User's Manual Part 2

Tech-X Flex
®
(NG2) Tech-X Flex User Guide - Firmware v06.50
7-35
Intro
Overview
Wi-Fi
Ethernet
System
IP/Video
MoCA
RF
Specs
7.7.4 MoCA functional overview
A MoCA network is designed for self-maintenance and dynamic adjustment for optimal performance.
The following sections briefly describe the physical and media access layers (PHY and MAC,
respectively), along with some details on the communications involved with network maintenance,
especially as related to testing with the MoCA module. For more information, including descriptions of
traffic not mentioned here, see the technical specifications related to MoCA.
MoCA physical layer
At the physical layer, a MoCA network operates on one of several available RF channels between 500
and 1650 MHz, with each channel spanning 50 MHz (MoCA 1.1) or 100 MHz (MoCA 2.0) in its spectrum
usage. The available channels are organized into bands, with only one channel per band permitted for
use on any physical network. In some cases, though, two channels from different bands may be active
on the same cable to allow the transport of two different logical networks, such as the case where a WAN
and LAN are operating on the same cable. In this case, there are effectively two different MoCA networks
on the same cable. For more information, see Example physical MoCA network on page 7-32.
MoCA uses the adaptive constellation multi-tone (ACMT) technique to transform layer 2 (MAC) data and
control frames, as well as physical layer probe frames, into QAM symbols. These symbols are modulated
onto a channel's subcarrier frequencies to create a serial bitstream. The MoCA specifications allow a
subcarrier to be modulated using any mode of QAM, up to 256-QAM which carries 8 bits per symbol
(MoCA 1.1) or up to 1024 QAM which carries 10 bits per symbol (MoCA 2.0). The actual maximum bits
per symbol that any given subcarrier can carry, however, depends upon the physical conditions which
affect the ability of the line to reliably transport the subcarrier frequencies.
For any coaxial network, the physical path between any two devices can vary widely due to line length,
reflections, splitter configuration, and other conditions. Aside from overall attenuation, these
characteristics also have a different effect on different frequencies, even subcarrier frequencies that are
close on the spectrum. To account for these variances, member nodes regularly send physical-layer
“probe” messages to each other to evaluate the physical network between them. Using this messaging,
the nodes are able to construct an appropriate “modulation profile” for transmission to each other, which
includes how many bits that each subcarrier should attempt to carry (known as the bit-loading). A profile
may determine that some subcarriers can not reliably carry the maximum number of bits and mandate a
lower number, or even restrict the transmission on certain subcarrier frequencies altogether. In any given
network, each node has a separate profile for communication with each other node, which maximizes
network performance.
With its statistics-gathering features, the MoCA module can produce bit-loading graphs that plot how
effectively subcarriers are carrying data to and from the unit. For more information, see Node Stats page
(MoCA Network Statistics) on page 7-13.
Preliminary issue - Limited distribution only!