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*To*: "'stds-802-3-hssg@xxxxxxxxxxxxxxxxxx'" <stds-802-3-hssg@xxxxxxxxxxxxxxxxxx>*Subject*: Scramblers, Error Multiplication, and Error Detection (Re: CRC ch eck indication of bad fiber)*From*: "Tim Armstrong" <tim@xxxxxxxxxxxxxxxxxx>*Date*: Fri, 21 May 1999 13:01:39 -0400*Sender*: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx

Scrambler Types --------------- There are three general types of scramblers used in transmission systems, Frame Synchronous Scramblers (FSS), Distributed Sample Scramblers (DSS), and Self Synchronous Scramblers (SSS). The FSS is synchronized by periodically initializing to an agreed state. The initialization occurs at a time defined by a frame marker. This has the advantage of quick and robust synchronization (as quick and robust as the frame marker detection). The disadvantage is some vulnerability to emulation attacks (made worse by shorter sequences) unless coupled with another randomization mechanism. The DSS is synchronized by periodically sending samples of the scrambler state to the descrambler. The advantage is better randomization when compared to the FSS. The disadvantage is the time required to acquire synchronization and the potential for false synchronization due to random errors in the transmitted samples. The SSS is synchronized by essentially conveying the scrambler state within the scrambled data. The advantages are simplicity, very fast synchronization, robustness against false synchronization, and robustness against emulation attacks. The disadvantage is the associated error multiplication. SSS Error Multiplication ------------------------ When a transmission error occurs, the SSS will introduce additional errors equal in number to the number of feedback taps the scrambler employs. For example, the 1 + x^43 SSS used in ITU-T ATM over SDH/SONET and IETF PPP over SDH/SONET has a single feedback tap. This means that each transmission error will result, after descrambling, in EXACTLY one additional error EXACTLY 43 bit periods after the original error. It is easy to understand this when you consider what the 1 + x^43 descrambler consists of: it is just a 43-bit shift register whose input is fed by the received data and whose output is modulo-2 added to the receive data by an XOR gate. When it is received, the original error proceeds through the 'data' input of the XOR gate at the output of the descrambler. At the same time, it is fed into the shift register. That error will propagate through the shift register and 43 bit periods later will show up at the 'descrambler' input to the XOR gate. The corresponding received data bit at the 'data' input of the XOR gate will be summed modulo-2 with the error to produce a second error. The error multiplication is now complete. There are no further errors generated by that original transmission error because the output of the XOR gate is not fed back to the descrambler. (Only the scrambler has feedback.) Effect of Error multiplication on Error Detection ------------------------------------------------- Regarding the effect on the detection capabilities of the CRC-32 when SSS error multiplication occurs, the third reference below contains a good discussion. This was in the context of PPP over SDH/SONET. Their conclusion: CRC-32 detection capability is not weakened at all provided that the bit order of CRC calculation, scrambling, and transmission are aligned. Other error detection mechanisms could be provided by the PHY layer that would permit performance monitoring of the fiber. These error detection mechanisms would have different characteristics to those of the CRC. References ---------- Useful references on scramblers for transmission systems and their effect on CRC error detection capability: 1. S.C. Kim and B.G. Lee,"Low-Rate Parallel Scrambling Techniques for Today's Lightwave Transmission," IEEE Comm. Magazine, April 1995, pp. 84-95 This gives a good, quick overview of scrambler types, their characteristics, and examples of parallel implementations. 2. S.C. Kim and B.G. Lee,"Recent Advances in Theory and Applications of Scrambling Techniques for Lightwave Transmission," Proc. of the IEEE, Vol. 83,No. 10, Oct 1995, pp. 1399-1428 This is a more in-depth treatment of the subject of scramblers with a focus on the theory behind the design of parallel implementations of the three general scrambler types. 3. D. Ferguson and R. Cherukuri,"Self-Synchronous Scramblers For PPP Over Sonet/SDH: Some Analysis," IETF <draft-ferguson-pppsonet-selfsync-00.txt>, Juniper Networks, November 1997 Section 3.4 discusses the effect of the 1 + x^43 SSS on the error detection capabilities of the HDLC CRC-32. This same polynomial is used in Ethernet, and ATM AAL5. --------------- Tim Armstrong Nortel Networks

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