Transmission mode Transmits bursts in time slots (TDM) with Continuous stream of data received reserved and contention time slots by all modems Table 17.2 relates DOCSIS 2.0 to the seven-layer OSI reference model. The physical plant itself is often called "layer 0." Original CATV networks were entirely based on 75-S2 coaxial cable signal delivery systems. Once fiber-optic cable technology matured, one excellent application was to use fiber-optic cable to bring the CATV signal spectrum from the headend to the first node by fiber optics. A number of advantages derived from this conversion. Among these are: Improved signal quality at the end-user because there are less coaxial cable amplifiers in tandem Improved reliability (availability) Extended coverage area The CATV industry uses the abbreviation "CMTS" for cable modem termination system, which is housed in the CATV headend and CM (cable modem) for the interface device that resides in the customer end of the system. 17.7.2 Layer 1 - Physical Layer The physical layer specifies the interfaces to the transmission medium and describes the methods of signal transmission. In the case of DOCSIS, there are separate standards for the upstream portion and the downstream portion of the system. See Table 17.2. DOCSIS and especially DOCSIS 2.0 have a very large information transmission capac- ity. In the upstream direction two different multi-access protocol formats are specified: Time Division Multiple Access (TDMA) and Synchronous Code Division Multiple Access (S-CDMA). Under the DOCSIS 2.0 TDMA protocol, the maximum RF bandwidth was increased to 6.4 MHz, and three new higher-order modulation techniques were included. These are 16-QAM, 32-QAM, and 64-QAM, resulting in theoretical bit packing efficiency of 4 bits/Hz, 5 bits/Hz, and 6 bits/Hz, respectively. Figure 17.18 is a typical HFC network upgraded with DOCSIS. The network is a point- to-multipoint, tree-branch access network in the downstream direction and is a multipoint- to-point bus access network in the upstream di