TECHNOLOGY BRIEF

T-1 and T-3 Circuits Provide LAN to WAN Interconnection

The use of T-1 and T-3 Telco circuits to interconnect LANs and other applications is growing rapidly. This article explains how these cost-effective circuits work and why they are so popular.

Inter-network Growth Continues

Local area networks (LANs) are getting larger, faster and becoming more complex. Campus, metropolitan, and wide area networks are becoming more common. Network administrators want one local network to communicate with others across campus, across town or in another city. Branch office systems periodically transfer data to or from headquarters locations.

Local and long distance telephone carriers have historically been the first choice for internetwork transmission. These carriers commonly use existing multiplex switching technology, referred to as T-1 and T-3. These are digital time-division-multiplexing (TDM) methods, developed many years ago by AT&T to increase the voice channel capacity of circuits within their central offices. After the breakup of AT&T, these technologies became ANSI industry standards, now designated DS-1 and DS-3 respectively, and widely used for inter-network connectivity. T-1 or DS-1 and T-3 or DS-3 terms are interchangeable within this article.

These circuits are also widely used for cellular phone switching and cellular antenna installations. The current upgrade to “3G” (Third Generation) cellular systems is consuming hundreds of thousands of feet of these cables.

How TDM Systems Work

Simply put, TDM equipment processes multiple voice or data channels by first converting them from analog to digital, then giving each a unique–but very brief–timing sequence. The TDM electronics allow each channel to transmit its message or data during its allotted time slot. At the other end of the circuit, a demultiplexer breaks apart the rapid, multiple-channel, digital data stream and reassembles it again into discrete voice or data channels.

TDM technologies essentially multiply the capacity of copper cable pairs. Instead of one pair for each channel, multiple channels can be run through a single cable pair. The following chart summarizes the capacity of each.

Designation Number of Channels (DS0) Capacity (Mbps)
T-1 or DS-1
T - 3 or DS-3
24
672
1.54
44.74

The beauty of these Telco switching technologies is that they are digital, they are available and they are effective for high speed data transmission. T-1 provides 24 channels of 64 Kbps bandwidth (DS0), totalling 1.544 Mbps. (The slightly larger bandwidth is due to timing and framing requirements.)

Fractional and Multiple T-1

Fractional portions of a T-1 circuit are also available from the local telephone carrier in increments of a single channel. Thus, a network can lease 4 or 5 channels if needed; however, the economics currently work out to be break-even for 8-10 channels, as compared to the cost of a full T-1 line equal to 24 channels. T-1 and fractional T-1 circuits are currently most prevalent for multi-LAN interconnection, supporting voice, video, data and FAX.

Although T-1 circuits are most common, T-3 circuits are also being used because of the extra bandwidth. T-3 or DS-3 provides the equivalent of 28 T-1 circuits, or 44.7 Mbps of bandwidth. T-3 circuits can be used to provide ultra high-speed Switched Multimegabit Datervice (SMDS) and even asynchronous transfer mode (ATM) network interconnection. Flicker-free video, exotic image processing, desk-top teleconferencing and other cutting edge computer technologies require this super bandwidth.

T-1 Connection Methods and Devices

Many cabling types and methods may be used to connect networks to DS-1 circuits. The most common is copper cabling, but fiber optic, coaxial, microwave and even satellite transmission links may be used. The best solution for a given installation depends on point-to-point distances, the local telephone carrier's equipment, bandwidth requirements and budget constraints. Of all interconnect options, copper and fiber are the most commonly used.

Fiber offers advantages over copper with regard to EMI immunity, maximum distances and potential bandwidth. However, copper is often already in place, and the total connection cost is less because the DS-1 devices that are required for copper are simpler and less expensive than fiber devices. Thus, copper cabling is often the most practical choice.

When T-1 service enters a commercial building, it is routed through various types of patching and interface equipment. Customer service units (CSU) or digital service units (DSU) process both voice and data channels. These devices convert the digital T-1 transmission signal to a LAN-acceptable digital data signal or voice channel, and the reverse for outgoing traffic. The diagram below illustrates how T-1 or DS-1 Telco service interfaces between different LAN and voice premises.

Copper cable used to transmit T-1 and T-3 signals has been available for many years. A typical design has been 2-pair 22AWG with foam skin insulation, each pair shielded, with 100 ohm impedance. These are commonly known as ABAM telephone cables. The physical size of these cables requires termination with DB-9 connectors, DB-15 connectors, punch-down hardware or mechanical terminal blocks. These older cable designs and their termination systems were developed for T-1 applications within a telephone company's central office facility.

Now that T-1 and T-3 circuits are finding their way into LAN wiring closets and other confined spaces, the requirements have changed. Most new DS-1 system interface devices utilize either shielded or unshielded RJ 45 Telco modular connectors. The older ABAM cables are too large to fit these relatively inexpensive and widely used connectors.

Cabling Confusion

The local Telco service supplier usually provides T-1 service into the entrance facility or to the “Demarc” point, and then the LAN owner or installer must take it from there. Since many LAN installers and owners are unfamiliar with the requirements for T-1 cabling, many use the wrong cabling. For short runs (less than 30 or 40 feet), just about any 100 ohm shielded cable will work, but longer runs often cause equipment to fail. Installers have tried using Category 5e or Category 6 UTP cable, 50 ohm security cable, cash register cables, or anything that will fit into a modular connector. Most do not realize that the cabling must support specific DS-1 system requirements that are defined by an ANSI specification. These include bit error rates, EMI shielding, a pulse shape definition and many others. The scope trace below illustrates an actual DS-1 data pulse and the limiting minimum and maximum pulse-mask shape. In short, DS-1 systems are far different from LAN Ethernet systems.

Scope Trace

Actual DS-1 pulse with min/max mask

Quabbin Wire's New T-1 Cable

Quabbin Wire has developed several cable designs that meet both the electrical requirements for DS-1 data and also terminate to an RJ 45. These cables employ a unique two-layer dielectric system, which provides a very stable 100 ohm characteristic impedance, low capacitance, low loss and a high velocity of propagation. By removing the outer insulation layer, the inner will fit into either a shielded or unshielded RJ 45 for an efficient crimp termination. The cables provide T-1 interconnection for cutting edge LAN systems and can be terminated using all customary methods, including the cost and labor saving RJ 45 modular connector system. For details on terminating Quabbin's dual-insulated T-1 cables to modular plugs, see our T-1 Installation Support Guide.

For a detailed review of the performance characteristics and dimensions of these cables, check out P/N 9720 and P/N 9745 in the Cable Finder. Both cables are identical electrically, however P/N 9745 has a zip cord jacket design, which permits the transmit and receive leg to be separated yet contained within its own jacket. This feature is important for some installations when these circuits are physically separated by some distance on the equipment. 

For more information on these products or to comment on this article, e-mail engineering@quabbin.com .

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