Fiber Optic Latency Calculator

Fiber latency can cause problem for networks where high availability and low latency are require. The latency that occurs for fiber optic cable is due to the signal travelling through the glass of the fiber optic cables at a slower rate than the speed of light. The latency for every kilometer of fiber optic cable is small, but the further the data signal have to go across a network, the more higher the latency that will be experienced.

The latency for fiber optic cables is important in networks where even a few milliseconds difference in latency can be the difference between a trade being executed or a video streaming service buffer. The speed of the signal through the fiber optic cable is dependent upon the refractive index of the glass of the fiber optic cable. The refractive index for standard single mode fiber optic cables is approximately 1.468, which means that the speed of light through the fiber is approximately 2/3 the speed of light in a vacuum.

What Causes Delay in Fiber Optic Networks

Multimode fiber optics is used within buildings and have a slightly slower rate of propagation for data signals than single mode fiber optics. Hollow core fiber optics technologies aim to create fiber optics where the refractive index is closer to 1, allowing light to travel closer to the speed of light in a vacuum. However, hollow core fiber optics are still relatively rare.

Since the refractive index of the fiber optic cable does not change with distance, you can calculate the latency of the fiber optic cable based off the type of fiber optic cable. All other latency will be added to this calculation. The routes of fiber optics are rarely laid out in perfectly straight lines.

Furthermore, the fiber optic cable often has to take turns around corners in buildings, and the fiber optic cable often has to go through cabinets where service providers lay there equipment. Additionally, there are slack loops at the splices of the fiber optics where technicians may need to make adjustments to the fiber optics. Additionally, there are also service coils that are installed around corners.

There is typically a three percent to five percent allowance for the length of fiber optics to be laid out in these situations. Thus, a distance of two kilometers may become 2.1 kilometer of actual fiber optics. This adjustment for slack is accounted for in the fiber optic latency calculator.

The calculator allow users to input the length of the route in kilometers, the percentage of slack to add to the route, and it calculates the propagation number for that route. Network equipment will also add to the latency of the fiber optic cable. Cut through switches can begin to forward data packets before the entire packet has entered the switch.

Cut through switches can only introduce delays of a few hundred nanoseconds. Store and forward switches will introduce delays of low microseconds as they must wait for the entire data packet to enter the switch. Furthermore, data routers will typically introduce additional latency to the network than switches do.

Equipment delays can be higher than fiber latency for small networks and links. For instance, a five kilometer link may take longer than a fifty kilometer link with only two network hop. Serialization will add to the latency of the network if the rate of the line is low or if the size of the data frame is large.

At a data rate of one gigabit, it can take more than twelve microseconds for a 1500 byte data packet to exit the port. At a rate of one hundred gigabits, the same data packet takes only a fraction of a microsecond to exit the port. The calculator accounts for frame size and line rate, and multiplies that value by the number of store and forward switches in the network.

Thus, serialization delays are factored into the total latency calculation. The round trip time for data is the one way time delay times two. Thus, it takes twice as long for a data packet to travel to the destination and for the reply to travel back to the originating device.

Most network monitoring tools will report the round trip time. However, it is beneficial to view the one way time delays to find the source of latency. If the propagation delays account for the majority of the latency for the network, then the solution is to reduce the length of the route or change the type of fiber optics.

However, if the majority of the delay is from the network equipment or serialization, then changes to the network will require the use of fewer network hops or faster network ports. The difference between these various forms of latency becomes critical when comparing two different networks. For instance, a regional backhaul network link may experience four milliseconds of fiber latency, but the use of coherent transponders at each end of the link can add two additional milliseconds of digital signal processing latency.

Furthermore, a link between two data centers might experience the same total latency but over a shorter distance of fiber optics. In both of these situations, if the latency for each of the components of the network link was not separate, it would appear that both links have the same latency with a latency calculator. The latency calculator is constructed in a way that allows the different components of network latency to remain visible for the user.

For instance, the users can easily make changes to the type of fiber optics, the number of network hops, or the line rate. Each of these changes can be viewed in how they alter the latency of the network. Thus, the latency calculator allow for a single number to become several different factors that relate to and can be adjusted during the planning of the network.