Daisy Chained Fiber Explained
Fiber is a mainstay for networking cameras run far apart. The number of cameras seldom matches the fiber available, and having two strands to run multiple cameras can be a showstopper. In this case, 'daisy chaining', or running multiple cameras in series is a common solution, but it can be a major headache when equipment breaks. In this note we look at fiber 'daisy chaining', discussed why it is used, and what options for improving reliability are available.
Definition
In early 'peer-to-peer' networking, 'daisy chaining' was a common method of stringing together a network of devices in series. Early networking protocols like System Bus used the same method, where each device forms a 'link' in the chain, and the signal from one device carries down until being received at the destination.
The image overviews the daisy chaining approach:
With fiber, a break in one of the chain links results in multiple cameras going out. Unlike modern ethernet networks using 'Star' or 'Fully Connected' topology, fiber optic cable frequently is pulled in a single pair. Because a single pair of fiber can traditionally handle many times the bandwidth of copper pairs, less fiber is presumed needed, and the inevitable result is 'not enough fiber' available to connect multiple cameras.
The Daisy Chaining Necessity
While a single fiber pair may be able to handle the sum bandwidth of many cameras, each device needs its own connection into the network. Because running more fiber is costly or situationally restricted, surveillance system designers are frequently forced to run multiple cameras on a single pair of fiber. Additionally, the needed switch gear and terminations required for fiber can quickly grow into tens of thousands, even for a small surveillance system.
Ideal is Expensive
Running fiber using the same topology as copper is ideal in terms of reliability. Having a multichannel 'node' gather together several channels of fiber, with one pair to each camera, is the most dependable approach. If one segment fails, the others are unaffected. However, this approach is significantly more costly and requires much more fiber than a chained bus.
Alternatively, using a ROADM allows multiple fiber channels to combine into a single pair. However, aside from costing thousands, it requires delicate sizing and fine tuning wavelengths from each source beyond the skill of typical network installers. Unlike the 'plug & play' nature of ethernet cabling, fiber signals must be finely adjusted to avoid interference with each other in a ROADM, or significant image quality problems result.
The image below illustrates over $80,000 in fiber, equipment, and terminations for a ~35 camera system run using a ROADM 'Star' fiber channel topology:
Out of cost necessity (or design oversight) the minimum amount of fiber is run for connecting remote cameras. Frequently, applications that cover vast outdoor spaces (eg: Parks, Playgrounds, Parking lots, Municipal Systems) have only a single pair of fibers to work with.
The image below (repeated) details the least expensive and most common approach:
Schematically, "Daisy Chaining" requires only a single pair of fiber. Using a switch with SFP ports allows for fiber to be directly interfaced via GBIC card into copper ethernet. Likewise, at every camera location, the fiber connection is broken and bridged into a 'media converter', that allows an IP camera requiring electrical (copper) signals to use a optical (fiber) network.
Each Media Converter needs two fiber interfaces, one for 'upstream' traffic, and the other for 'downstream' connectivity. With 'daisy chaining', each subsequent link depends on the links behind it for backhaul; in the example above, if Segment 2 is broken, only one camera is affected. However, if Segment 1 goes down, both cameras, and any other devices in the chain are affected.
'Best Practice' should limit linking the minimum number cameras with daisy chained fiber, however the 'design maximum' is only limited by the media converter used and the type of fiber connecting them - sometimes thirty or more cameras.
Improving Reliability
The fragility of Daisy Chaining can be minimized with a few simple additions. If an additional fiber pair can be run to the 'last' link in the chain, 'Spanning Tree', or 'Rapid Spanning Tree' can often be implemented instead. The image below shows this adjustment:
The additional fiber pair forms a 'ring' of devices. Specifying 'IEEE 802.1D (STP)' support in the switch results in a degree of 'multidirectional' failover, should a link fail. Even if 'Segment 2' fails in the above system, the second camera still has a good backhaul connection to the man switch.