Study on feasibility of SATCOM for railway communication

Final Report
Table 55: Inmarsat 4 / Inmarsat 6 assessment
C.3.1
C.3.1.1
Additional information useful for the assessment
Delay study
Despite Inmarsat 4 is a narrowband system, it can achieve a throughput of 492 kbps. In this example, we assume a transmission data rate in the return
link of 64 kbps, a rate more appropriate to a narrowband system. To calculate the transmission time the following approximation is done:
Where,
T tx is the transmission time for a given packet
T up is the propagation delay from ground to the overhead satellite
T down is the propagation time from the satellite to the ground
In order to provide an example, following data is assumed:
Data packet: 120 bytes
Transmission in return link: 64 kbps
The end-to-end transmission packet delay in return link is:
T tx = packet_size / throughput = 120 bytes / 64000 bps = 15 ms
T up = T down = sat_distance / speed = 36000 km / 300000 km/sec = 120 ms
Doc.Nº: SRAIL-FNR-010-IND
Edit./Rev.: 1/1
Date: 16/02/2017
Page 237 of 285