Duplex?

samuelnw53 · 04-10-2004, 20:35

Swapping my Samsung stb for SACM tomorrow what duplex setting works best ie auto detect ,full 10 or 1/2,or full 100 or 1/2. thanks, Sam

Web-Junkie · 04-10-2004, 21:02

WinXP has defaulted to 'TP Half Duplex' with my Realtek NIC card and Motorola SB4200 and that works no problem! I can't set any duplex options in the CM anyway!

TheBlueRaja · 04-10-2004, 21:03

You should always leave it set to auto unless otherwise specified as this is what the other end will be set to provide. If you change this then the auto detection will probably default to either 10 or 100 HALF Duplex on the other side and you will experience packet loss.

samuelnw53 · 04-10-2004, 21:06

Thanks for your quick response ,will leave it as it is at auto detect.

altis · 04-10-2004, 21:25

Sir Robin doesn't agree with you there...

Quote:

It has been verified by experiment that certain auto-negotiating ethernet cards (both in PCs and in Macs), when connected to certain models of cable modem, sense the ethernet duplex setting incorrectly. Such a cable modem has ethernet hardware which is capable of operating only in half-duplex mode, at 10 Mbps, yet some auto-negotiating ethernet cards incorrectly sense the cable modem as full-duplex. If an auto-negotiation card incorrectly starts operating in full-duplex mode, there can be collisions between ethernet packets being transmitted in both directions at the same time, leading to packet loss, and repeated re-transmissions. There is no visible evidence of this apart from poor performance. This problem can be avoided by manually configuring your ethernet card to be 10 Mbps half-duplex.

taken from:
http://homepage.ntlworld.com/robin.d.../ethernet.html

Nemesis · 05-10-2004, 06:11

and from experience I'd agree with him

DeadKenny · 05-10-2004, 07:28

Only applies to very old CMs though? Most these days have proper 10/100 support and will detect 100Mbps anyway. Though that's not what duplex is about, so why Robin mentions 10Mbps I'm not sure (does that somehow imply it's an old modem only capable of 10Mbps half-duplex?). Duplex is about being able to send whilst receiving.

TheBlueRaja · 05-10-2004, 08:38

Quote:

Originally Posted by altis

Sir Robin doesn't agree with you there...

taken from:
http://homepage.ntlworld.com/robin.d.../ethernet.html

Sir Robin does, as Sir Raja wrote - "unless otherwise specified". Always helps to cover ones shiny metal ass.

And ok, assuming that he has a very old cable modem - what should he set it to? How would he determine how it should be set and what the cable modem defaults too? Anyone know? RTFM perhaps? (Seriously)

If it is a new one it should be set to auto IMO, as Robin mentiones that the cable modem is limited to 10mbps half-duplex on these old modems, so one way to tell might be the following, that if you plug in your cable modem and you get a 100Meg connection on auto detect then it is a newer modem and not an old one?

And therefore if you do the same thing with an old modem - would you auto detect 10Mbps and then force it to 10 Meg half-duplex on the PC?

Nemesis · 05-10-2004, 08:41

From my experience, NIC cards set as Auto don't just have a problem negotiating with Cable Modems, I have seen this happen with Cisco Switches too.

I try Auto, and if it doesn't work, generally go for 10 Mbps Half Duplex or 100 Mbps Half Duplex. Try them all and see what works best.

It seems to be a problem with the Nic cards, and not the Modems/switches.

TheBlueRaja · 05-10-2004, 08:51

I have seen problems with auto only when the other side is not Auto as well. In that case you MUST set your connection speed manually to equal what you are connecting too, otherwise it will probably default to X Speed Half Duplex.

I myself have done a lot of testing with this, not on Cable modems mind you but with VoIP Network testers.

Chrysalis · 07-10-2004, 12:53

I have ambit 120 and I had to force to 10 half duplex, it slows down in 10 full duplex mode.

BBKing · 07-10-2004, 13:23

Worth remembering that 10Mb/half duplex is capable of speeds way faster than any ntl cable modem or STB is able to deliver at present. The half duplex bit means that you can't send electrical signals in opposite directions at the same time, but this has no effect on being able to send actual data in opposite directions at the same time, the length of time each direction is blocked for is too tiny to notice.

TheBlueRaja · 07-10-2004, 13:44

Quote:

Originally Posted by BBKing

Worth remembering that 10Mb/half duplex is capable of speeds way faster than any ntl cable modem or STB is able to deliver at present. The half duplex bit means that you can't send electrical signals in opposite directions at the same time, but this has no effect on being able to send actual data in opposite directions at the same time, the length of time each direction is blocked for is too tiny to notice.

Are you sure BBKing,
Half - duplex IIRC means that you can transmit or receive information (aka packets) but not at the same time, full duplex on the other hand means you can transmit and receive information at the same time.

You are right though that 10Mb is way faster than necessary.

BBKing · 07-10-2004, 13:59

Quote:

can transmit or receive information (aka packets)

Not aka packets at all, we're talking waaay below the IP layer, this is purely the physical 1s and 0s as electrical impulses on the cable. That you choose to transmit IP packets over the top of these electrical impulses is completely irrelevant.

10Mb/half also puts less strain on the quality of your cabling.

TheBlueRaja · 07-10-2004, 14:47

LOL,
Reading a bit too much into what i wrote BBKing, i was technically wrong in specifing packets - your right - i was trying to keep it simple.

I think your going too deep here and possibly confusing people with your statements, i doubt his cable will be any worse at half or full duplex - in fact i wouldnt even concider the cable at all unless it was falling apart at the seams.

The simple fact of the matter is this-

Always run AUTO unless it is specified otherwise or you KNOW that the connected equipment is forced at a specific speed or duplex. In this case your speed and duplex setting should be set to match that of the connected equipment.