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This month we talked with Anton Monk, CTO of the Multimedia over Coax Alliance (MoCA™) about how coaxial lines in a home can be used to distribute data and multimedia content throughout the home. MoCA is an open, industry driven initiative, promoting distribution of digital video and entertainment through existing coaxial cable in the home.
MoCA essentially allows consumers to utilize existing coaxial lines as a broadband backbone through which to distribute rich content or data throughout their homes.
The goal of MoCA is to create specifications and certify products that will tap into the vast amounts of unused bandwidth available on the in-home coax without the need for new connections, wiring, point of entry devices or truck rolls. Figure 1.x shows a picture of a set top
box that has MoCA capability. |
The MoCA specification was designed from the ground up specifically to handle video. Key requirements for video are low packet error rate and low latency. This is achieved through the use of an extremely robust physical layer and a fully scheduled MAC layer that operates in a contention-free manner. Packets are scheduled over 1msec frame boundaries to keep latency extremely low.
Quality of Service (QoS) is handled through the use of Layer 2 prioritization mechanisms. 802.1p Ethernet priorities are mapped to MoCA priorities. Nodes on the network request bandwidth with associated priority and the centralized Network Coordinator schedules the traffic accordingly. This means that MoCA supports true network-wide QoS. The MoCA system uses DES encryption to ensure privacy
between networks. |
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MoCA was designed to operate with existing home coaxial networks without requiring changes. Home coaxial networks commonly are set up as a tree structure and there may be several signal splitters used in the tree. To operate without any changes, signals between devices in the MoCA network must travel backwards through a splitter ("splitter jumping"). In addition, the signal passes easily through the splitter in the reverse direction, typically bounces off an 'upstream' splitter or multitap port, and returns through the forward path of the splitter. This can result in very large echoes in addition to the potentially large attenuation from the port-to-port isolation of the splitter. Fortunately, the channel is fairly static and the modulation scheme has been designed to take advantage of this. |
MoCA deals with this challenging environment by using a variation of OFDM modulation known as Adaptive Constellation Multitone
(ACMT). All paths between all nodes are fully characterized through the use of probes. Signals are then pre-equalized through the use of OFDM
bitloading. The network is periodically re-characterized to allow for slow channel variations.
Figure 1.2 shows how a MoCA system uses a coaxial distribution system to dynamically interconnect devices by connecting devices to coax and using frequencies above 860 MHz. This example shows that the MoCA system is designed to operate in an existing cable television system by allowing signals to jump across and through existing splitters. |
