Interference and Retransmission

Interference is anything that disturbs a signal carrying data, so the data arrives damaged or not at all. Wireless connections are the most exposed: their signals travel through the air as radio waves, sharing that space with every other wireless device nearby.

What causes interference

Other wireless networks are a common cause: on a busy high street, a café's free Wi-Fi, the shop next door's network and the bus stop's Wi-Fi all cover the same patch of pavement, each one adding noise to the others. Individual wireless devices do the same — cordless phones, wireless doorbells and wireless speakers all send out radio waves of their own.

Interference can corrupt data, so the bits that arrive are not the bits that were sent. It also reduces the effective bandwidth — how much useful data actually gets through — and shortens the range the signal covers, because with noise competing for the same airwaves the signal weakens sooner. A fitness tracker that syncs in seconds at home can take far longer in a crowded gym full of the same kind of device.

Cables and interference

Copper cable carries data as an electrical signal, so nearby motors, machinery and power cables can interfere with it. Fibre optic cable carries data as pulses of light, which electrical noise cannot disturb at all. Wireless is affected most of all, so a wired connection is generally less affected by interference than a wireless one.

Why retransmission slows a network

If data is lost or damaged in transit, it must be retransmitted — sent again. That uses up bandwidth that could have carried new data, and it uses up time, so performance drops even though the connection's own bandwidth has not changed. If one chunk in every ten is damaged, about a tenth of what the link carries ends up as repeats, so the file arrives later than it would over a clean connection.