How Do You Want Your DS3? Scrambled?

DS3 (also known as T3) scrambling is not an issue that comes up very often. It's something every network administrator should know about, though. After all, it just might save your bacon.

Some physical layer protocols depend on the transition from 1 to 0 in a frame to maintain clocking. Therefore, a frame that has too many 1s or 0s can cause the physical layer of a DS3 to go into alarm. Generally, this results in errors on the link, but it could result in downtime.

Scrambling was designed to randomize the patterns of 1s and 0s in a frame to ensure physical layer protocols are not tripped up by long sequences. The randomization is done in hardware, so it offers no hit in terms of performance. Once it's enabled, nothing further is required.

It is important to note that scramble must be enabled on both sides of the link. Otherwise, the link will suffer rolling <acronym title="Cyclic Redundancy Check">CRC</acronym> errors and you'll suffer some embarrassment!

To enable scrambling, add the scramble command under the interface configuration:

SLAP(config)# interface Serial 4/1
SLAP(config-if)# scramble

REMINDER: Do that on both sides!

Personally, I went 10 years without needing to enable scrambling on an interface. But, after suffering through the doldrums of figuring out the problem one time, I'm convinced that it should be enabled every time. Since there is no performance hit, and it ensures a circuit that is free from timing slips (caused by long sequences of 1s and 0s, at least), it's well worth it.

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