Date: Sat, 17 Jul 1999 02:12:38 -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.4.05.9907170206040.331-100000@venus.GAIANET.NET> In-Reply-To: <3.0.2.32.19990717015406.0095f670@storm.digital-rain.com>
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On Sat, 17 Jul 1999, Tim Baird wrote: > > 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. > > Ahem....I am not sure what this means......It sounds a little bit like > those articles that you read in that audiophile magazine "The Absolute > Sound" where people claim that they can hear the difference in the > acoustics of their listening room depending on where they have situated > their teacup......but I digress.....see below.... It all depends on the equipment too since remember the wire is the weakest link in the chain and every cable sounds different. I'm sure no wire is 100% perfect. > 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. > > This is not a very accurate description of what is happening in a twisted > pair situation....... > > There are two factors influencing the design of twisted pair transmission > line. > > First there is the need to control the consistency of the characteristic > impedance of the line. This happens by controlling the distance (radius) > between the two conductors as well as the dielectric material that is > present in their vicinity. For coax, this is a special compound between > the braided outer conductor (shield) and the inner conductor. For twisted > pair, this is usually air (the dielectric properties of the insulation are > made to be virtually the same as air). The dielectric only affects the > capacitive element by controlling permittivity, the inductive element is > essentially controlled by the permeability of free space. The > characteristic impedance has nothing at all to do with the DC resistance of > the conductors used...it (CI) is a combination of the two reactive effects > inherantly present along the line...namely capacitance (storage of electric > charge) measured in Farads/meter and inductance (storage of magnetic > energy) measured in Henries/meter. These two effects or "reactances" will > always be equal and opposite at some frequency, the idea is to have an > effective cancellation happening over as broad a range as possible so that > they do not limit the bandwidth of the transmission line. The values of > the two reactances are positive and negative imaginary numbers. When you > take the square root of the ratio of the two values, you end up with a real > number in the unit of "ohms".....ie the characteristic impedance......... Yes, but how consistent the ohms rating is what is important. > BTW This is EXACTLY analogous to "index of refraction" with respect to > light....... > > As the spectral components of your signal increase in frequency, there are > some interesting effects which occur...one is "skin effect". This effect > is the tendency for currents on the line to travel to the outside of the > conductor as their self induced electromagnetic force creates a higher flux > density at the center of the conductor...this changes the effective > cross-sectional area of the conductor and thus the resistance. The result > is a more "lossy" line....Other losses are found in the dielectric > material, and of course the natural resistance of the copper.... > > > Second, to minimize the effect of electromagnetic interference (EMI) which > is both "crosstalk" and other stray emag signals incident on the > transmission line, the twisted pair design is used on the assumption that > common mode signals are effectively rejected by the reciever...ie any > currents induced along the line that are EQUAL will be cancelled out by the > differential reciever. If you twist the cable pair as it travels through > space, you will have any incident signal affect BOTH conductors equally > (hopefully)....thus the EMI induced currents will be effectively cancelled > out by the commond mode rejection characteristics of the differential > receiver....This is why you twist your antenna wire that comes from your TV > antenna on your roof (if anyone still uses these)...otherwise your TV will > have a ghosted image...one signal from the antenna, and another signal > (shifted in time) coming in along the antenna wire. You have a good point. > As for these issues in the audio realm, I can assure you that high > frequency effects such as skin effect, characteristic impedance > considerations, and the like have NO real influence on your sound > quality...anyone who says so is shoveling cow cookies, or has a high bit > error rate in the cerebral cortex It all depends again on the equipment on both ends of the chain. It's like garbage in/garbage out if the equipment is garbage in the first place. > For input stages like microphones etc, there is the need to block out EMI > so that you don't record your guitar along with the 60 Hz hum from your > electrical system, the brush noise from your neighbour's drill etc.....This > is where using a combo of sheilded and twisted cable is preferred....... Yep, that's true. > As for data transmission, you want to maximize the data rate, maximize the > distance, minimize the error rate, simplify the installation procedure, > minimize the cost etc....Therefore, you want to have all of these design > considerations outlined above built in to the whole system. Also, you > don't want to have to rip out the network cable every time you upgrade your > data pumps...You want as good a cable as you can afford, that way the data > pumps (ethernet cards) are able to operate at their full capacity without > wasting time doing retrans....Most of the time though, a reasonably > conscientous cable installation will not be found to be the weak link in > your system. Usually it will work great or really badly, and the cause - > if it is bad cable - is usually found easily by isolating different > segments and testing.... > > I hope everyone is benefitting by these simple facts....I hate to see > needless misunderstanding > > BTW, if this is too far off topic for this list, please feel free to > express your concern (anyone) I am benefiting from it for sure. I guess what I was asking originally was if the higher frequency rated cables will give it more headroom since the 100BaseTX ethernet does push CAT5 to the limit. 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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