STP vs UTP for Video Surveillance TutorialAuthor: Brian Rhodes, Published on Aug 06, 2012
For many video system designers, deciding which ethernet cabling to use is a quick decision: go with the cheapest. However, this overlooks he possibility the cable, and the video it carries, needs extra protection against common electromagnetic interference.
Is the difference between cable types that significant? In this note, we examine shielded cable, look at how it can prevent video problems, and compare it to nonshielded alternatives.
Inside this note, we explain:
- How Electrical Interference Affects Video Quality
- What Shielded Cable Looks Like Compared To UTP
- Physical Differences Between Cable Types
- Other Names For STP
- Typical STP vs UTP Costs
- Use STP Against Common Sources of Interference
- Practical Use Is Limited, But Axis Disagrees
Electrical Interference Affects Video Quality
In the video surveillance world, the emphasis is often on cameras and NVRs, but little attention to the cabling in between. When video quality problems arise, it can be a frustrating exercise to swap camera and tweak settings, only to discover problems are still present.
However, taking a hard look at cabling can resolve maddening issues. Take the example in the image below:
Electrical interference in the cabling itself cause this type of problem. Not only does the cable transmit intended data streams, but it also can attract and transmit unwelcome 'noise'. A jacketed cable can serve as an 'ad-hoc antenna' helps emphasize why cable shielding is sometimes critical.
'STP' Is Integrated Cable Shielding
A quick physical comparison between nonshielded UTP and STP, or 'shielded twisted pair' cabling reveals the primary differences. The image below shows the additional metallic foil shielding surrounding wire pairs in STP:
'Shielding' should not be confused with 'cable screening' where a single layer or metallic foil or mesh covers the entire bundle of wires. While the decision to individually wrap pairs versus gross wrap the entire bundle shares some of the same benefits, they are not equivalent to one other and result in different performance.
Other Names For STP
The common abbreviation for shielded twisted pair cable, 'STP', is sometimes called 'U/FTP' for 'Unscreened/Foil-shielded twisted pair' or 'S/FTP' for 'Shielded/Foil-screened twisted pair' instead. Especially for structured cabling specifications, some may use these alternate abbreviations to describe shielded cabling. However, in general use, the 'STP' abbreviation is most widely used.
Physical Differences Between Cable Types
The following list summarizes the tangible differences between UTP and STP.
- Metallic Foil Shield: A thin layer of foil, commonly aluminum, surrounds wire pairs. This layer is often called the 'drain', and must be properly terminated. Failure to adequately ground the drain can amplify the problems that STP attempts to mitigate.
- Thicker Jacket: The added layers of foil increase the weight add diameter to the cable bundle. As a result, a thicker plastic insulating jacket is needed, which adds rigidity. Overall, STP is heavier and thicker compared to UTP, and may be more difficult to install as a result.
- Cores, Pull Strips, and Groundwires: Depending on the exact manufacturer and brand of STP, other features may be present not commonly found in UTP cabling. This includes plastic divider 'core' sprues, strings to aid stripping the jacket, and additional electrical grounding wires. Below IS an outdoor rated cable with a center core that separates the pairs into quadrants within the jacket.
STP vs UTP Functional Differences
Those additional physical features provide STP with properties that UTP does not possess.
- EMI resistance: The primary advantage of shielding is protection from environmental electromagnetic interference, or EMI. Because each pair is individually wrapped, the ability for ambient interference to permeate and carry down the cable is significantly minimized or eliminated.
- Isolated line noise: Interference can be a 'two-way street' in that unshielded cabling is a source itself of interference. In some applications, like sensitive medical imaging or manufacturing, normal ethernet cabling can be a uncontrolled conduit for interference that throws those instruments off. Again, the addition of pair shielding minimizes or eliminates this problem.
Using STP adds between $20 to $40 per camera compared to UTP cabling, assuming cable runs of 150 feet, based on STP cable costs ~40% more than UTP and depending on how much additional labor or larger conduit is needed for the larger, heavier and more rigid STP.
Use STP Against Common Sources of Interference
Simply, STP should be used where interference could be a problem. To firm up where these places commonly are found, here are some places where interference could impact video quality:
Adjacent to High Voltage Wiring
Power wiring can interfere with data transmission even when run parallel to each other. Even wiring run a compliant distance apart within a grounded raceway can be a source of video interference.
While no hard specification exists for when to use STP in this situation for video, best practices in data networking design follow that any data cabling sharing the same raceway, regardless of how it is contained in EMT or conduit, must be run using STP cable.
Near Inductive Devices
Data cabling run near common electromechanical features like electric motors, power transformers, magnetic coils, or solenoids can introduce significant EMI. Especially for industrial facilities, where these devices are common, shielding cable runs is an important protection.
These sources are characterized by their 'inductive' properties, or their reliance on magnetic fields for operation. Cable proximity to devices like HVAC equipment, ventilation fans, door maglocks, electrical switchgear, and industrial machinery can generate enough interference to degrade video quality.
GSM Devices/Walkie Talkies
Common low powered communication radios disrupt data transmission. While a token handset may not be significant enough to be a problem, locating data runs near high powered repeaters or transmitters should be run using STP to eliminate problems.
Fluorescent Light Fixtures
One of the biggest sources of EMI are common light fixtures. Given the common occurrence of ethernet cables running overhead of these fixtures, if cabling cannot be run in formal cable trays on Conduit, it should be run using STP cable.
Even worse, there are many occasions where ethernet 'installed by others' is to be used for video. In many cases, cabling previously installed is the source of video quality problems that remain unfixed until the cable network is corrected.
Practical Use Is Limited
In our experience, the vast majority of all surveillance ethernet cabling has been run using UTP, and we have no significant issues to report. Since specifying networks with the appropriate shield is imperatively acknowledged in datacenter and networking design "best practices", it is generally suitable for video network design.
Axis: Mandatory STP Use Outdoors
Industry giant Axis Communications recently declared use of STP mandatory for outdoor cameras per the following whitepaper:
"Our recommendation is to deploy an STP network cable in demanding electrical environments. Examples of demanding indoor environments are where the network cable is located in parallel with electrical mains supply cables or where large inductive loads such as motors or contactors are in close vicinity to the camera or its cable. It is also mandatory to use an STP cable where the camera is used outdoors or where the network cable is routed outdoors."
This is a bold statement given that ~40% of all cameras are installed outside. Observing Axis' recommendation may drive a significant increase in overall project cost and is likely overkill relative to common problems faced.
Rather, our experience disagrees with the generalized application of STP. The smartest use of STP is where ethernet is run in the 'high risk' areas identified above.
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[Note: This tutorial was originally published in 2012 and substantially revised in 2017]
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