The following is a standard Layer 1 802.1Q frame where we can see the 20 bytes header(IFG, Preamble and SFD) included.
The same packet if it is in Layer 2, the Layer 1 header is not included, and it looks like this:
So the first concept is Layer 1, Layer 2, Layer 3 packet length. The Layer 1 packet length is the whole packet length included in the Layer 1 header, the Layer 2 packet length will exclude the Layer 1 header and the Layer 3 packet length will exclude both Layer 1 and 2 headers.
For example, if you want to send a packet length Layer 1 of 128 bytes, then the OSN will receive only 128 bytes packet.
If you want to send a Layer 2 packet length of 128 bytes, the OSN device will receive 128+20= 148 bytes packet, then the throughput can`t reach 100%, because an OSN device we have the Layer 1 header which occupies bandwidth.
Regarding the MPLS encapsulation: on the OSN device we configured the VPLS service, then when the packet reaches OSN device, the MPLS header will be added to the packet. The MPLS header has 26 bytes like below:
The whole original packet will be put in the payload field in the MPLS packet, then the header will occupy another bandwidth.
Now we calculate the throughput on a OSN device, with the next formula:
Packet length/real packet length in OSN = Throughput / Physical bandwidth.
When a tester is used on the Layer1 traffic, the packet length of 128 bytes will be:
Throughput = (128byte/(128byte+MPLS header))*physical bandwidth = 128/(128+26)*1Gbps = 831.2Mbps
When the tester is used on the Layer 2 traffic, the packet length of 128 bytes will be:
Throughput = (128bytes/(128bytes + MPLS header + Layer 1 header))*physical bandwidth = 128/(128+26+20)*1Gbps = 735.6Mbps
Conclusion: the packet send by the end user has some other encapsulation on our OSN device, then the throughput for the end user can`t be 100%. The longer length end user has, the bigger throughput he will have, as the encapsulation is a fixed length.