Subject: [Hubmib] Point-to-point ethernet interfaces

From: Francis Arts <[email-masked]>
Date: Wed, 04 Apr 2001 07:10:42 +0200

Hello,

I am looking for all the differences between ethernet interfaces used as
shared media and ethernet interfaces used as point-to-point links. Could
someone give me a pointer where I can find more information about these
differences and about ethernet interfaces used as point-to-point links
in particular?

Thanks in advance for any help,

    Francis.

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From: "Geoff Thompson" <[email-masked]>
Date: Wed, 04 Apr 2001 11:41:13 -0700

>Thanks in advance for any help,

Francis-

The technical shift that enabled point-to-point links in Ethernet was the 
move, many years ago, away from coaxial cable. Coax allowed users to hook 
multiple machines onto a single piece of cable with no central electronics. 
In this environment all users shared the single coax media.

The next step was the shift to star wiring using UTP cabling. This 
effectively moved collision detection and access resolution from the coax 
into the hub (formally called a multi-port repeater in the IEEE 802.3 
Ethernet Standard). This moved to point-to-point cabling but still involved 
CSMA/CD and the sharing of a single channel.

Step 3 was to change the hub/repeater to a switch. This can be done 
transparently to the end station. The difference between a hub/repeater and 
a switch is that a hub is single channel bit store and forward device, a 
switch is a packet store and forward device. It is not constrained to be a 
single channel. If it isn't fast enough to keep up with worst-case traffic 
it described as having an "over-subscribed" switch fabric. Most low speed 
switches these days (i.e. 10/100) can keep up with things.

Step 4. Once all of the hubs are gone but you have switches everywhere you 
are still doing CSMA/CD between the end station and the switch, i.e. half 
duplex. All of the reason is gone for half duplex at this point and you can 
switch over to full duplex. This doesn't make much difference for the 
current application set at end stations. It might when we have interactive 
voice and video. Full duplex does make a difference on backbones.

I hope this answers your questions.

Sincerely

Geoff