Editing Time-division multiplexing (section)

==Multiplexed digital transmission==
In circuit-switched networks, such as the [[public switched telephone network]] (PSTN), it is desirable to transmit multiple subscriber calls over the same transmission medium to effectively utilize the bandwidth of the medium.<ref name="hanrahn">
{{cite book
| last = Hanrahan
| first = H.E.
| title = Integrated Digital Communications
| publisher = School of Electrical and Information Engineering, University of the Witwatersrand
| location = Johannesburg, South Africa
| year = 2005
}}</ref> TDM allows transmitting and receiving telephone switches to create channels (''tributaries'') within a transmission stream. A standard [[DS0]] voice signal has a data bit rate of 64&nbsp;kbit/s.<ref name="hanrahn" /><ref name="ericsson">{{cite web|website=[[Ericsson]]|title=Understanding Telecommunications|url= http://www.ericsson.com/support/telecom/index.shtml|archive-url=https://web.archive.org/web/20040413074912/http://www.ericsson.com/support/telecom/index.shtml|archive-date=April 13, 2004}}</ref> A TDM circuit runs at a much higher signal bandwidth, permitting the bandwidth to be divided into time frames (time slots) for each voice signal which is multiplexed onto the line by the transmitter. If the TDM frame consists of ''n'' voice frames, the line bandwidth is ''n''*64&nbsp;kbit/s.<ref name="hanrahn" />

Each voice time slot in the TDM frame is called a channel. In European systems, standard TDM frames contain 30 digital voice channels (E1), and in American systems (T1), they contain 24 channels. Both standards also contain extra bits (or bit time slots) for signaling and synchronization bits.<ref name="hanrahn" />

Multiplexing more than 24 or 30 digital voice channels is called ''higher order multiplexing''. Higher order multiplexing is accomplished by multiplexing the standard TDM frames. For example, a European 120 channel TDM frame is formed by multiplexing four standard 30 channel TDM frames. At each higher order multiplex, four TDM frames from the immediate lower order are combined, creating multiplexes with a bandwidth of ''n''*64&nbsp;kbit/s, where ''n'' = 120, 480, 1920, etc.<ref name="hanrahn" />