Date: Fri, 16 Jul 1999 23:16:22 -0700 From: Tim Baird <tim@storm.digital-rain.com> To: freebsd-hackers@freebsd.org Subject: Re: poor ethernet performance? Message-ID: <3.0.2.32.19990716231622.007e2100@storm.digital-rain.com> In-Reply-To: <Pine.BSF.4.05.9907162005320.331-100000@venus.GAIANET.NET> References: <199907170220.TAA25948@apollo.backplane.com>
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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). > >> I'm not sure what the gigabit copper ethernet people are doing, but there >> are other ways as well. > > I wonder if any Gigabit ethernet cards exist that uses UTP because >everything seems to be pointing to the fiber direction. > >> Basic ethernet uses baseband which is quite noisy even with the >> preattenuation, so there was lots of room to go faster. > > Yep, it would have been nice if the original spec was shielded but >anything shielded would have a degrade in signal... It's the insultaion >whether it is PVC, Teflon or even the new air chambers introduced by >Matthew Bond at Tara Labs using Regtangular Solid Core cable capable of >handling 2.5Ghz. > > >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 > > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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