Bandwidth - Capacity or Speed???

Oct 20th, 2002

I am trying desperately to understand some fundamental concepts regarding Bandwidth. I do not have an Engineering degree (although I think it may help at this point), but here is my question:

I am confused when we say that a current travels at the speed of light on copper...how does that relate to bandwidth?? More fundamentally, how does the measurements of bandwidth relate to Windowing? MTU? MSS?

Here is an example and why I get confused:

Computer A and Computer B hooked up with 100mbps Ethernet.

This says I can send 100 million bits in one second. so this is the capacity of the link...but what is the propagation speed for those 100 million bits???

Here is the REAL question I have. If I send a 64K file between two computers, one at 10 mpbs and one at 100 mpbs.....will it get there faster on the 10 mbps link or the 100 mbps link??? I would say it shoud not matter and it will arrive at the same time (given there is no other traffic) because 64K consumes so little of the capacity of this link in either case. So what determines the SPEED at which this 64 K travels???

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Replies

uniemeyer Mon, 10/21/2002 - 00:01

OK,

to answere your last question first: your 64 KBYTE file will travel in fact 10 times fast on the 100 Mbit/s link than on the 10 Mbit/s link.

time = ( BIts to transfer / link speed)

But for your example with the 64 KByte File a human being wouldn't really

notify the difference between theoretical 51,2 ms (10 Mbit/s link) and 5,12 ms

Now we come to the other questions. The Bandwidth describes how fast

we can write the Bits on the wire (clocking). This has nothing to do with the speed of the electro-magnetical wave ( around (6 * speed of the light) for copper) along the wire.

Windowing, MTU and Maximum Segment Size are parameter used during the transfer of the Bits.

MTU: Maximum transfer Unit: maximum packet size allowed on a Medium

(default for ethernet 1500 Byte)

Window size: With the window size a receiving hst can tell the sending host how much bytes the receiving host can receive in the incomming buffer. That

means for the sending host, he can send that number of bytes without waiting

for an acknowledgment. So the window size is used for flow-control within the

TCP protocol.

MSS: Maximum segment size: is nearly the same as the MTU but without

the TCP (20 Byte) and the IP (20 bytes) overhead. The MSS will be discovered

during the open sequence (3-Way handshake) of the TCP protocol.

Hope this brought some light in all the parameter.

regards Ulrich Marzoli

www.netaid.de