HP auroraTempo User Manual page 223

LAPD was originally developed to convey signalling
information between the elements of an ISDN network.
Signalling, however, does not use all the available bandwidth
thus freeing up the remainder for the transmission of user
data.
Frame Relay uses the capabilities of the LAPD protocol for
the transmission of all the data.
The following diagram shows a typical Frame Relay frame:
• The flag indicates the beginning and end of a frame.
• The FCS (Frame Check Sequence) is used to ensure
frame integrity.
• The information field contains the user data which
is to be passed between devices over the Frame
Relay network. This field is of variable length - in
theory it can be up to 8192 bytes in length; in
practice the maximum is vendor specific.
• The header contains addressing and congestion
control information. The header may contain 2, 3
or 4 bytes.
The structure of the header is shown in the following
diagrams:
2 Byte Header
Issue 1a - 10/99
Introduction to Frame Relay
8-9