The Speed of Hubs and Shared Collision Domains

Question

I am watching a video course from Brocade. The instructor says that "all devices on a single collision domain must operate with the same speed parameters -- for example, 100 megabits; full duplex. This lowers the speed of your network to the slowest device on the segment. When a bridge is used to separate collision domains, each segment can operate a different speeds." In the slide is a picture of two bus topologies separated by a bridge. Is what he said accurate? If I connected two PCs to a hub -- one with a 10 Mbps NIC and another with a 100 Mbps NIC -- would the switch force the 100 Mbps NIC to operate at 10 Mbps? Or is it possible he's referring to really old coaxial Ethernet setups rather than the twisted-pair hubs?

Answer 1

An ethernet hub is nothing but a single cable inside a box with multiple RJ-45 connectors just like this:

The devices connected to the hub are connected among them using a single cable (technically it's called a single domain).

They must be on the same speed, otherwise the network won't work. Network cards nowadays can use 10/100/100 Mbps per second (through autosensing they can use the same speed as the network), but there are older cards that are fixed to 10 Mbps or 100 Mbps so in that case the card must be in concordance with the desired speed on the network.

Devices on a hub must use half duplex because they need to be aware of possible collisions on the cable. Full duplex can be used on switched environments where there is no need of collision-hearing.

If you use a bridge to connect two single domains then each domain can be in a different speed and the bridge controls the speed difference, like this:

Answer 2

A standard hub is single speed. I.E. it is either a 10Mbs hub or a 100Mbs hub, so:

• if you connect a 10Mbs NIC on a 100Mbs hub it will not work at all
• if you connect a 10/100Mbs NIC on a 10Mbs hub, the NIC will operate at 10Mbs

So in practice, yes on a true hub all stations operate at the same speed.

I said "standard" or "true" hub, because it exists some more advanced hub that allow both 10 and 100Mbs, but those hubs actually embed a 2 ports switch (I.E. bridge) to connect the 10Mbs domain to the 100Mbs domain, so in practice you have a 10Mbs hub + a bridge + a 100Mbs hub.

Answer 3

I am watching a video course from Brocade. The instructor says that "all devices on a single collision domain must operate with the same speed parameters

True.

for example, 100 megabits; full duplex.

Devices on a collision domain must operate in half duplex mode. If your instructor gave this as an example he either made a mistake or doesn't have a good understanding of how Ethernet works.

This lowers the speed of your network to the slowest device on the segment.

Afaik with a pure hub a device will either link at the speed the hub is set up for or not link at all. Automatically downgrading the whole network would seem like a very undesirable behaviour to me.

Note that there were/are many "dual speed hubs" around. These are logically two separate hubs connected by a two-port bridge. The dual speed hub then negotiates speed on each port and connects it to the appropriate logical hub. At the time This was cheaper to implement than a full switch.

When a bridge is used to separate collision domains, each segment can operate a different speeds." In the slide is a picture of two bus topologies separated by a bridge. Is what he said accurate? If I connected two PCs to a hub -- one with a 10 Mbps NIC and another with a 100 Mbps NIC -- would the switch force the 100 Mbps NIC to operate at 10 Mbps?

It depends what exactly the device you connect those two PCs is.

• If it is a pure 10 Mbps hub then most likely* both NICs will operate at 10 Mbps half-duplex.
• If it is a pure 100 Mbps hub then most likely* the 100 Mbps device will link at 100 Mbps half duplex and the 10 Mbps device will not link at all.
• If it is a "dual speed hub" then most likely* the 100 Mbps device will like at 100 Mbps half duplex and the 10 Mbps device will link at 10 Mbps half duplex.
• If it is a "switch" then most likely* the 100 Mbps device will link at 100 Mbps full duplex while the 10 Mbps device will like at 10 Mbps half duplex.

Or is it possible he's referring to really old coaxial Ethernet setups rather than the twisted-pair hubs?

Standardised coax Ethernet only ever ran at 10 Mbps.

* This assumes that the "10 Mbps NIC" only supports 10 Mbps half duplex and does not have auto-negotiation while the "100 Mbps NIC" support auto-negotiation with 100 Mbps half, 100 Mbps full, 10 Mbps half and 10 Mbps full.

P.S. True hubs operate at a level above the wire-level encoding of the bits but below the medium access control layer. So from the perspective of medium access control protocols all devices connected to a true hub can be treated as if they were on a single wire even though physically they are not.