Follow the signal through the network

An uplink travels from a ground station to a spacecraft; a downlink travels from the spacecraft toward the ground. A communications payload receives and relays information, while the terminals must support the frequencies and signal format used by the particular service. Merely pointing a dish at a satellite is not the entire compatibility check.

Anik generations changed the available communications capabilities. CSA’s chronology describes Anik B in 1978 combining conventional communications with higher-frequency work following the separate Hermes experiment. A family name therefore does not guarantee that every generation supports the same terminal or frequency arrangement.

Keep a service specification attached to its spacecraft and period. An older overview may show a predicted service life or label a satellite active. That is evidence of what the page said at its date; it is not a guarantee of today’s capacity or availability.

For an original simplified network, assume three serial links can sustainably carry 18, 12 and 8 megabits per second. A continuous end-to-end transfer cannot exceed the slowest of these links: 8 megabits per second. Adding the numbers to get 38 would confuse serial stages with parallel paths.

Now send an invented 800-megabit file through this route. Ignoring startup delay, overhead, retransmissions and sharing, 800 / 8 = 100 seconds is the idealized transfer time. The calculation uses bits in both numerator and rate; substituting a file size in bytes without conversion would change the answer.

Improving the 18-megabit link alone does not remove the 8-megabit bottleneck. This is why a satellite capacity upgrade need not produce the same gain at every endpoint. The local distribution path, shared resources and actual traffic conditions also affect delivered performance.

A serial route is limited by its narrowest link

Coverage asks whether a location can be reached under the stated system conditions. Capacity asks how much traffic the service can support. Availability asks whether the needed parts are functioning when the user needs them. These properties are related, but none is a substitute for all the others.

In a fictional allocation, four users share a total 20-megabit-per-second link equally and simultaneously. Each would receive 5 megabits per second in the simplified model. If one user becomes idle, a particular scheduler might redistribute capacity, but that requires a stated scheduling rule rather than an assumption.

A useful acceptance test therefore checks the intended user route and workload. A successful command link to the spacecraft establishes a different capability from a working customer data path. Distinguishing those paths makes fault reports and service claims more informative.