Definition of Terms
- Virtual-circuit switching is a data link layer technology in which links are shared.
- A virtual-circuit identifier (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.
- Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type of WAN in the late 1980s and early 1990s.
- Frame Relay operates only at the physical and data link layers.
- Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.
- To handle frames arriving from other protocols, Frame Relay uses a device called a
Frame Relay assembler/disassembler (FRAD). A FRAD assembles and disassembles
frames coming from other protocols to allow them to be carried by Frame Relay frames.
- Frame Relay networks offer an option called Voice Over Frame Relay (VOFR) that
sends voice through the network.
- Local Management Information (LMI) is a protocol added recently to the Frame Relay protocol to provide more management features.
- One of the nice features of Frame Relay is that it provides congestion control and quality of service (QoS).
- Both PVC and SVC connections are used in Frame Relay.
- The data link connection identifier (DLCI) identifies a virtual circuit in Frame Relay.
- VCIs in Frame Relay are called DLCIs.
- 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).
- Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.
- ATM eliminates the varying delay times associated with different-size 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.
- ATM technology can be adopted for use in a LAN (ATM LAN).
- In a pure ATM LAN, an ATM switch connects stations.
- In a legacy ATM LAN, the backbone that connects traditional LANs uses ATM technology.
- A mixed architecture ATM LAN combines features of a pure ATM LAN and a legacy ATM LAN.
- Local-area network emulation (LANE) is a client/server model that allows the use of ATM technology in LANs.
- LANE software includes LAN emulation client (LECS), LAN emulation configuration server (LECS), LAN emulation server (LES), and broadcast/unknown server (BUS) modules.
The ATM standard defines three layers:
- a. Application adaptation layer (AAL) accepts transmissions from upper-layer services and maps them into ATM cells. The AAL is divided into two sublayers: convergence sublayer (CS) and segmentation and reassembly (SAR).
- 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.
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 datagram approach).
- d. AAL5 for packets requiring no sequencing and no error control mechanism.
Frame Relay network
Three address formats
Virtual connection identifiers in UNIs and NNIs
An ATM cell
Note: You can proceed to take the multiple choice exam regarding this topic. Virtual-Circuit Networks: Frame Relay and ATM – Set 1 MCQs
List of Data Communications Lectures
credit: Behrouz A. Forouzan©2013 www.PinoyBIX.org