A method and a communication modem for broadband communication over power transmission lines. The modem includes a coarse level synchronization mapping unit which maintains a regularly updated coarse level clock synchronization map of neighboring communication units with which it is likely to exchange communications; and a second level synchronization unit which utilizes session handshakes and session data capacity to increase the synchronization level with a neighboring communication unit to allow a communication session to be held at a higher modulation level than the coarse level synchronization is able to support.