A T1 connection is usually a direct fiber optic connection from your internet service provider to your office or business. A T1 connection can also alternatively be served over a regular copper line in some cases. Speeds of a T1 are synchronous, both download and upload speeds are 1.536 Mbps, faster than many cable and DSL connections.
T1 connections are very reliable and have a service level agreement that states if there is any amount of downtime, you will get refunded some of the monthly price. Unlike cable and DSL, a T1 connection is not shared between multiple subscribers. With this in mind, speeds are usually constant and do not fluctuate, even when shared by a couple of hundred people web browsing in an office.
The upload speeds of DSL and cable cannot compare to a T1's, some only offer upload speeds of 512 kbps, three times less than a T1 connection. A T1 has a direct line to the ISP, skipping over other slowdowns to the central office.
Some features that one should look for in a T1 CSU/DSU card are power saving features, high reliability, and support for multiple transport protocols. A couple reliable manufacturers of T1 cards are Cisco, Linksys, and Samsung. T1 cards should also have ways of easily diagnosing the problem in order to save time, this can be done through LEDs that light up an indicator.
Another feature that is useful in a T1 card. would be easy management, through a shell or command line interface. The type of T1 CSU/DSU card you choose for your T1 router can be a major determining factor in the speed and consistency of your connection.
A T1 connection with a good T1 card is more reliable and faster than a cable or DSL connection. If you own a business and have high connectivity needs, a T1 will support hundreds of users at one time, with no congestion in the network at all.