MoCA, Multimedia over coax basic operations Page 2

	MoCA Explained, Chapter 2 MoCa device verification test To determine whether or not a MoCA node has a defective interface, the technician connects a MoCA tester directly to the MoCA connector on the device. The tester also emulates a MoCA device and interacts with the node under test to effectively create a two-node MoCA network (tester and device). The technician then tests the device’s MoCA interface without including the home’s coaxial cabling. Under these conditions, the MoCA data rates between the device and the tester should be approximately 240 Mb/s to 250 Mb/s. If the data rates are significantly less, the device should be considered defective. If the device passes this test, the other node on the problematic communications path is tested in the same fashion. Coaxial cabling test If devices are not the sources of the problem, then the technician should “divide and conquer” and progressively test the individual coaxial segments between the outlet and the other MoCA node to isolate the issue. A number of other MoCA metrics can be used to assist the technician with troubleshooting the coaxial cabling. For example, the technician can read MoCA transmit and receive power levels to determine whether or not excessive attenuation exists along the MoCA communications path. Additionally, the technician can monitor the MoCA channel for bit errors based on corrected or dropped MoCA packets. While examining the home’s coaxial cabling, the technician should look for well-known problems that can affect MoCA transmissions: • Excessive attenuation caused by too many splitters or long cable runs

	Coaxial cabling test con't • Un-terminated splitters or outlets • Poor crimping or other connector issues • A drop amplifier that does not bypass the MoCA spectrum Cable faults or other conductor flaws Noise and interference test If no problems are found in the physical plant, the final suspect is noise and interference affecting the MoCA channel. To determine this, a technician views the MoCA bit-loading graph to determine the number of bits that subcarrier frequencies are carrying across the MoCA channel (7 bits maximum for 128 QAM). MoCA networks dynamically adjust the number of bits per subcarrier, loading each subcarrier to the maximum that probe exchanges have determined is feasible for reliable transmission . Therefore, a bit-loading graph (essentially a granular view of the channel’s frequency response) displays all available subcarriers and the number of bits each carries. The technician looks for frequencies carrying fewer than six bits, which indicates noise and interference on that frequency. Technicians can then attempt to locate and remove the source of noise or interference, including corroded cable connections. An improved bit-loading graph will show that the interference has been removed, and improved data rates between the nodes should be evident.