Almost two years ago, I posted an article about how downlink traffic over my DSL line is severely impacted when at the same time I am sending a large amount of data in the uplink. This is due to the fact that acknowledgments are held up by other uplink data which slows down the traffic in downlink direction. I also mentioned then that some DSL routers are capable of prioritizing traffic such as TCP acknowledgments and VoIP packets to reduce this impact. Now two years later, I bought myself a Fritzbox DSL router and could finally put it to the test myself. Seeing is believing!
And indeed, the difference to a standard DSL router is quite amazing. The first picture below shows how the speed of an ongoing data download is severely reduced while I sent an e-mail with a large file attachment. Once the e-mail was sent, the speed returns again to what my DSL line is capable of, about 6 MBit/s. The same test with the Fritzbox shows quite a different behavior as shown in picture 2 below. While one can see a slight impact once the e-mail transfer starts, but the overall data rate remains pretty much the same as during times without the uplink being fully loaded (600 kbit/s).
Next on the test list was a VoIP call while both uplink and downlink were fully used. To my surprise both the standard DSL router I have and the Fritzbox managed to handle the SIP call both from my Nokia N95 and via a VoIP soft-client on the PC I used for the download without a glitch. Voice quality in both uplink and downlink direction to a PSTN line via a media gateway in the Internet was flawless, no packet loss and also no perceptible increase in delay. Quite a surprise indeed, I was expecting some problems with my standard DSL router in uplink direction. However, there were none which means those VoIP UDP packets must have sneaked through well despite the high load.
Picture 1: No QoS