[Info-vax] OS implementation languages

[Johnny Billquist]

On 2023-09-01 12:06, Bob Gezelter wrote:

> Johnny,
> 
> I am somewhat busy today, but the "With ethernet it was shown that it performs just fine up to
> about 70-80% utilization." is somewhat apples/oranges.

Well. That might be. But in more recent development, that number just 
goes up. It don't go down. But we're going into very fine details here.

> You are correct that Aloha is the first, or at least the first well-known discussion of the phenomenon. However, the cited lecture notes miss several points.
> 
> - Aloha network nodes do not have guaranteed mutual visibility (a side effect of being radio-based on islands).

There are multiple reasons why Aloha basically drops in performance when 
you get above around 30% saturation. But it's an aspect of Aloha, and is 
not applicable to ethernet.

> - The comparable Ethernet analysis presumes 10Base5 or 10Base2 coax with possible repeaters, not switches, with a network diameter maximum computer to guarantee that the signal of a transmitting node is detectable within the minimum packet size.

Yes. And ethernet have a defined maximum diameter for that specific 
reason. (And I said wrong, it's 64 bytes, not 64 bits). But the point 
is, with a defined maximum diameter, and a defined minimum packet size, 
you are guaranteed to know if there is a collision by that time, and 
beyond that moment in time, you will not be getting collisions. And this 
allows the performance throughput to get much higher than the 30% you 
naively might think.

> The presence of switches, which are inherently store-and-forward at a packet level, change the analysis dramatically. For the record, I recall that there were 10BaseT hubs, but the cost saving was not significant, so they are rare.

True. With switches it becomes a different ballgame. But it's a 
ballgames that allows you to reach higher utilization, since the 
collisions go away, except if you are dealing with half duplex, where 
you can collide if both sides start talking at the same time.
With switches and full duplex, you no longer get any collisions at all.

With 10Base (even -T) hubs were not uncommon. I think I still have some 
lying somewhere. I think there were still a few by 100Base, but they 
basically went away around that point, and beyond that, hubs were no 
longer even available, or allowed.

> With care and careful design, one can get higher utilization, but that is a different story. For reference, look up Digital's CI, a higher performance CSMA/CD scheme, but with some twists to run at higher utilization percentages.
> 
> IEEE 802.11 (aka WiFi) is a different conversation altogether.

I don't even want to go there... :-P

   Johnny