Subject: What is the meaning of 100 Mb/s connection?

From: Oleg Goldshmidt <[email-masked]>
Date: 20 Feb 2001 11:42:18 +0200

Hi hackers,

I have a stupid (read "basic") question. When people talk about
an (ethernet) connection of N Mb/s, what exactly does this mean?

Does this mean that at any time I have a total of N Mb/s for both
upstream and downstream? Or do I have N Mb/s each way independently?
Or is there a fixed (configurable?) division between upstream and
downstream, e.g. 75/25, and even if I don't utilize all of the
upstream I cannot use more than the allotted downstream bandwidth?

Does anyone know for sure? Can anyone point me to a good (web)
reference or piece of documentation where it is spelled out?

Thanks,

-- 
Oleg Goldshmidt | [email-masked] 
"... We work by wit, and not by witchcraft;
 And wit depends on dilatory time." [Shakespeare]

---

From: Gilad Ben-Yossef <[email-masked]>
Date: Tue, 20 Feb 2001 14:03:55 +0200

> reference or piece of documentation where it is spelled out?

A Full Duplex 100Mb Ethernet means you have 100Mb data to pass in each 
direction. Take into acocunt though that:

1. The Ethernet preamble takes 64 bits per frame transmitted.
2. The Ethernet inter frame gap takes another 96 bits per frame.
3. Ethernet headers take their toll (14 bytes me thinks).

This means that if you consider a 64 bytes UDP packet (including all 
Ehternet and above headers, and of course data) you can move close
to 148,800 such packets from point to point, in each direction.

This is the theoretical limit. I say theoretical, because if you hooked 
up a standard issued PC + Linux + good NIC (most Tulip chip set based 
cards are highly recommended) and tried to IP forward this traffic you'd 
discover you can't reach this limit - you'll die off (loose packets) a 
lot sooner because of interrupt live lock.

There are ways around this problem but I'll leave it as an exercise to 
the alert reader ;-)

Standard disclaimer applies, of course.

Hope this helped,

Gilad.

-- 
Gilad Ben-Yossef <[email-masked]>
http://benyossef.com :: +972(54)756701
"Anything that can go wrong, will go wrong, while interrupts are disabled. "
	-- Murphey's law of kernel programing.