Over the past couple of days I've experienced some problems with large file downloads over Wifi. Regularly, download speeds decreased to almost zero for a couple of seconds before recovering. Today I found the culprit: My new Bluetooth USB stick in combination with the Nokia PC suite.
The 100 mW Bluetooth stick I bought a couple of days ago is used by the Nokia PC suite every now and then to check if one of my phones is in the neighborhood. As Bluetooth and Wifi use the same frequency band they can interfere with each other. The PC suite tries to establish a connection to the phones about every 20 seconds. When the Bluetooth stick attempts to create a Bluetooth connection the Wifi connection is pretty much wiped out. On the first picture on the left it can be seen that the Wifi throughput measured by NetMeter drops to 0 just when the HCI Create Connection Event is sent to the BT stick.
Picture two shows the situation traced with a Wifi network tracing tool. At the time of the interference the access point tries to deliver a packet (4043) but does not get an 802.11 acknowledgment frame from the notebook. It tries three more times before giving up. It then tries to send the next packet (4047) which equally fails. The third packet (4051) gets an ack from the notebook so that transmission was o.k. That doesn't help much, however, as two packets are missing so the TCP stack immediately stalls and requests a retransmission from the source (not shown in the picture).
I can break the pattern by transmitting a file to the phone over Bluetooth. While the file transfer is in progress the Wifi download returns to normal. Once the file transfer has finished the Wifi connection is interrupted again in the regular pattern. This is most likely because once the Bluetooth connection is established power control kicks in which reduces the Bluetooth power and thus the interference to the Wifi transmission. As both my Bluetooth stick and the remote device are BT 2.0 capable it is also possible that the BT stack recognizes the Wifi interference and starts excluding the Wifi band from the hopping sequence. Maybe it's even both, hard to tell.
Two remedies come to mind: For the moment I have deactivated Bluetooth in the PC Suite which promptly brings my Wifi transmissions back to normal as the Wifi stick no longer attempts to connect to the phones every 20 seconds or so. I think I'll bring the 100mW stick back to the shop and see if a 10mW stick creates less interference during connection establishment. I'll keep you posted.