Frame Relay Polling, Error Handling, and Specification Enhancements - Page 2

 By Cisco Press | Posted Jan 25, 2002
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Frame Relay Error Handling
Frame Relay uses the Cyclic Redundancy Check (CRC) method for error detection. Frame Relay services perform error detection rather than error checking; error detection is based on the premise that the underlying network media is reliable. Frame Relay error detection uses the CRC checksum to determine if the frame is received by the Frame Relay networking device (router or switch) with, or without, error. Error correction is left to the upper-layer protocols, such as the TCP (of the TCP/IP protocol suite).

NOTE:   Error detection detects errors, but does not make attempts to correct the condition. Error correction detects errors and attempts to correct the condition, usually under control or direction of a higher-layer protocol. The termination node performs error detection.

Frame Relay Frame Format
Figure 15-13 illustrates the standard Frame Relay frame format.

Figure 15-13: Frame Relay Standard Frame Format
Click image for larger view in a new window
(Click image for larger view in a new window)

Table 15-7 presents a description of each of the Frame Relay standard frame fields.

Field Description
Flags Delimits the beginning and end of the frame. The value of this field is always the same and is represented as hexadecimal 7E or as binary 0111110.
Address Contains the following information:
  • DLCI -- The 10-bit DLCI is the most significant part of the Frame Relay header. This value identifies and represents the VC* between the FRAD and the Frame Relay [network service provider] switch. Each VC that is multiplexed onto a physical channel will be represented by a unique DLCI. The DLCI values have local significance only, meaning they are only significant to the physical channel on which they reside. Devices on each end of a VC can use different DLCIs to identify the same VC.

  • Extended Address (EA) -- Used to indicate whether the byte in which the EA value is 1 is the last addressing field. If the value is 1, then the current byte is determined to be the last DLCI byte. Although current Frame Relay implementations all use a two-byte DLCI, this capability does allow for the use of longer DLCIs in the future. The eighth bit of each byte of the Address field is used to indicate the EA.
    MPLS labels use the extended address field of the Frame Relay frame header.
  • C/R -- The C/R (Command/Response) bit that follows is the most significant DLCI byte in the Address field. The C/R bit is not currently defined.
  • Congestion Control -- Consists of the 3 bits that control the Frame Relay congestion-notification mechanism. These are the FECN, BECN, and DE bits; they are the last 3 bits in the Address field.
  • Forward-Explicit Congestion Notification (FECN) -- A single-bit field that can be set to a value of 1 by a switch to indicate to an end DTE device (router) that congestion was encountered in the direction of the frame transmission from source to destination. The primary benefit of both the FECN and BECN fields is the capability of higher-layer protocols to react intelligently to these congestion indicators. Currently, DECnet and OSI are the only higher-layer protocols that implement these capabilities.
  • Backward-Explicit Congestion Notification (BECN) -- A single-bit field that, when set to a value of 1 by a switch, indicates that congestion was encountered in the network in the direction opposite of the frame transmission from source to destination. Explicit congestion notification is proposed as the congestion avoidance policy. It tries to keep the network operating at its desired equilibrium point so that a certain quality of service (QOS) for the network can be met. To do so, special congestion control bits have been incorporated into the address field of the Frame Relay: FECN and BECN. The basic idea is to avoid data accumulation inside the network.
  • Discard Eligibility (DE) -- Set by the Frame Relay networking device (router) to indicate that the marked frame is of lesser importance relative to other frames being transmitted. Frames that are marked as DE should be discarded before other frames in a congested network. This allows for basic prioritization in Frame Relay networks.
Data Contains encapsulated upper-layer data. Each frame in this variable-length field includes a user data or payload field that will vary in length up to 4096 bytes. This field serves to transport the higher-layer protocol data unit (PDU) through a Frame Relay network.
Frame Check Sequence (FCS) Ensures the integrity of transmitted data. This value is computed by the source device and is verified by the receiver to ensure integrity of the data transmission.

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