Concepts of ATM

Connection oriented service. In connected oriented service, over a virtual circuit, the data stream from origin to destination follows the same path. Virtual circuit (a type of packet switching) operate on the same concept as packet switching, but the routing of packet is specified before transmission. Data from different connections is distinguished by means of virtual path identifier (VPI) and virtual channel identifier (VCI). By virtual circuit, cells in the same connection reach the destination in the order they are sent. It eliminates the need for sequencing numbers and buffering packets. Also, each cell incurs an overhead corresponding to the length of VPI/VCI which is less than the length source/destination address nedded. VPI and VCI are called connection identifiers.

ATM Services. ATM forum specifies five types of services. They are:

1. Constant bit rate (CBR). This is used for emulating circuit switching. The cell rate is constant with time. Telephone traffic, videoconferencing and television are the examples that use CBR.

2. Variable bit rate-non real time (VBR-NRT). This service allows users to send traffic at a rate that varies with time depending on the availability of user information. Multimedia email is an example of VBR-NRT.

3. Variable bit rate-real time (VBR-RT). This service is similar to VBR-NRT, but it is designed for applications that are sensitive to cell delay variation. Examples for real time VBR are voice with speech activity detection (SAD) and interactive compressed video.

4. Available bit rate (ABR). This service provides rate based flow control and is aimed at data traffic such as file transfer and e-mail.

5. Unspecified bit rate (UBR). This class is widely used today for TCP/IP.

Traffic parameters:

1. Peak Cell Rate (PCR). PCR is the reciprocal of the minimum time between two cells.

2. Sustained Cell Rate (SCR). Long term average cell rate.

3. Initial Cell Rate (ICR). Rate at which a should send after an idle period.

4. Cell Delay Variation Tolerance (CDUT). Measures permissible departure from periodicity of the traffic.

5. Burst tolerance. Maximum number of cells in a burst of back-to-back cells.

6. Minimum Cell Rate (MCR). Reciprocal of the maximum time between two cells.

Quality of Service Parameters:

1. Cell loss ratio (CCR)

2. Cell delay variation (CDV)

3. Peak-to-peak cell delay variation (peak-to-peak CDV)

4. Maximum cell transfer delay (Max CTD)

5. Mean cell transfer delay (Mean CTD).

Two types of connection:

1. Permanent virtual connection (PVC). PVC is a connection that is setup and taken down manually by a network manager. A set of network switches between the ATM source and destination are programmed with predefined values for VCI/VPI.

2. Switched virtual circuit (SVC). SVC is a connection that is setup automatically by a signalling protocol. SVC is more widely used because it does not require manual setup, but it is not reliable.

ATM switch operation : ATM switch process cells at an extremly high rate or speed. Fig. 11.27 illustrates the basic operation of ATM switch.

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