PPP was designed as a standard method of communicating over point-to-point links. Initial deployment has been over short local lines, leased lines, and plain-old-telephone-service (POTS) using modems. As new packet services and higher speed lines are introduced, PPP is easily deployed in these environments as well.
This specification is primarily concerned with the use of the PPP encapsulation over SONET/SDH links. Since SONET/SDH is by definition a point-to-point circuit, PPP is well suited to use over these links.
Real differences between SONET and SDH (other than terminology) are minor; for the purposes of encapsulation of PPP over SONET/SDH, they are inconsequential or irrelevant.
STS-12c/48c/192c payload C-4-4c/16c/64c
2. Physical Layer Requirements
PPP in HDLC-like framing presents an octet interface to the physical layer. There is no provision for sub-octets to be supplied or accepted.
The octet stream is mapped into the SONET STS-SPE/SDH Higher Order VC, with the octet boundaries aligned with the SONET STS-SPE/SDH Higher Order VC octet boundaries.
Scrambling is performed during insertion into the SONET STS-SPE/SDH Higher Order VC to provide adequate transparency and protect against potential security threats (see Section 6). For backward compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), the scrambler MAY have an on/off capability where the scrambler is bypassed entirely when it is in the off mode. If this capability is provided, the default MUST be set to scrambling enabled.
For PPP over SONET/SDH, the entire SONET/SDH payload (SONET STS-SPE/SDH Higher Order VC minus the path overhead and any fixed stuff) is scrambled using a self-synchronous scrambler of polynomial X**43 + 1. See Section 4 for the description of the scrambler.
The proper order of operation is:
detection -> PPP -> IP
The Path Signal Label (C2) indicates the contents of the SONET STS-SPE/SDH Higher Order VC. The value of 22 (16 hex) is used to indicate PPP with X^43 + 1 scrambling.
For compatibility with RFC 1619 (STS-3c-SPE/VC-4 only), if scrambling has been configured to be off, then the value 207 (CF hex) is used for the Path Signal Label to indicate PPP without scrambling.
The Multiframe Indicator (H4) is unused and MUST be zero.
PPP does not require the use of control signals. When available, using such signals can allow greater functionality and performance. Implications are discussed in.
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4. X**43 + 1 Scrambler Description
Note: While the HDLC FCS is calculated least significant bit first as shown:
A B C D
(that is, the FCS calculator is fed as follows: A, A, … A, B, B, etc…), scrambling is done in the opposite manner, most significant bit first, as shown:
A B C D.
That is, the scrambler is fed as follows: A, A, … A, B, B, etc…
The scrambler operates continuously through the bytes of the SONET STS-SPE/SDH Higher Order VC, bypassing bytes of SONET Path Overhead and any fixed stuff (see Figure 20 of ANSI T1.105 or Figure 10-17 of ITU G.707). The scrambling state at the beginning of a SONET STS-SPE/SDH Higher Order VC is the state at the end of the previous SONET STS-SPE/SDH Higher Order VC. Thus, the scrambler runs continuously and is not reset per frame. The initial seed is randomly chosen by the transmitter to improve operational security. Consequently, the first 43 transmitted bits following startup or reframe operation will not be descrambled correctly.
5. Configuration Details
Other than the FCS length (see below), the standard LCP sync configuration defaults apply to SONET/SDH links.
The following Configuration Options are RECOMMENDED for STS-3c-SPE/VC-4:
No Protocol Field Compression
For operation at STS-12c-SPE/VC-4-4c and above, Address and Control Field Compression and Protocol Field Compression are NOT RECOMMENDED. The Magic Number option remains RECOMMENDED.
Regarding the FCS length, with one exception, the 32-bit FCS MUST be used for all SONET/SDH rates. For STS-3c-SPE/VC-4 only, the 16-bit FCS MAY be used, although the 32-bit FCS is RECOMMENDED. The FCS length is set by provisioning and is not negotiated.
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