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Re: More 100Mbit
>> The cable for 10BaseT Ethernet is *not* flat. It is made of Twisted
>> Pairs: each *pair* of leads is twisted within itself. It is the most
>> important attribute of the cable.
>
>Yes, but the 2 twisted pairs are usually molded in flat plastic envelopes
>when used with RJ 45. Thus people confuse them with the modular wires
>sometimes used for short connections on 10 MBPS (and these are not twisted
>- max length 1.5 meters).
Please. You are *not* supposed to use a non-twisted-pair cable for
10BaseT Ethernet (10Mb/s), not at *any* length. If you are lucky
enough to have it work at all, you will have an extremely high
packet corruption rate (bad CRC and such). And do not believe
the green light that turns on (10BaseT "link") - this one is DC,
and will go over anything.
>> Indeed, the requirements for 100Mb/s are higher than those for 10Mb/s.
>> The commonly used standard is EIA/TIA-568A, and it categorizes the cables
>> into 5 categories; only two of them (3 and 5) are actually relevant
>> when we talk about Ethernet and Fast Ethernet. 10Mb/s can safely run
>> on cat. 3 cables; 100Mb/s requires cat. 5.
>
>You have the standards handy ;) The cat.5 wiring is mentioned everywhere,
>but reading through that standard will upset anyone's stomach. Could you
>perhaps quote the essentials ?
I don't have it handy right now; however the main parameters this standard
quantifies are impedance, attenuation, NeXT (Near end crosstalk), and
capacitance. Other parameters would be pair unbalance (resistance/capacitance),
SRL (Structural Return Loss), and Propagation Delay.
>The point I was trying to make was, that unmatched cable will work over
>short distances, IFF not a whole multiple of lambda/4 at the baseband
>modulation speed. For wiring under 1 meter, I have even used twisted phone
>pairs. (obviously for testing only), and I have used 'link' cables in 900
>MHz circuits made in the same way.
The point I was trying to make was, that at short cable lengths,
100BaseTX transceivers tend to cause interference with one another
and hence have operational problems. At 100BaseTX, you do *not* want
to have too short a cable.
>> The IBM Token Ring does not use any coax. Its cable is two-pair, twisted,
>
>Well, something did use such a cable, and I had one once. It had a special
>connector like BNC but with 2 pins inside it. And it was 120 ohms (I
>measured). I thought it was some form of old ethernet or token ring.
What you have seen is called Twinax. It was (and still is, at some places)
used to connect IBM 5250 terminals to midrange systems - S/36 and S/38
in older times, AS/400 in more recent times. Twinax is no LAN cable
(although there are BALUNs that can make use of it in LANs, if you have
an existing installation of such cables).
Doron Shikmoni