Packet over SONET / SDH

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Packet over SONET / SDH ( POS ) is a transmission protocol , the inside of the ISO / OSI model Layer 2, the data link layer is to incorporate. POS is based on the point-to-point protocol .

The user data are packaged in a HDLC- like manner and inserted into higher-order virtual containers (VC-4) octet-by- byte synchronously .

In order to prevent user data (payload) or intentionally look randomly through manipulation as SDH control data (overhead), the data is through a self-synchronizing scrambler sent.

Scrambler

The scrambler works with a linear feedback shift register that is linked with the user data via XOR .

Channel:

                                              Unscrambled Data
                                                    ↓
      +-----------------------------------------+ +---+
    +→|        → 43 bit Schieberegister →       |→|xor|
    | +-----------------------------------------+ +---+
    |                                               |
    +-----------------------------------------------+
                                                    ↓
                                              Scrambled Data

Receiver:

                                              Scrambled Data
                                                    |
    +-----------------------------------------------+
    |                                               ↓
    | +-----------------------------------------+ +---+
    +→|      → 43 bit Schieberegister →         |→|xor|
      +-----------------------------------------+ +---+
                                                    ↓
                                              Unscrambled Data

In order to generate a certain bit sequence in the data stream, both the 43-bit start value of the shift register and all data sent so far must be known. The probability of correctly guessing the current shift register content is around 1⁄127. In addition, the probability that the bit sequence is in the correct place in the VC is ¹ ⁄ ₁₂₇ . This results in an overall probability of the occurrence of special bit sequences that could be confused with control characters of approx. 9 · 10 ⁻¹⁶ . ${\displaystyle {\frac {1}{2^{43}}}}$

Transmission in detail

Channel

IP → → PPP FCS generate → bit stuffing → → Scrambling SONET / SDH framing

receiver

SONET / SDH framing → Descrambling → Remove stuffing bits → Check FCS → PPP → IP

Web links

• RFC 1662 - Point-to-Point Protocol in HDLC-like framing
• RFC 2615 - PPP over SONET / SDH