abstract view
layer N+1	application layer	[PDUv(N+1)] protocol data unit aka message (the v means subscript)
layer N		transport layer		[PCIv(N)|SDUv(N)] [protocol control information|service data unit]; aggregately known as TCP segment
layer N-1	IP layer		[PCIv(N-1)|SDUv(N-1)]

encodings:
recover clock signal
data encoding

NRZ - one level for 0, a different one for 1
long periods of unchanging signal are bad for clock recovery, this happens to NRZ
solutions:
	NRZI: 0 - old level kept, 1 - transition forced; long strings of 0 are still an issue
	manchester code: 0 - transition from low to high, 1 - transition from high to low
	4b/5b encoding table 2.4,p83: uses an extra bit, just a lookup table for us, probably something complicated when it was thought of, express in NRZI
	gray code

modulation - stream of bits across a link, generally lose HF components, so need special encoding called modulation, performed by a modulator/demodulator (modem)
	takes the form of an amplitude-modified carrier signal
	you can only pack so many bits into a given time frame because of complexity and SNR constraints
	distro of amp of received signal - normal distribution
	we're going into probabilities...hate...
	figure out probability of error based on distribution of incoming amplitudes