digital hierarchy multiplexes or muxes four T-1 signals together to derive a T-2 or DS-2 (6.312 Mbps channel) and seven DS-2 pipes together to create an even higher-speed multiplexed signal called a DS-3. DS-3 is the term most commonly used to refer to the 44.736 (usually rounded to 45) Mbps pipe that is used for high-capacity transmission. The digital multiplex system we have just described has largely been replaced by Synchronous Optical NETwork (SONET) transmission systems. Europe uses a system similar to T-1 that is known as E-1/CEPT( (typically referred to as E-1). E-1 is a 32-channel pipe in which two channels (13 and 30) carry only network signaling and synchronization. Phone companies base their networks on digital carrier. That is to say, they connect their switching centers together using channels packaged in accordance with the digital hierarchy specification. The specification for all digital networks is D4 framing; it describes the basic DS-1 signal including the framing bit. Coding and Compression We digress from audio and video coding specifically into networks for good reason. T-1 pipes are often installed in conjunction with videoconferencing room systems when T-carrier is not already installed for voice or data services. Multiples of the 24 channels (six is typical) are commonly used for video communication applications. The remainder is available for other services such as outbound and inbound long distance or access for various data communications services. 8 bits 8 bits 8 bits Ones and Ones and Ones and Ones and zeros zeros zeros zeros Channel 1 Channel 2 Channel 23 Channel 24 Framing bit Figure 6-2. T-1 Frame Including Framing Bit (Michel Bayard). The public switched telephone network was designed for audio signals. Dealing with audio is easy; human speech possesses characteristics that permit efficient coding. The range of frequencies is narrow and sounds occur at the lower end of the electromagnetic spectrum. Coding audio information is easy in comparison to coding video signals. VIDEO ENC