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Figure 4 shows the general NetWare Link/Frame Relay data frame format.
Figure 4
Data Frame Format
For more information about data frame format, refer to:
The High-level Data Link Control (HDLC) flags are the first and last octet, and indicate the beginning and end of the frame. If there is only one flag between two consecutive frames, the closing flag of the first frame serves as the opening flag of the next frame.
The Address field consists of the NetWare Link/Frame Relay control and management fields. These fields specify the virtual circuit numbering, flow control, and frame discard eligibility.
The Data Link Connection Identifier (DLCI) is a 10-bit routing address. The DLCI consists of two noncontiguous bit fields in the header, the most significant bit (MSB) or high order bit field and the least significant bit (LSB) or low order bit field. The DLCI is the address of the virtual circuit at either the User-Network Interface (UNI) or the Network-Network Interface (NNI). It allows the user and network management to identify the frame as being from a particular PVC. The DLCI is used for multiplexing several PVCs over one physical link. The router DLCI specifies a local virtual circuit.
For both ANSI T1.618 and the LMI standards, the DLCI addressing space allows 1,024 values at each local interface. Because some DLCIs are used for signaling, management, and future specification, 992 of 1,024 DLCIs (16 through 1,007) are available to address frame relay virtual circuits at each local interface, as shown in Table 3 and Table 4.
Table 3. ANSI T1.618 Numbering
Table 4. LMI Rev 1.0 DLCI Numbering
The Command/Response (C/R) is not used in this industry-standard implementation. It is always set to 0.
By enabling the NetWare Link/Frame Relay header to extend to either 3 or 4 octets, the Extended Address (EA) allows for a DLCI longer than 10 bits and greatly expands the number of possible addresses.
Forward Explicit Congestion Notification (FECN) is set by the frame relay network to indicate that it has experienced congestion in the packet forwarding direction of the frame.
When this bit is set to 1, the frame relay network notifies the user receiving the frames that congestion is occurring in the direction in which the frame is being sent.
Backward Explicit Congestion Notification (BECN) is set by the frame relay network to indicate that the network has experienced congestion in the reversed packet forwarding direction of the frame.
When this bit is set to 1, the frame relay network notifies the user sending the frames that congestion is occurring in the direction opposite to that in which the frame is being sent.
The Discard Eligibility (DE) bit is set by the end node and, when set and supported by the frame relay network, allows frames to be discarded in preference to other frames when a network is congested.
The frame relay network edge node might discard transmitted data exceeding the Committed Information Rate (CIR) on a PVC. (The CIR is the data rate at which the frame relay network agrees to transfer data.) Internally, the frame relay network might prefer to discard data with the DE set when it encounters congestion. If the congestion condition persists after discarding all frames with the DE set, the congested node can start discarding frames with the DE cleared.
Network edge nodes can also set DE bits in response to user data that exceeds the committed burst size during a fixed measured interval.
The Information field (also called the Data field) contains the protocol data packet being transmitted. The field can contain a maximum of 4,520 octets; however, the 16-bit Frame Check Sequence (FCS) is more effective with frames smaller than 4K. You should ensure that the network can handle the maximum frame size sent by the router. If not, you must adjust the Maximum Frame Size parameter in NIASCFG.
Different network and frame relay switches are expected to support varying sizes. However, the maximum size of 4,520 octets should accommodate most LAN traffic and frame relay network variations. The maximum information field size is configurable on a per-port basis. To avoid or minimize segmentation and reassembly of higher-level PDUs, you should choose an optimal frame size.
Novell supports the multiprotocol encapsulation scheme described in RFC 1490 to multiplex multiprotocol LAN traffic over a single frame relay link. This means that higher-level PDUs must be encapsulated so that receiving nodes can interpret and demultiplex them properly.
The Frame Check Sequence (FCS) is the standard 16-bit cyclic redundancy check (CRC) used by HDLC. This field detects bit errors that occur in the bits of the frame between the opening flag and the FCS. The WAN board performs a 16-bit CRC to ensure data integrity.
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