ETS Ethernet Monoline Balun/Converter

February 1999

Background
Ethernet 10BaseT was designed as a broadcast, star based, two pair, local area network protocol. It is based upon a collision detection methodology where the Receiver listens while transmitting to assure that no other node is also transmitting. To accomplish this, 10BaseT Transmits its signal on one pair while simultaneously monitoring for any Receive signal on a second pair of wires. If a signal is detected during transmission, all stations are notified of a collision. This is done by broadcasting a collision signal through the network so that all stations receive it.

The design for 10BaseT was predicated on taking advantage of the widely installed telephone style 100ohm unshielded twisted pair (UTP) wire from the user's PC to a hub in the wiring closet. The topology is a star based system with all the users connected to a hub or a hierarchy of hubs. Only after the IEEE 802.3 10BaseT standard was issued, did the newly formed Telecommunications Industry Association (TIA) develop the 568 standard for premise wiring. It defined categories of cable with different levels of performance and required four pairs of UTP, run directly from the wiring closet to each user's room with a maximum length of 100 meters. 10BaseT uses pairs 2 & 3 (pins 1,2 and 3,6) on the 8-pin modular telephone jack (often referred to as an RJ45) that is standard in the TIA document.

Where other wiring is installed (such as the IBM cabling system, a two pair 150ohm shielded wiring system) it is a relatively simple task to convert the signals pair to pair for the other medium. However, if the existing wiring is coaxial cable ( such as 93 ohm RG62U), the issue is more complex. With only one wire path instead of two, then a method of coexisting the two signals, Receive and Transmit, without the transmitted signal destroying the collision detection function is needed. The ETS Monoline Balun is designed just for that purpose: to allow both signals on one line.

ETS also has a Monoline Converter which allows the use of a single pair of UTP. This device is useful for applications where adding new wire is difficult or expensive. Examples would be in homes or hotels that have phones that were installed with two pairs. The idle pair will support 10BaseT with a pair of converters. For large installations, such as the hotel application, ETS supplies the converters in the form of keystone wall jacks and 24-port patch panels.

Theory of Operation
The ETS Monoline Balun/Converter is designed to allow the Receiver to listen to the line while the Transmitter is transmitting but without the Receiver being able to hear the transmission. The only requirement is to maintain channel separation; on the 10BaseT side, the device must see an impedance of 100ohms +/- 20 %. Since the wire is specified at 100ohms +/-15% and most terminations are +/- 1%, there should be no problems with a proper installation. These devices will work with typical inside wire installed prior to the introduction of categories of data cabling in the TIA 568 standard. It may not work with "quad" wire where neither impedance nor crosstalk were controlled by design.

The technique used to accomplish this feat is a splitter/combiner (also called a hybrid in telecommunications) which offers signal combining for the cable run while maintaining isolation between the two signals on the 10BaseT compliant end of the device. With the transmitter connected to one line and the receiver to the other, they both have access to the line exiting the device but do not interfere with each other. Finally, the Monoline Balun/Converter impedance matches and balances the signal from the coaxial cable/UTP. Two Monoline Baluns/Converters are necessary, one at each end of the coaxial cable/UTP.

Because they combine/split two lines, splitters reduce the signal available to each line. The 10BaseT signaling levels were chosen prior to the 100 meter limitation of the TIA 568 standard, so that the signal strength after going through the balun/converter is still adequate.

Troubleshooting
First, determine if everything else is working properly by physically taking the PC to the hub (or running two pair UTP from the PC to the hub) and checking for standard 10BaseT operation.

If not properly operational, proceed to uncover the failure, which is not with the Monoline link. If successful over two pairs of UTP, then run a short piece (under 10ft. or over 20ft.)) of the installed cable (UTP or coax, depending on the device) between a pair of Monoline Baluns/Converters and recheck. Note that short pieces (10 to 20ft.) of marginal cable ( poor impedance) are susceptible to failure due to reflection reinforcement (a resonance condition). This condition disappears with cable over 20ft. long.

If the short link does work but the installed link does not, then there is most likely something wrong with the installed wire. A time domain reflectometer (TDR) should be used to expose shorts, opens and bridge taps. The TDR will also allow checking the link distance. A calibrated TDR can also verify impedance limits.

If the short link does not perform as expected, then the balun/converter may be the wrong device for the wire used. It is also possible that the device is faulty. Verify correct usage, replace and retest.

During installation, caution is recommended when interpreting hub lights. If the other end of the link is not properly terminated with either a device and its PC, or with a terminating resistor, the hub link light may indicate a successful link. Without proper termination, the splitter function fails and the receiver hears its own transmitter and believes it has a valid link.

Older 25 pair cable can exhibit significant crosstalk, especially with disparate signals from switching hubs, that may interfere with the one pair 10BaseT solution. We recommend only home run wiring without consolidation into multiple pair cable. An exception is with Category 5 or better cable. System level cross talk is not easily field tested and makes it difficult to field verify the source of crosstalk induced failure.