Date: Sat, 17 Jul 1999 00:28:18 -0700 (PDT) From: Vincent Poy <vince@venus.GAIANET.NET> To: Tim Baird <tim@storm.digital-rain.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: poor ethernet performance? Message-ID: <Pine.BSF.4.05.9907170021220.331-100000@venus.GAIANET.NET> In-Reply-To: <3.0.2.32.19990716231622.007e2100@storm.digital-rain.com>
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On Fri, 16 Jul 1999, Tim Baird wrote: > At 08:08 PM 16/07/99 -0700, you wrote: > >On Fri, 16 Jul 1999, Matthew Dillon wrote: > > > >> : I know, I'm just wondering how did they get more frequency out of > >> :wire of the same size. I can understand it if the wire was a larger > >> :guage. > >> > >> For twisted pair, Less power == less crosstalk. Plus the higher > >> bandwidth transceivers use better receivers and better pre-attenuation > >> of the signal. > > > > Cross talk is only one of the variables I think... There is > >balancing and ACR and how well it can keep a true 100 Ohms to the cable. > > Now for a brief tutorial on transmission line / gigabit ethernet...... > > > Ensuring that the entire transmission path maintains a consistant > characteristic impedance is the most difficult task in cable manufacture. > It has typically the most influence on the quality of the signal > transmission. Crosstalk is always present in a multi pair cable system, it > is a matter of degree i.e. how much cross talk (undesirable signal) is > present as a percentage of desired signal. The level of crosstalk is > proportional to dv/dt .... the rate of change in signal voltage with > respect to time. This is different than "frequency". A 1 Hz signal that > approaches a square wave (or rectangle) can have HUGE crosstalk at the > edges because of the large dv/dt at those points. Care must be taken to > optimize the dv/dt with respect to the desired baud rate. (baud == state > changes per second) > > As a cable length increases, losses increase, (these can be compensated for > by increasing drive level ... and thus dv/dt) but a potentially worse bogey > man plays a role....propagation delay. Most of the physical problems > previously indicated can be improved upon, but no one has found a way > around the limits of the speed of light (most transmission line allows > propagation at about 70 to 80% of c). The time is not far off where this > will be by far the most significant limit on information exchange for > everyday communication. > > STP is better to use over shorter lengths where high level of EMI > compromise the common mode rejection of the reciving system. The downside > is that STP typically has rotton characteristic impedance > consistency....not because of the plastic jacketing etc, but because of the > varying distance (radius) between the cable pairs and the sheild over the > length of the cable. Going to coax is usually the choice here for standard > ethernet. > > Gigabit ethernet obviously creates a new level of awarenes about all of > these factors. From a great article at > http://www.gigabit-ethernet.org/technology/whitepapers/gige_11.97/how.html.. > ..... > > * Use existing 4-pair Category 5 cable. To ensure proper operation at full > link lengths, the cable must conform to the requirements of > ANSI/TIA/EIA-568-A (1995). > > * Use all four pairs in the cable to keep symbol rate at or below 125 Mbaud. > > * Use PAM-5 coding to increase the amount of information sent with each > symbol. (similar concept to analog modem methods) > > * Use 4D 8-state Trellis Forward Error Correction coding to offset the > impact of noise and crosstalk. (ie. reciever has the ability to correct the > error without requesting retransmission) > > * Use pulse shaping techniques to condition the transmitted spectrum. (i.e. > limit dv/dt etc) > > * Use state-of-the-art DSP signal equalization techniques to manage the > problems of noise, echo and crosstalk interferences, and to ensure a bit > error rate of 1 x 10 exp(-10). Thanks for the article and for the brief. I just have a little comment on shielded versus unshielded for both analog and digital audio cables, not sure if this applies to data cable but digital audio is data: Cables are of the "nude" (unshielded) style, which has generally been perceived to sound "faster" and less "colored" than conventional fully shielded cables. As it turns out, there is good reason to think so since properly designed, un-shielded cables are much less reactive to the signal than their fully shielded counterparts. At audio frequencies and with reasonably short lengths of cable, a shield typically does more harm than good and is otherwise necessary only for Radio Frequency transmission and/or into extremely high gain inputs such as microphone and phono pre-amps. Instead, properly braided or twisted conductors effectively reduce susceptibility to induced noise, especially inductively coupled interference (EMI) while angular crossing weakens the field effects of opposite polarity conductors on each other. The mechanism for the self-shielding/field controlling design is to divide the signal into several separate runs in a continually changing orientation such that only a small fraction of either polarity is ever in the ideal orientation to the wave front. This has most relevance to electromagnetic fields either internal or external, which especially require an optimal angular component to induce the greatest opposing current flow. Cheers, Vince - vince@MCESTATE.COM - vince@GAIANET.NET ________ __ ____ Unix Networking Operations - FreeBSD-Real Unix for Free / / / / | / |[__ ] GaiaNet Corporation - M & C Estate / / / / | / | __] ] Beverly Hills, California USA 90210 / / / / / |/ / | __] ] HongKong Stars/Gravis UltraSound Mailing Lists Admin /_/_/_/_/|___/|_|[____] To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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