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What types of High Speed Facilities do the US use?
Article Id #: 273
 The US uses a T1 (message oriented or robbed bit) line. A message oriented line is a PRI (Primary RateInterface) ISDN line with 23B channels and 1 D channel. Instead of the phone company sending the ring voltage in-band with the data, the digital packet with the hook status is sent in its own channel (D channel) as an out-of-band signal (Common Channel Signaling). The gain from losing one channel (D channel which happens to be channel 24 on a T1) is that the 23 B channels are full 64K able DS0s and are not interrupted by calls coming in on the line. This is the line you want to use if you have users using a ISDN BRI (Basic Rate Interface) and a Terminal Adapter. This line also will tell you information like who is calling and what number was dialed. Robbed bit signaling is a type of in-band signaling (Channel Associated Signaling) used in T1 when the D channel is buried with the B channels, using the least-significant bits to indicate the hook condition. The least significant bits are "robbed" from each DS0 leaving a throughput of 56kb per second. Robbed bit signaling leaves you with 24 DS0s rather than 23 (remember you can only make ISDN BRI calls on a PRI that's the advantage, because the robbing of the bits only allows each DS0 56K and there is no digital channel to send digital packets on).
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