On Mon, 2008-02-04 at 17:00 +0900, John Summerfield wrote: > Alan Cox wrote: > > On Sun, 03 Feb 2008 13:54:26 +1030 > > Tim <ignored_mailbox@xxxxxxxxxxxx> wrote: > > > >> On Sat, 2008-02-02 at 11:57 -0800, Les wrote: > >>> SCSI is a serial system, or at least it can be. > >> Pardon? Usually, when one has a data bus for several parallel data > >> lines at once, one refers to it as parallel. > >> > >> Serial - one data line, that sends bits sequentially. > >> Parallel - several data lines, that send bits simultaneously. > > > > Of course its never that simple. Most "serial" busses are actually > > multiple wires which send information in parallel.. > > More likely > txdata > rxdata > Maybe a pair for the other direction (eg RS0232C) > Ground > > For a modern cable that has fair throughput, view your ethernet cable: > http://www.ertyu.org/steven_nikkel/ethernetcables.html > Actually Ethernet cable is only modest throughput. Twisted pair limits frequency due to included inductance. The bandwidth in bits is due to the encoding which is moderately complex. And as the encoding envelope gets more and more complex and higher in bandwidth, the relevant transmission effects, radiation loss, envelope delay, phase errors introduced by lumped changes in impedance and so forth, become limiting factors in transmission distance. That is one of the reasons for the change to PoE which is to allow the use of modular repeaters inserted into the line. Encoding is a somewhat exciting area of research these days, with things like PRML (Partial Response Maximum Liklihood) and RLE (Runlength Encoding), giving way to analog Quadradure Amplitude Modulation (QAM) in one of several forms. WLAN and Cellphones are using Orthogonal Frequency Division Multiplex (OFDM) and within the OFDM envelope are several standards of BiPhase, and QAM modulation. Modern DSP techniques allow envelope recovery, phase leveling (wideband signals tend to have a phase creep with distance, even when sent wirelessly), and frequency tracking using pilot frequencies within the envelope. In addition, digital frequency synthesizers coupled with some fancy DSP allow phase shift control at low jitter points on the signal (zero crossing and peaks) which lessen the intermodulation products, minimizing the inherent noise, and maximizing the signal to noise ratio in a controlled environment like cable. Further improvements like coherent signal tracking allow predictive reception, lowering the bandwidth of the receiver, which in effect reduces the reception of radiated noise yeilding signals that can actually be plucked from below the open band noise floor. It is an arcane area of study, but bandwidth, transmission means and noise studies for modern signal processing is really growing by leaps and bounds. Lots of things that have been known to theorists for years are now finding practical and useful application. The real cutting edge is in light manipulation, and means of applying these DSP techniques to light, yielding greater transmission distance and lots more bandwidth than can ever be obtained by RF. Sorry, I get carried away a bit on this stuff. Regards, Les H
