Solutions
- T-1 & T-3 Circuits
Feature - Future-proof Media May No Longer Be a Reality
Explanation - The Importance of Cable Capacitance
in Electronic Applications
Expert Profile - Chris Burr
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T-1 & T-3 Circuits |
| for LAN to WAN Interconnection |
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| Cabling used to connect a T-1 circuit. |
Because local area networks are growing larger, faster and more complex, and campus, metropolitan and wide-area networks are becoming more common, the need for multi-channel, high speed data transmission is increasing.
Local and long distance telephone carriers have historically been the first choice for internetwork transmission. They use existing multiplex switching technology, commonly referred to as T-1 and T-3. These are digital time-division-multiplexing (TDM) schemes, 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.
Connection
Methods & Devices
Many cabling types and methods may be used to connect networks to DS-1
circuits, including copper cabling, fiber optic, coaxial, microwave
and even satellite transmission links. 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 of these 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 the most practical choice, as it is often already in place. The total connection cost is less because the DS-1 devices required for copper are simpler and less expensive than fiber devices.
When T-1 service enters a 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 incoming 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 is a 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 when they were confined within a telephone company's central office facility.
Now that T-1 and T-3 circuits are finding their way into LAN wiring closets, the requirements have changed. Most new DS-1 system interface devices utilize either shielded or unshielded RJ45 Telco modular connectors. The older ABAM cables are too large to fit these relatively inexpensive and widely used connectors.
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Cabling
Confusion
The local
Telco service supplier usually provides T-1 service into the entrance
facility or to the "Demarc" point, and the 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 5 UTP, 50 ohm security cable, cash register
cable, 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, pulse shape definition and many others. DS-1 systems
are far different than LAN Category 5.
Where
to Find the Right Cables
Ekris Cable has several cable assemblies that meet both the electrical
requirements for DS-1 data and also terminate to an RJ45. These cable
assemblies employ a unique 2-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 layer will fit into either a shielded or unshielded
RJ45 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 RJ45 modular connector
system. For a detailed review of the performance characteristics of
these cables, call and ask for specifications on P/N 9720 and P/N 9745.
Return to Spring, 2001 Newsletter Directory
Future-proof Media May No Longer Be a Reality
For many years, proponents of optical fiber solutions for information transfer have hailed fiber as the ultimate future-proofing media. The TIA/EIA standard offered 62.5/125-micron multi-mode optical fiber as one of the three recommended horizontal media. Neither the distance limits nor the bandwidth capacity had been challenged by high-speed applications until the advent of 1,000 Mbits/second Ethernet.
Studies now show that 62.5/125-micron multi-mode fiber's information-carrying capacity and its power-coupling efficiency are insufficient to meet the distance requirements of the application. Some in the industry suggest using 50/125 fiber for Gigabit Ethernet.
Users now have to revisit the standard recommendations to evaluate their relevance to the future needs of the user's own network. In deciding between 62.5/125 and 50/125 fiber, keep in mind the difference between the two. 62.5/125 fiber was designed with a core exactly half the size of the whole fiber, since it makes the fiber less sensitive to bending losses. It has a numerical aperture (NA) of 0.26, which means it accepts light in a cone of about 30° angle. 50/125 fiber has a smaller NA of about 0.21, which means it accepts light in a smaller cone of about 25°.
The
higher order modes in 50/125 fiber do not have as large a path variation
as in the higher NA 62.5/125 fiber, reducing the modal dispersion. The
smaller acceptance cone and smaller core diameter make it easier to
make 50/125 fiber more precisely compensate for modal dispersion, yielding
a fiber that is easier to build to higher bandwidth specifications.
The diagram below illustrates the coupling difficulties or power penalty
that can occur when mixing these two types of fiber in one installation.
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| 50/125
to 62.5/125 The smaller core of the 50/125 easily couples to the 62.5/125 and is very insensitive to offset and angular misalignment. |
62.5/125
to 50/125 The larger core of the 62.5/125 overfills the core of the 50/125, creating excess loss. |
For now, it appears that no universal future-proof media exists. Although some manufacturers have developed a newer 62.5/125 fiber with bandwidth equivalent to 50/125 fiber, the newer 62.5/125 fiber will be more costly than single-mode fiber. Existing 62.5/125 fiber cannot sufficiently meet the requirements of Gigabit Ethernet, and 50/125 fiber is bend-sensitive and has greater loss at connectors. Single-mode fiber networks are thought to be more costly than multi-mode networks, due to light source and connectors, but new technology may, in fact, lower the total cost below that of systems with the newer 62.5/125 fiber.
Overall, 50/125-micron fiber optic cable performs better than 62.5/125-micron fiber optic cable. However, studies suggest that 50/125 fiber is not an absolute necessity for 1,000 Mbits/second Ethernet. Instead, 62.5/125 fiber is a much more viable option. The newer 62.5/125 multi-mode fiber can support 1,000 Mbit/second Ethernet signals without error for distances 150-200% of expectation.
With all of this information in mind, the message to many network managers is "buyer beware." Giving thought to where a network is heading, as well as what is in place right now, must be evaluated before moving ahead with the fiber networks.
Return to Spring, 2001 Newsletter Directory
The Importance of Cable Capacitance in Electronic Applications
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| Ekris stocks a full line of low capacitance cables, including this 3-pair cable with PVC jacket. |
When computer systems were first introduced decades ago, they were all relatively large, slow-speed devices that were incompatible with each other. Today, national and international networking standards have established electronic control protocols that enable different systems to "talk" to each other. The Electronic Industries Alliance (EIA) and the Institute of Electrical and Electronics Engineers (IEEE) developed standards that established common terminology and interface requirements, such as EIA RS-232 (equipment to modem interface) and IEEE 802.3 (Ethernet-based network). If a system designer builds equipment to comply with these standards, then the equipment will properly interface with other systems.
Many of these standards specifically identify cable constructions and electrical performance necessary to meet the system data speeds and transmission distances. If a certain cable design is specified by a system manufacturer, it is done so for a reason: any cable with less than the specified performance criteria will degrade the system. If using a cable with inferior performance, the output signal can deteriorate too much, causing the device at the other end to not recognize it, or, worse, to record false data.
While there are four characteristics important in the performance of an electronic cable (impedance, attenuation, shielding and capacitance), we are going to look at the role capacitance plays.
Capacitance in cable is usually measured as picofarads per foot (pf/ft). It indicates how much charge the cable can store within itself. If a voltage signal is being transmitted by a twisted pair, the insulation of the individual wires becomes charged by the voltage within the circuit. Since it takes a certain amount of time for the cable to reach its charged level, this slows down and interferes with the signal being transmitted. Digital data pulses are a string of voltage variations that are represented by square waves. A cable with a high capacitance slows down these signals so that they come out of the cable looking more like "saw teeth", rather than square waves. The lower the capacitance of the cable, the better it performs at higher frequencies.
Minimizing cable capacitance can be accomplished by increasing the insulation wall thickness, decreasing the conductor diameter and using an insulation with a lower dielectric constant. Because insulation wall thickness and conductor diameter cannot be altered without considering several other variables, insulation material often becomes the critical variable.
Years ago, most computer cables were insulated with PVC (dielectric constant of about 6.0) and capacitances of 40 pf/ft for a pair component were adequate. Today, with more stringent system requirements, most true low capacitance cables have capacitance values of approximately 12.5 pf/ft. Some systems now being designed will require capacitances as low as 9 pf/ft or lower. Using an insulation material such as foamed polypropylene or polyethylene (dielectric constant of about 1.6) can help to achieve true low capacitance.
We at Ekris Cable stock a full line of low capacitance cables, from two-pair through 18 1/2-pair constructions. This ensures that our cable assemblies meet most system requirements for true low capacitance. These cables utilize polyethylene insulation with a flexible PVC jacket to provide ease of termination and installation for our customers.
Return to Spring, 2001 Newsletter Directory
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Chris Burr |
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Chris Burr joined EKRIS Cable's sales force in the summer of 2000. He previously worked in the insurance, real estate and graphic design industries, fields in which computer technology and data communications play such an important role. His customer service and sales experience, along with Ekris Cable's extensive training program, give Chris the tools needed to meet all of your connectivity requirements. |