Test Your EQ (Engineering Quotient)

EQ #6

In a data communications system, what is a “killer packet”?

There are many types of data communications systems, which, in lieu of using a self-clocking code, rely on there being enough one-zero and zero-one transitions in the data itself to provide a frequency reference. Long strings of ones or zeros can make such a system fail temporarily while it tries to recover timing, so the designers use what are called “data scramblers” (pseudorandom number generators) to randomize the data.

This works really well when the payload is, for example, a set of multiplexed audio (telephone) channels. However, in these days of purely digital packet-based (e.g., Internet) traffic, it is possible for the payload of a packet to match relatively long sequences in the data scrambler, producing long strings of ones or zeros even after scrambling.

In fact, if one knows the characteristics of the scrambler being used — and this is usually specified in the relevant public standard — it is possible to deliberately design a “killer packet” that will achieve this. Even though the alignment of the packet with the scrambler is still random, sending enough of them will disrupt a typical network within a few seconds (or less).


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