RE: [ethmac] FW: exchanging data trough the Ethernet card.

["NewsGroup" <nbase@nbase.com>]

24-Aug-00

   Hi,

        One small comment in respect to your saying that "If you use full
duplex, you get all this bandwith 'guaranteed' because there is no
collision."

I wish this was true as than we all could move to full duplex and pass voice
and video with data without and QoS and other "ugly" staff we need to do, to
get as close as possiable to the "guaranteed" bandwidth (of course you can
run 1M over a 100M and get this "guaranteed" but 100M over 100M is never
"guaranteed" in a real network)

while in full duplex there is no collision as you mention there is flow
control and this mean that if your reciveing side is slower that the
trasmiting side it will start generating flow control packet.

and here the "bad news", a flow control packet is simple a regular packet in
size and I belive everyone use size 64 for it, so in worse case you get get
in full duplex to about 50% assuming you get into case where you send one
packet "real" and one packet flow control !.

MOST likely in the senario the originator mention this will not happen but
still this can happen and in real network it does happen.

this is also why in many cases the flow control is disable as it is better
to loss few packet than reduce your system bandwidth to half

have a nice day

   Illan


-----Original Message-----
From: owner-ethmac@opencores.org [mailto:owner-ethmac@opencores.org]On
Behalf Of Jean-François Larin
Sent: Wednesday, August 23, 2000 7:37 AM
To: ethmac@opencores.org; Andrea.Aizza@epfl.ch
Subject: Re: [ethmac] FW: exchanging data trough the Ethernet card.


> Dear OPENCORE friends,
>
> I have never worked with Ethernet protocol but I need about 15Mbps from an
FPGA
> device and a PC. One idea I thought today to implement a `null modem`
based
> connection from the PC to the device (just two devices - no hubs or
friends) I
> have just 2 questions:
>
> 1) Assuming they claim 10/100Mbps on those Eth cards, can you really reach
> those speeds?

Ethernet works in half duplex and in full duplex.  If you are working in
half
duplex, then only one device can speak on the wire at the same time, so you
get
10/100 Mbps.  But when 2 device start to transmit on the wire on the same
time,
there is a collision and both have to retry.  If there is litte traffic,
there is
little collisions.  When more traffic gets in you get more collision.  I've
heard
that you reach at peak at about 30% of utilisation (3/30 Mbps)  If you try
to use
more bandwidth, you get too much collision and the actual bandwidth
decrease.

But since you only use 2 devices, you can use full duplex (if you use a
unshielded
twistet pairs (similar to telephone jack in north america).  Full duplex is
not
supporter on co-axial cable (afaik).  Then you get a real 100Mbs.

In fact, you never get 100 percent of the 100Mbs.  Every packet has a 12
bytes of
addresses, 2 bytes for type and 4 bytes for CRC.  The total packet lenght
must be
between 64 bytes or 1518 bytes.  between each packet, there is some overhead
equivalent to 20 bytes (12 for inter-packet gap and 8 for pre-amble).  So if
you use
large packets (because you actually have lot of information to send), the
overhead
(12+2+4+20) is negligible compared to 1518 bytes, so you get almost 100Mbs
of true
bandwidth.  If you use full duplex, you get all this bandwith 'guaranteed'
because
there is no collision.  that is as far as Ethernet is concerned.  You Eth
Card must
be of good design.  Maybe the cheap (old) cards use I/O access to transfer
the
packets.  Then your CPU is going to be the limitations.

Note also that Ethernet is not like a null modem. You have to send data in
packets.
You can't just send a single caracter on the wire.

good luck


--

************************************************************
Jean-Francois Larin ing. stag.
FPGA design
Network Products Group
Matrox Electronic Systems
1055 St. Regis Blvd.
Dorval, Quebec, Canada
H9P 2T4
Tel:(514)685-7230 ext. 2041
Fax:(514)822-6272
E-Mail: JeanFrancois.Larin@matrox.com
************************************************************