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

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.

See Also

The Author — Johannes Helmig

Dr.Johannes Helmig is working as Director, Technical Knowledge Management in the Belgium office of Gerber Technology where he is involved in Customer Service and internal training, with special interest in Networking.


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