I’ve been using a faithful DG834GT router for as long as I’ve had an ADSL connection, but it is now showing its age. I recently bought a Time Capsule with an inbuilt 802.11n router. Unfortunately, it caused my internet to drop out sporadically, especially under heavy loads. I suspected a protocol mismatch between the ADSL modem and the Time Capsule. After hours on Google, I discovered the problem was inherent to all Apple routers.
It seems Time Capsules are designed by Americans for Americans. The majority of Americans connect to the internet via cable modems, rather than ADSL. While otherwise indistinguishable, the PPPoE ADSL standard requires 8 bytes of overhead for each packet. The standard maximum transmission unit (MTU) is 1500 bytes, which is apparently hard-coded into Apple routers, whereas ADSL only handles packets up to 1492 bytes. Under heavy use, it seems this minor mismatch befuddles the Time Capsule.
Unfortunately, there is no way to adjust the MTU of Apple routers (for reasons I can’t understand), so instead I limited the MTU within the network settings of each of my devices. To maximise throughput, the MTU should be kept as large as possible. This post suggests an optimal size of 1452 bytes for ADSL connections. I opted for 1400 as a conservative setting; beware though, setting the MTU too low affects streaming applications like Skype, causing an unreliable connection.
I have been operating with an MTU of 1400 for months and haven’t seen any dropouts or router freezes, so hopefully this information is useful to others suffering the same problem.
Update: The simpler solution is to switch your ADSL connection to PPPoA, if your ISP allows it, as PPPoA does not have an 8-byte overhead.