Frame Relay

Frame relay is a WAN service that is designed to connect two points that require only intermittent communications. 
  • It was first designed for transport across ISDN (Integrated Services Digital Network) infrastructure.
Each frame relay PDU (protocol data unit) has a field:
  • Flag Field - high level data link synchronization
  • Address Field - Occupy either octet 2 to 3, 2 to 4, or 2 to 5
  • Information Field - maximum number of data bytes that a host can pack into a frame
  • Frame Check Sequence Field - implement error detection to avoid wasting bandwidth due to the transmission of erred frames
Congestion Control
  •    Admission Control - principal mechanism used in frame relay to ensure the guarantee of resource requirement once accepted
  • Committed Information Rate (CIR) - average rate (in bit/s) at which the network guarantees to transfer information units over a measurement interval T. This T interval is defined as: T = Bc/CIR
  • Committed Burst Size (BC) - maximum number of information units transmittable during the interval T
  • Excess Burst Size (BE) - maximum number of uncommitted information units (in bits) that the network will attempt to carry during the interval