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| Misreading specifications of transmission products can lead to making the wrong selection |
Manufacturers’ product specifications are the main source of determining product purchases for video security users. In many cases, the ability to pretest a product under the actual operating conditions is almost impossible. Often the choice is between several different product manufacturers. The time and expense it would take to install and evaluate each one in the actual application would be cost-prohibitive. Some product categories have advantages. A camera’s low light capability, for example, can be somewhat evaluated by decreasing the input light in any location. A video management system (VMS) can be evaluated by operating it under similar conditions. In both cases, the selection process can be based on a matter of opinion. For the camera, the opinion is the user’s view of quality. For the VMS, how comfortable are the operations?
Transmission Products’ Specifications
None of this applies to transmission products. The ability of moving signals is a matter of physics that is usually black and white. No manufacturer tries to use specifications that are untrue; however, your misreading can lead to making the wrong selection. With transmission products, it is a good idea to go beyond the product specifications and also review the product’s operating manual. What specifications are not included? Taking these additional steps can reveal factors that have substantial effects on your system performing as expected.
How To Choose A Network Switch?
Let’s start with a network switch. Its specification might indicate a port speed of 1Gbps and the ability to handle jumbo frames with packet sizes up to 9600 bytes. True; however, what you have overlooked is the connection from the network camera requires a port speed of 100Mbps, and misalignment of port speeds can lead to dropping of frames. At a 100Mbps port speed, packet sizes are limited to 1518 bytes. Potential video quality problems can exist with cameras greater than 3 megapixels.
For determining if the switch will meet your application requirements, you need to focus on the PoE budget. If the manufacturer doesn’t provide it, the safest approach would be use 75 percent of the specified power and divide it by the number of ports. |
The switch specification states a port power of 30 watts is compliant to 802.3at. True, but is doesn’t state that all ports can provide 30 watts at the same time. The likelihood is they cannot. The statement is true; however, it still may not meet your application requirement.
The switch specifications provide a figure for operating power. You divide it by the number of ports or the number of ports you need to provide PoE and determine there is enough power for your application. True, well maybe, as you haven’t taken into account the power required to operate the switch. Not all switch manufacturers provide both a total power specification and a separate PoE budget. For determining if the switch will meet your application requirements, you need to focus on the PoE budget. If the manufacturer doesn’t provide it, the safest approach would be use 75 percent of the specified power and divide it by the number of ports.
You have applied an operational power overhead to the switch, but in actual operation it might still not provide the port PoE power you need. The reason might be the switch’s operation with regard to how PoE is applied, which is not included in the product specifications. The switch may only provide the ability to turn all the ports on and off; dividing PoE equally between all the ports or PoE is applied only based on port priority.
Complications While Selecting Transmission Medium
From the switch we move to the transmission medium. Here it is important to note that all specifications are based on the IEEE transmission standards using UTP cable. For our applications, that would be Cat 5 and above. If your application has to travel over Cat 3, and the product specifications doesn’t show a Cat 3 specification, it is a good idea to check with the manufacturer. For any application that transmits over distances greater than 328 feet or on a medium other than Cat 5, you are dealing outside the normal specifications, and performance is only decided by the equipment manufacturer. This is where things start to get complicated.
As long as the power at the end of the longest distance meets the lowest figure, the specification itself is true even if it fails to properly power your camera. |
The manufacturer claims their product transmits at 100Mbps. True, but do they mean 100Mbps in both directions or do they mean 50 Mbps in both directions? The statement would be true in either case. The manufacturer claims an operating distance of 5,000 feet. True, but in the case of bandwidth, what hasn’t the manufacturer told you? They have omitted the bandwidth available at the highest stated distance. Even if that bandwidth is only 1Mbps, a level that would not be acceptable for most IP cameras, the statement is still true. Nowhere is there any indication within the specification of the ability to handle jumbo frames. Keep in mind that most networking products are tested and claims made based on the ability to handle 64byte, a figure that doesn’t come close to the lowest video packet size of approximately 1024 bytes. These can be embedded in any number of specification claims. What packet size is the manufacturer basing its claim on?
Bandwidth and packet size take on more importance when it comes to large systems that use multicasting. This feature of cameras and video management systems helps to reduce bandwidth when dealing with large groups of cameras. The key is the system bandwidth must be large, at least at 100Mbps and must be maintained throughout transmission distance. Decreases in bandwidth will render multicasting inoperative.
PoE Selection:
The same approach applies to PoE. The specifications state it provides 802.3af power, but at what class? Keep in mind PoE power is defined in classes, each of which has a range of power. That statement can be true even if the power at the longest distance is under 4 watts. If the specification states the extender is capable of 802.3at power, that statement is true if the power level exceeds 15.4 watts. It doesn’t have to be at any power level above that. As long as the power at the end of the longest distance meets the lowest figure, the specification itself is true even if it fails to properly power your camera. What level of power within a PoE class claimed within a product specification is actually produced?
Determining Cable Performance
Finally there is the question of the cable itself. If the extended medium is UTP, are the associated specifications based on the more commonly used Cat 5e, or based on the more expensive Cat 6 that has better performance characteristics? Also, don’t make the mistake of assuming the performance specification stated for Unshielded Twist, is the same for Shield Twist. The latter has very different characteristics that affect the transmission of both bandwidth and power. When it comes to single pair, twisted or untwisted, or coax, there simply are no standards for IP and PoE transmission. Here, also, it is important to understand the association between the stated specification and the cable you require. In most cases conversions of existing analogue video security system to IP will involve standard RG59 cabling. Within the family of RG59 cabling, there are several different types of cable resistance. Fortunately, most of the common cable used will have a cable resistance that closely matches that of Cat UTP used for determining 802.3 performance. However, there are exceptions. Most often you may find a manufacturer stating coax maximum distance performance based on either RG11 or RG6 coax cable. As the main reason for using Coax to Ethernet extenders is to avoid pulling cable and the associated costs, it makes no sense to have your requirement met if you do have replace the cable in order to achieve your distance goals.
Role Of Power Source In Deciding Performance
The question of power also brings up the question of the power source. In an ideal installation you should be able to use the existing power from a PoE network switch. What power source is the performance specification based on? In some cases the ability of a cable extender to achieve its stated maximum distance is based solely on the use of its own power supply, usually at extra cost. Or it can be based on the requirement that the extender at the camera location have a power supply adding cost not only for the power supply but also for the need of an outlet. However, in both cases statements relating to distance and the ability to provide a particular, usually high, power level are true.
Nowhere is the subject of PoE over coax more confusing than when it is applied to the need for 60 watts. Physics dictate the higher the power level the shorter the distance. Additionally, coax has more a resistance to power than UTP (Ethernet). In most cases, the actual ability to transmit 60 watts over coax provides a very short distance with the rest of the product carrying either multiple ports of 30 watts or a combined port of a value over 30 watts on Ethernet cable. However the claim of “We can transmit 60 watts over coax” remains true, it just overlooks the subject of distance or the practicality of your actual applications.
