A T1 circuit is a dedicated digital data carrier transmitted over copper wiring and is used to carry multiplexed voice and data at speeds up to 1.544 million bits per second (1.5Mbps) using 24 "channels" or timeslots. T1's are most commonly installed as dedicated links between two office locations or from a customer to the Internet.

  1. Description of a T1
    1. Full T1
    2. Burstable T1
    3. Channelized T1
    4. Fractional T1
  2. Physical Specifications
    1. General
    2. Electrical
    3. Cabling


A T1 circuit is a dedicated digital data circuit transmitted over copper wiring and is used to carry voice and data at speeds up to 1.544 million bits per second (1.5Mbps) and may be fractionalized into 24 "channels" or timeslots. T1's are most commonly installed as dedicated links between two office locations or from a customer to the Internet, but this has changed. The complete end-to-end T1 is composed of two halves: the local loop (often called the "Last Mile", but usually not literally a mile long) and the carrier circuit. This two-part setup is due to United States law regarding monopolies and the operations of telecom carriers. Local telephone companies own the 'right of way' and regional carriers such as long distance and Internet providers must purchase the local circuit from the local telephone company to get their customer connected to their network. Telecom carriers refer to the connection points between the customer and the local telco as a 'demarcation point' or simply 'demarc' and the connection point between the local telcom and long-distance carriers and ISP's as 'exchanges', 'points of presence' or 'POPs'.

The local loop is provided by the local telephone company because they are the ones that put the wire that is conncted to the site into the ground. If the T1 is for data Internet service, the other end of the circuit will be run to the local phone company's "point of presence" to hand off to the telecommunications or Internet provider of your choice. Thus, your Internet T1 contains a local phone company portion of the circuit closest to you and an Internet provider part of the circuit owned by your ISP, even though you purchased the line from Sprint, MCI, AT&T etc. Always remember that when troubleshooting an Internet T1, you need to get both the local phone company who provides your local loop (last mile) circuit, and the Internet provider on the phone at the same time to do any work on the circuit. If they're not working in a conference call, you're probably going to be down for hours or days while they point fingers at each other and play the blame game.

In some places, long distance companies (who are usually the big ISP's) also own the local circuits and are the local telecommunications provider. This is very rare. More often when you buy a T1, you are buying a T1 where the local phone company's part of the circuit is being resold by the ISP to you as a complete T1. This is a better way to purchase it if you can, because you can hold the ISP accountable for all problems in the local phone company portion of the circuit. However, it will be more expensive.

A T1 is sold with several types of service: as a full dedicated T1, a burstable T1, a channelized T1 and fractional T1.

Full T1 Service

A full T1 service is usually sold as a complete circuit of up to 1.544Mbps total speed. This communications channel is often referred to as a digital trunk line. A T1 is a dedicated circuit from the telecommunications provider to the customer and is not shared with any other customers. This is why it is often referred to as a 'dedicated' line. The full circuit can be either data or voice, but not both.

Burstable T1 Service

This is one of two services: a full T1, sold with some sort of measuring (QoS/CoS) technology attached at the ISP's end designed to charge you more if you "burst" to the full speed of the T1. You purchase an amount of bandwidth that you receive each month and pay a premium when your line exceeds this level of data.

Channelized T1

A channelized T1 contains 24 individual channels, each capable of carrying voice or data. The full set of channels has the same speed as a full T1, but the individual channels may be split into voice lines or data lines using a device called a Channel Service Unit/Data Service Unit or CSU/DSU. The CSU/DSU is used to split off the voice channels from the data channels, allowing the voice channels to be connected to a phone system or PBX. The data lines are then connected to a router serial interface and often are used to provide Internet connectivity. The key advantage here is to purchase a single T1 to provide voice and data services over the same connection.

Fractional T1

A fractional T1 is one or more channels bundled together and sold to a customer as a set to provide various desired speeds and cost savings over a full T1. This allows a consumer to purchase less than a full T1's bandwidth at a lower cost. The price of T1's has fallen significantly, and fractional T1 service is becoming ever more rare, being replaced by other faster communications technologies including cable Internet, and fiber optic service (Verizon FIOS). Like the channelized T1, individual channels within a Fractional T1 can be voice or data and a CSU/DSU is used to split the channels, however less than the full set of 24 channels is available to the customer for use.



A T1 circuit is an always on connection. It in no way resembles a voice circuit which only generates expenses when a call is placed. For this reason, T1's are sometimes referred to as private lines or dedicated data lines because they are always in use (whether data is being transmitted or not) and thus, can never be used for carrying traffic from multiple subscribers. It is because T1's are dedicated circuits that they cost more than ISDN or Frame Relay circuits which assume that the customer's connection is shared with other subscribers. T1's are 'always on' and thus must be a dedicated data line.

A T1 circuit is the first multiplexed level of the PDH digital signalling multiplexing scheme. T1s use what is called a Stratum 3 clock to maintain what is called clocking on the line. Devices supporting a T1 estimates its synchronization with the T1 based on the data received over the link. Thus, T1 communication is not always fully synchronous (this is one cause of frame slips). T1's are more properly called pleisiosynchronous connections.

T1s are often referred to as DS1 (Digital Signalling level 1). A T1's speed can reach as high as 1.544 Mbps on the circuit. T1's are the most common high speed circuit provided by telecommunications carriers because a T1 can be regenerated and extended to reach any location. Other solutions such as DSL do not have the same total reach as standard T-carrier services. T1's carry 24 channels of digital information and must maintain a certain level of 1's density in order to assure the clock is maintained at both ends of the circuit.


A T1 uses a bipolar signalling method where voltage states range between +/-12 volts. A binary zero is signaled with zero voltage; binary ones are signalled using either positive or negative voltage. This positive and negative value is refered to as polarity. When a series of 1's are received on a T1 link, the voltage is alternated from positive to negative voltage. From this change in voltage, the receiving equipment can detect and synchronize with the remote device's clocking. Zeroes are indicated by zero voltage on the line. Because clocking is derived by detecting changes in voltage, a long stream of zeroes from an inactive line would cause problems. Modern T1's therefore perform zero substitution to prevent a long string of zeroes and maintain the clocking.


Within the communications network copper twisted pairs are used. One pair for transmit, and another for receive making four wires for each T1. This allows T-carrier systems to transmit and receive simultaneously in both directions at full speed (full duplex). T1 trunk cables are made in bundles of 25 pairs of 22 AWG copper wires designed for carrying multiple T1's. Most carriers today (as of 2004) use fiber optics wherever possible between central offices and have been using T3's for many years. This type of physical plant is slowly fading out of use and will probably be completely gone by the turn of the next century; at least at the rate most carriers currently perform upgrades. The trunk cables carry multiple bundles of 25 pairs and the standard method used for carrying transmission is to separate transmit from receive on each pair into separate bundles to reduce cross talk. Occasionally a few extra pair are added to these bundles for line management functions such as fault location or provisioning.

Buyer Beware

Companies that provide T1's multiplex the T1's into higher capacity trunk circuits (T3, OC1, OC3, OC12, OC48). Those higher capacity circuits usually do not have enough capacity to carry the total data rate of all the customers of the T1's sold that use that higher capacity trunk line circuit. Carriers oversubscribe their networks. Its the same concept as the airlines, they overbook on the assumption that not everyone will be using the full rate of their service at all times. Buying a dedicated T1 doesn't gurarantee you will receive a full 1.544 Mbps end to end unless it was wired directly between two locations with no carrier equipment in-between, which is hideously expensive.

Carriers can only make money if they oversubscribe their network. They are selling more bandwidth than they can actually provide. Data communication is bursty, so most people never notice that they don't get the full 1.544 Mbps at all times. Furthermore the carrier increases storage buffers in the equipment to allow the data to get stored and forwarded later when there is capacity on the trunk line. The data is buffered and forwarded a fractions of a second or a few seconds later, and the customer is usually none the wiser. However, don't expect you will get the full 1.544 Mbps rate at all times.

Before you purchase a T1, make sure you look at the contract and see what the Committed Information Rate (CIR) or

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