
Configuring Dial Services
3-4 114062 Rev. B
Unlike other data link layer protocols, LAPD can handle multiple users on the
same multiaccess interface. Figure
3-3 shows a LAPD frame.
Figure 3-3. LAPD Frame
The fields in the LAPD frame are as follows:
• The opening flag and closing flag fields are High-level Data Link Control
(HDLC) flags that separate one frame from the next.
• The address field contains two kinds of information. It provides a data-link
layer entity; more specifically, a logical point that defines the data link
between the user and the network. This portion of the address is the service
access point identifier (SAPI). The address field also contains a number to
identify the terminal equipment connected to the ISDN network. This is the
terminal endpoint identifier (TEI). Either the network assigns a unique
number to each device or you set this number at the device.
• The control field identifies the type of frame. This field also contains
sequence numbering.
• The information field contains data to set up the link.
• The frame-check sequence is a cyclic redundancy check (CRC).
Opening flag
Address
Control
Information
Closing flag
Frame-check sequence
DS0009A
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