T-Carrier Facilities (Networking)

T-carrier is a type of digital transmission system employed over copper, optical fiber, or microwave to achieve various channel capacities for the support of voice and data. The most popular T-carrier facility is T1, which is implemented by a system of copper wire cables, signal regenerators, and switches that provides a transmission rate of up to 1.544 Mbps using digital signal level 1 (DS1). In Europe, the UK, Mexico, and other countries that abide by International Telecommunication Union (ITU) standards, the equivalent facility is E1, which provides a transmission rate of 2.048 Mbps.

T-carrier had it origins in the 1960s. It was first used by telephone companies as the means of aggregating multiple voice channels onto a single high-speed digital backbone facility between central office switches. The most widely deployed T-carrier facility is T1, which has been commercially available since 1983.

Digital Signal Hierarchy

To achieve the DS1 transmission rate, selected cable pairs with digital signal regenerators (repeaters) are spaced approximately 6,000 feet apart. This combination yields a transmission rate of 1.544 Mbps. By halving the distance between the span-line repeaters, the transmission rate can be doubled to 3.152 Mbps, which is called DS1C. Adding more sophisticated electronics and/or multiplexing steps makes higher transmission rates possible, creating a range of digital signal levels, as follows:

North American Digital Signal Hierarchy

International (ITU) Digital Signal Hierarchy

For example, a DS3 signal is achieved in a two-step multiplexing process whereby DS2 signals are created from multiple DS1 signals in an intermediary step. DS1C is not commonly used, except in highly customized private networks where the distances between repeaters is very short, such as between floors of an office building or between buildings in a campus environment. Some channel banks and multiplexers support DS2 to provide 96 voice channels over a single T-carrier facility. DS4 is used mostly by carriers for trunking between central offices.

Quality Objectives

The quality of T-carrier facilities is determined by two criteria: performance and service availability. The performance objective refers to the percentage of seconds per day when there are no bit errors on a circuit. The service availability objective refers to the percentage of time a circuit is functioning at full capability during a three-month period. If these objectives are not met, the carrier issues credits to its users. Each carrier has its own quality objectives for T-carrier services, which are based on circuit length.

The quality objectives for AT&T’s Fractional T1 service, for example, is nine errored seconds per day, which translates into 99.99-percent error-free seconds per day, four severely errored seconds per day, and 99.96-percent service availability per year. According to AT&T, “severely errored” means that 96 percent of all frames transmitted in a second have at least one error.

A related measure of performance is “failed seconds,” defined by AT&T as the time starting after 10 consecutive severely errored seconds and ending when there have been 10 consecutive seconds that are not severely errored. Channel Service Unit/Data Service Unit (CSU/DSU) at each end of the circuits collect and issue reports on this type of information.

T1 Lines

As noted, the most popular T-carrier facility is T1, which is a digital line or service providing a DS1 transmission rate of up to 1.544 Mbps. T1 lines are used for economical and efficient voice and data transport over the wide-area network. The available bandwidth is divided into 24 channels operating at 64 Kbps each, plus an 8-Kbps channel for basic supervision and control. Voice is sampled and digitized via Pulse Code Modulation (PCM).

Economy is achieved by consolidating multiple lower-speed voice and data channels over the higher-speed T1 line. This is more cost-effective than dedicating a separate lower-speed line to each terminal device. The economics are such that only five to eight analog lines are needed to cost-justify the move to T1.

Efficiency is obtained by compressing voice and data to make room for even more channels over the available bandwidth. Individual channels can also be dropped or inserted at various destinations along the line’s route through the use of an add-drop multiplexer (ADM) or digital crossconnect system (DCS). Network management information can be embedded in each channel for enhanced levels of supervision and control.

A T1 multiplexer usually provides the means for companies to realize the full benefits of T1 lines, but channel banks offer a low-cost alternative. The difference between the two devices is that T1 multiplexers offer higher line capacity, support more types of interfaces, and provide more network management features than channel banks.

D4 Framing

T1 multiplexers and channel banks transmit voice and data in D4 frames. These frames are bounded by bits that perform two functions: they identify the beginning of each frame and help locate the channel-carrying signaling information.

D4 frames consist of 193 bits, which equates to 24 channels of 8 bits each, plus a single framing bit. Each frame contains framing bits or signaling bits in the 193rd position, which permits management of the facility itself This is done by robbing the least significant bit from the data stream, which alternatively carries information or signaling data. Another bit is used to mark the start of a frame. Twelve D4 frames comprise a su-perframe.

Extended Superframe Format

Extended Superframe Format (ESF) is an enhancement to T-carrier, which specifies methods for error monitoring, reporting and diagnostics. Use of ESF allows technicians to maintain and test the T1 line while it is in service, and often fix minor troubles before they adversely affect service. ESF extends the normal 12-frame superframe structure of the D4 format to 24 frames. By doubling the number of bits available, more diagnostic functionality also becomes available.

Of the 8-Kbps bandwidth (repetition rate of 193rd bit or framing bit) allocated for basic supervision and control, 2 Kbps are used for framing, 2 Kbps for Cyclic Redundancy Checking (CRC-6), and 4 Kbps for the Facilities Data Link (FDL). With CRC, the entire circuit may be segmented so that it can be monitored for errors, without disrupting normal data traffic. In this manner, performance statistics can be generated to monitor T1 circuit quality Via FDL, performance report messages are relayed to the customer’s equipment, usually a Channel Service Unit (CSU), at one-second intervals. Alarms can also use the FDL, but performance report messages always have priority.

ESF diagnostic information is collected by the CSU at each end of the T1 line for both carrier and user access. CSUs gather statistics on such things as clock synchronization errors and framing errors, as well as er-rored seconds, severely errored seconds, failed seconds, and bipolar violations. A supervisory terminal connected to the CSU displays this information, furnishing a record of circuit performance.

Originally the CSU compiled performance statistics every 15 minutes. This information would be kept updated for a full 24 hours so that a complete one-day history could be accessed by the carrier. The carrier would have to poll each CSU to retrieve the collected data and clear its storage register By equipping the CSU with dual registers—one for the carrier and one for the user—both carrier and user alike have full access to the performance history

Today, the CSU is not required to store performance data for 24 hours. Also, the CSU no longer responds to polled requests from carriers, but simply transmits ESF performance messages every second.

ESF also allows end-to-end performance data and sectionalized alarms to be collected in real-time. This allows the carrier’s technical staff to narrow down problems between carrier access points and on interoffice channels, and to find out the direction the error is occurring.

Monitoring Services

Carriers offer optional services that monitor private T1/T3 networks, providing customers with protection against unexpected service impairments. Such services include:

■ On-site network monitoring

■ Configuration monitoring

■ Fault (alarm) monitoring

■ T1 performance monitoring

■ Trouble ticket management

■ Onsite supervisory workstation

End-to-end performance monitoring is implemented using ESF to collect error data from the circuits. The carrier will usually report on three ESF parameters for a T1 circuit: errored seconds, severely errored seconds, and failed seconds. This data is updated on an hourly basis, then compared to the circuit error thresholds set by the carrier. When recorded ESF errors exceed 80 percent of the carrier’s preset 24-hour thresholds, an alarm is triggered.

When problems arise, both visual and audio indicators alert the customer of the event. Color-coded changes on the workstation indicate the severity of the alarms. At the same time, alarm text is automatically output to the workstation’s printer. At the workstation, the customer can view detailed circuit data such as circuit number, alarm type, start date and time of alarm, direction for the alarm (whether “from” or “to” transmission direction), plus the circuit segment where the alarm occurred— either an interoffice circuit or access circuit. The customer can view or print historical alarm monitoring information later, using an alarm record that is maintained on each circuit on a rolling, 31-day basis.

Meanwhile, the carrier’s technical staff acts immediately to resolve the alarm conditions. Any changes are reflected graphically at the supervisory workstation, where the customer can stay informed of T1/T3 performance and the carrier’s problem resolution process. In addition, the customer has access to the trouble ticket manager to electronically open, close and track trouble tickets. The customer can even use this tool to contest tickets closed by the carrier.

Last Word

T-carrier underlies just about every type of carrier facility available today, including T1 and T3, and their fractional derivatives—dedicated or switched. These facilities, in turn, support such services as frame relay and ISDN, and provide access to SMDS, ATM, and virtual private networks. Through multiplexing and compression techniques, companies can subdivide T-carrier facilities to achieve greater bandwidth efficiency and cost savings.