 1.544, PRI can carry 4 HO channels or 3 HO + D; at 2.048 it can carry 5 HO + D. H11 dialing combines 24 DSOs into a channel-less arrangement with an aggregate bandwidth of 1.536 Mbps. H12 combines 30 channels into a channel-less arrangement. In 1991, AT&T became the first U.S. carrier to offer commercial HO service, by making it available on a dedicated-access (both ends) basis. AT&T offers H11 service, as well. MCI WorldCom also offers HO and H11 service. An even more recent development, ISDN multirate, also known as Nx64 or switched fractional T-1 service, lets users combine, on a call-by-call basis, between 2 and 24 B channels. Video Communications: The Whole Picture INVERSE MULTIPLEXERS Multirate ISDN has never become ubiquitous. An alternative is to use hardware to achieve variable-rate bandwidth, or bandwidth on demand. To dial up multiple 56 or 64 Kbps channels, a customer can use an inverse multiplexer (I-MUX). An I-MUX breaks a high-bandwidth videoconference into multiple lower-speed channels for transport across a switched digital network. I-muxes were introduced in 1990. They break a high-speed data stream (such as that produced by a codec) into N (where 'N' typically indicates some number between 4 and 24) lower-speed switched digital channels. I-muxes use the EIA RS-366 standard to provide dialing commands to the network. They keep a signal synchronized SO that a compatible device on the other end can rebuild a single high-speed signal from the multiple lower-speed signals it receives. Synchronization is necessary because a switched digital network is a mesh of circuits. Many different paths can be taken to get between two points, and calls are routed across this mesh independently. If a high-speed videoconference were subdivided into lower-speed fragments, each fragment could take a different route to its destination. Routing disparities can cause data stream synchronization problems. I-muxes perform delay synchronization. In other words, they hold a frame open until all its fragments reach their d