Networking Question

[Ben Beeson]

Warren,

	I'm not too sure about the vernacular here, but my understanding is that 
the way you wire ethernet cables for directly connecting two computers is 
different from those cables used to connect to a hub or router.  The 
technician that built my cable for me called it a "half-rolled CAT 5 
cable."  Basically he crossed two wires on the terminating jack in the 
same manner that one would do to build a null-modem cable.  That way, the 
two ethernet cards believe that the circuits are completed.  The 
following is extracted from the Ethernet HOWTO at 
http://www.linuxdoc.org/HOWTO/Ethernet-HOWTO-5.html



5.2 Twisted Pair 

Twisted pair networks require active hubs, which start around $50, and 
the raw cable cost can actually be higher than thinnet. You can pretty 
much ignore claims that you can use your existing telephone wiring as it 
is a rare installation where
that turns out to be the case. 

On the other hand, all 100Mb/sec ethernet proposals use twisted pair, and 
most new business installations use twisted pair. Also, Russ Nelson adds 
that `New installations should use Category 5 wiring. Anything else is a 
waste of your
installer's time, as 100Base-whatever is going to require Cat 5.' 

If you are only connecting two machines, it is possible to avoid using a 
hub, by swapping the Rx and Tx pairs (1-2 and 3-6). 

If you hold the RJ-45 connector facing you (as if you were going to plug 
it into your mouth) with the lock tab on the top, then the pins are 
numbered 1 to 8 from left to right. The pin usage is as follows: 

        Pin Number              Assignment
        ----------              ----------
        1                       Output Data (+)
        2                       Output Data (-)
        3                       Input Data (+)
        4                       Reserved for Telephone use
        5                       Reserved for Telephone use
        6                       Input Data (-)
        7                       Reserved for Telephone use
        8                       Reserved for Telephone use

If you want to make a cable, the following should spell it out for you. 
Differential signal pairs must be on the same twisted pair to get the 
required minimal impedance/loss of a UTP cable. If you look at the above 
table, you will see that 1+2 and
3+6 are the two sets of differential signal pairs. Not 1+3 and 2+6 !!!!!! 
At 10MHz, with short lengths, you *may* get away with such errors, if it 
is only over a short length. Don't even think about it at 100MHz. 

For a normal patch cord, with ends `A' and `B', you want straight through 
pin-to-pin mapping, with the input and output each using a pair of 
twisted wires (for impedance issues). That means 1A goes to 1B, 2A goes 
to 2B, 3A goes to 3B and
6A goes to 6B. The wires joining 1A-1B and 2A-2B must be a twisted pair. 
Also the wires joining 3A-3B and 6A-6B must be another twisted pair. 

Now if you don't have a hub, and want to make a `null cable', what you 
want to do is make the input of `A' be the output of `B' and the output 
of `A' be the input of `B', without changing the polarity. Tha means 
connecting 1A to 3B (out+ A to
in+ B) and 2A to 6B (out- A to in- B). These two wires must be a twisted 
pair. They carry what card/plug `A' considers output, and what is seen as 
input for card/plug `B'. Then connect 3A to 1B (in+ A to out+ B) and also 
connect 6A to 2B
(in- A to out- B). These second two must also be a twisted pair. They 
carry what card/plug `A' considers input, and what card/plug `B' 
considers output. 

So, if you consider a normal patch cord, chop one end off of it, swap the 
places of the Rx and Tx twisted pairs into the new plug, and crimp it 
down, you then have a `null' cable. Nothing complicated. You just want to 
feed the Tx signal of one
card into the Rx of the second and vice versa. 

Note that before 10BaseT was ratified as a standard, there existed other 
network formats using RJ-45 connectors, and the same wiring scheme as 
above. Examples are SynOptics's LattisNet, and AT&T's StarLAN. In some 
cases, (as with
early 3C503 cards) you could set jumpers to get the card to talk to hubs 
of different types, but in most cases cards designed for these older 
types of networks will not work with standard 10BaseT networks/hubs. 
(Note that if the cards also have
an AUI port, then there is no reason as to why you can't use that, 
combined with an AUI to 10BaseT transceiver.)   


Note the section for a normal patch cord.

Hope this helps,

Ben 


>>>>>>>>>>>>>>>>>> Original Message <<<<<<<<<<<<<<<<<<

On 7/7/01, 9:32:29 PM, "Warren Togami" <warren at togami.com> wrote regarding 
[luau] Re: Networking Question:


> What is a roll over cable anyway?

> ----- Original Message -----
> From: "Dusty" <dusty at sandust.com>
> To: "Linux & Unix Advocates & Users" <luau at list.luau.hi.net>
> Sent: Saturday, July 07, 2001 8:00 PM
> Subject: [luau] Re: Networking Question
> > 5. Is your cross-over cable configured correctly?  A rollover is 
different
> from a crossover.  On standard ethernet we only use wires 1


> ---
> You are currently subscribed to luau as: beesond001 at hawaii.rr.com
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