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Label Switching

As the cells arrive at the ATM, their VPI/VCI is modified by a forwarding table and the VPI/VCI is changed for the cell's trip.

Label Writing

The above mentioned process is called rewriting. ATM, therefore, is called a label writing or label switching system.

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An Example ATM Network

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Permanent Virtual Circuits

ATMs can provide customers with virtual circuits that look like traditional leased digital circuits.
Such Parmanent Virtual Circuits last as long as the customer pays periodic fee for em.
The forwarding table entries are statically configured and are automatically restored after the power of equipment fails. The term used for this is called provisioning.
Provisioning requires 2 steps: 1. To determine each switch that will be taken in the complete path. 2. To choose appropriate VPI/VCI for each step of the path.

Switched Virtual Circuits

Establishing an SVC

The computer sends a connection request to the switch it is attached to, which later finds a network path to destination and sends along the connection request.
Each switch in the path chooses a VPI/VCI to establish a path to the destination and after that, a message is sent back to the original PC indicating that the SVC is ready.
If any of the switches fails, an error message is sent back to the original PC.

Signaling

The networking is not only used for data communication but there is also control messages communicated. The term used for such communication is called signaling.

Quality of Service

Providing Desired QoS

These are specified at the time of connection setup.
The switches then reserve resources and hence are never altered.
If a switch fails to reserve resources, an error message is sent back.

ATM QoS Specification

There are 3 types of these:

Constant Bit Rate

Used in audio and video since they have predefined maximum data rates.

Variable Bit Rate

Used in compressed audio and video because the data rates depends on the level of compression being used

Available Bit Rate

Used in typical applications where data rate is unknown and whatever is available at the time, is used.