Some of them are very familiar. Others are part of emerging technology that promises high-speed, accurate data transfer at relatively low cost. A very simple WAN device is a modem on a telephone line. By connecting a computer to another computer, via the Public Switched Telephone Network, we can create a very simple WAN that meets the criteria of the definitions we laid out earlier.
Modems are far from being obsolete in the development of wide-area networks.
Modems do not usually require significant cost in terms of initial outlay or maintenance. Unfortunately, modems are relatively slow devices. They're not particularly good at transmitting large quantities of data, given their speed limitations. Worse, modems are not very efficient at transmitting data. One whole phone line is used per connection, whether the modem is transmitting a steady data stream or not.