Communications Systems is split into two parts: Analog Communications and Digital Communications.
Analog Communications covers AM, DSB-SC, SSB, VSB, FM, PM (?) and noise.
Digital Communications covers some information theory (entropy, Huffman encoding, Hamming distance, etc) as well as baseband (PAM) and passband digital communications (FSK, PSK, etc.).
before 2002, CommSys was split separately into Communications and Information Theory and Coding