Line density, lateral connector strain, cable radius & equipment airflow management enable the success of wireless systems.By Dale Reed, Trompeter Electronics
Network architects talk broadly about their world as "edge" and "core". In the case of cellular voice traffic the "edge" is the individual cell site transmitting to and receiving from the cell phone, and the "core" is the public telephone company that manages the voice call to its destination via conventional land lines. Wireless signal transport has proven to be a viable option for service at the edge of the network (see Figure 1). For convenience and portability, wireless technology cannot be beat. It is essential for applications such as marine radio, GPS, or targeting. With frequency stacking, use of a broader band of higher frequencies, and other innovations, wireless technology is moving nicely into broadband applications including Internet access.
A new miniature BNC connector series developed by Trompeter achieves an improvement of 40% more interconnects per given area, while retaining all the attributes of the standard series telco BNC that are critical to reliable network performance.
In the wireless support system many of the network edge, switching and core access elements are copper-based wireline technology. Examples include wiring bays in telephony central offices (CO), CATV headends and broadcast studios are substantially coaxial cable. This technology still carries significant advantages in cost, ease of use, high data rate/frequency signal clarity and bandwidth. Despite tremendous capacity available in fiber technology and ongoing attempts (such as xDSL) to utilize the vast installed-based of twisted pair copper, coax is still a favored and proven technology with low risk and extremely low line failure rates.
As line density has risen, however, so has the amount of cable attached to the back side of these bays. In the case of DSX cross-connect switching or video patch jacks, the number of individual connectors can be enormous. As a result, several problems arise which need addressing.
Back side of a standard HDTV patch panel rack, featuring nested 45 degree and 90 degree 75 ohm BNCs.
First, the weight of the cable acts as a lateral force on the RF connectors that interface the cable to the back side of the equipment. In the case of a BNC connector, the constant lateral downward pressure can, over time, deform the wave washer that provides the mating force unique to the BNC design. Use of higher quality "telco central office type" BNC's can eliminate this particular risk. This type of BNC connector features byrilium copper in the wave washer, a more costly material choice which provides wonderful compression set resistance over time for a positive electrical engagement as well as a mechanical resiliency.
Further, this downward lateral force on the connector is of greater significance as cable "fall" increases consequently equipment located higher in the bay (rack) is impacted more than equipment on the bottom. While cable-tying the bundles of hang wire may create an organized appearance, it does not necessarily provide weight relief. Additional considerations include the weight on the jacks and the panel front.
Perhaps the most important issue at stake with respect to signal integrity is the radius (bend) that the cable takes in making the transition from horizontal (in line with the jack and BNC) to the vertical (gravity) (see Figure 2). To the extent that the cable deforms over time, this radius reduction can alter the critical dielectric spacing in the cable at the bend, negatively altering transmission line performance. The higher the frequency, the more pronounced this effect becomes.
Exceeding the recommended bend radius of the cable can produce return loss or signal reflection problems (also called VSWR) where the inbound signal is returned, altering the incident wave form. In the worse case scenario, the wave form reflected is equal in amplitude to the incoming wave and is out of phase by 180 degrees, producing an effect that makes the inbound wave appear that it is not even present. This plays havoc with data transmissions, even if it is all about ones and zeros!
All of the mentioned problems are complicated by the use of lower loss (larger diameter) cable for longer runs or higher bandwidth/frequency capabilities. In addition, as space limitations are encountered, stations and switching facilities are demanding greater equipment density. New connector designs such as the Trompeter M-BNC (mini BNC) solve this problem by enabling up to 40% increased density per given area, which by itself, further exacerbates the cable volume and weight predicament.
An additional issue that surfaces as a result of high cable density is "curtain effect" blockage of airflow in the "depth" direction of the rack units. This airflow blockage can have a potentially serious impact on the service life of sensitive heat-producing electrical equipment. Thermal degradation of electrical equipment due to insufficient air movement and radiant cooling has been well documented. Simply stated, blockage of air movement due to a curtain of hanging cable is not a good thing for reliable network performance.
An elegant solution to these problems involves the use of 45 degree connectors, which typically cost the same as 90 degree BNCs. Using 45 degree BNCs, plugs can be nested on top of each other, directing the associated cable horizontal to the side of the rack unit. At this point, the cable can be tied off to the rack, alleviating the weight strain regardless of the height of the equipment. Most importantly, the approach permits maximum uninterrupted airflow in the z-direction of the cabinet for full radiant cooling of the terminated equipment.
Wireless technology depends on a good bit of conventional copper-based wireline technology to enable a systems level of overall functionality. Coax transmission lines are a robust and proven solution for many of these applications. Line density, lateral connector strain, cable radius issues, and equipment airflow management are all factors that, when done well, enable the success of the wireless system.
Dale Reed is the vice president of marketing for Trompeter Electronics. The company is located in Westlake Village, CA. Questions or comments on this topic can be directed to firstname.lastname@example.org.