T-1 101 for the TAS: Answers to Common Questions

By Allen Kalik

The mere buzz word “T-1” brings tremors of the unknown to many TAS owners and managers. The following information will help to answer most of the commonly asked questions about this technology and its applications in your TAS.

What is T-1 service?: T-1 is a type of telephone service capable of transporting the equivalent of 24 conventional telephone lines, using only 2 pairs of wires.

Who uses T-1?: The telephone companies have used T-1 for decades to economize on runs between central offices. In the last ten years, T-1 has become commercially available for high volume telephone service users.

What is a T-1 circuit?: T-1 is a high speed 4-wire data circuit with 2 wires used for transmitting and 2 wires used for receiving. The T-1 is capable of transmitting and receiving data at the rate of about 1.5 million bits per second. For comparison, the rate of data transmission in a T-1 is over 100 times faster than a PC modem operating at 14,400 bits per second.

How can one data circuit turn into 24 telephone lines?: At one end of the T-1 (the central office, for example), each of the 24 phone lines is encoded to a digital format much like a CD recording. Then the packets of data from each line are transmitted in sequence order into a single data stream. A device called a channel bank is responsible for this process.

At the other end of the T-1 (for example, the TAS), another channel bank reverses the process by separating the data stream into the original 24 distinct data packets representing each phone line. The data is then decoded from digits back into the 24 telephone (voice) lines.

What are the main advantages of T-1 to the TAS?: Cost savings over the equivalent service on regular analog lines. Another advantage is that T-1 is a digital transmission and is less prone to loss and interference than regular phone lines.

What are the main disadvantages of T-1?: The main disadvantage is that it requires a channel bank, a multiplexer, or a digital switch to convert the signal to telephone lines. While T-1 is very reliable, another disadvantage is that an outage could take out all 24 lines at once.

What are the main applications of T-1 in TAS?: There are three main applications:

  1. Replacement of Local DID trunks: Your local DID trunks can easily be replaced by T-1 service from your local telephone company. In most areas a 24 line T-1 is about the same cost as 12 DID trunks. (Rates can vary greatly on both DID trunks and T-1 costs, so first check with your local telephone company or T-1 vendor).
    All of your existing DID numbers can be moved to the T-1, where they are called DNIS digits. (Pronounced “Dee-niss;” stands for Dialed Number Identification Service).
  2. Long Distance Carrier Dedicated Service: T-1 service can be purchased from AT&T, MCI, Sprint, or other long distance carriers as a dedicated connection for taking 800/888 calls and making outbound long distance calls. The main advantage of this service is that the long distance company provides highly discounted rates on T-1 dedicated service.
    The decision t o use this service should be based on your projected savings versus your monthly T-1 charges and added equipment cost. If your long distance usage is over 20,000 minutes per month, it is probably worth investigating a long distance T-1 connection. Unfortunately, long distance T-1 service cannot be used to carry local DID. Also, the long distance T-1 can only handle traffic from the specific carrier (i.e., MCI) that provides the T-1.
  3. Point-to-point connection between offices: T-1 can also be used to connect the lines between two locations. For example, a T-1 circuit can be used to provide 24 off premise extensions of lines ringing at a remote location. Because T-1 is only one circuit, the mileage fees are significantly less than the mileage fees on 24 individual lines.

T-1 solutions such as this allow an operation to close down are mote TAS office by transporting the lines to an alternate location. Depending on the location of the terminating locations, this type of T-1 service may be provided by the local telephone company, a long distance company, or an alternative carrier.

Can T-1 be used to link two office networks together?: T-1 is capable of transporting data about 100 times faster than most PC modems. As such, it can be used to create a wide area network between two offices. Keep in mind, however, that T-1 is only one-sixth as fast as most standard office computer networks.

Can T-1 be used for both telephone (voice) and data communication?: With a piece of equipment called a multiplexer (similar to a channel bank), the T-1 can be distributed into data circuits and voice channels. For example, a T-1 could be used to carry 12 telephone lines, plus 2 data circuits at 386K baud.

What equipment do I need to utilize T-1 telephone service?: If you have a digital switch or TAS equipment with digital capability, you only need to purchase a T-1 card for your system.

T- 1 cards usually have a 24-line capacity and can be directly connected to the T-1 circuit. If you do not have digital telephone equipment, you need to rent or purchase channel bank equipment.

What is a channel bank?: A channel bank is a small digital telephone system with an input for T-1 and 24 outputs, one for each telephone line. The trunk cards in a channel bank must be compatible with the type of lines being used.

For example, if all the lines were coming in as DID, the cards must be compatible with this type of service. As a rule, channel banks are incredibly sturdy pieces of equipment with virtually a zero failure rate. There are no moving parts to break, no keyboard controls to mistype on, and no disk drives to fail.

If the channel bank has a critical role in your operation, spares or even a spare channel bank would be advisable to be safe. Channel banks can be purchased new in the $5,000 to $7,000 range, and in the used market for about half that price.

What is a CSU?: Another piece of equipment, called a CSU or customer service unit, is sometimes required by the telephone company. The CSU is a protective device similar to a modem that is wired between the T-1 connection and the channel bank. The CSU typically has diagnostic and status lights to help identify any problems with the T-1 circuit. A CSU can be purchased new for about $500.

Are you T-1 equipment-phobic?: If purchasing and maintaining T-1 equipment is an intimidating thought, consider renting from the long distance carrier or the telephone company. They will provide, install and maintain both CSU and channel banks at your location, delivering to you the familiar 24 lines. The cost of renting equipment is usually a few hundred dollars per month as opposed to the thousands required to purchase the same equipment.

Conclusion: To T-1 or not to T-1?: T-1 is a reliable, manageable technology for most TAS bureaus offering the possibility of greater services at a cost savings while expanding their coverage territory. The final decision should depend on the economics of the situation.

[From Connection Magazine, November 1997]

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