TDM access versus ATM access A clear benefit of TDM is its ability to carry any type of service. ATM offers the same service independence while also furnishing valuable data-link information such as cell counts and error counts, which provide knowledge about utilization and additional troubleshooting capabilities. TDM provides no visibility in the data-link layer. However, a more careful examination of most customers' requirements reveals a different story. The fact that most SNA traffic still traverses TDM facilities, not frame relay networks, underscores this reality.
Frame relay is actually very good at only one thing, connecting routers across a wide area network.
ISDN works well for voice and video, but it shows its limitations with data. With TDM T1 ATM's cell structure, then, becomes the best available solution. ATM cells' packet-like nature permits statistical multiplexing for efficient bandwidth utilization, while their fixed length forms the basis for service level guarantees. It is the combination of these effects that gives ATM its power. Ten years ago carriers could distinguish their TDM services by offering availability guarantees.