I dont know where to ask this question. So i think its best suited for networking community.

As fiber optic transmit data using light. So we are getting/sending data at the speed of light ??

I did some simple distance and speed calculations but whatever the pings that we get does not add up to the pure calculations.

So, what are the hurdles ? What slows down Internet ?

  • 2
    Yes, the pulses of light travel (unsurprisingly) at the speed of light. But the rate at which those pulses are emitted from the transceiver is the bottleneck. Commented Nov 12, 2019 at 2:36
  • 5
    The speed of light in fiber optics is around a third slower than light in a vacuum.
    – Ron Trunk
    Commented Nov 12, 2019 at 2:45

4 Answers 4


From Wikipedia:

A typical singlemode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.444 at 1500 nm, and a core of doped silica with an index around 1.4475.[48] The larger the index of refraction, the slower light travels in that medium. From this information, a simple rule of thumb is that a signal using optical fiber for communication will travel at around 200,000 kilometers per second. To put it another way, the signal will take 5 milliseconds to travel 1,000 kilometers in fiber.

By the way, pings are not a reliable way to measure latency. The process on a host that responds to echo requests is usually a low priority process, so there can be significant delays on a busy device.

  • I have toyed around with the idea of creating an ultra-high priority process to respond to a special test protocol for a custom ping application. It, unfortunately and ironically,, is a low priority for me, and other stuff keeps getting in the way, so it times out. :)
    – Ron Maupin
    Commented Nov 12, 2019 at 3:00
  • That's what loopback plugs are for.
    – Ricky
    Commented Nov 12, 2019 at 4:02

As fiber optic transmit data using light. So we are getting/sending data at the speed of light ??

The speed of light in fiber is not the same as the speed of light in free space. Typically the speed of light in fiber is about two thirds of the speed of light in free space.

So, what are the hurdles ? What slows down Internet ?

Many many things, but two main ones.

Firstly your traffic rarely goes in a straight line from A to B, networks operate in a limited number of locations, interconnected by a limited number of paths, furthermore those paths themselves. The paths themselves are also likely to not be straight. In general Internet providers route for cost and bandwidth, not for minimum latency.

Secondly your packet may spend some time waiting in queues to be forwarded or processed.

There is also the time to actually serialise the packet onto the fiber, but at modern network speeds this is fairly small. At 1 gigabit per second a max-size packet is about 2.4km long, at 10 gigabit per second this drops to about 240 meters long.


You seem to be confusing some things.

Yes, the light in a fiber optic cable travels at the speed of light in the medium (glass fiber), which is slower than the speed of light in a vacuum.

Where I think you are confused is that a sending device will only serialize data onto the fiber at a given rate. A 1 Gbps interface will serialize 1,000,000,000 bits of information in one second, so the first bits are arriving at the destination while the last bits have not yet been serialized on the fiber.

When sending data on a network, the data must be framed so that the receiver knows where the data start, and where they end. The data must then go up through the network stack at the receiver, be processed, and a reply generated and sent back in the same way the data were sent in the first place. All that adds a lot of time when comparing the latency of a ping request/reply.

The request data end up going through two network stacks and the length of the cable, being serialized/deserialized at whatever speed the interface does, and travel the length of the fiber. The request is processed, a reply is generated, and it must do the same thing the request did. The reply is processed, and the time difference noted. The speed of light plays only a very small part in all that.

Also, using copper cabling will result in basically the same time as fiber optic cabling with the same serialization rate because electrons move through the copper at the speed of light in the medium, and that is very close to the speed of light in a glass fiber.


The speed that the packets/frames/whateaver travels on the fiber optics cable its about 68% of the speed of light on the vacuum, do you calculations again with this number. Here follows a link for the presentation of Tiago Setti a specialist in fiber optics and dwdm: https://www.youtube.com/watch?v=95iZIoOcb7g. In that presentation he clarified a lot of things about fiber optics. Hop that helps.

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