README.md: add FAQ

README.md

@@ -1,4 +1,48 @@
 # pydtm
-Python (Euro)DOCSIS Traffic Meter
+Python (Euro)DOCSIS (3.0) Traffic Meter
 
-This is work in progress.
+## Overview
+
+This tool uses a DVB-C capable video card (e.g. a cheap USB stick) to measure
+the EuroDOCSIS 3.0 traffic per frequency, allowing you to venture an educated
+guess about your local segment's utilization.
+
+Data is written to a UDP socket in [graphite](https://graphiteapp.org/)
+format.
+
+## Requirements
+
+This was tested with cPython 2.7.13 and cPython 3.5.3 using a Hauppauge WinTV
+soloHD USB stick on a Raspberry Pi 3 running Raspbian/stretch.
+
+It should however work with most Python versions as long as the DVB-C card
+is supported by your kernel and it's driver complies with the DVBv5 API.
+
+
+## FAQ
+
+### How does this work?
+
+EuroDOCSIS 3.0 uses standard DVB-C mechanisms to transport it's data: It's
+encoded as a standard MPEG Transport Stream on [PID]
+(https://en.wikipedia.org/wiki/MPEG_transport_stream#Packet_Identifier_\(PID\))
+8190 with either 64- or 256-[QAM]
+(https://en.wikipedia.org/wiki/QAM_\(television\))
+modulation with a symbol rate of 6952ksyms/s. Since cable is a shared medium,
+determining the total amount of data transferred and comparing this to the total
+amount possible after
+[FEC](https://en.wikipedia.org/wiki/Forward_error_correction) (which is about
+51Mbit/s for 256-QAM and 34 MBit/s for 64-QAM) will show you how much capacity
+is used.
+
+### Wait, I can read my neighbours data with this?
+
+No, you can't.
+
+### Why Python?
+
+I wanted to learn Python.
+
+### Why output data to UDP? Why not text files, sockets, ...?
+
+You can easily use tools like `netcat` to capture the data.