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Purpose driven life pdf download. There’s one last type of wireless network we need to discuss, and it’s one with which you’re probably intimately and constantly familiar. I’m talking about the ubiquitous wireless network employed by the humble cell phone—or, more common today, the high-tech cellular-data network used by iPhones and other smartphones to connect not only to each other, but also to the Internet.
How does cellular networking work—and what does it have to do with the other wireless networking you employ on a daily basis? Good questions, and two of many that are answered in this chapter.
Cellular phones work much the same way as do the other wireless devices we’ve been discussing. Signals carrying voice, text, and digital data are transmitted via radio waves from one device to another. In the case of cellular networks, the data is transmitted not to a central hub in a small network of devices (as it is with Wi-Fi) or even directly from device to device (as it is with Bluetooth), but through a global network of transmitters and receivers.
What’s interesting about mobile phone networks is their cellular design. (Hence the terms “cellular network” and “cellular phone.”) By that, I mean that a mobile phone network is divided into thousands of overlapping geographic areas, or cells. A typical cellular network can be envisioned as a mesh of hexagonal cells, as shown in Figure 4.1, each with its own base station at the center. The cells slightly overlap at the edges to ensure that users always remain within range of a base station. (You don’t want a dropped call when you’re driving between base stations.)
Figure 4.1. Cells in a cellular network.
The base station at the center of each group of cells functions as the hub for those cells—not of the entire network, but of that individual piece of the network. RF signals are transmitted by an individual phone and received by the base station, where they are then re-transmitted from the base station to another mobile phone. Transmitting and receiving are done over two slightly different frequencies.
Base stations are connected to one another via central switching centers which track calls and transfer them from one base station to another as callers move between cells; the handoff is (ideally) seamless and unnoticeable. Each base station is also connected to the main telephone network, and can thus relay mobile calls to landline phones.
All this transmission within a cellular network originates with the handheld cell phone. A mobile phone is actually a two-way radio, containing both a low-power transmitter (to transmit data) and a receiver (to receive data).
When I say low power, I mean low power—really low power. The typical cell phone includes a dual-strength transmitter, capable of transmitting either 0.6-watt or 3-watt signals. In comparison, a larger AM radio station will typically broadcast a 50,000-watt signal; even smaller AM stations broadcast 5,000-watt signals. A cell phone’s 3-watt signal is puny in comparison.
The reason mobile phones can get by with such low-power transmitters is that they’re transmitting within a relatively limited range—within the current network cell. It’s not necessary or desirable for a phone’s signal to extend beyond the current cell; this way, the same broadcast frequencies can be used by multiple cells without danger of interference.