Server 20003 is TCP/IP, which is a connectionless protocol. As such, TCP/IP cannot offer some of the advantages that a connection-oriented, virtual circuit, packet-switching technology, such as ATM, can. Unlike most connectionless networking protocols, ATM is a deterministic networking system — it provides predictable, guaranteed quality of service.

ATM LOGICAL CONNECTIONS

• ATM provides fast packet switching and so can be used for multimedia communication. In voice and video applications, the packets have to reach the destination in sequence and without variable delay. In other words, the quality of service is of paramount importance for supporting multimedia services. To achieve this objective, in ATM networks the concepts of virtual channel connection and virtual path connection are introduced.
• The logical connection between two end nodes is called virtual channel connection (VCC). A bundle of VCCs that have the same end points is called the virtual path connection (VPC).
• The logical connection between two end nodes is called a virtual channel connection (VCC). This is analogous to a virtual circuit in X.25. VCC is set up between two end users, a user and a network (for control signaling), or network to network (for network management and routing).
• A bundle of VCCs that have the same end point is a called virtual path connection (VPC). All the cells flowing over all of the VCCs in a single VPC are switched together. The concept of virtual channel and virtual path is illustrated in Figure below.

                ATM virtual channels and virtual path.

Using the virtual path has the following advantages:

• Once a virtual path is established, establishing new virtual channel connections is easy. capacity can be reserved in anticipation of later call arrivals. No call processing is required at intermediate switches, so adding virtual channels involves minimal processing. Hence, the processing is reduced and connection setup time is much less.
• The switching is very fast and, as a result, network performance and reliability will increase.
• The network architecture will be simplified.