Duplex

synner · 18-08-2008, 20:16

Hi All,

Just returned to Virgin Media, received my Ambit 256 modem, all connected up and running ok... Just one question...

I have my modem connected to an Apple Airport Extreme which is broadcasting wireless N (130Mbit/s)

When I set my Airport router WAN port to 100Mb full duplex my downloads come in at around 5-6Mb/s mark, when I set to 100Mb half duplex my downloads come in at the 9Mb/s mark.... !?!

Just as a test I plugged in my old Netgear router and exactly the same thing happens !

Is this normal ?

Bri

Joxer · 18-08-2008, 22:30

No idea, but if it is normal it does explain why people keep getting rubbish speeds on wireless.....

Hom3r · 18-08-2008, 22:33

what speed do you get when you connect to the modem directly?

---------- Post added at 22:33 ---------- Previous post was at 22:32 ----------

What speed are you paying for?

ccarmock · 18-08-2008, 22:40

If you fix the speed and duplex of only ONE end of a connection - ie the router but not the cable modem you will always get a duplex mismatch. Ie the router will be fixed at 100 / Full but the modem will then negotiate 100 / Half and you will end up getting collisions on the link between the two causing performance issues.

Duplex auto-negotation at 100 Mb/s only works correctly if both ends of link are set to auto.

This is why you are seeing better performance with the router at auto as then both it and the cable modem will negotiate correctly.

Since the cable modem doesn't have a fixed speed/duplex option the device connectd to it shoudl always be set to auto negotiate.

If you want to use fixed speed/duplex settings then you have to ensure both ends of a given connection are fixed.

Matth · 18-08-2008, 22:52

http://homepage.ntlworld.com/robin.d....html#ethernet

If the Ambit 256 is similar to the others then you may need to configure the full duplex option accordingly, or rely on autonegotiation.

If the settings are mismatched, then the full duplex interface will not attempt to avoid transmit collisions with the half duplex interface, as full duplex allows simultaneous transmit and receive.

Joxer · 19-08-2008, 01:25

OK fair points but did anyone else notice that iworks better on HALF than on FULL?

Ignitionnet · 19-08-2008, 02:07

Yep... If the modem is set to autonegotiate while the other side is hard set it will always end up at half duplex, and depending on the autonegotiation it may even fall back to 10Mbit half duplex. Hard setting one side to full results in duplex mismatch as well as a possible fallback to 10Mbit.

Hard setting both sides, or ideally neither side and letting it autonegotiate is the best plan.

synner · 19-08-2008, 11:56

Hi All,

Thanks very much for that, interesting stuff ! It could explain the speed issues some people have..

I am on the 10Mb package, plugged my Mac straight into the modem and got 9779KB/s, plugged my routers in and got between 6-7Mb/s.

As I said, once I changed from 100Mb/s Full Duplex to Half duplex I got 9777KB/s over wireless !

I'll change the Airport WAN to autosense when I get home and see how it goes.

Thanks

Bri

Matth · 19-08-2008, 22:44

Depends if it can tell you what it is autosensing...

If it goes slow, then it's probably an autosense mismatch, if it goes fast, it may be a successful negotiate of full duplex both ends, or a negotiated half duplex.

My router only tells me if it is 10 or 100.

ccarmock · 19-08-2008, 22:56

On kit that doesnt' give full informtion or optiosn for fixing both speed & duplex at both ends I would have both ends set to auto sense, as you are far more likely to get a correct match then.