CCNA Practical Studies (Cisco Certification & Training)

To understand much of the functionality of Frame Relay, it is helpful to understand the structure of the Frame Relay frame. Figure B-4 depicts the basic format of the Frame Relay frame, and Figure B-5 illustrates the LMI version of the Frame Relay frame.

Flags indicate the beginning and end of the frame. Three primary components make up the Frame Relay frame: the header and address area, the user -data portion, and the frame-check sequence (FCS). The address area, which is 2 bytes in length, is comprised of 10 bits representing the actual circuit identifier and 6 bits of fields related to congestion management. This identifier commonly is referred to as the data-link connection identifier (DLCI). Each of these is discussed in the descriptions that follow.

Standard Frame Relay Frame

Standard Frame Relay frames consist of the fields illustrated in Figure B-4.

The following descriptions summarize the basic Frame Relay frame fields illustrated in Figure B-4.

• Flags Delimits the beginning and end of the frame. The value of this field is always the same and is represented either as the hexadecimal number 7E or as the binary number 01111110.

• Address Contains the following information:

  - DLCI The 10-bit DLCI is the essence of the Frame Relay header. This value represents the virtual connection between the DTE device and the switch. Each virtual connection that is multiplexed onto the physical channel is represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection.

  - Extended Address (EA) Indicates whether the byte in which the EA value is 1 is the last addressing field. If the value is 1, the current byte is determined to be the last DLCI octet. Although current Frame Relay implementations all use a two-octet DLCI, this capability does allow for longer DLCIs to be used in the future. The eighth bit of each byte of the Address field is used to indicate the EA.

  - C/R Is the bit that follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined.

  - Congestion Control This consists of the 3 bits that control the Frame Relay congestion-notification mechanisms. These are the FECN, BECN, and DE bits, which are the last 3 bits in the Address field.

  - Forward-explicit congestion notification ( FECN ) is a single-bit field that can be set to a value of 1 by a switch to indicate to an end DTE device, such as a router, that congestion was experienced in the direction of the frame transmission from source to destination. The primary benefit of the use of the FECN and BECN fields is that higher-layer protocols can react intelligently to these congestion indicators. Today, DECnet and OSI are the only higher-layer protocols that implement these capabilities.

  Backward-explicit congestion notification ( BECN ) is a single-bit field that, when set to a value of 1 by a switch, indicates that congestion was experienced in the network in the direction opposite of the frame transmission from source to destination.

  Discard eligibility (DE) is set by the DTE device, such as a router, to indicate that the marked frame is of lesser importance relative to other frames being transmitted. Frames that are marked as "discard eligible" should be discarded before other frames in a congested network. This allows for a fairly basic prioritization mechanism 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 16,000 octets. This field serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network.

• Frame Check Sequence Ensures the integrity of transmitted data. This value is computed by the source device and is verified by the receiver to ensure integrity of transmission.

LMI Frame Format

Frame Relay frames that conform to the LMI specifications consist of the fields illustrated in Figure B-5.

The following descriptions summarize the fields illustrated in Figure B-5.

• Flag Delimits the beginning and end of the frame.

• LMI DLCI Identifies the frame as an LMI frame instead of a basic Frame Relay frame. The LMI-specific DLCI value defined in the LMI consortium specification is DLCI = 1023.

• Unnumbered Information Indicator Sets the poll/final bit to 0.

• Protocol Discriminator Always contains a value indicating that the frame is an LMI frame.

• Call Reference Always contains zeros. This field currently is not used for any purpose.

• Message Type Labels the frame as one of the following message types:

  - Status-inquiry message Allows a user device to inquire about the status of the network.

  - Status message Responds to status-inquiry messages. Status messages include keepalives and PVC status messages.

• Information Elements Contains a variable number of individual information elements (IEs). IEs consist of the following fields:

  - IE Identifier Uniquely identifies the IE.

  - IE Length Indicates the length of the IE.

  - Data Consists of 1 or more bytes containing encapsulated upper-layer data.

• Frame Check Sequence (FCS) Ensures the integrity of transmitted data.