RFC 1618 (rfc1618) - Page 2 of 6
PPP over ISDN
Alternative Format: Original Text Document
RFC 1618 PPP over ISDN May 1994
Transmission Rate
PPP does not impose any restrictions regarding transmission rate,
other than that of the particular ISDN channel interface.
Control Signals
PPP does not require the use of control signals. When available,
using such signals can allow greater functionality and
performance. Implications are discussed in [2].
Control signals MAY be required by some of the framing techniques
described, and is outside the scope of this specification.
Encoding
The definition of various encodings and scrambling is the
responsibility of the DTE/DCE equipment in use, and is outside the
scope of this specification.
While PPP will operate without regard to the underlying
representation of the bit stream, lack of standards for
transmission will hinder interoperability as surely as lack of
data link standards. The D-channel LAPD interface requires NRZ
encoding at the T reference point. Therefore, as a default, it is
recommended that NRZ be used over the B-channel interface at the T
reference point. This will allow frames to be easily exchanged
between the B and D channels.
When configuration of the encoding is allowed, NRZI is recommended
as an alternative in order to ensure a minimum ones density where
required over the clear B-channel, with caveats regarding FCS [2].
Historically, some implementations have used Inverted NRZ (merely
switching the sense of mark and space), in order to ensure a
minimum ones density with bit-synchronous HDLC. The use of
Inverted NRZ is deprecated.
Automatic Detection
Implementations which desire to interoperate with multiple
encodings MAY choose to detect those encodings automatically.
Automatic encoding detection is particularly important for
Primary Rate Interfaces, to avoid extensive pre-configuration.
Only simple encodings are currently distinguished.
The only reliable method of detection available is to switch
modes between the supported encodings. Transmission of the LCP
Simpson