Nelson's log

WiFi protocol notes

I’ve been doing Wikipedia reading associated with my wireless experiments on how 802.11n works. Here’s some brutal summaries, note I’m totally ignorant of signals processing and believe taming the electromagnetic field is basically witchcraft. I’m sure I made a mistake somewhere.

802.11n notes

MCS Index

Most of the 802.11n options are summarized in a single number called the MCS Index. See also this table. MCS Index specifies the number of spatial streams, the modulation type, and the coding rate. Those together define how much data can be stuffed in a single subchannel. Combine that with your bandwidth (20Mhz or 40MHz) and your guard interval and you get a max data rate.

In practice my iMac mostly hangs out at MCS 7 or MCS 12. MCS 7 is a very dense modulation and coding, but only one spatial stream, and it’s reporting 73Mbps. MCS 12 is a less dense modulation but 2 spatial streams, and reports 78Mbps.

Misc other observations

The upstream bandwidth from my link is 12Mbps, so I don’t really care about any of these higher speeds. I’m much more interested in reliability.

I had my RT-N16 configured to use 40MHz but my Mac wasn’t using that. (From a quick search, Apple doesn’t support 40MHz at 2.4GHz.) So I knocked it down to 20MHz in the router and now I think I’m getting a stronger signal. -60dBm instead of -65dBm. I suppose that’s possible? I didn’t measure carefully.

I’ve got two wireless devices connected to the router; my iMac and an old WRT54GL running 802.11g. I tried making both of those wireless links busy, about 5Mbps each, and didn’t see any obvious contention. My iMac didn’t even drop down to 1 spatial stream which I was sort of naively expecting. That’s all as it should be: 802.11g promises 54Mbps bandwidth and I wasn’t getting near that. Nice that it really works.

Google’s new OnHub router does 802.11ac and 802.11n in a 3×3 configuration.

My 2013 iMac’s wifi antenna is behind the Apple logo in the case. Why? Because it’s the only part of the back that’s not made of aluminum.