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T1 Lines | At 1.5Mbps and over $200/month, why have they remained relevant? - Fiberguide

T1 Lines | At 1.5Mbps and over $200/month, why have they remained relevant?

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T1 Lines | At 1.5Mbps and over $200/month, why have they remained relevant?

T1 lines (or E1 lines in EMEA) and their variants such as T2,T3…T5 have been around for a long time – from as far back as the 1960s. Despite relentless pressure from emerging higher bandwidth, and low cost/bit technologies T1 lines remain relevant for certain applications and in certain market segments. In this article T1 lines and the reasons for their survival are briefly reviewed.

About T1 Lines

A T1 line can be defined as a physical transmission medium capable of transmitting 1.544Mbps (1.536Mbps + 8 bits for channel control). The European equivalent, E1 is capable of 2.048Mbps. When initially developed by Bell Labs a T1 line consisted of a twisted copper pair but today T1 signals can be transmitted over optical fiber and other media, even wireless.

Since T1 lines were originally designed for voice transmission, their capacity was defined in terms of the number of voice channels. In the old Plesiochronous Digital Hierarchy (PDH) and later SONET/SDH, a voice channel was referred to as DS0 (Digital Signal Zero) equivalent to 64Kbps. Thus, a T1 line is capable of carrying 1.536Mbps/64Kbps or 1536Kbps/64Kbps = 24 voice channels. The corresponding E1 circuit carries 2.048Mbps or 32DS0.

A T1 line can be configured to carry voice, data or a combination of both. When used to carry voice, 23 channels are used for simultaneous voice calls and the 24th channel is used for overhead, including caller ID.

Although T1 is most widely deployed, higher T-carrier signal levels are available as follows:



T1 lines can also be bonded or combined together, resulting in multiples of T1s.

Why are T1 Lines still relevant today?

Despite their modest capacity of 1.5Mbps and relatively high cost per bit, T1 lines are still widely used. High availability, guaranteed bandwidth and ubiquity are some of the factors that have kept T1 lines relevant for so many years.

T1 line availability and guaranteed bandwidth

The 1.5Mbps offered by T1 line providers is usually guaranteed, i.e. any time you access the connection, you always get the same bandwidth. This is contrary to some DSL and HFC (or cable) Internet access solutions whose bandwidth can vary significantly with distance from the central office or number of subscribers on the network. In some cable business Internet access Service Level Agreements (SLAs), there is a clause that reads something to the effect that actual speeds may vary and are not guaranteed.

Availability or uptime is a measure of the time the network is operational as opposed to downtime when the network is not functioning. Most T1 line providers offer an uptime of 99.9% making the service suitable for mission critical applications such as connecting email and web servers. Other competing business packages (cable and DSL) may come with business essential services such a fixed IP address, multiple emails and hosting but may not include an availability guarantee.

T1 line ubiquity

Carrier grade Ethernet is one service that is disrupting T1 lines. T1 lines cost about $200/month or ~$130/(Mbps.Month) but the same monthly rate can get you a symmetric 10Mbps dedicated business Ethernet package which translates to $20/(Mbps.Month).

Obviously anyone would go for the later package if only it was available in their area. Business Ethernet services are mostly available in large metropolitan areas while T1 lines can be provisioned wherever there is a phone line. They remain the most logical choice for businesses in undeserved areas needing business class connectivity.

To learn more about communication technologies, especially optical networking, sign up for one of our optical networking training events.


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