Frame Relay

1. A virtual-circuit technology that provides low-level (physical and data link layers) service in response to the following demands.
   • Higher Data Rate at Lower Cost -- provides T1 and T3 service at lower cost.

   • Bursty data serviced by bandwidth on demand

   • Less overhead due to improved transmission media -- does not provide error checking or require acknowledgment in the data link layer. Instead, all error checking is left to the protocols at the network and transport layers, which use the Frame Relay services.

2. Advantages over WANs (X.25) and T lines
   • Operates at higher speed (T1 and T3)

   • Operates in just the physical and data link layers

   • Allows bursty data

   • Allows a frame size of 9000 bytes, which can accomodate all local area network frames

   • Less expensive than traditional WANs

3. Disadvantages
   • Even T3 speeds (44.376 Mbps) is not high enough for protocols with higher data rates

   • Allows variable length frames

   • Because of varying delays it is not suitable for sending delay sensitive data such as real-time voice or video.

4. Provides permanent virtual and switched virtual connections (PVC and SVC). The switches that route the frames through the network are DCEs. It is usually used as a WAN to connect LANs or mainframes. A router or bridge can serve as the DTE and connects, through a leased line, the LAN to the Frame Relay switch.

   • A virtual circuit is identified by a number called a data link connection identifier (DLCI).

   • A PVC is established between two DTEs by the network provider.

   • A SVC needs the services of another protocol that has a network layer and network layer addresses (such as ISDN or IP).

   • DLCIs are assigned not only to define the virtual circuit between a DTE and DCE, but also to define the virtual circuit between two DCEs (switches) inside the network.

5. Frame Relay Layers
   • Physical layer -- supports any of the protocols recognized by ANSI.

   • Data link -- employs a simplified version of HDLC called core LAPF.

6. Congestion Control
   • Congestion may occur if users send data into the network at a rate greater than that allowed by network resources.

   • For congestion avoidance Frame Relay uses two bits in the frame to explicitly warn the source and the destination of the presence of congestion.
     • The backward explicit congestion notification (BECN) bit warns the sender of congestion in the network.

     • The forward explicit congestion notification (FECN) bit is used to warn the receiver of congestion in the network.

7. Traffic control
   • Access Rate depends on the bandwidth of the channel connecting the user to the network. The user can never exceed this rate.

   • The commited burst size is the maximum number of bits in a predefined period of time that the network is committed to transfer without discarding any frame or setting the discard eligibility (DE) bit.

   • The committed information rate (CIR) is similar in concept to committed burst size except that it defines an average rate in bits per second. It can be defined as CIR = B_c/T bps.

   • The excess burst size is the maximum number of bits in excess of B_c that a user can send during a predefined period of time.

   • If the user never exceeds B_c, the network is committed to transmit the frames without discarding any. If the user exceeds B_c by less than B_e (that is, the total bits are less than B_c + B_e), the network is committed to transfer all of the frames if there is no congestion.