The first edition of APUE included a chapter on modem communication. Because very few applications today communicate directly with a modem, I removed this chapter. After adding three new chapters and entirely rewriting one example chapter, the book was pushing almost 1000 pages, so this seemed like a reasonable way to prevent the book from getting too big. So that the information wasn't lost, I updated it and include it here as a stand-alone chapter. You can download it as a PostScript file, or as a PDF file.