Ethernet Frame and Calculation of Throughput

Ethernet Speed and throughput is one topic which i see many people getting confused and searching for answers.

When we say Ethernet speed is 100Mbps, this does not mean that it's data throughput is 100Mbps, it is to convey that the maximum link capacity is 100Mbps , including header and everything else like Inter frame gap.

The RFC 1242 and 2544 talks about what is throughput and how we should test it.

We have many Devices using which we can calculate the throughput for Ethernet frames. Smartbits and IXIA are mostly used devices.

These devices generally gives data in packets/sec and we should know how it calculates the packets/sec

Ethernet Frame :

TDM frame rate frequency 8000 Hz

Fast Ethernet LAN capacity 100 Mbit/s

Ethernet Frame
Preamble---------------->7 bytes
Start Frame------------->1 bytes
Destination MAC Address->6 bytes
Source MAC Address------>6 bytes
Type/Length Field------->2 bytes
Minimum payload size---->46 bytes
Frame Check Sequence (FCS)->4 bytes
Frame inter-packet gap---->12 bytes

Minimum Ethernet frame size 64 bytes
Minimum Ethernet frame total 84 bytes

For every Ethernet packet of 46 byte payload, the total byts on wire is 84 bytes.


Calculate throughput:

For 100 Mbps, 64 byte, : 100000000 / ((64+20)*8) =148809.5 Frames per second.

For 100 Mbps, 1500 byte : 100000000/((1500+20)*8)= 8223.6 Frames per second .

So Ethernet payload data rates are :

1500 / (38+1500)  = 97.5293 % without VLAN tags.


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