Q & A on Analog modems, voice circuits and T1 circuits Q: Is it true that a home modem is sending over twisted pairs of copper? A: Yes. When you use a home modem, you are sending over the standard telephone connection between your home and the telephone company. Connections to residences consist of twisted pairs of copper wire. Q: Do the other ends of the phone lines (connected to residences) connect to T1 lines. A: No. The phone line from your house connects to a telephone switch (e.g., ESS#5). T1 lines are digital circuits used to interconnect telephone switches. For example, if you live in a town with more than one telephone switching center, there may be ``digital trunks'' between the switching centers that use T1 technology. Q: Can a standard T1 line transport more than one phone call. A: Yes, T1 is designed to transport 24 simultaneous voice channels (the capacity is 1.544 Mbps). By the way, the second edition of the text describes both local loop and T1 connections. Q: Using TCP/IP over modems, how many Internet clients' communication can be sent over two twisted pairs of copper? A: Because TCP/IP uses packets, arbitrary numbers of client-server communications can pass over a single pair of copper wires. Of course, throughput slows down as the number increases. Thus, the practical answer is to take the throughput of the wires and modems, divide by the number of connections, and ask whether users will tolerate the slow speed or merely give up. Q: How many TCP/IP connections move across a T1 line? A: Again, an arbitrary number of connections are possible, but the apparent rate for one connection depends on the number of simultaneous active uses. In practical terms, medium-size corporations that have thousands of employees often choose a T1 line as the connection from the corporation to the Internet.