 may also be made up of multiple circuits temporarily connected for the duration of a con- versation or transmission. One of the challenges is to make the distant locations as accessible as if they were in the same office next door. The information being transmitted may be a voice, written information, still pictures, moving pictures (video) or some combination of these. Here are some of the things to think about, relating to tele- communications transmission. Transmission Medium First is the transmission medium. The communications sig- nal is a low-voltage electrical signal that must have some means of getting from one point to another. The medium may be a physi- cal connection such as a cable. There are different types of cable made out of different materials, but most common is copper (twisted pair and coax) or glass (fiber optic cable). A cable can be used to connect devices several feet from each other. Cable is also run on the ocean floor to connect the continents. The second most commonly used medium is space. Commu- nications signals sent via microwave or satellite are transmitted through the air or through space outside the atmosphere. Water is also a communications medium, such as with sub- marine-to-submarine communications, but it is not used to carry your average business communications signals. The Telecom Handbook 84 235 TRANSMISSION Typically telecommunications signals either travel through a cable or through space. The ones that go through space have of- ten started out in a cable and may end up in another cable before they reach their final destination. As wireless communication becomes more widespread, the need for cable is diminishing, but it is not likely that it will go away. All of this cable running through the office, under the street and around the Earth is useless as a transmission medium for communication signals unless there is some hardware at both ends of the circuit and more in the middle. Synchronous and Asynchronous Transmission At each end of the circuit there must be some type 