Integrated Services Digital Network
A set of communications standards allowing a single wire or optical fibre to carry voice, digital network services and video. ISDN is intended to eventually replace the plain old telephone system. There are different kinds of ISDN connection of varying bandwidth:

DS0 = 1 channel PCM at 64 kbps
T1 or DS1 = 24 channels PCM at 1.54 Mbps
T1C or DS1C = 48 channels PCM at 3.15 Mbps
T2 or DS2 = 96 channels PCM at 6.31 Mbps
T3 or DS3 = 672 channels PCM at 44.736 Mbps
T4 or DS4 = 4032 channels PCM at 274.1 Mbps