Cable length

[RVilhelmsen]

Hi,

The MS120-8 has a Marvell PHY 88E1680 transceiver build in. Reading the specs for the 88E1680, it says, it supports up to 170m cat5/cat5e cable length.

I though the maximum length was 100 meters according to rfc´s but maybe this is a Marvell add-on?

Questions:

1. So can we use up to 170m cables?

2. Is this only a specific Marvell technology?

/Robert

[PhilipDAth]

Do not exceed 100m if you want it to work reliable.  There is a lot of complex physics involved, and the system as a whole (including the remote NIC) would need to be able to support it.

The concept of "bit time" is where the distance restriction comes from.  I found this article which does a reasonable job of describing it.

http://www.informit.com/articles/article.aspx?p=21320&seqNum=4

If you want to go 170m I would recommend you use OM4 fibre (multi-mode) and SFPs.  Either that or split the copper run in two by adding another switch in the middle.  As a bonus, OM4 would be able to do 10Gbe at 170m.

[RVilhelmsen]

Thanks, i have no need to go beyond 100m. It´s more the question, why/where Marvell came up with the 170m distance and where to use it. As you said the remote NIC also has to support it.

I guess the magnetics modules, which is soldered on the pcb, would also have to support this distance, not only the transceiver itself.

I have never had the need for at fiber connection between 2 locations with only 8 available ports to use. I always wonder why those small switches has fiber support, more likely the change is, i would be using a fiber to cobber converter in the last to ports for client connectivity.

[PhilipDAth]

The last time I used an MS120-8 with fibre was in a factory.  I needed to connect about 3 x PLC controllers which were around 400m away from the rest of the network.

[Uberseehandel]

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@PhilipDAth wrote:

The last time I used an MS120-8 with fibre was in a factory.  I needed to connect about 3 x PLC controllers which were around 400m away from the rest of the network.

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When I started automating production plants "a very long time ago", we had to use fibre because of all the interference, safety, and environmental issues. Still we see folk resisting the use of fibre, mostly because the industry has done a really poor job of explaining how to deploy it, and what to use under different use cases. Also most of the time people who haven't used fibre before expect it to be much more expensive than it actually is.

For those that are green inclined, I rather suspect that fibre is more environmentally friendly than copper (subject to sheathing material).