Network Speed and Access Method

by Johannes Helmig [Published on 27 March 1999 / Last Updated on 27 March 1999]

What speed can we expect from a 10base2 or 10baseT network ?
These networks are rated for 10 MBit : Bits, not Bytes !
With 8 Bits per Byte, the maximum theoretical throughput is 1.25 MByte,
but this value cannot be reached in reality.

Lets make a test and use System-Monitor to check it (assuming a proper cable installation, otherwise re-transmittions will slow the network down):

2 systems on the network.

test #1:
copy of one 30 Mbyte file
approx. 600 KByte/second


test#2:
copy of 1900 Files (18 MByte)
approx. 300 KByte/second


Reason #1:
Protocol overhead: In addition to the data itself, control-information is transmitted,
like to verify that the data has correctly arrived on the destination system.
That is also the reason, by test#2 has a lower throughput, because transferring
multiple files has a higher overhead than just a single file.


Reason #2:
10base2 and 10baseT Ethernet is based on CSMA/CD:
Carrier SenseMultiple Access / Collision Detection

Network throughput can also be impacted by Registry settings
(like with TCP/IP Registry settings)

Carrier Sense:
Before a system can start transmitting on a Network, it 'listens' on the cable for
a carrier signal (very much the same as when you pick up the phone and listen to
the dial-tone). Only when the cable is not busy with another data-transfer, it will
start the transmission.

Multiple Access:
As long there is no 'busy-signal' on the cable, any connected station can start
transmitting immediately.


Collision Detection:
It can happen, that 2 or more stations start transmitting at the same time, which causes
then a collision of the signal, which is then detected causing the transmitting systems
to abort, wait a little (length is randomly determined) before the systems try to access
the network cable again.



These collision with the need for re-transmissions is the main reason, why a 10base2
or 10baseT network cable cannot carry more than 70-75% of the theoretical throughput.
(to get more thoughput, you will need to use Twisted Pair Ethernet (100baseTX/100BaseT4)

In general, you could say, that Ethernet works like having a disciplined discussion between
multiple people:
* you only start talking, if nobody else is talking
* if 2 people start talking at the same time, both stop, wait a little and start again to talk.

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