In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations, worldwide standards have been created and deployed.
The European Conference of Postal and Telecommunications Administrations (CEPT) originally standardized the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T).This is now widely used in almost all countries outside the USA, Canada and Japan.
The E-carrier standards form part of the Plesiochronous Digital Hierarchy (PDH) where groups of E1 circuits may be bundled onto higher capacity E3 links between telephone exchanges or countries.
This allows a network operator to provide a private end-to-end E1 circuit between customers in different countries that share single high capacity links in between.
In practice, only E1 (30 circuit) and E3 (480 circuit) versions are used. Physically E1 is transmitted as 32 timeslots and E3 512 timeslots, but one is used for framing and typically one allocated for signalling call setup and tear down.
Unlike Internet data services, E-carrier systems permanently allocate capacity for a voice call for its entire duration.
This ensures high call quality because the transmission arrives with the same short delay (Latency) and capacity at all times.
E1 circuits are very common in most telephone exchanges and are used to connect to medium and large companies, to remote exchanges and in many cases between exchanges.
E3 lines are used between exchanges, operators and/or countries, and have a transmission speed of 34.368 Mbit/s.
