> 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 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu