r Distribution Coaxial cable or optical Coaxial fibre cable Figure 9.17 Cable TV architecture Frequency 200 kHz 6 MHz to 6.8 MHz (8 MHz) 0 to 48 MHz 50 MHz to 1 GHz or higher Figure 9.18 Cable TV frequency allocation Broadband Telecommunications Technologies and Management Distribution Cable Cable headend modems Figure 9.19 Cable TV frequency allocation the data. In the upstream, packets are scheduled by the cable headend and are transmitted in a time division multiple access fashion (Figure 9.19). Downstream and upstream transmissions use quadrature phase shift keying (QPSK), 8- QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM and in the latest release of the standard 4096-QAM modulation techniques. The maximum transmission rates can be calcu- lated from the available bandwidth and the modulation technique. Example 9.2 Calculate the downstream transmission rate for a North American cable system which uses 256-QAM modulation. (Note this is not Shannon capacity.) Answer Bandwidth = 6 MHz, supporting a 5.36 mega symbol/s transmission rate. Modulation = 256-QAM or 8 bits/symbol Transmission rate = 8 X 5.35 = 42.88 Mbps. (This is the channel level transmission rate. If overheads are considered, the data transmission rate is reduced to 38 Mbps.) The DOCSIS 3.1 standard release specifies usage of up to 24 channels in the downstream and 8 channels in the upstream, yielding 1029.12 Mbps and 245.76 Mbps transmission rates, respectively, with a 256-QAM modulation. Case Study 9.2: Comcast Comcast is the largest cable broadband operator in the United States by coverage area, offering a Content and services layer range of connectivity plans [13]. The entry of Comcast into the broadband market dates to 1996 Connectivity retail layer when @Home its joint venture with several other cable TV companies offered home internet and IP telephony services. However, the joint venture Infrastructure layer did not fare well, and was finally taken over by Comcast in 2002. Meanwhile Comcast started an independent ISP which has grown to serve so