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Virtual-Circuit Networks: Frame Relay and ATM

* Virtual circuit switching is a data link technology in which links are shared.
* A virtual circuit identifer (VCI) identifies a frame between two switches.
* The three phases in virtual circuit switching are setup, data transfer, and teardown.
* The setup phase can use the permanent virtual circuit (PVC) approach or the switched virtual circuit (SVC) approach.
* Frame Relay is a relatively high-speed, cost-effective technology that can handle bursty data.
* Both PVC and SVC connections are used in Frame Relay.
* The data link connection identifier (DLCI) identifies a virtual circuit in Frame Relay.
* Asynchronous Transfer Mode (ATM) is a cell relay protocol that, in combination with SONET, allows high-speed connections.
* A cell is a small, fixed-size block of information.
* The ATM data packet is a cell composed of 53 bytes (5 bytes of header and 48 bytes of payload).
* ATM eliminates the varying delay times associated with different-sized packets.
* ATM can handle real-time transmission.
* A user-to-network interface (UNI) is the interface between a user and an ATM switch.
* A network-to-network interface (NNI) is the interface between two ATM switches.
* In ATM, connection between two endpoints is accomplished through transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs).
* In ATM, a combination of a virtual path identifier (VPI) and a virtual circuit identifier identifies a virtual connection.
* The ATM standard defines three layers:
a. Application adaptation layer (AAL) accepts transmissions from upper-layer services and maps them into ATM cells.
b. ATM layer provides routing, traffic management, switching, and multiplexing services.
c. Physical layer defines the transmission medium, bit transmission, encoding, and electrical-to-optical transformation.
* The AAL is divided into two sublayers: convergence sublayer (CS) and segmentation and reassembly (SAR).
* There are four different AALs, each for a specific data type:
a. AAL1 for constant-bit-rate stream.
b. AAL2 for short packets.
c. AAL3/4 for conventional packet switching (virtual circuit approach or data-gram approach).
d. AAL5 for packets requiring no sequencing and no error control mechanism.