mines the received SNR and therefore the channel capacity and transmission rate. A typical VSAT terminal is illustrated in Figure 11.8. Figure 11.8 A typical VSAT terminal antenna connection. Source: Axlsite (own work) CC BY-SA 3.0 (https://commons.wikimedia.org/wiki/File%3ATooway_satellite_antenna_drawing.png) Satellite Communications Very Small Aperture Terminal Multiple Access Methods Multiple users access the resources of a satellite though multiple access protocols. These function typically belong to layers 2 and 3 of the OSI model, or the media access control (MAC) and logical link control (LLC) layers. These specifications determine how the limited resources of a satellite (i.e. the available spectrum) may be simultaneously used by many VSAT users. VSAT architecture as shown in Figure 11.9 requires a central hub which deter- mines which users transmits to the satellite and at what time. The process requires a user to first send a request to the satellite when it needs to transmit data in the uplink. In response the satellite hub allocates resources in the time and frequency domains SO this transmission will not collide with other users' traffic. Resource allocation, or channel multiplexing, may be done in a number of ways, but it generally follows four domains: frequency, time, space and code. The first two are discussed below, space multiplexing was shown in Figure 11.4, and code multiplexing will be described in the next chapter as it is more commonly used in terres- trial mobile telecommunications. Frequency Division Multiple Access In Frequency Division Multiple Access (FDMA), a portion or the whole of a transponder bandwidth (Figure 11.6) is allocated to a VSAT subscriber to use. This set-up is useful for major hubs which need constant connectivity to a network, and which themselves divide this connectivity among a number of local users (this may be a wholesale buyer who provides services to its retail subscribers.) Internet access Internet Internet access with VolP Figure 11.9 VSAT system architectur