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Telephony: Cable Telephony Comes of Age
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In this column nearly five years ago, I conducted a verbal "tour" of a telephone company’s central office, to give our industry’s technical people some insight into how telephony could change a typical headend. With all the activity in cable telephony in year 2000, I thought it would be interesting to revisit that tour side-by-side with a tour of a headend that is actually equipped for cable telephony. It’s kind of fun to see how far we’ve come, and in many ways, how much better we look than our telco counterparts.

Realize, however, that this tour is a visit to a circuit-switched telephony headend. Sometime in the future, we can do this all over again for Internet protocol (IP) telephony. That doesn’t mean this tour is irrelevant—it just reflects the dynamic nature of telecommunications in our industry. In the meantime, however, there’s a whole bunch of cable headends that are becoming their own version of a telco central office.

So, here’s the side-by-side tour. The old central office tour from my May 1996 column is in italics. Today’s cable telephony tour is in regular type.

Telco: The telco central office is readily recognizable as a telephone building by the brown brick exterior, the reception-less entry door, and relative lack of windows. There’s typically a parking area for telco service trucks. So far, not too different from the headend.

Cable: Some things never change.

Telco: Now, let’s walk into the building. We start our tour in the basement. This is where cables containing the twisted pair copper wires enter the building. In a metropolitan location, almost everything enters from underground cable ducts. Typically, the cable diameter is 2 or more inches. The room we enter is pressurized and the door is alarmed to ensure it remains closed. That’s because toxic "sewer gases" could also enter via the cable entry points.

Cable: Hybrid/fiber coax is a lot more efficient media for the "last mile." There’s no need for a pressurized room to house thousands of cables entering the building. Even considering splitters and combiners, our link to the subscriber is a lot more compact.

Telco: Outside the cable room we find the power for the central office equipment. Of course, commercial AC enters the building and is converted to -48 volts by a bank of rectifiers. What we don’t expect is the nearly 50-foot long shelf holding 3-foot high glass cylindrical lead acid storage batteries. This power reserve is connected to a bus to the commercial power line and central office equipment. This is the last course of electrical reserve, which must power the office for at least eight hours. They are the last choice for powering the office, and hopefully, they will never be used. They are continually being charged, however, to maintain readiness.

Close by, in another room there is a diesel generator, which is an alternative source of AC power and the first alternative in the event of commercial power failure. Diesels generate toxic fumes, so this subbasement room must be rigidly ventilated to the outside with a system of fans, ducts, and filters.

Cable: The headends I have visited typically have the diesel generator outside the building, or rely on natural gas to power it. Like the telco, we are equipped with battery reserve, which is usually inside the headend building. We engineer to the same standards as the telcos for hours of outage.

Telco: Our next stop is on the ground floor, where we find the distributing frames. This is the cross-connect point for the lines from the outside world to the central office equipment. Common to all systems of cross-connects is that one side of the cross-connect goes to the wire pair to the subscriber, and the other side goes to the central office equipment, typically a switch line card. The frame has two sides: a vertical side, where the lines from the cable entry room are terminated, and a horizontal side, where the equipment is terminated. As part of the line installation, the craftsperson makes the crossconnection between the two.

Cable: We bring coax cables to the host digital terminal (HDT) frame, where demodulation occurs. Connection to the path to the switch is done by internal electronics, within the Next Generation Digital Loop Carrier (NGDLC) part of the HDT. The link to the switch is over DS1 lines, each transporting 24 separate subscriber calls.

Cable management has markedly improved. The headends I have visited have color-coded coax for telephony, which is neatly bundled along the rear side of each frame, up to the point of termination for each cable. It’s a visual pleasure to see an HDT frame that has been properly cabled.

Telco: Now that we’ve seen the interfaces to the outside world, we’re excited about getting to the business end of the offices—the switch and the transmission plant. The switchroom is somewhat of a disappointment. In the far corner of the room are four rows of what looks like computer equipment, but is actually the digital switch. It’s quiet in here. All you hear is the hum of the equipment cooling fans.

On closer inspection, we see that the rows of equipment are arranged by functions. The row closest to the wall is the processor portion of the switch, where the system software resides, and where the printer and display terminal are connected. This row also houses the hard drives with billing records.

The other rows contain frames of switching modules. Some of them hold circuit cards for line circuits (the equipment connected to the lines on the distributing frame). Others have trunk circuits (similar equipment connected to the rest of the public switched network, possibly to interexchange carriers), and service circuits for announcements, testing and signaling, such as ringing the line.

In some central offices, other equipment for special service offerings is put in the switch lineup. One example might be a voice messaging system, to provide the subscribers with an alternative to an answering machine.

Cable: Our telephony switches are often remotely located from the headend. In this case, the only headend interface is the DS1 lines from the HDT to the switch, which are often multiplexed to OC-x rates and carried over fiber to the switch location. The headend tech will therefore also be responsible for maintaining the multiplexing equipment and connections.

The switch itself is not much different than its telco counterpart—which is one reason we can offer telephone service which is the same or better quality than our competition. We follow all the same rules for electrical distribution, especially grounding, and physical environment.

Our tour of these two worlds has been completed. Like I said at the beginning, it’s fun to see how far we’ve come. Obviously, as an industry, we still have a lot of work to be done if we are to be accepted by subscribers as a viable alternative to the telephone company. Much of that work is training our personnel to maintain the headend telephony equipment and the associated services, especially the HDTs and their interfaces. We’re moving in that direction too, but that subject needs to be saved for another column.

Justin J. Junkus is president of KnowledgeLink, Inc., applications engineering director for ANTEC, and Communications Technology’s telephony editor. Upi may reach him by e-mail at .

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