of tele- communications device to send a signal and another one at the other end to receive it. This may be a telephone, a fax machine, a terminal or computer (PC) with a modem, a multiplexer or a video transmission device. It is becoming increasingly common to have a personal com- puter as the device at the end of the communications circuit. Terminals with no built in intelligence, sometimes known as dumb terminals used to be the most prevalent type. They were connected to a central mainframe computer where both the intel- ligence and information resided. The characters traveled to the mainframe individually as they were typed in by the user. This type of transmission is called asynchronous transmission. The signals are sent over the line in a random fashion with added signals called start bits and stop bits. These start and stop bits tell the device at the distant end when to start and stop sampling the data coming across the circuit. The information is transmit- ted in digital form (different combinations of ones and zeroes representing all letters, numbers and symbols). Teletype and TWX machines were examples of dumb terminals. They are still in use in some countries. There is also synchronous transmission. In this case, digital information is sent between two devices that are in a specific time relationship, synchronized by a master clock. This is 236 & The Telecom Handbook CHAPTER 11 typically used between mainframe computers. It enables blocks of information to be sent, rather than individual characters one at a time. While some dumb terminals are still in use, most have long ago been replaced by PCs (personal computers). PCs communi- cating with each other or with mainframes use an asynchronous type of transmission. The terms synchronous and asynchronous transmission are becoming somewhat dated. Instead, one refers to a particular tech- nology such as ATM or to a protocol such as TCP/IP to describe the how the digital signals are being sent over the circuit. ATM or Asynchronous Transfer Mode is a very hig