6.4 Satellite Delay Analysis
Satellite delay, in the tactical GIG, is a primary challenge that applications and transport layer protocol developers face. We will analyze this satellite delay in order to better grasp its effects on networks. For geostationary orbit satellites (which are used for communications), the propagation delay from the Earth terminal to the satellite could be as long as 120 ms. For a packet to be transmitted to the satellite and back to the Earth terminal, a delay of 240 ms is expected. If we add the queuing delay from the NCW TDMA architecture, atop the propagation delay, we can expect a total satellite delay of nearly one second.
Example: TDMA over NCW
Let us assume that the frame period in TDMA, used with NCW, is 400 ms. Let us also assume that a large network is formed, such that the NC assigns most NCW nodes a single timeslot per frame. The nodes (assigned one timeslot per frame) will transmit for 1.5625 ms every 400 ms (we assume that 400 ms can be divided into up to 256 timeslots, since NCW can accommodate up to 255 nodes in a single network).22 Figure 6.20
a shows how a TDMA frame time of T
= 400 ms results in the node having an active transmission period of t
= 1.5625 ms and an idle period of T
= 398.4375 ms. The active period is when the node transmits at the full transmission capacity of the NCW uplink subnet and the inactive transmission is when the node does not transmit.
Figure 6.20 Delay characteristics of a single NCW link.
If we ...