Transmission can go as high as Gigabits per second and have two or more conversations happening at once. T1 have capable speeds of 1536 Kbps due to the 24 channels multiplexed into 64K bps channels. Synchronous and asynchronous are two types of time division multiplexing. Synchronous multiplexing has a system where a mechanism determines where each bit belongs whereas asynchronous has information in packets that have a label as to where to send the data. One data bit has to be used to determine the end and beginning of each packet.
Multiplexing is used for data, voice and a combination of both.
It can be transmitted over fiber and is better for longer runs to eliminate noise and repeaters on the line. 672 channels can be achieved with a T3, which is equivalent to 28 T1s. In the CO either one or two interface cards are used to receive the 24 individual telephone circuits. The T1 cards have the ability to convert the analog to digital with the 8.000 per second sampling required to fit the 8 bit compressed sample. After it is converted to digital and it is transmitted it has to be converted back to analog in order for the DTMF signaling to be effective.