Network topologies, AV transmission, and testing

Feb. 11, 2021

By Ariel Marcus, MSolutions

AV over IP has opened previously unseen freedoms in how systems integrators and their end-user customers manage their AV operations. Aside from the immediate interoperability benefits of moving AV systems to the network, AV over IP is a gamechanger for systems flexibility and scalability.

These points are especially true given that AV over IP theoretically does not limit the customer to a fixed number of sources and destinations for connectivity. For example, an AV over IP switching and distribution system offers a flexible and cost-efficient platform for adding new inputs and outputs. In comparison, a legacy matrix switcher limits the user to a fixed I/O count; if more are needed, it’s time to buy another box.

That said, HDBase-T is not going anywhere. For many facilities, HDBase-T remains the ideal solution, especially when scalability is not a concern. There are also plenty of I/O capacity options, and many integrators will buy “a size up” to leave some open connections for new sources and destinations. Think of your typical huddle space, small classroom, or home AV systems. In most cases, your matrix switcher will only need to connect to a handful of AV components. HDBase-T continues to be the preferred option in these types of environments.

The commercial systems integrator or residential custom installer is required to test, commission and, when needed, troubleshoot their installations on behalf of their customers. While many contractors are still limited in their need (or desire) to install AV over IP systems, they will at least need to self-educate on the testing parameters. And even with AV over IP here to stay, HDBase-T still has plenty of miles in the tank.

Straight line action

HDBase-T represents the serial transmission of signals, whereas AV over IP follows a more-nonlinear path. Unlike older analog systems, AV over IP transmission can be applied to a more IP-based network topology, such as a star network. This is very rare however, as it requires an unusually large—and therefore expensive—switch to manage the transmissions.

HDBase-T systems are therefore mostly deployed using a linear bus topology. A linear bus topology supports carriage of the video transmission from source to the first end node. The signal continues onto the second node, third node and onward in a straight-line manner. Each device’s repeater board receives the signal, extracts the data, and presents an image.

While advancing through these nodes, each destination or sink device, such as a display, extracts the data assigned to it. If there is a scaler in the display, the scaler receives the specific information it needs, and presents the data. That data will then be displayed onto a single or combined image. For example, the image might be standalone on a single display, or be a separate image on a menu board. Or, that data may be combined into a big continuous image across a video wall.

The integrity challenge with this topology is rooted in that first transmission phase. The first node will be unable to extract the correct image to present without perfect delivery. As that signal moves to the next node, the signal will further deteriorate if not perfectly extracted at the first node. That deterioration will continue as the signal moves to each subsequent node. In an AV over IP environment, that deterioration might be at least partially attributed to latency—but not with HDBase-T, where latency is always below 10 microseconds, which is essentially not noticeable.

With latency removed from the mix, any signs of signal deterioration can be localized to the quality of the transmission. This is most often associated with the structural integrity of the cables, and the presence of enough overhead in the connection to carry the signal. A quality HDBase-T test device can immediately confirm the following.

  • HDBase-T terminations are exacting, with no differentiations between twisted pairs
  • A 300-Mbit/sec (4K UHD) overhead presence to support carriage of high-resolution signals over HDBase-T
  • HDMI cables are of high quality, and in perfect condition from source to destination (i.e. no missing wires, sharp bends, kinks or damage beneath the shielding)
  • Activation of links to transmit AV signals through the cable

In a linear bus topology, these simple confirmations will eliminate corruption of the extracted video from the HDBase-T system.

A variety of topologies

Whereas HDBase-T will nearly always follow the linear bus topology, the nonlinear nature of AV over IP distribution clears the way for varied topologies. Naturally, each topology is generally suited for systems of a certain scale. Two of the topologies are most suited for AV environments.

On the lightest end of the spectrum, modest AV over IP deployments can cleanly operate in a single logical switch environment. While technically also a star network, these are simpler, lower-cost systems that can carry AV signals without bandwidth issues, but are generally limited in scalability. And, if the switch goes down, so goes the system.

Star networks represent the more common choice for AV over IP systems. These systems bring the scalability and redundancy lacking in single-switch configurations. A star network will typically comprise a core switch with a series of supporting switches, or access switches, that offer different traffic routes for AV signals.

The result of receiving, extracting and presenting data to produce images is essentially the same as with HDBase-T. The destination will generally always be a smart device, such as a system-on-chip (SoC) digital signage display or smart TV.

The key difference is the varied network paths of the star topology. The network switches interconnect, with all active ports sending data over category cable. These signals move over the IP network, and that data can be sent separately to each receiving node. That means ensuring that all displays and other nodes are full synchronized, and that each port has a specific virtual LAN configuration to guarantee bandwidth 

Unlike HDBase-T, latency is a concern with AV over IP. If there is latency on one of the lines, the same image presented on different screens may be delayed on one or more screens. While AV over IP test and analysis software will troubleshoot latency problems, latency can be properly addressed in the initial switch configuration by doing the following.

  • Confirm that all VLANs are configured correctly, and in accordance with IT department policies if applicable.
  • Confirm that fast leave capabilities are enabled to synchronize all screens.
  • Understand the concepts of multicast propagation delay and bandwidth capture.

Multicast transmits a single source over dedicated category cables to each display, and the propagation of the signal through the cables must be exact for each. With accurate signal propagation and switch configuration, the repeater board on each device will extract the proper data to create the required image at the same time, if the same image and resolution is to appear on every screen.

However, many AV systems will have different content, and perhaps different resolutions, for various screens. A star topology does not require that all screens be synchronized. If one display is presenting in full high definition (FHD) and another in 4K, the appropriate propagation delay (for latency) and bandwidth (for resolution) must be allocated on the switch for each port. Otherwise, a bottleneck forms with one image, while latency affects the other.

The job is not over for the integrator once the switch is properly configured and the appropriate measures of latency and bandwidth are added. The commissioning process will require AV over IP software to confirm all network and switch information and settings are accurate, and certify that IP network conditions and connections are in good operating order. Some of the important parameters to confirm and certify include:

  • Identification of network configuration faults,
  • Identification of network port status (open or blocked) and present network nodes,
  • Confirmation that pertinent multicast settings are correctly enabled,
  • IGMP (Internet Group Management Protocol) snooping, including VLAN setup and fast-leave supports, and
  • SNMP (Simple Network Management Protocol) compatibility for network status queries

This information will allow integrators to quickly confirm if network settings fit AV over IP performance requirements, and that the switch configuration can support smooth video streaming.

Common ground

While HDBase-T and AV over IP follow two different network architectures, there are common issues that can affect the performance of both. DC resistance is one that can and will cause problems for signal transmission across linear and star topologies.

DC resistance represents the pure quality of twisted-pair wires. In an optimal situation, the integrator will have a full copper implementation of twisted pairs inside the category cable. However, these are not always optimal situations; often, the core of the wire is copper-coated steel or aluminum (CCS, CCA). The copper coating is created through a specialized electrolytic process, leading to high resistance and diminishing the quality of the cable.

Ethernet cables are measured for 330 feet/100 meters. The leading manufacturers producing their own cables will reflect 5 to 8 ohms over that distance. With CCS/CCA cables, the reflections rise to 18 to 20 ohms per meter. The high resistance of the twisted pairs will result in HDBase-T transmission failures, as the artifacts in the signal will degrade the quality of the signal from point to point.

The problems are aggravated when the transmission also must pass through the termination on the receiving end (where the HDMI is extracted), and each subsequent node. The DC resistance per 100 meters will continue to increase, and therefore reduce the quality of the signal, and the ability to transmit HDBase-T, on a linear topology.

The same concepts apply to DC resistance and category cabling in an AV over IP star topology. IP networks are also sensitive to resistance within twisted pairs, and a high resistance can initiate retransmissions of the signal, leading to potential traffic fails and network roadblocks.

There is much more to explore on the actual processes of AV signals on HDBase-T and AV over IP, as well as the HDMI cables that live within both worlds. Understanding the differences of how these signals are transmitted over the two topologies will provide installers with a better understanding of what to test for, and how to troubleshoot these issues by understanding how they correlate with the network architecture.

Ariel Marcus is chief technology officer of MSolutions.

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