Virtual-Circuit Networks Frame Relay and ATM Study Notes

  • 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.
  • 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-size packets.
  • ATM can handle real-time transmission.
  • A user-to-network interface (UNI) is the interface between a user and anATM switch.
  • A network-to-network interface (NNI) is the interface between two ATM switches.
  • In ATM, connection between two endpoints is accomplisheCl 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.
  • TheAAL 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. AALI 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.
  • 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.


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