The E carrier system has been created by the European Conference of Postal and Telecommunications Administrations (CEPT) as a digital telecommunications carrier scheme for carrying multiple links. The E-carrier system enables the transmission of several (multiplexed) voice/data channels simultaneously on the same transmission facility. Of the various levels of the E-carrier system, the E1 and E3 levels are the only ones that are used.
More specifically E1 has an overall bandwidth of 2048 kbps and provides 32 channels each supporting a data rate of 64 kbps. The lines are mainly used to connect between the PABX (Private Automatic Branch eXchange), and the CO (Central Office) or main exchange.
The E1 standard defines the physical characteristics of a transmission path, and as such it corresponds to the physical layer (layer 1) in the OSI model. Technologies such as ATM and others which form layer 2 are able to pass over E1 lines, making E1 one of the fundamental technologies used within telecommunications.
A similar standard to E1, known as T1 has similar characteristics, but it is widely used in North America. Often equipment used for these technologies, e.g. test equipment may be used for both, and the abbreviation E1/T1 may be seen.
The life of the standards started back in the early 1960s when Bell Laboratories, where the transistor was invented some years earlier, developed a voice multiplexing system to enable better use to be made of the lines that were required, and to provide improved performance of the analogue techniques that were used. The step of the process converted the signal into a digital format having a 64 kbps data stream. The next stage is to assemble twenty four of the data streams into a framed data stream with an overall data rate of 1.544 Mbps. This structured signal was called DS1, but it is almost universally referred to as T1.
In Europe, the basic scheme was taken by what was then the CCIT and developed to fit the European requirements better. This resulted in the development of the scheme known as E1. This has provision for 30 voice channels and runs at an overall data rate of 2.048 Mbps. In Europe E1 refers to both the formatted version and the raw data rate.
The E-carrier standards form part of the overall Synchronous Digital Hierarchy (SDH) scheme. This allows where groups of E1 circuits, each containing 30 circuits, to be combined to produce higher capacity. E1 to E5 are defined and they are carriers in increasing multiples of the E1 format. However in reality only E3 is widely used and this can carry 480 circuits and has an overall capacity of 34.368 Mbps.
Physically E1 is transmitted as 32 timeslots and E3 has 512 timeslots. Unlike Internet data services which are IP based, E-carrier systems are circuit switched and 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. Nevertheless it does not allow the same flexibility and efficiency to be obtained as that of an IP based system.
In view of the different capacities of E1 and E3 links they are used for different applications. E1 circuits are widely used to connect to medium and large companies, to telephone exchanges. They may also be used to provide links between some exchanges. E3 lines are used where higher capacity is needed. They are often installed between exchanges, and to provide connectivity between countries.
An E1 link runs over two sets of wires that are normally coaxial cable and the signal itself comprises a nominal 2.4 volt signal. The signalling data rate is 2.048 Mbps full duplex and provides the full data rate in both directions.
For E1, the signal is split into 32 channels each of 8 bits. These channels have their own time division multiplexed slots. These are transmitted sequentially and the complete transmission of the 32 slots makes up a frame. These Time Slots are nominated TS0 to TS31 and they are allocated to different purposes:
Time slot 0 is reserved for framing purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and match up each channel in turn. The standards allow for a full Cyclic Redundancy Check to be performed across all bits transmitted in each frame.
E1 signalling data is carried on TS16 is reserved for signalling, including control, call setup and teardown. These are accomplished using standard protocols including Channel Associated Signalling (CAS) where a set of bits is used to replicate opening and closing the circuit. Tone signalling may also be used and this is passed through on the voice circuits themselves. More recent systems use Common Channel Signalling (CCS) such as ISDN or Signalling System 7 (SS7) which sends short encoded messages containing call information such as the caller ID.
Several options are specified in the original CEPT standard for the physical transmission of data. However an option or standard known as HDB3 (High-Density Bipolar-3 zeros) is used almost exclusively.