me X 8000 frames/s, or 1.544 Mbits/s. In an era of increasing use of broadband communications, such as digital subscriber lines (DSLs), the use of T1 and E1 circuits to inter- connect a central office to a DLC RT located within a neighborhood represents a problem. That problem is one of capacity. A DSL typically provides a transmission rate of a minimum of 640 kbits/s in the downstream direction from the telephone company central office to the subscriber, while transmission in the uplink direction from the subscriber to the telephone company normally occurs at a minimum rate of 160 kbits/s. To obtain this transmission capacity, a DSL modem modulates data at frequencies beyond the 0- to 4-kHz range used by human voice. Today telephone companies support over 10 types of DSL products, with most operating by splitting the bandwidth of the twisted-pai wire into two distinct channels beyond the 0 to 4 kHz used to transport Fiber in the Neighborhood human voice. Figure 7.4 illustrates the general frequency allocation on the local loop to the subscriber for one type of DSL service referred to as asymmetrical digital subscriber line (ADSL). The name of this DSL ver- sion indicates one key characteristic of its operation, namely, that the data transfer is asymmetrical. In examining Figure 7.4, note that when ADSL operates on a sub- scriber line, there are actually three channels formed by frequency. One channel, which represents approximately 4 kHz of bandwidth, is allocat- ed for the existing voice telephone operations. A second segment of fre- quency, which begins at approximately 30 kHz, is allocated to an upstream data channel, while a third segment of frequency is allocated to a downstream data channel. The third segment of frequency is much wider than the second segment to provide a downstream operating rate that is higher than the upstream operating rate. The reason for this asymmetrical design is that for Web surfing, transmission upstream is primarily in the form of relatively short Uniform Resource Locator (URL) 