Editing ISDN (section)

=== Digital lines ===

Since its introduction in 1881, the [[twisted pair]] copper line has been installed for telephone use worldwide, with well over a billion individual connections installed by the year 2000. Over the first half of the 20th century, the connection of these lines to form calls was increasingly automated, culminating in the [[crossbar switch]]es that had largely replaced earlier concepts by the 1950s.{{sfn|Cioffi|2011|p=30}}

As telephone use surged in the post-WWII era, the problem of connecting the massive number of lines became an area of significant study. [[Bell Labs]]' seminal work on digital encoding of voice led to the use of {{nowrap|64 kbit/s}} as a standard for voice lines (or {{nowrap|56 kbit/s}} in some systems). In 1962, Robert Aaron of Bell introduced the T1 system, which carried {{nowrap|1.544 Mbit/s}} of data on a pair of twisted pair lines over a distance of about one mile. This was used in the Bell network to carry traffic between local switch offices, with 24 voice lines at {{nowrap|64 kbit/s}} and a separate {{nowrap|8 kbit/s}} line for signaling commands like connecting or hanging up a call. This could be extended over long distances using repeaters in the lines. T1 used a very simple encoding scheme, [[alternate mark inversion]] (AMI), which reached only a few percent of the [[Noisy-channel coding theorem|theoretical capacity of the line]] but was appropriate for 1960s electronics.{{sfn|Cioffi|2011|p=31}}

By the late 1970s, T1 lines and their faster counterparts, along with all-digital switching systems, had replaced the earlier analog systems for most of the western world, leaving only the customer's equipment and their local [[Class-5 telephone switch|end office]] using analog systems. Digitizing this "[[Last mile (telecommunications)|last mile]]" was increasingly seen as the next problem that needed to be solved. However, these connections now represented over 99% of the total telephony network, as the upstream links had increasingly been aggregated into a smaller number of much higher performance systems, especially after the introduction of [[fiber optic]] lines. If the system was to become all-digital, a new standard would be needed that was appropriate for the existing customer lines, which might be miles long and of widely varying quality.{{sfn|Cioffi|2011|p=31}}