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The E carriers (E1, E3, and so on), like the North American T carriers, are communication systems and standards that provide multiplexed, multichannel, point-to-point communication links. It has been used by carriers to link remote telephone switching offices and is commonly used for customer-to-carrier access links.

The E carriers are defined by the European CEPT (Conference of Postal and Telecommunications Administrations) and are used in Europe, Mexico, and South America. The T carriers are part of the NADH (North American Digital Hierarchy). The CEPT format defines how digital information is formatted and multiplexed into the transmission system.

Conceptually, the two standards provide a similar type of service: transmission links that provide multiple time division multiplexed (TDM) channels. Traditionally, multiple voice calls are carried over the service, but data is also supported. The service is digital, meaning that analog phone calls piped into a TDM channel are first converted to digital signals. PCM (pulse code modulation) is used for analog-to-digital conversion.

The carriers originally used the services to connect outlying remote telephone switches with central office switches or to interconnect central offices. For example, if a new subdivision was built, a remote switch was installed near the subdivision to provide a connection point for new subscriber telephone lines (local loop copper wires). The subscriber lines then were multiplexed onto a single cable that connected the remote switch with the central office.

The basic channel in both systems is the 64-Kbit/sec channel that supports one voice call or data circuit, depending on how the customer wants to use it. But this is where to two systems diverge. The E carriers allocate the full 64-Kbits/sec channel to the user, while the T carriers rob some of the bits in the basic channel to use for signaling. The end results are the same, but the digital hierarchy is different. The E carrier hierarchy is listed here.

Note that an E1 carries 30 channels, while a T1 carries 24 channels. Since E1 circuits carry more channels than T1 circuits (24 channels), repeaters are required every 6,000 feet when copper wire is used. See "T Carriers" for information about the T carriers and "NADH (North American Digital Hierarchy)" for information about the signaling methods. See "TDM Networks" for information about building networks with E1/T1 links.

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