Fritz!Box 7530 on Zen FTTP 300/50 not achieving full upload

I am using a Fritz!Box 7530 running Fritz!OS 7.25 on an FTTP 300/50 connection provided by Zen Internet.

Within the router, I have set the download/upload values to 300 (download) and 50 (upload), but when I do a speedtest, I get about 307Mbit down and only 47Mbit up.

However, if I change the upload value to 53Mbit, I am then able to achieve a 50Mbit upstream result on the speed test.

Is there a reason why the Fritz!Box behaves in this way?

As a wild stab, the FB bandwidth limits may be wire/raw (unframed) Ethernet speeds. Once framing etc overhead is taken into account the ‘net’ throughout / speed - what you see on speed tests etc - is typically about 94% of the wire speed (based on standard size Ethernet frames). Jumbos the efficiency is around 99%

As @pheasant stated there are protocol overheads. PPP uses 8 bytes per frame for its headers , then there are tcp/ip overheads on top of that.

The other factor, which is probably in play is the Fritzbox has Quality of Service turned on my default which reserves a small amount of upstream bandwidth for VoIP traffic. Telling the Fritzbox you have more bandwidth will have affected this calculation.

Go to internet, filters , prioritization, there will probably be an entry under Real Time Applications for Internet telephony.

In reply to a post by Reado:

I am using a Fritz!Box 7530 running Fritz!OS 7.25 on an FTTP 300/50 connection provided by Zen Internet.

Within the router, I have set the download/upload values to 300 (download) and 50 (upload), but when I do a speedtest, I get about 307Mbit down and only 47Mbit up.

However, if I change the upload value to 53Mbit, I am then able to achieve a 50Mbit upstream result on the speed test.

Is there a reason why the Fritz!Box behaves in this way?

Those values are probably used to feed into some QOS engine like codel (this is a general principle, rather than experience of this exact router). When a router has such a feature, they usually don't allow the connection to fully saturate, as that causes the opposite of a quality service.

Your connection will be better behaved with the QOS engine reserving some headroom than not. I'd suggest setting the value to the speed of the connection, and not above it, as you're bypassing the very logic that is understood to give a good service.

...unless you care more about Speed test banners than the actual in use performance of the connection

In reply to a post by Reado:

I am using a Fritz!Box 7530 running Fritz!OS 7.25 on an FTTP 300/50 connection provided by Zen Internet.

Within the router, I have set the download/upload values to 300 (download) and 50 (upload), but when I do a speedtest, I get about 307Mbit down and only 47Mbit up.

However, if I change the upload value to 53Mbit, I am then able to achieve a 50Mbit upstream result on the speed test.

Is there a reason why the Fritz!Box behaves in this way?

Further to Jimbof's reply above is this perhaps interrelated to the application prioritisation that you posted on StackExchange a few weeks ago:

https://serverfault.com/questions/1058308/fritzbox-7...