[SI-LIST] Re: PHY PCS
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 7 Feb 2006 18:10:35 -0500
> The other thing I did not get is why use different coding techniques for
> different bit rates like Manchestar encoding for 10 base-T, MLT-3 for
> 100base-TX and PAM-5 for Gig??
There are probably a handful of reasons for this, and others more
experienced and knowledgeable than I who can answer better. But I'll take a
stab at a partial answer.
10Base-T was one of the earliest Ethernets. Manchester encoding is
sometimes considered "wasteful" of bandwidth because it sends two states per
bit, in order to guarantee a transition in the middle of the bit-time, which
makes it easier to do clock recovery and re-sync the data in the receiver.
At the time, it was good enough (the wasted bandwidth was OK, and Manchester
encoding made the electronics simpler.
Other methods are more bandwidth-efficient by having no more than one edge
per PHY symbol. Some versions also use more than two states per symbol,
further improving bandwidth efficiency.
As Ethernet progressed to faster speeds where frequency dependent cable
attenuation became severely limiting, these more advanced coding schemes
were used. You can't push 1 or 1.25 Gb/s down a twisted pair using
Manchester encoding and have it get very far.
Regards,
Andy
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