Another metallic transmission medium which offers a large bandwidth, is the coaxial cable. Such cables consist of an outer protective shell made of copper tube, less than one inch in diameter, which actually carries messages. There are two types of coaxial cables- the base band and the broad band. Base band cables use digital transmission and are suitable for fairly short distances. These have a data rate of about 50 Mb/s in half-duplex mode. Broad band coaxial cable on the other hand, is more familiar as CATV and is suitable for long distance (up to 15 KM) transmission. It can support data rates of over 100 Mb/s. Coaxial cables, with their better electrical properties, can transmit at much higher frequencies and support wider bandwidth than the twisted pairs. They are suitable, therefore, for high quality bulk transmission of data at high speeds as well as text, picture and voice. For example, one such cable can support the equivalent of up to 10800 telephone (voice) channels.