ation system, installed at the HEs (see Figure 6.23). The 54- to 550-MHz frequency band is allocated for broadcast TV channels of 6 MHz each [6]. One or more of these 6-MHz channels is reserved for downstream data and voice. Upstream data carrying data or Introduction to Telecommunications Network Engineering Fiber feeder Fiber Fiber Satellite Coaxial distribution station Unidirectional amplifier Local channels, Fiber feeder news and commercials Fiber Fiber Drop point Figure 6.22 Traditional cable TV plant. Customer Satellite Internet premises station Twisted pairs Cable Servers gateway router Coax. modem Head- LAN switch demodulator modulator Bidirectional conversion Fiber Downstream amplifiers TV program traffic Combiner Upstream traffic Fiber feeder Local channels Drop point Figure 6.23 Cable TV plant modified for cable modem service. voice use 6-MHz channels in the 5- to 42-MHz frequency range. Major modifications are required in the network to carry upstream traffic. First, strands of optical fiber must be allocated for upstream signals. The HE has to be equipped with a modulator and combiner for downstream and receiver Data Communications and demodulator for upstream signals as shown in Figure 6.23. Second, fiber nodes and coaxial cable amplifiers have to be changed to bidirectional devices. A customer premises network interface unit (NIU) splits up voice/data signals and TV broadcast channels. Data between the LAN switch at the HE and the cable modem at the customer premises are transmitted in standard 10BaseT/Ethernet frames. In the downlink direction 64 QAM or 256 QAM with 6-bit or 8-bit symbols, respectively, is used and data rates around 30 or 40 Mbps are achieved through each 6-MHz downlink channel. Note that all users of the channel share this capacity. Uplink frequency band is noisier because of the branching structure, which adds noise from all branches, when you approach the HE. In the uplink direction robust modulation scheme QPSK is used, restricting the total data rate via one 6-MHz channel to 