T3 Line Definition | The Interactive Glossary

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A T3 line is composed of 28 bundled T1-level circuits. Each T1 circuit operates at 1.544 megabits per second (Mbps), for a total connection speed of 44.736 Mbps. A T3 line is also often referred to as a Digital Signal 3 (DS3) connection.

T3 lines are a popular choice for businesses that require the most dependable Internet connection possible. Both T1 and T3 lines establish a physical point-to-point connection between an Internet Service Provider (ISP) and the organization purchasing the connection. As a result, outages that affect normal customers often don't affect customers connected by a point-to-point line. In addition, in the event of a major outage, such as following a natural disaster, ISPs are required by law to repair point-to-point connections before moving on to general Internet connections. As a result, a typical T1 or T3 connection experiences only 15 minutes of downtime annually.

T3 circuits are typically limited to connections within a single building due to the need to have a repeaters to boost the signal no more than 600 feet apart, and possibly as little as 225 feet apart, depending on the type of cable used to establish the connection.

Due to the short distances which can be spanned by a T3 line, when a customer orders a T3 line the loop between the customer and ISP is typically made on a SONET circuit – a modern protocol for transferring data over fiber optic cable which overcomes the technical limitations of the T3 line, and maintains the direct point-to-point nature of a T3 line.

Also See: T1, Bandwidth, Broadband, ISP

Frequently Asked Questions

Why does the cost of a T3 line vary so much from one geographic area to another?

When a T3 connection is established, a physical connection must be made between the customer's network and the ISP's nearest point of direct connection. As a result, T3 lines often encompass several hundred, or even thousands of, feet of buried lines dedicated to a single customer. The cost of the connection will vary depending on the distance between the customer and the nearest point of connection, and the amount of competition between ISPs in a given market area.

Why are T3 lines so expensive?

A T3 line is capable of transmitting data at approximately 45 Mbps at a cost that may range from $1,000 to $5,000 per month. However, the speed of a T3 line is not incomparable to a broadband Internet connection which can be had for significantly less than $100 per month in most markets. Why would anyone consider a T3 line when the same sort of connection speed is available at much lower price?

The difference is that T3 connections are private and backed by a Service Level Agreement (SLA). General broadband connections, including DSL and Cable, makes use public networks and shared bandwidth, both of which will be affected by other customers. In addition, due to the public nature of general-use broadband connections, frequent and extended outages are not unusual.

While a T3 line may not be any faster than a broadband Internet connection made over a fast network with low traffic, the difference is that the T3 line is a dedicated connection that does not carry the traffic of any other customers. In addition, the SLA that backs up the T3 line stipulates the maximum downtime that is acceptable before the ISP is hit with financial penalties, and the customer is due financial remuneration.

In short, when an organizations pays for a T3 line they aren't paying for speed as much as for the utmost in reliability and uptime.