[ILUG] Followup 1: Naive ADSL/WiFi questions (long (sorry!))?

[Michael Watterson]

Brian Foster wrote:
>  
>  2nd, a possibly trickier question:  it's a c.12 metre
>  cable run from the phone socket to the computer.
>  should I go for a 10m cable from socked to ADSL modem
>  (and then a 5m(?) cable to the computer), or visa-versa
>  (5m to the modem and 10m to the computer)?  the latter
>  would position the modem in an inaccessible location,
>  so I'm inclined to go 10m phone-modem and 5m modem-PC.
>  ( I can get all the implied 5m and 10m cables easily,
>   which is why I am restricting my question to those
>   two lengths.  to the best of my knowledge (my notes
>   are incomplete ;-\ ), all the cables are shielded. )
>
>  3rd, my (current) test plan — prior to arrival of the
>  modem — is to use my printer (an HP 6180) as the peer.
>  is there any difference between a cable (cat 5,
>  I presume?) that connects PC-printer to one connecting
>  PC-modem?   ( I'm thinking in RS-232 terms here, where
>  there _is_ a difference:  PC-printer is DTE-DTE (hence
>  needs a null) while PC-modem is DTE-DCE (no null). )
>
>   
The ADSL modem can be up to 5km to 10km from exchange depending on 
technology. The wiring is mostly CAT3. So have whatever length you like 
of phone cable to ADSL modem.

The Ethernet can only be up 100m. Straight through cables are the norm 
unless you are connecting two hubs, even then, most have reverse or 
upstream switch or port. Newer switches detect the cable and can thus 
use straight cable for everything.

Normally you use a switch or hub and don't connect two ethernet devices 
direct. If you do want to plug the printer DIRECT to the PC (A switch is 
very cheap, and the ethernet ports on ADSL modem/router will work even 
if you don't have ADSL service), then you do need either a cross over 
adaptor (cheap and simple) or a cross over cable (harder to find and 
tricky to make for a beginner).

The pairs on an ethernet 8 way plug are not sequential. One pair 
(normally unused) is the centre two pins. The live phone wires would be 
here if you plugged the smaller RJ11 plug into an RJ45 socket.
Then a pair has a wire either side (used). Then a pair on either end 
(one of these used).  So a cross-over cable is disobvious... unless you 
know which of the 4 pairs is TX pair and RX pair.

Even with a suitable cross-over adaptor or cable, if the NIC is on auto 
negotiation,  you may have a problem compared with plugging both devices 
into a hub or switch. Possible modes are normally 10Mbps half duplex, 
100Mbps half duplex and 100Mbps full duplex. With a cross over adaptor 
or cable try both devices at 10mbps half duplex. Auto is usually fine on 
a Switch or Router.



-- 
Mike