Alcatel Carrier Internetworking Solutions 1000 ADSL manual System Overview, Hdlc Framing

HDLC Framing

PPPpackets arriving via a PPTP tunnel and PPP packets encapsulated on ATM connections differ in format. The PPP format on AAL5 is shown in figure 43.

P_ID Information

Padding

Figure 43. PPP ATM Format

The PPP format within a tunnel is shown in figure 44. This format has two additional bytes (FF and 03) in front of the packet, inherited from another encapsulation method (PPP in HDLC-like framing).

Address

Control

P_ID Information

Padding

FF 03

Figure 44. PPP/PPTP Tunnel Format

The ADSL modem adapts to the different formats based on the connection type. If problems with operation occur, the modem can be configured to handle the differing formats as follows:

∙When you set this field to never, the modem ensures that FF-03 is never placed in front of a PPP packet encapsulated on an AAL5/ATM connection. This is the default setting.

∙When you set this field to always, the modem ensures that FF-03 is always in front of a PPP packet encapsulated on an AAL5/ATM connection. Although not standard, some equipment may rely on this format.

∙When you set this field to keep, the modem does not alter the PPP packet arriving via a tunnel; it keeps the two bytes in front of the PPP packet when it performs the encapsulation.

System Overview Page

This page provides an instant overview of all ADSL channels on the ADSL modem. It is a summary of the information provided on the Bridge page and the PPP/PPTP page.