sslug-teknik team mailing list archive

[Mogens Kjaer <mk@xxxxxx>]

"Anders S. Johansen" wrote:

> Jeg tror i virkeligheden ikke vi er uenige her....

Hver gang nogen skriver: "Jeg har mange kollisioner på
mit netværk, hvad er der galt?" tænker jeg på følgende
citat:

If I had to do it over again, I would have called `collisions' something
else.
    -- Bob Metcalfe, inventor of Ethernet

Det han mener med dette er, at ordet "kollisioner" hentyder til noget
destruktivt, hvad det ikke er.

Silicon Graphics har en udemærket artikel ang. ethernet
kollisioner, desværre er den kun tilgængeligt for folk med serviceaftale
:-(

De har et (tænkt) regneeksempel: 

citat:

The effects of collisions on throughput can also be examined from a
mathematical perspective.
Data moves on the wire at an average 1.25 bytes per microsecond, which
is 1.25 Mbytes/s or 10 Mbits/s. Collisions are 64 bytes; theoretically,
the time that the Ethernet chip is not transmitting is
64 bytes divided by 1.25 bytes per microsecond, or 51.2 microseconds (in
practice, this number 
varies depending on numerous factors such the interframe gap/pause and
nonstandard back-off algorithms).

It has been determined that if bulk data transfers are performed on an
IP network on which 100% or
greater collision rates are common, about two-thirds of the
transmissions are 1500-byte frames
(the maximum transmission unit (MTU)) and one-third of the transmissions
are 64-byte frames.
Therefore, the average packet size is approximately 1,000 bytes
((1,500+1,500+64) divided by 3). 

Ideally, an Ethernet chip will send 1,250 packets per second (1,250,000
bytes per second divided
by approximately 1,000 bytes in a packet). If the same Ethernet chip has
a 100% or greater
collision rate (collision volume of 1,250 collisions per second), and
each collision uses
51.2 microseconds of time that could be used to transmit data, then
theoretically 80,000
bytes per second are lost (1,250 collisions multiplied by 51.2
microseconds per collision
multiplied by 1.25 bytes per second). However, this loss is not serious:
80,000 bytes
lost divided by 1,250,000 bytes per second is 6.4% lost bandwidth. 6.4%
lost bandwidth at a
100% collision rate is not a significant amount of bandwidth to lose.

citat slut.

Summa Summarum: Antallet af kollisioner på et netværk kan ikke bruges
til noget, man
skal se på performance.

Mogens

-- 
Mogens Kjaer, Carlsberg Laboratory, Dept. of Chemistry
Gamle Carlsberg Vej 10, DK-2500 Valby, Denmark
Phone: +45 33 27 53 25, Fax: +45 33 27 47 08
Email: mk@xxxxxx Homepage: http://www.crc.dk