a digital signal enters a communications channel in a format that is properly shaped into square pulses and precisely timed. In addition, it manages other OSI layer-one signal characteristics such as longitudinal balance, voltage isolation, and equalization. Nearly all CSUs allow the carrier to test the line by performing a loopback test. The customer is always responsible for buying, installing and maintaining both the CSU and the DSU (or a combined version of the two). Carrier Gateways Each carrier structures the signaling and management portion of its digital networks a little differently. These networks, after all, were developed during a time when competition was fierce. Carriers did not exchange network information because it was considered proprietary. The legacy of the 1980s lives on today, when it comes to digital network interconnection. If dedicated access service will be used for video communication, bear this in mind: While the IXCs' analog networks are universally interconnected, their digital networks are not. This presents a problem when two different companies want to set up a videoconference when both use dedicated digital access, but each uses a different IXC. Video Communications: The Whole Picture Placing a call through a carrier gateway is still the most reliable way to bridge between carrier's digital networks when dissimilar dedicated access causes connection problems. The cost for using these gateways varies widely, based on speed, location, type of service, and carriers. All the major long distance carriers offer these gateways (AT&T, MCI WorldCom). Some gateways are one-way. In other words, carrier A's customers may be able to gateway to the network of carrier B, but customers of the carrier B may not be able to gateway to the network of carrier A. When a customer of carrier B needs a videoconference with a customer of carrier A, he must arrange for that party to initiate the call. Since the customer of carrier B initiated the call, they are billed for it. Moreover, carrier A's customer