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T1 Circuit Resources

T1 Circuits are 4 wire copper lines. T1 Circuits offer point-to-point or direct internet service. T1 is designed for voice, video and data communications. Learn more about T1 circuits below.

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What are T1 Circuits?

Operating at a slower 1.544 Mbit/s T1 Circuits are a private digital line used to carry voice, video and data communication. T1 was initially developed to transport voice calls from one location to another location. Circuits like T1 are popular because they are an industry standard. The  T1circuit is flexible and highly reliable with an excellent Service Level Agreement (SLA).

T1 might seem slow compared to fiber optic lines. While Fiber optic internet offers higher bandwidth and speed, Fiber isn’t always available in all locations. Because T1 utilizes copper lines, T1 is still a widely-deployed technology. Ethernet Internet continues to overtake T1 deployments.

T1 Circuit History

Developed in the 1960’s by AT&T the T1 circuit or Transmission System 1, was designed to transport 24 simultaneous calls over a copper pair wire. The initial deployment for T1 circuits were for connecting telephone switching offices together. T1 circuits haven’t changed much in the last 50 years.

The T1 carrier is primarily transported over copper wire during the last mile of connection. T1 is also transported over Fiber and Microwave radios in a longer haul configuration. Older business telephone systems and key systems still use fictionalized T1 circuits to transport voice calls. In this configuration a channel bank is placed at the customer site. T1 circuits are modulated down into 24 channel DID lines by the channel bank.


AT&T T1 Circuit History

T1 Connection Speeds

1.544 Mbit/s is the max speed for the T1. The circuits are symmetrical connections, meaning the upload speeds are as fast as the download speeds. It is possible to bond T1’s into groups increasing there bandwidth capacity. The circuit is ordered as either an internet connection or a point to point line.

The term fractional T1 refers to using only part of the circuit. If you only purchase half of the channels or bandwidth on a T1, you’ve purchased a fractional T1. This allows a lower cost for small businesses. With more and more data traffic going out to the cloud, using the entire T1 circuit is normal.

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T1 Clear Channel/Unchannelized

All T1’s have the same structure, which includes 24 embedded DS0’s (Digital Signal 0, a 64 kbit/s channel).  All T1’s are channelized. The way a T1 is configured determines how channels are utilized. The concept of “unchannelized” simply means the sender and receiver have configured their equipment to use all 24 DS0’s as one serial stream of data. In this configuration, the channels only technically offer a 56 Kbps rate and the stream is only 1.344 Mbps = 56kbps x 24 DS-0’s. The lost data rate is due to robbed bit signaling.

Unchannelized T1 configuration are common for router connections. The circuit can be configured to use some DS0 paths for voice and others for data. This all depends on the needs of your organization. If you do configure traditional POTS voice lines on a DS0 and you wish to use the other DS0’s for data, be prepared to have the proper equipment at your office. If you are using Voice Over IP (VoIP) on the T1, you will configure your T1 as unchannelized.

T1 Frame Diagram
T1 Voice Connections

T1 Voice Connections

The practice of using a T1 for analog voice communications continues to decline. In this configuration, the T1 is channelized so it can be connected to an older PBX or telephone Key System. Depending on how the circuit is ordered, depends on how it is channelized. In the basic form, the circuit is sliced into 24 digital channels called DS0’s containing 64 Kbps each. Channels are then multiplexed into the one circuit.

VoIP over T1 is also a popular means of transporting voice communications. This is a more modern method of using the T1 carrier circuit as a clear channel digital only connection. Digital only clear channel T1’s pass voice, video and web traffic over a router. T1 enabled routers send VoIP calls to a local VoIP PBX or directly to a VoIP office phone.

T1 Disadvantages

One of the main reasons many organizations still select T1 is the reliability factor. This circuit offers one of the best Service Level Agreements (SLA). SLA is the contract agreement between your business and the T1 provider. The SLA will set the Mean Time to Repair (MTTR). MTTR is the response time and repair time for the T1 circuit. This is a very important factor. These circuits offer one of the fastest repair time responses in the telecommunication industry.

The main disadvantage of T1 is cost. The overall cost for the T1 is high compared to the speed of the circuits. Other disadvantages of T1 are ageing copper wires and lack of copper wire maintenance. Telecommunication providers continue to phase out T1 for more modern Fiber Optic connections.

T1 Disadvantages

T1 Alternatives

Alternatives to T1 are Cable Internet, Fiber Optic Internet, Ethernet Internet or Microwave. Cable offers much higher speeds on both, upload and download, but a limited Service Level Agreements (SLA). Cable internet is a shared resource, which isn’t a positive. During peak hours, cable internet speeds begin to slow as more users utilize the cable connection. T1 is a dedicated technology and will guarantee internet speeds consistently, regardless of peak periods.

Satellite companies place very low limits on the amount of data you can download monthly. Ethernet Internet is the latest technologies and could be considered the replacement for T1. Ethernet still uses the same copper lines as T1. We recommend using Ethernet Internet over T1 if it’s available in your area.

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