Ethernet over Coax: Preserving Legacy Systems?
Because of the widespread use of composite-based video surveillance, coaxial cable is still prolific across the majority of sites. Whilst it can be argued that installing new coaxial infrastructure is a path best avoided, it equally makes little sense to simply rip out and replace all legacy infrastructure. Can the employment of Ethernet-over-Coax technology exploit any remaining value?
[dropcap]C[/dropcap]oaxial cable is pretty much synonymous with video surveillance, and it would be fair to say that more miles of coax have been pulled for security applications than any other type of cable. Coax is still widely used, and BNC connections have – for many years – been the industry standard when delivering composite video. You could be forgiven for thinking that anything so popular must surely be the best option.
The reality is that coax isn’t the best solution for transporting composite video. Performance tests in the real world have proved that point, and comparisons relating to the time and expense of installation have highlighted the weaknesses of coaxial cable too. Indeed, it is probably quite widely accepted by experts within the surveillance sector that coax isn’t the best option. Despite this, coax is still prolific.
The dominance of coaxial cable has less to do with its suitability, and more to do with a traditional approach to the installation of video surveillance. When surveillance was in its infancy, unshielded twisted pair (UTP) wasn’t an option. Coax was the natural choice, and so manufacturers standardised on BNC connections for their products. As the number of surveillance solutions increased, so the need for compatible devices also grew, and the BNC connection was pretty much cemented in as a ‘must-have’ element of any video product.
As time passed, the increased availability of UTP offered a wide range of benefits, but the BNC – and coax – was too well established. A few manufacturers did offer products with UTP connections, but these never proved as popular as the more traditional devices, predominantly because they often used additional integral converters (with an associated cost increase), and so were soon dropped.
The result is today’s legacy of coaxial infrastructure, and given that cabling is a significant element of any solution, it seems futile to not utilise at least some part of that previous investment where possible.
Coaxial cable might be a mainstay of traditional surveillance solutions, but it does have limitations, both with regard to what it can support, and to its overall suitability. Coax is not the easiest cable to work with. Because of its construction it can be difficult to pull in small conduits or voids, it has a low bend radius, and it is vulnerable to a number of sources of interference.
Coax is predominantly restricted to use with composite video systems, which themselves are restricted in terms of resolution. Some existing coax can also be used with HD-SDI systems. However, this does rely on the coax being of correct specification and a high enough quality, and the run lengths must also not exceed the performance limitations of such systems. These are around 100 metres. Therefore, if coaxial runs are in excess of this distance, HD-SDI may not be a viable option.
Both composite video and HD-SDI video systems share one characteristic, in that devices need a dedicated cable for every video channel. If a site has legacy coaxial cable in place it could potentially be re-used to deliver a new composite video system with a similar performance to the old one, or to maybe allow an upgrade to HD-based video in some circumstances. However, any expansion of the system, or change to the overall topology, would probably require more coax being installed, and for many that is viewed as a regressive (and sometimes costly) step!
The alternative is to utilise some of the existing coaxial infrastructure as cabling in a LAN infrastructure. It must be noted that employing coax in such a way isn’t always ideal, dependent upon the site’s requirements, the way in which the original cabling was installed, and the design of the new system. There are site-specific challenges to be overcome as well if such a move is to be effective.
One last point is that good site management practice often dictates that obsolete cable be removed, rather than simply abandoned in voids! In some sites the removal of old cable can more than double the cost of installing new infrastructure.
Because coax was dependent upon a point-to-point cabling topology, in that all devices needed a link to management and archiving solutions, legacy coax will often connect different parts of a site. Where the connected areas require a LAN connection, the cabling may be able to support such a link.
Before assessing the suitability of EoC, it’s worth touching on a few basics. Convertors are available from a number of manufacturers, but as with anything, not all convertors are the same. The most common type of EoC devices feature a single BNC connection, and a single RJ45 port. One device is used at either end of a coaxial cable run, and these allow connection to the LAN. However, there are also convertors that allow a single unit to receive data from a number of other convertors, thereby allowing a significantly higher degree of flexibility. The multiple units each use individual coax runs, but these can be joined via simple T-Connections in the cabling.
It is also worth considering throughput and range. Typical LAN connections using UTP are limited to 100 metres before repeaters are required. Approximately the same range limitations apply to HD-SDI connections made using coax. However, when used with suitable EoC convertors, the coax can deliver full rate network connectivity over two or even three times that distance.
One final consideration is that many EoC convertors can also supply power. Because the coax is effectively an Ethernet connection, PoE (power over ethernet) can be supported by suitable devices.
When designing an EoC solution, it is important to understand that the system can be EoC throughout – which is an unlikely scenario because of the way legacy systems were designed – or hybrid. When we say hybrid, the solutions can be a mix of EoC and traditional UTP-based LAN cabling, or even a mix of EoC, LAN and coax-based composite video. EoC does allow for parts of a system to be upgraded to networked video, enabling the use of HD and megapixel devices, whilst also retaining much of previous investment in composite video.
Often, HD or megapixel video is only required where positive identification of individuals is needed. The legacy composite video system will then be more than good enough, as much of the video footage will be used for more general surveillance and to provide continuity. Evidential video for identification can be taken from the higher quality cameras.
EoC is a very good example of how an existing technology can help bridge a technological gap in the surveillance sector. It can deliver real benefits with immediacy, allowing solutions to exploit the highest quality video without a need for a total replacement of infrastructure. In short, it is proven to enhance solutions that have legacy coax!