On Nov 22, 9:14, Eric J. Korpela wrote:
> They don't need to. Nothing stops machines from talking 10 Mbps over
> 10baseT. That's what the 10 stands for. As long as you've got
appropriate
> hardware, they can commingicate with a 100 Mbps machine elsewhere on the
net.
> I've got 4 machines on the desk behind me on a 10 Mbps hub. The hub is
> connected to the 100 Mbps switch that the machine on my desk is connected
to.
> I have no problems with my 100 Mbps full duplex machine talking to 10
Mbps
> half duplex machines. I don't claim to know how the switch negotiates
> rates and duplex. I just know that it does.
It's done by a protocol involving exchange of patterns of link pulses when
the link is first established. Full duplex and 100Mb capability are
offered (separately, and there are a few devices that support full duplex
10Mb although it's not part of the standard) by one side of the link; if
there's no approriate response from the other end, the link remains at 10Mb
half-duplex (ie, normal 10baseT).
> If you've got 10base2 nothing stops you from connecting it to a 100baseT
> network through a repeating hub or switch.
Well, strictly speaking, such dual-speed hubs are actually 2-segment
repeaters that are very like simple switches (or a bridge). They buffer
the data between the fast and slow segments, and on the cheaper ones that
can reduce the performance significantly.
> But at $80 a new 100 Mbp hub doesn't break the bank. (I also have never
had
> a hub or switch fail.
Unless you only have a few, you've been lucky. I look after around a dozen
serious switches and about 3 dozen hubs, and in the last four years we've
had around half a dozen failures. Of course, one or two have been simple,
like dead fans causing a hub to overheat, and we've had at least three
instances of a single port failing. That's a lot fewer failures than we
have on PCs, but then we have a lot more PCs!
This is comparable to the rates seen by my colleagues across campus,
incidentally. You should also bear in mind that ethernet is amazingly
tolerant, and there can be intermittent failures, dropouts, links that fall
back to half duplex etc which you won't notice unless your management
system is fairly good.
> On a related note, are there standards for wiring an RJ45 for phone use?
> For localtalk? (I would assume that it would be possible to put both
> in one.)
There is, at least in the UK. I don't know if it would apply in the
States, since you use two wires (one pair) for the phones and we use three
(a pair and a half, but we normally also connect the fourth wire between
nodes). Anyway, 10baseT and 100baseT use pins 1,2,3, and 6; the phones use
3,4,5, and 6; ISDN uses 3,4, 5 and 6 for data and carries power on the
other two pairs. You normally plug a little white box containing a phone
socket and PBX termination into the RJ45 wall socket. ISDN uses RJ45 plugs
so nothing extra is required (unless it's a long run or multiple sockets
and you need a terminator on the end).
You can get little doublers that have one RJ45 plug on a short lead
connected to a small box with two RJ45 sockets, wired in such a way that
you can plug a 10[0]baseT into one side and a phone line into the other;
connect the plug to a patch panel, and reverse the process at the other end
(wall socket) to separate the signals again. They're meant for places
where there's not enough "horizontal" wiring (the
under-floor/overhead/behind-walls runs from patch panel/wiring closet to
wall socket). You can also get ones that double up 2 x 10[0]baseT onto one
cable, using all four pairs instead of two.
> It would be nice to have the wired such that a misconnection
> (ethernet into phone) won't fry anything.
The phone won't be hurt by the net devices, but it's possible the net
devices might be surprised by phone/ISDN voltages. However, as far as I
know, we've never had a mishap (touch wood).
> It would also be nice to sprinkle
> RJ45 sockets around the house and decide what to use them for later.
It's called "structured wiring", and that's what patch panels are for :-)
All our ethernet, serial, ISDN, and POTS signals are carried on the same
wiring and patch panels (except for the fibre, of course). We have about
800 RJ45 wall and floor sockets, not counting the patch panels, in our
building for exactly that reason. I have about a couple of dozen at home,
but I also have quite a few DB25s, BNCs, and phone sockets :-)
--
Pete Peter Turnbull
Network Manager
Dept. of Computer Science
University of York
Received on Wed Nov 22 2000 - 20:33:30 GMT