Alcatel Carrier Internetworking Solutions SpeedTouch Speed Touch Home Asymmetric Digital Subscriber Line (ADSL) Modem - Manual (Page 55)

7 Speed Touch Home Local Configuration
55 / 88
3EC 17058 AAAA TCZZA Ed. 02
The following fields are part of the 'PPTP Connections' table:
"
Name
Indicates the Phonebook name of the PPTP entry.
"
Encaps
Encapsulation/Decapsulation refers to the encapsulation/
decapsulation of PPP packets in/from ATM Adaption
Layer (AAL)5/ATM.
The STHome is compliant with Request For Comments (RFC)
2364 PPP Over AAL5" and supports both the Logical Link
Control (LLC)/Network Link Protocol Identifier (NLPID) method
and the VC MUltipleXing (MUX) method. By default the
encapsulation/decapsulation method is set to VC MUX.
"
HighĆlevel Data Link Control (HDLC) Framing
PPP packets arriving via a PPTP tunnel and PPP packets
encapsulated on ATM connections differ in format. The PPP
format on AAL5 follows RFC 1661 PointĆtoĆPoint Protocol"
(See figure 31) whereas the PPP format within a tunnel follows
PointĆtoĆPoint Tunneling Protocol" (See figure 32).
The latter format has two additional bytes in front of the packet
(FF 03) inherited from another encapsulation, i.e. RFC 1662
PPP in HDLCĆlike framing".
adsl 0760/ 01
P_ID Information Padding
Figure 31 PPP ATM Format (RFC 2364:PPP over AAL5).
adsl 0761/ 01
Address Control P_ID Information Padding
(FF) (03)
Figure 32 PPP/PPTP Tunnel Format.
In order to cope with these PPP packet differences, the Speed
Touch  Home adapts to the different formats on a 'per
connection' base . Although RFC2364 is quite clear with regards
to PPP format on AAL5, the STHome offers a configuration
possibility if interoperability problems should arise.