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The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links. This document describes the use of PPP over Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) circuits.

1.  Introduction

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.

For the convenience of the reader, we list the equivalent terms below:
       SONET                   SDH
   SPE                         VC
   STS-SPE                     Higher Order VC (VC-3/4/4-Nc)
   STS-1 frame                 STM-0 frame (rarely used)
   STS-1-SPE                   VC-3
   STS-1 payload               C-3
   STS-3c frame                STM-1 frame, AU-4
   STS-3c-SPE                  VC-4
   STS-3c payload              C-4
   STS-12c/48c/192c frame      STM-4/16/64 frame, AU-4-4c/16c/64c
   STS-12c/48c/192c-SPE        VC-4-4c/16c/64c

   STS-12c/48c/192c payload    C-4-4c/16c/64c

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The only currently supported SONET/SDH SPE/VCs are the following:
       SONET                   SDH
   STS-3c-SPE                  VC-4
   STS-12c-SPE                 VC-4-4c
   STS-48c-SPE                 VC-4-16c
   STS-192c-SPE                VC-4-64c

2.  Physical Layer Requirements

PPP treats SONET/SDH transport as octet oriented synchronous links. SONET/SDH links are full-duplex by definition.

Interface Format

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:

When transmitting:
         IP -> PPP -> FCS generation -> Byte stuffing -> Scrambling  -> SONET/SDH framing

When receiving:
         SONET/SDH framing -> Descrambling -> Byte destuffing -> FCS

         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.

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.

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3.  Framing

The framing for octet-synchronous links is described in “PPP in HDLC-like Framing”.
The PPP frames are located by row within the SONET STS-SPE/SDH Higher Order VC payload.  Because frames are variable in length, the frames are allowed to cross SONET STS-SPE/SDH Higher Order VC boundaries.

4.  X**43 + 1 Scrambler Description

The X**43 + 1 scrambler transmitter and receiver operation are as follows:

Transmitter schematic:

scrambler transmitter

Receiver schematic:

scrambler receiver

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[0], A[1], … A[7], B[0], B[1], 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[7], A[6], … A[0], B[7], B[6], 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.

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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:

Magic Number

      No Address and Control Field Compression

      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.



Books you may interested

Books on SONET/SDH, ATM, MPLS and Optical Networks Books on SONET/SDH Books on SDH/SONET, ATM, xDSL, and Synchronization Networks  Books on PPP Books on PPP Books on CCIE Routing and Switching 



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