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:
    from reference

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
Network Latency

The total delay experienced by a frame in the course of its transmission between two points in a network, measured from the time that the first bit in the frame passes the first point (P1) to the time that the last bit in the frame passes the second point (P2). Refer to §3.120 of IEEE 802.1Q-2022.[2]

1.2. Assumptions

  • Assume that the network switch on a Starlink satellite operates in cut-through mode, meaning that the switch buffers the frame for 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 802.11 WiFi datagram, the unit would record/buffer the first 10 octets (80 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.

  • Find the average orbital altitude h of the operational Starlink satellites.

2. References

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

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

https://findstarlink.com/

https://satellitetracker3d.com/track

1. Clearly understand the difference between propagation delay and latency.
2. From https://www.ieee802.org/3/dm/public/adhoc/082224/Latency%20&%20delay%20definition%20802.3dm.pdf