Web page addresses, while transmission in the downstream direc- tion consists primarily of Web pages. Because the number of bytes required to transport graphical images and text on a Web page consid- erably exceeds the number of bytes in a URL, bandwidth for the down- stream data channel considerably exceeds the bandwidth allocated for the upstream channel. This results in a higher data transmission rate being obtainable on the downstream channel, which should satisfy the operational requirements of a vast majority of Web surfers. If we turn our focus back to Figure 7.3, we will note that the T1 mul- tiplexer uses a fixed format for allocating digitized voice channels onto a T1 line. That allocation results in twenty-four 64-kbit/s time slots becoming available for use. In this modern era of broadband communi- cations where a single DSL-capable subscriber line can use all or most Figure 7.4 ADSL frequency allo- cation. upstream downstream voice channel channel Frequency kHz Chapter Seven of the capacity of a T1 or E1 transmission line, the use of a DLC RT is insufficient and represents a bottleneck. Thus, the existing infrastruc- ture of local telephone companies must be rebuilt to support broad- band communications, a topic that we will focus on later in this chap- ter. Now that we have an appreciation for the local telephone company infrastructure that provides connectivity to subscribers via local loops, let's turn our attention to the cable television (CATV) infrastructure. The Original Cable TV Infrastructure In this section we discuss the original cable TV infrastructure, which remained essentially unchanged for almost 40 years. We will also learn how information flows on the original cable TV network infrastructure and why that infrastructure requires modification to support modern two-way communications used by cable modems. Overview Cable TV was originally developed as a mechanism to provide unidirec- tional video signals to subscribers. To accomplish this task, the wiring infrastructure was designed in 