Starlink can have a lower network latency than a fiber optic link. [1]

1. Data drag race

starlink fiber
Figure 1. Starlink and terrestrial fiber paths

Compare the delay experienced by a network datagram sent over a terrestrial fiber optic link versus a one sent via a 2-satellite hop.

This problem setup is very similar to using free-space communications (through air: laser or microwave) versus fiber without the space component. The only material difference is that a terrestrial link’s antenna is on a tower, and therefore the maximum distance between points is limited by the tower height. Think of this Starlink setup as really tall antenna towers.

1.1. Information

The line-of-sight distance from a point h above the surface (e.g. a tall tower or orbiting satellite) to the horizon is given by:

\[d_{\mathrm{los}} = \sqrt{2 R_E h}\]

Remember that the distance between two towers is double this distance — the wave just grazes the surface at the path’s midpoint.

  • Look up the typical propagation velocity of light in single-mode fiber.

Table 1. Frame for IEEE 802.11 (WiFi) datagrams
Field Control Duration Dest. Address 1 …​

length (octets)

2

2

6
Table 2. Frame for IEEE 802.3 (Ethernet) datagrams (Physical layer 1)
Field Preamble Start Dest. Address …​

length (octets)

7

1

6

1.2. Assumptions

  • Assume that the network switch on a Starlink satellite operates in cut-through mode, meaning that the switch buffers the frame by only the time needed to determine the destination address of the datagram and immediately begins re-transmitting the frame (before receiving the rest of the datagram’s content). For example, for an Ethernet datagram, the unit would record/buffer the first 14 octets (112 bits) before it starts the output transmission.

  • Assume the bit rate of the link is 10 Gb/s (10×109 bit/s).

1.3. Questions

  • Compute the delay for an arbitrary fiber distance df (use km units).

  • Compute the latency of an IEEE 802.3 Ethernet frame with a payload size of the MTU of 1500 bytes for arbitrary distances (e.g. df).

  • At what distance does the end-to-end delay of fiber equal the delay of two cut-through switches but transmission in air/vacuum?

  • A point-to-point microwave link can have a switch delay approaching zero (since there need not be a network switch to route the datagram to the appropriate output port). What is the delay of a p-p microwave link compared to a direct fiber link?

  • Compute the maximum total (source → starkinkA → space → starlinkB → destination) distance if the Starlinks are orbiting at an altitude of h km.

  • Find the combinations or joint ranges of h and df where sending an 802.3 Ethernet frame with a 1500 byte payload over Starlink has a lower latency than when sent over fiber.

2. References

https://en.wikipedia.org/wiki/Starlink

https://celestrak.org/NORAD/elements/table.php?GROUP=starlink&FORMAT=tle — Current orbital information for all Starlink satellites.

https://starlink.sx/ — A neat realtime plot of the Starlink system’s satellites and ground stations.

1. Clearly understand the difference between propagation delay and latency.