The MoCA Technology

The MoCA physical layer uses 50 or 100 MHz channels located in the spectrum of 775- 1525 Mhz or 475–1675 MHz. The MoCA standard organizes the channels into multiple bands across the available spectrum. Only one channel per band is used on a physical network, though multiple MoCA networks may be formed over the same coaxial cable plant using different bands. The drawing below describes the MoCA frequency plan. Note how the frequency plan allows MoCA networks to coexist with CATV services sharing the same cabling. For this reason, additional installation and maintenance procedures must be considered when MoCA is deployed.

At the physical layer, MoCA uses a technique called adaptive constellation multi-tone (ACMT)—modelled after OFDM—to carry data between nodes. Units of data named ACMT symbols are mapped onto 224 or 480 discrete orthogonal sub-carriers that occupy the 50 or 100 MHz MoCA channel bandwidth. Each of the subcarriers is modulated independently using 1–8 bits per symbol (BPSK–256 or 1024 QAM). The MoCA standard allows the bit loading of each sub-carrier to be controlled based on the physical characteristics of the path between the transmitting and receiving nodes. This is accomplished using probe messages, which are exchanged periodically, to determine the physical characteristics between each pair of MoCA nodes communicating over the network. Based on the probe messages, MoCA nodes are able to develop modulation profiles that specify the bit loading attributes between the nodes. MoCA modulation profiles are designed to optimize throughput between nodes while assuring a low bit error rate. MoCA also provides additional protection using Reed-Solomon FEC. The theoretical maximum physical-layer bit rate is today 1.4 Gbps.