Digital Circuit

The ISDN has a bandwidth of 128Kb/s
T1 has a bandwidth of 1500Kb/s

ISDN is basically 2 phone lines used in tadem to increase internet connection speed. The ISDN still dials in to connect to the ISP. The phone lines operate up to at a speed of 56KB (actually no more than 53KB) and with 2 tied together the speed is doubled. The speed 128KB is rarely actually achieved since the phone lines have issues with noise and other problems.

T1 lines are simular in that lines are also tied together but at a speed of 64KB. T1 service comes with either 12 and up to 24 phone lines connected together. Often with T1 service some of the lines are used for phones and the remaining lines are used for internet access. Newer technology allows for the unused phonelines to be reallocated to the internet connection. Sometimes customers are disapointed because if they have most of the lines used up for phones they really don't get that fast of a internet connection.

Hope this helps