Network Topology

Topology is how the physical elements of a network are arranged and connected. Entertainment control networks are arranged around one or more Network Switches in a variety of valid topologies.

Network Width

Every time data passes through a switch, a small amount of latency is added. To maintain near real-time data transfer in an entertainment control network, the cumulative latency across all devices and components of a network is important.

• Link - the physical or logical connection between two nodes or segments. This includes any installed cable runs within walls or conduit, as well as the patch cables on both sides. The maximum lengths of the wire are determined by the cable type, which are rated for different distances (see Network Wiring). Exceeding these distances will cause a drop in signal strength, can cause erratic behavior in the network, or potentially prevent data reaching its destination entirely.
• Segment - a connection between two NICs that goes through at least one switch.
• Hop - a segment where data is passed from one device to another. This could be a console to a switch, a switch to a device, or a switch to a switch. A hop count (how many nodes data passes through between source and destination) that is acceptable for near-real-time control is limited by a network's topology, as well as the node hardware involved.
• Network Width - the highest number of segments or hops between nodes in a network. Acceptable network width is also limited based on network topology and hardware, and excessive network width can introduce latency or prevent communication entirely.

The network width describes the maximum number of switches data can pass through between any two nodes, and connection lengths between switches must not exceed the requirements of the cable being used.

The following segment connects an Eos device to the nodes on the right:

Any additions or alterations to a network can affect the hop count and network width, especially when connecting additional switches. Plugging a switch into the backstage left port may seem like an easy way to add extra ports to your system, but can easily lead to unintended impacts on the entire network.

Many fixtures now offer direct wired network connectivity via onboard RJ45 ports. Since these fixtures have internal switches, they count as nodes and contribute to the node and hop limit for the network. This is especially important to consider when daisy-chaining fixtures together; running each fixture directly back to separate switch ports is preferred.

Since DMX is not subject to the same hop limitations, it is often recommended as a "last mile" protocol to connect fixtures to network infrastructure via a DMX gateway.

Network Loops

With any topology, care should be taken to avoid network loops, where cabling creates multiple paths to a switch. In this scenario, the data forwarded out of the switch reenters the switch through the loop and is duplicated and redistributed indefinitely. Eventually, the increase in traffic can impact other communication in the network, or, in extreme cases, cause a broadcast storm which disables the network entirely.

Loops most commonly occur when more than one connection to a switch is established. In a single-switch network this is typically because the switch has been connected to itself, one port to another.

Multi-switch networks, particularly when changed or reconfigured, have a higher potential for loops, which typically occur when two switches are connected by more than one cable run. If enabled within the network, Rapid Spanning Tree Protocol (RSTP) can help manage switch connections.

Single-Switch Network Topologies

In a single-switch topology, all nodes in the network are connected through a single network switch.

This topology is generally used to arrange small, simple networks. Permanent small networks should follow Basic Managed Network Requirements, while portable small networks may follow Unmanaged Network Requirements.

Multi-Switch Network

Larger, more complicated networks will generally make use of one or more topologies that utilize multiple network switches. See Basic Managed Network Requirements and Advanced Managed Network Requirements.

Star Topology

Star topology consists of nodes connected to multiple edge switches, each of which is connected to a central core switch. Nodes can also be connected to the core switch directly.

Devices are connected by either of the following wiring segments:

• End device segment - a home run from a node to a switch.
• Trunk segment - a home run from an edge switch to a core switch.

In some Advanced Managed Networks, the core switch can be a group of connected switches specially configured to behave as a single device.

Star topology design minimizes the overall network width and number of hops necessary to get from one point in the network to another. It also minimizes the impact of wiring or equipment failures, as a node, switch, or wiring segment issue will only affect a small portion of the network and be easier to troubleshoot. As the core switch is critical to the operation of the network, many switches offer redundant core capabilities which can mitigate the risk of failure.

Caution should be taken to avoid branches or spurs, decentralized offshoots created when a switch is connected to an edge switch. This can often occur when a new switch is added to an existing network. Branches increase the network width and hop count between nodes and can have detrimental effects on the network.

ETC recommends star topology with no loops and the minimum number of branches as the ideal topology for entertainment control networks; other topologies may function but are generally more complex to configure and maintain.

Linear Topology

In a linear topology, multiple switches are connected sequentially.

Hop count and network width must be carefully observed. In the pictured example, there are 3 or 3 hops between Eos and any of the connected nodes, which would increase to 5 hops if Eos was connected to the first or last switch in line.

Ring Topology

Ring topology is an advanced arrangement that intentionally introduces a network loop between switches. One switch is specially configured to close one of the network segments at a time. This switch also evaluates the flow of traffic around the network, checking the health of the ring using the Ethernet Ring Protection Switching (ERPS) protocol.

The two switches connected by the closed segment communicate the long way around through the normal segments; in terms of network traffic, acting as a linear topology.

If the ERPS switch detects a disconnect in transmission across a normal segment between switches, it will open the closed segment to maintain the flow of data within the network.

Despite its self-healing properties, ETC does not recommend ring topology for entertainment control networks. As well as being difficult to configure and troubleshoot, miswiring the system or misconfiguring the ERPS switch could induce a network loop, introducing latency or taking the network down entirely.

Compared with star topology, ring topology can also adversely increase the overall network width, with a high number of hops between the switches connected by the closed segment. Even when correctly configured, it can be easy to increase the hops between a sender and receiver, which could have adverse playback effects during a live event.

Imagine a scenario where an Eos console is connected to one of six switches in a ring topology, controlling a fixture connected through the adjacent switch, totaling 3 hops to pass data from console to fixture.

If these two switches are disconnected and the closed segment heals the network, console instructions must now pass through all other switches in the network to reach the fixture, increasing the hop count to 7.

If ring topology must be used, ETC recommends testing failover scenarios frequently, and during event-like conditions, to ensure that network communication remains possible. Any network using ring topology is subject to Advanced Managed Network Requirements. ERPS protocol tends to conflict with Rapid Spanning Tree Protocol (RSTP) when both are present in a ring network.