Is VM Cable Broadband Half-Duplex?

[Sephiroth]

Can we knock this one on the head please?

1/
From the Cable Modem perspective, is the ethernet port FULL or HALF duplex? And if it's half-duplex, is that the case on all the services? (10 mbps/20/50)?

2/
Can it be confirmed that if the Cable Modem is half-duplex, the WAN ethernet port should be set to the highest bit rate (that both Router & Modem can handle) half-duplex?

3/
The PCs can be set full duplex because they are talking on the LAN segment and the Router handles all the protocol with the WAN segment? I ask this just for completeness of topic.

4/
If all the above are true, might this be a potential cause of many speed problems due to packet collisions between the Router and the Modem?

[Ignitionnet]

1/ It's just an ethernet port, it autonegotiates unless told otherwise. The modem acts as a 'media converter' and switch for the most part. This has been the case for all modems VM have supplied for quite some time, capable of 100/full on LAN side.
2/ See 1/.
3/ Correct. VM only supply a single IP address to residential customers so all comms between PCs and the outside world get NATed or NAPTed - router acting as a layer 4 proxy so no direct communication between PCs and modem. Any router or switch would have the same effect from the ethernet point of view, both end the layer 1 domain where autonegotiation resides. Even if one had a switch connected to the modem it would still bridge between the PCs and the modem at layer 1.
4/ No - not unless there's issues with autonegotiation or where possible links have been hard set wrongly - same as every other ethernet link. There have been some issues where some modems and network cards / routers struggle to autonegotiate appropriately so it's worth checking just incase. As you probably know autonegotiation can be somewhat hit and miss with some chipsets.

[Sephiroth]

So, where we see the 50 Mbps modem status reporting the ethernet port at 100 mpbs half-duplex then:

1/
Is the above likely if the status is read by a PC directly connected to the modem by ethernet with Autodetect or Autonegotiate set in the PC?

2/
If the router WAN port connected to the modem was set to full duplex is there any circumstance whereby the modem status would read half-duplex?

You'll appreciate that at least one current thread is reporting the modem as 100 Mbps half-duplex.

I'm trying to have it made absolutely clear as to what the nominal the half/full duplex status should be on the modem.

[Ignitionnet]

Should be full duplex, only circumstances when that may not be the case are the same as for other ethernet, either some confusion between the two while negotiating, or where one side is hard set and the other still on auto - they tend to go to the lowest common denominator or even just pick something random and stick with that.

Full duplex always though. Half duplex belongs on ye olde hubs, oh and some BT NTEs.

[ccarmock]

It might report half duplex on the modem if whatever is connected to it is set to 100/FULL as opposed to auto negotiate.

Devices will rarely if ever work correctly at 100 Mb/s if one end is set to auto negotiate and the other fixed at 100 FULL. Typically the other end will negotiate 100 HALF in this scenario. While this will pass traffic it will work badly with excessive collisions.

So set both ends to 100/Full or both ends to auto/auto but never a mixture.

[Sephiroth]

Which VM cable modems are user configurable as regards full/half duplex?

[Ignitionnet]

Which VM cable modems are user configurable as regards full/half duplex?

The DOCSIS 3 modem is not, I don't think the newer ones are either, but you would need to get more input from the owners of the modems.

I suspect the Motorola kit Telewest were providing is configurable, and the Scientific Atlantas, but not the Ambits.

[Sephiroth]

The DOCSIS 3 modem is not, I don't think the newer ones are either, but you would need to get more input from the owners of the modems.

I suspect the Motorola kit Telewest were providing is configurable, and the Scientific Atlantas, but not the Ambits.

So, any talk of setting both sides the same is fruitless.

I think we can say that current VM cable modems will negotiate full/half duplex acording to its partner's setting (glitches cited by BB apart).

That's the definitive position I wanted to reach. Thanks.

[Ignitionnet]

So, any talk of setting both sides the same is fruitless.

I think we can say that current VM cable modems will negotiate full/half duplex acording to its partner's setting (glitches cited by BB apart).

That's the definitive position I wanted to reach. Thanks.

:tu: