Frame Relay -- Summary - Page 2

Two types of congestion-notification mechanisms are implemented with Frame Relay:

• Forward explicit congestion notification (FECN) -- The FECN bit is set by a Frame Relay network to inform the Frame Relay networking device receiving the frame that congestion was experienced in the path from origination to destination. Frame relay network devices that receive frames with the FECN bit will act as directed by the upper-layer protocols in operation. The upper-layer protocols will initiate flow-control operations, depending on which upper-layer protocols are implemented. This flow-control action is typically the throttling back of data transmission, although some implementations can be designated to ignore the FECN bit and take no action.

• Backward explicit congestion notification (BECN) -- Much like the FECN bit, the BECN bit is set by a Frame Relay network to inform the DTE that is receiving the frame that congestion was experienced in the path traveling in the opposite direction of frames. The upper-layer protocols will initiate flow-control operations, depending on which upper-layer protocols are implemented. This flow-control action is typically the throttling back of data transmission, although some implementations can be designated to ignore the BECN bit and take no action.

Frame Relay VCs, both permanent and switched, have three configurable parameters that must be agreed upon between each end node and the Frame Relay network provider:

• Committed information rate (CIR) -- This is the amount of bandwidth that will be delivered as best-effort across the Frame Relay backbone network.
• Discard eligibility (DE) -- This is a bit in the frame header that indicates whether that frame can be discarded if congestion is encountered during transmission.
• Virtual circuit identifier
• Data-link connection identifiers (DLCIs) for PVCs -- Although DLCI values can be 10, 16, or 23 bits in length, 10-bit DLCIs have become the de facto standard for Frame Relay WAN implementations.
• X.121/E.164 addressing for SVCs -- X.121 is a hierarchical addressing scheme that was originally designed to number X.25 DTEs. E.164 is a hierarchical global telecommunications numbering plan, similar to the North American Number Plan (NANP, 1-NPA-Nxx-xxxx).

The formulae in Table 15-17 can be used to determine the number of VCs required to enable each associated network topology.

Table 15-17: Summary of Network Topology Formulae

Network Topology	Formula Note:   N is the number of locations
Fully meshed	[(N (N - 1)) / 2]
Partial-mesh	(Approximation) [N2 / (N - 1)] (Guideline) [((N (N - 1)) / 2) X (N - 1)]
Hub-and-Spoke	[N - 1]

Frame Relay uses the cyclic redundancy check (CRC) method for error detection. Frame Relay has no inherent error correction mechanisms, leaving error correction to the management and control of the upper-layer protocols.