Frame Relay Components, part 2 - Page 3

By Cisco Press | Posted Jan 16, 2002
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FECN and BECN

Congestion is inherent in any packet-switched network. Frame Relay networks are no exception. Frame Relay network implementations use a simple congestion-notification method rather than explicit flow control (such as the Transmission Control Protocol, or TCP) for each PVC or SVC; effectively reducing network overhead.

Two types of congestion-notification mechanisms are supported by Frame Relay:
  • FECN
  • BECN
FECN and BECN are each controlled by a single bit in the Frame Relay frame header.


NOTE:   A Frame Relay frame is defined as a variable-length unit of data, in frame-relay format, that is transmitted through a Frame Relay network as pure data. Frames are found at Layer 2 of the OSI model, whereas packets are found at Layer 3.

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

Much like the FECN bit, a Frame Relay network device sets the BECN bit, usually a switch, to inform the Frame Relay networking device that is receiving the frame that congestion was experienced in the path traveling in the opposite direction of frames encountering a congested path. The upper-layer protocols (such as TCP) will initiate flow-control operations, dependent on which protocols are implemented. This flow-control action, illustrated in Figure 15-11, is typically the throttling back of data transmission, although some implementations can be designed to ignore the BECN bit and take no action.

Figure 15-11: Frame Relay with FECN and BECN
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(Click image for larger view in a new window)


NOTE:   The Cisco IOS can be configured for Frame Relay Traffic Shaping, which will act upon FECN and BECN indications. Enabling Frame Relay traffic shaping on an interface enables both traffic shaping and per-VC queuing on the interface's PVCs and SVCs. Traffic shaping enables the router to control the circuit's output rate and react to congestion notification information if it is also configured. To enable Frame-Relay Traffic shaping within the Cisco IOS on a per-VC basis, use the frame-relay traffic-shaping command.

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