3 ms·

The article talks about how the 1500 byte MTU came about but doesn't mention that the problem of clock recovery was solved by using 4b/5b or 8b/10b encoding whe

by smoyer 7y ago

The article talks about how the 1500 byte MTU came about but doesn't mention that the problem of clock recovery was solved by using 4b/5b or 8b/10b encoding when sending Ethernet through twisted-pair wiring. This encoding technique also provides a neutral voltage bias. EDIT: As pointed out below, I failed to account for the clock-rate being 25% faster than the bit-rate in my original assertion that Ethernet over twisted-pair was only 80% efficient due to the encoding (see below)

mchristen 7y ago
There has to be something else going on here because I routinely achieve > 800mbps on my gigabit network over copper.
- jws 7y ago
  Yes, the wire symbol rates are higher. For instance, 100mbit Ethernet has a 125 million symbols per second wire rate.
- adrianmonk 7y ago
  I'm not a hardware engineer, but from some quick research it appears that 100 megabit ethernet ("fast ethernet") transmits at effectively 125 MHz. So the 100 megabit number describes the usable bit rate, not the electrical pulses on the wire. Gigabit Ethernet is more complicated, and it uses multiple voltages and all four pairs of wires bidirectionally. So it is not just a single serial stream of on/off.
Unklejoe 7y ago
> Ethernet through twisted-pair wiring only provides 80% of the listed bit-rate Actually, they already accommodated for this in the advertised speed. In other words, a 1 GbE SerDes runs at 1.250 Gbit/s, so you end up with an actual 1 Gbit/s bandwidth. The reason you don't actually hit 1 Gbit/s in practice is due to other overheads such as the interframe gaps, preambles, FCS, etc.
- smoyer 7y ago
  You're absolutely correct ... it's been a long time since I was designing fiber transceivers but I should have remembered this. Ultimately efficiency is also affected by other layers of the protocol stack too (UDP versus TCP headers) which also explains why larger frames can be more efficient. In the early days of RTP and RTSP, there were many discussions about frame size, how it affected contention and prioritization and whether it actually helped to have super-frames if the intermediate networks were splitting and combining the frames anyway.
- blattimwind 7y ago
  > The reason you don't actually hit 1 Gbit/s in practice is due to other overheads such as the interframe gaps, preambles, FCS, etc. Actually Gigabit Ethernet is highly efficient; it can actually give you 98-99 % of line rate as the payload rate.

by smoyer 7y ago

mchristen 7y ago

There has to be something else going on here because I routinely achieve > 800mbps on my gigabit network over copper.

jws 7y ago
Yes, the wire symbol rates are higher. For instance, 100mbit Ethernet has a 125 million symbols per second wire rate.
adrianmonk 7y ago
I'm not a hardware engineer, but from some quick research it appears that 100 megabit ethernet ("fast ethernet") transmits at effectively 125 MHz. So the 100 megabit number describes the usable bit rate, not the electrical pulses on the wire. Gigabit Ethernet is more complicated, and it uses multiple voltages and all four pairs of wires bidirectionally. So it is not just a single serial stream of on/off.

Unklejoe 7y ago

> Ethernet through twisted-pair wiring only provides 80% of the listed bit-rate Actually, they already accommodated for this in the advertised speed. In other words, a 1 GbE SerDes runs at 1.250 Gbit/s, so you end up with an actual 1 Gbit/s bandwidth. The reason you don't actually hit 1 Gbit/s in practice is due to other overheads such as the interframe gaps, preambles, FCS, etc.

smoyer 7y ago
You're absolutely correct ... it's been a long time since I was designing fiber transceivers but I should have remembered this. Ultimately efficiency is also affected by other layers of the protocol stack too (UDP versus TCP headers) which also explains why larger frames can be more efficient. In the early days of RTP and RTSP, there were many discussions about frame size, how it affected contention and prioritization and whether it actually helped to have super-frames if the intermediate networks were splitting and combining the frames anyway.
blattimwind 7y ago
> The reason you don't actually hit 1 Gbit/s in practice is due to other overheads such as the interframe gaps, preambles, FCS, etc. Actually Gigabit Ethernet is highly efficient; it can actually give you 98-99 % of line rate as the payload rate.