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ISDN, T1 and E1: What are the differences?

T1 and E1 network links have been used in networks for years, as has ISDN in some countries. Our expert looks at the differences among them and new networking options taking their places.

T1 and E1 lines are equivalent North American and European standards, respectively, for point-to-point digital...

data transmission based on the use of two pairs of copper wire -- one for transmitting data and one for receiving it. Comparatively, Integrated Services Digital Network, or ISDN, is a digital transmission standard for transmitting voice and data over a single copper-wire PSTN line. All three communications transmission technologies can be used to carry voice, data and video.

T1 and E1 lines

The T1 -- also called DS1, or digital signal 1 -- is the North American specification for a dedicated 1.544 Mbps line, first developed by AT&T Bell Laboratories in 1962. T1s can transmit up to 24 PSTN voice calls over a single line simultaneously. The T in T1 stands for terrestrial, which originally distinguished the line from a satellite network.

E1 is the equivalent European digital transmission format of the DS1 that was devised by the ITU Telecommunication Standardization Sector and named by the European Conference of Postal and Telecommunication Administrations. An E1 offers dedicated 2.048 Mbps and can simultaneously carry 32 voice calls of 64 Kbps each.

T1 and E1 lines offer enterprises dedicated internet access that includes service-level agreements that guarantee at least 99.9% of the bandwidth available 99.9% of the time. T1 and E1 channels can be split into any combination for voice and data transmission.

Both T1s and E1s carry DS1 signals and can interconnect with each other globally.

The use of T1 and E1 lines for voice channels has been greatly reduced with the advent of internet connections that offer more bandwidth at lower cost. Today's voice over IP (VoIP) technology doesn't use the concept of voice channels because VoIP calls are calculated on how much bandwidth they use, which depends on the voice codecs applied to transmit it.

AT&T later developed specs for T2 and T3 circuits that carry multiple T1 channels that are multiplexed tooffer transmission rates of up to 44.736 Mbps.

ISDN lines

First defined in 1988, the ISDN standard integrated voice and data on one circuit-switched telephone line with a maximum of 128 Kbps of bandwidth upstream and downstream. It can provide voice, video and text transmission between individual desktop video conferencing systems and group video conferencing systems. It is used in Europe, the U.K., Australia, India, some countries in Asia and, to a much lesser extent, in North America. Overall, ISDN was surpassed by packet-switched technologies.

Since T1, E1 and ISDN were released, other digital networking technologies have made transmitting voice, data and video more efficient and cost-effective by lowering the cost of bandwidth and increased speeds that dwarf those of the T1 and E1. Additional bandwidth is now offered and available via technologies such as Carrier Ethernet, software-defined WAN, bandwidth on-demand services and network-as-a-service options.

Next Steps

Using SDN as UC's killer app for voice and video

Using broadband internet links for branch WAN connectivity

What building a cloud network really means

Considering SD-WAN for voice and video

Dig Deeper on VoIP QoS and Performance

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Why did your organization decide to include T1 or E1 lines in its network?