CSMA/CD network protocol (Ethernet)

Question

I hope I'm in the right stackexchange subsite (can't seem to find a networking related subsite..).

I've got 2 questions concerning the CSMA/CD network protocol:

1. Before sending some (say 5) messages over the bus, is there some kind of formula how long it will take to transmit all messages succesfully (so without collisions)? Or is the time unknown when starting to transmit?

2. If (1) is no -> Is it possible to calculate the maximum time of the transmitting? So for example -> 'it'll take max. 10ms before all the messages are transmitted succesfully'.

I hope my questions are clear to you! If not, I'm glad to clearify when needed!

Thanks for your time!

Answer 1

In case you're on your own and otherwise empty collision domain the calculation should be the following for Ethernet:

Inter fram gap = 96 Bit / link speed (in MBit/s)
Packetsize = Preamble (8 bytes) + Ethernetframe (68-1522 bytes)
Time = Packetsize / Link speed (either 10 MBit/s, 100 MBit/s, 1 GBit/s, 10 GBit/s) + Inter frame gap

So to transfer a full size standard frame of

8 bytes preamble + 22 bytes header + 1500 bytes payload = 1530 bytes

over a 1 GBit/s connection

Time = 1530 bytes / 1 GBit/s = 12.24 µs

and the inter frame gap of 96 ns makes 12.336 µs.

Do this calculation for each packet and add the durations.

This is the minimum timespan needed as, as soon as you are not on your own collision domain, there is no way to calculate the time needed, as it could be that you are even unable to send it at all.

Answer 2

CSMA/CD is inherently not fully predictable, because you don't know what the other stations on the network will do. You can calculate the minimum time (as bot47 did) but max time is not known in advance.