Not All Cables Are Created Equal
24 February 2021 / by Gareth Heywood
When it comes to AV signal switching and extension over standard copper category cabling, Semtech offers two high performance platforms: AVXT chipsets for point-to-point AV extension applications, and the BlueRiver® AVP series of devices for AV-over-IP and Software Defined Video Over Ethernet (SDVoE™) network-based applications. Both solutions take advantage of standard Ethernet technologies, typically operating at 10Gbps, to support transmission of high bandwidth 4K60 video with uncompressed image quality. However, when video bandwidth does not require the full 10Gbps, both AVXT and AVP can operate at the 2.5Gbps and 5Gbps Ethernet rates. The benefits at operating at these lower data rates includes significantly longer cable reach over high performance category cable, or reuse of existing network cable infrastructure designed to carry lower data rate Ethernet.
Benefits of Standard BaseT Technology
A great deal of R&D effort, money and brainpower has gone into the development of silicon technology to support high data rate transmission up to 10Gbps over standard category cabling. Advanced digital signal processing techniques are employed to maintain signal integrity, compensating for signal loss and crosstalk, while providing noise rejection. By using a fully IEEE-compliant 10G Ethernet copper PHY, both AVP and AVXT deliver a robust standards-based solution, taking full advantage of the huge investment in Ethernet technology. To learn more about the benefits of using standard Ethernet PHY technology, you can watch a demonstration here.
Standards also define the key characteristics of category cable and the expected distances when these are met. The following table shows the recommended cable type for each Ethernet data rate supported by AVP and AVXT to meet 100m maximum distance.
|Interface||Recommended Category Cable||Max. Distance|
However, I am often asked by customers: “Why must I use Cat6a cable with your solution?” Well, the quick answer is, you don’t! You can use whatever cable you choose, but you do need to manage your expectations when it comes to cable reach performance. You see, not all category cables are created equal, so careful attention must be paid to the specifications of the cable you choose. Ideally, choose cable from a well-known brand, follow industry standard installation techniques, using high quality connectors, but to quote the classic disclaimer car manufacturers always use, your mileage may vary!
A Little Bit of Physics
One of the major limiting factors for sending high speed data over copper cables is the skin effect, which results in increased attenuation as the frequency of the signal increases. This effect is due to signal currents flowing mostly on the surface (or skin) of a conductor at high frequencies. The skin effect is defined by the equation below, where w is the frequency of data transmission. As a result of the skin effect, the insertion loss of a cable increases as the square root of the frequency of the signal it carries.
Skin effect results in increasing attenuation as frequency increases
Other factors such as dielectric loss and impedance mismatches at connectors also limit cable length performance at high bit rates, but beyond about 20m or so at the frequencies being discussed here, losses are dominated by the skin effect.
In simple terms, at higher data rates, the cable becomes more “resistive”, which results in shorter cable lengths. Reducing the data rate, reduces the skin effect, resulting in longer cable lengths at lower data rates. The skin of a thicker copper conductor will have a greater surface area than that of a cable using thin conductors. Therefore, the thicker the wire gauge specification of the cable (more skin), the better reach performance it will provide. Making sense?
Why Specifications are Important
Let’s look at a real example of a Cat6a rated cable using 23 AWG solid copper conductors, with the insertion loss specification shown in the following table.
|Frequency (MHz)||Insertion Loss (Attenuation)|
The above table clearly illustrates the increasing loss, or signal attenuation, as the frequency increases. It’s important to understand which frequencies are of interest when studying the specification of your category cable of choice. The bandwidth required at the Ethernet data rates supported by Semtech’s AVP and AVXT chipsets, 2.5Gbps, 5Gbps and 10Gbps, are shown in the following table.
|Interface||Speed (Mbps)||Bandwidth (MHz)|
Since this cable is fully compliant to the category specifications to meet 100m cable length at 10Gbps, we can see that conductors must be a minimum of 23 AWG and approximately 40 dB/100m or better insertion loss is required for 10GBaseT operation. Converting from dB/100m to dB/m at 400MHz, we can then calculate the expected cable length performance when operating at both 2.5GBaseT and 5GBaseT nodes, summarized in the following table.
|Interface||Frequency (MHz)||Max. Insertion Loss (dB/100m)||Max Distance|
What this analysis is telling us is, a cable rated for 100m with 10GBaseT can provide twice the reach performance at 2.5GBaseT. In other words, if your video bandwidth means you can operate at a lower speed Ethernet node, you can get great reach performance with standard compliant cable.
Choose Cables to Meet Your Requirements
As we saw in an earlier table, by operating at the lower sub-10GBaseT speeds, you can also use different category cables, which are designed to operate at lower frequencies, such as Cat6 and Cat5e. Now let’s bring this back to world of Pro AV and talk about video and the implications this has on the choice of Ethernet data rate. The following table shows typical HD and 4K formats and how they map to the respective Ethernet interface and category cable.
|Interface||Recommended Category Cable for 100m Reach||Video Format|
HD formats still dominate in many Pro AV applications, from low cost digital signage to corporate AV. The bandwidth required for a 1080p source running at 60 frames per second is less than 2.5Gbps, whereas a similarly sampled 4K60 video source requires over 9Gbps. If your application only demands HD video extension or distribution, then configuring the AVXT or AVP device to operate at 2.5GBaseT means you have the following options to consider:
If cable distance is critical, use a 10GBaseT rated Cat6a cable and get 200m reach, or
If reusing existing Cat5e network infrastructure is required, 100m reach is possible
So based on the cost and performance requirements needed to meet your specific application, the choice of cabling is yours to make. Products based on both the AVP and AVXT chipsets provide the flexibility to meet a broad range of Pro AV applications, from the most demanding, performance critical AV distribution for medical operating rooms, to low cost point-to-point AV extenders.
You can learn more about the power and flexibility of BlueRiver solutions for AV-over-IP in the webinar, “BlueRiver ASIC: Enabling a New Class of SDVoE Products,” and why AVXT is the ideal choice for low-cost point-to-point AV extension in the “AVXT ASIC: Superior AV Extension” webinar.
Semtech, the Semtech logo and BlueRiver are registered trademarks or service marks of Semtech Corporation or its affiliates. SDVoE is a trademark or service mark of the SDVoE Alliance.