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Cable Modem System
Figure 1 is used to bring out points applicable to wireless. The client-server modem architecture provides the operator control of the modems (customers) from the head end.
Figure 1. Simplified System Diagram
Starting with the service provider, a connection to the Internet is required in addition to a number of local servers, such as e-mail and a caching server, to save frequently accessed pages instead of requesting them repeatedly over the network. Connected to this local-area network (LAN) is some form of downstream router. The digital output is converted by a 64–quadrature amplitude modulation (QAM) modulator which provides a 44-MHz intermediate frequency (IF) signal to the transmitter. This signal occupies a 6-MHz bandwidth. (The modulation type and channel size varies according to the vendor.) The customer receives the line-of-sight (LOS) signal, and the cable modem is connected by way of Ethernet to the computer or multiple computers on an Ethernet LAN. The cable modem return path in this diagram is over the dial-up telephone network to a modem bank and an upstream router. This return path carries both upstream requests and acknowledgments of the downstream packets. Connecting the return path directly to the provider's LAN in this way gives the fastest file transfer from the local servers. The modem banks could also be at a remote location connected by traditional digital facilities. (The modem banks could also be connected to another Internet access point. This makes no difference to the traffic from the Internet but would slow the transfer from the local servers due to Internet delay of ACKs.)
Each 6-MHz channel can support about 9000 subscribers. The number varies according to the type of traffic and the user. It can be increased by using directional instead of omnidirectional transmitter antennas so that different sectors are served by different transmitters.
Cable Modem
Wireless broadband modem service builds upon the simpler system architecture of wired cable service so it is helpful to first understand operation in a cable system with fewer variables. A modern CATV system has downstream channels from 50 MHz to 806 MHz. The cable modem tunes to all or some of these channels depending on its design. The signal level is nominally 0dBmV (1 millivolt in 75 ohms) at the interface to the customer and is stable, varying slowly with temperature and time. USA CATV channels are 6-MHz wide and with 6-MHz spacing. A 64–QAM modulator can fit 30–Mbps of data in a 6-MHz channel.
Figure 2. Simplified Block Diagram of a Cable Modem
A cable modem consists of a standard digitally controlled cable-ready television tuner, a 64–QAM demodulator and an Ethernet connection to the customer's computer. The return path from the computer carries ACK messages or upstream file transfers such as e-mail. The upstream connection may use an RS232 connection to a telephone modem or a modulated upstream signal such as quaternary phase shift keying (QPSK) for return over the cable system. An internal processor with memory controls the tuner and the return path as well as more complex functions such as downstream packet filtering.
There are some points to note as regards modem tuning:
- The ideal modem operates both for wired CATV and wireless, although wireless has some different requirements due to propagation and the manner in which the signal is received.
- A standard digitally controlled TV tuner has fixed tuning steps of 62.5 kHz.
- The cable-modem vendor makes a modem that tunes only to fixed frequencies. Cable modems use discrete digital, not analog tuning. Many modems tune in 6-MHz steps, which is not sufficient for all wireless bands.
- The 64–QAM demodulator also drives the tuner. The demodulator has to be able to correct frequency errors that occur in the television tuner/downconverter and the received signal. The input frequency stability is normally in the range of ±50 kHz.
