Twenty-first century transport: SNG over IP 223 using GPRS or 3G connectivity. `Wi-Fi' is capable of connecting a PC to a wireless router up to 100 m away at a theoretical speed of up to 108 Mbps, and is now a mature technology in its second generation. Finally, at the time of writing, `WiMAX' is a new technology that is designed for a range of 3­50 km and with a connection speed of up to 72 Mbps. In remote places, the connection may be via a small fixed satellite terminal called a VSAT (Very Small Aperture Terminal), and this is typically at connection speeds of up to 1 Mbps, although higher speeds are possible. Incidentally, an SNG uplink can be considered as a very rugged and portable type of VSAT (as long as it used in a bi-directional mode with a return path from the satellite and receive equipment). Inmarsat satellite phones can also provide connectivity to the IP network (Figure 5.2). In order to simplify our discussion, Figure 5.3 shows a `cake' with four different layers arranged in hier- archical fashion (there are in fact seven layers, but they are combined into four in this simplified version). Cake layer 1: the Ethernet At the bottom is the Ethernet, which is the physical data link, i.e. the mix of cable, fiber optic, wireless, satellite link and all the devices that connect these together that forms the transport medium ­ the phys- ical highway. Cake layer 2: IP The next layer is the IP layer, which forms the key element of the network. It is all very well having a number of devices connected together, but if the devices cannot be addressed, it is useless as a network ­ so this provides the road numbering, destination naming and overall map of the highway system.