What can make Full/100Mb/s be slower than Half/100Mb/s?

  • Thread starter Thread starter DeanB
  • Start date Start date
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DeanB

Hello all,

We have networking issues at work. Some computers are lightning fast.
Others are very slow to do any kind of network access / data
transfer.

On my test machine, I can change the NIC settings from Full Duplex /
100Mb/s to Half Duplex / 100Mb/s, and I get an 8-fold increase in
network connection speed (based on timings for an application that
downloads data from an oracle machine, and also on simple file
transfers between two computers). If I put it in auto mode, I get
almost as slow as the full duplex mode.

If anyone has seen this effect before or could guess where to look for
the problem, I would really like to know about it! The IT department
that is managing the network says that this means the computers that
are slow are running too much software to handle the bi-directional
flow of data (I guess they would be handling the incoming data instead
of sending out the outgoing data). But I look at the CPU usage and its
mostly idle before I do the test. I want to know the real reason.

Thanks and cheers for any help,

(ps The slower machines are all about 2-year old Dells desktops, while
the faster machines are all Lenova laptops with gigabit adapters,
though they are all using 100Mb/s.)
 
Hi
if you change the Duplex to half and it increases "Seed" it is a Network
compatibility issue with the NIC in the computer the computer might need a
newer NIC.
Otherwise, if a computer is too slow in every thing that it is doing then it
might be a slow old computer, or an infected computer.
However any computer that work reasonably OK do not affect Network traffic.
If it does it is usually the Network fault, and should be handles by IT.
Jack (MVP-Networking).
 
Hi
if you change the Duplex to half and it increases "Seed" it is a Network
compatibility issue with the NIC in the computer the computer might need a
newer NIC.
Otherwise, if a computer is too slow in every thing that it is doing then it
might be a slow old computer, or an infected computer.
However any computer that work reasonably OK do not affect Network traffic.
If it does it is usually the Network fault, and should be handles by IT.
Jack (MVP-Networking).











- Show quoted text -
Click to expand...

Hi Jack - thanks for the reply. What kind of compaitibility issue can
I mention to them, if I want to back up this recommendation? Just
being too old? The machines are between 1 and 3 years old.

But I guess it would be quite a simple task to change the NIC out!
 
Hello all,

We have networking issues at work. Some computers are lightning
fast. Others are very slow to do any kind of network access / data
transfer.

On my test machine, I can change the NIC settings from Full Duplex
/ 100Mb/s to Half Duplex / 100Mb/s, and I get an 8-fold increase
in network connection speed (based on timings for an application
that downloads data from an oracle machine, and also on simple
file transfers between two computers). If I put it in auto mode, I
get almost as slow as the full duplex mode.
Click to expand...

"Auto" mode tries to sense the correct network configuration but
often doesn't always succeed. When it fails, you see the problems
like you are seeing.

Half-Duplex is designed to be used in networks connected with a
"Hub". In these networks, every sent packet is simultaneously
distributed to every machines connected to the hub, including the
sending machine. If two machines send simultaneously, there is a
"collision" which is detected by noticing that the outgoing signal
doesn't match the incoming signal. After a collision is recognized,
the packet is immediately retransmitted after a very short delay.
This is by design.

In Full Duplex mode, (used when connecting with "Switches") the
outgoing packets and incoming packets are totally independent.
Outgoing packets are buffered by the switch and a totally different
incoming packet may be received at the same time. Since a switch
individually buffers packets for each connected device, collisions
are eliminated. In Full-Duplex mode, the collision-detecting
circuitry is disabled on your NIC because it is not needed.

If you have a Half-Duplex network and [for whatever reason] the NIC
is configured for Full Duplex, then the inevitable collisions are not
detected and retransmissions don't take place. Eventually there is a
retransmission when the sending device doesn't receive an
acknowledgement from the receiving device, but that can be a many
seconds downstream and as you have noticed, network speed suffers
significantly. If "Auto Detect" is not working for any NIC devices
on your network, then force them to the correct Duplex ... that's why
that setting is there.

HTH,
John
 
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