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GEORGE GEORGEVITS: Multimedia coaxing

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Network technology that uses existing hybrid fibre coax cable looks good at first glance, but is it better than Ethernet? George Georgevits compares the two.

PAGE 68 Troubleshooting COAXAn industry alliance of electronics companies has developed a home networking technology that runs over existing coaxial cabling.

The stated aim of the Multimedia over Coax Alliance (MoCA) is to provide a low-cost and reliable technology for accessing and distributing content throughout the home, with little or no cabling remediation work required.

Suitable content for an MoCA compliant network include HD TV, Ultra HD TV, gaming, large file transfers and general local area network traffic. MoCA technology can also be used to provide the copper link to a wireless router for extending the reach of wireless networks.

The alliance has 53 member companies, including some large entertainment electronics manufacturers (Samsung, LG), networking electronics manufacturers (Cisco, D-Link, Huawei, Netgear, ZyXel), cable TV operators (Verizon, Broadcom) and some chip manufacturers and distributors.

Only members have access to the technology, and MoCA-enabled devices are certified to work on MoCA networks. The alliance provides a list of certified products on its website:

www.mocalliance.org/products/index.htm

In essence, MoCA technology is another form of coax-based Ethernet. Those with long memories will recall the days of 10Base2 and 10Base5 coax-based Ethernet. The last version of this type of network ran at 10Mbps over 50W coax in a bus topology using BNC connectors.

MoCA uses 75W coaxial cabling with the familiar F-type screw connectors. This type is installed in the home to provide access to cable TV services (eg: Foxtel in Australia) or hybrid fibre coax (HFC) internet services (also known as ‘cable internet’).

 

Specifications

The alliance, established in 2004, approved the first MoCA specification in 2006 and this is now the legacy 1.0 version.

Two approved versions of the MoCA specification are available – 1.1 and 2.0. Both were ratified in 2010 and both support up to 16 networked devices on a single network.

MoCA 1.1 provides 175Mbps net throughput and operates in the 500-1500MHz frequency range.

MoCA 2.0 supports two performance modes – basic and enhanced ­– offering 400Mbps and 800Mbps net throughput respectively.

There is also a channel-bonded point-to-point ‘turbo mode’ that can provide 1Gbps net throughput.

The operating frequency range for version 2.0 is 500-1650MHz.

MoCA 2.0 also supports two low-power modes: sleep and standby. It is backward compatible with 1.1.

MoCA networking is intended to operate on the same cabling as the pay TV service.

In the United States, pay TV frequencies are outside the range of frequencies used by MoCA, and the two services can operate simultaneously on the same cable. A filter is still fitted at the point of entry, to ensure the MoCA network does not put any unwanted signals back into the pay TV network.

MoCA is not suitable for use where coaxial cabling is used for distributing a satellite service. The output signal from the low noise block on the dish occupies a band generally in the region 950Mhz to 2150MHz, depending on type, and this overlaps most of the MoCA signal bands.

In Australia, HFC cable uses a band from 5MHz to 700MHz (Optus), or up to 750MHz (Telstra), and the higher end conflicts with MoCA frequencies.

This can be overcome by programming the MoCA network to not use the conflicting channels, but it may be one reason why MoCA technology is not offered by pay TV carriers.

 

Network topology

The MoCA physical network topology is determined by existing coax cabling and is usually some form of tree topology, with splitters used for the branching.

Logically, the MoCA network is a bus topology, just like other types of Ethernet network.

Connections to Ethernet devices without a MoCA port (standard wireless router, Ethernet switch, etc) are made by connecting to the Ethernet port (typically 10/100Mbps) on a MoCA bridge, in much the same way as is done with an HFC cable modem or ADSL modem.

The bridge provides a gateway to other networks and is simply a MoCA network device, so a MoCA network can have a number of bridges.

In the US, the cable service provider usually provides the bridge to link the MoCA enabled cable network to the home owner’s MoCA network. Otherwise, the cable network simply connects to the home network via a splitter. This first splitter is known as the root splitter.

Some minor remedial cabling work may be required when establishing a MoCA network. Splitters and sometimes one or more amplifiers will be embedded in the existing coax wiring.

These can sometimes cause problems. Amplifiers and splitters are often only rated to 1000MHz (1GHz) or so, because this is all that is needed for conventional pay TV signals. MoCA on the other hand requires bandwidth to 1650MHz.

If the amplifier or splitter cannot perform to this higher frequency, it will cause significant performance degradation on the MoCA network.

Sometimes there are un-terminated tap points on splitters that will act as a source of signal reflections and must be eliminated.

 

MoCA advantages

The main advantage of a MoCA network is that it uses existing cabling to provide high-speed data communications to all access points.

It also offers the reliability and security of a cable-based network.

A MoCA network doesn’t have to be connected to the internet. Its prime function is to provide high-speed point-to-point connectivity within the home. It can work independently as a stand-alone network and still provide this connectivity.

For file transfers within your home that do not require internet involvement, MoCA offers a very satisfactory high-speed data service.

However, once you connect to a network outside your home things can change.

One of the claimed advantages of MoCA is that there are no delays associated with transferring streaming content, as can sometimes occur with internet TV or internet-based HD video downloads (such as movies).

This may be true if you are using MoCA in the US, where a pay TV operator is providing you with the content. However, it is not the case in Australia when streaming content accessed via the internet.

In Australia, pay TV companies do not provide a MoCA compatible service, so the only way to access streaming content is via the internet.

When you watch Internet TV, HD movies or some other form of streaming HD content in real time in Australia, you access it via ADSL, HFC cable or whatever internet access technology you have signed up for with your ISP.

Regardless of the technology, it will suffer from latency problems caused by the limited bandwidth of your service and the traffic-handling capabilities of your ISP’s equipment. The internet infrastructure in general can also be a problem, particularly at times of peak traffic.

In short, you will sometimes experience frame freeze and buffering delays, regardless of whether your home network is MoCA based.

Another of MoCA’s claims is that it is reliable. This is probably true, if the performance is compared with that of a wireless network. However, it is certainly no more reliable than the competing technology, namely Gigabit Ethernet over Category 5e twisted pair.

 

Disadvantages in Australia

MoCA seems to be gaining acceptance in the US, where a number of pay TV operators are providing MoCA based services.

The equipment is also available in the UK, but support is said to be poor. The technology is unheard of in Australia.

The reason (apart from incompatibility with Australian HFC services) is that using MoCA becomes cost effective only when there already is a coax point in every room of your home, or at least in every room where you require network access.

Unlike US dwellings, Australian homes are generally not wired throughout with coax for access to cable TV. Most houses have only one or two points, and MoCA is not cost effective if the wiring needs to be radically altered. Also, MoCA enabled devices are not cheap, even in the US.

MoCA 2.0 is touted as being fast and secure. Well, a Cat 5e cabled Gigabit Ethernet network will support a 1Gbps data rate to every port and is every bit as secure as a MoCA network.

Furthermore, twisted-pair Ethernet is not limited to 16 devices or to a maximum span distance of 85m. Gigabit Ethernet over Cat 5e cabling has a maximum link length of 100m, so if the hub is centrally located, it can reach ports up to 200m apart. The distance limit can be pushed even further if you use Cat 6 cabling.

Ethernet hardware is cheap and not restricted to a few select suppliers. Most entertainment devices these days are equipped with at least one Ethernet port.

 

Conclusion

Although MoCA seems to be a good idea at first glance, it is not suited to the Australian environment for the reasons outlined.

About George Georgevits

George Georgevits

George Georgevits, BE (Hons), manages his communications engineering consultancy Power and Digital Instruments, established in 1980. PDI specialises in lab and field testing and troubleshooting of cabling systems and components, as well as general electronics, communications and power engineering. Contact PDI on +61 2 9411 4442.

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