Cabling considerations for multimedia applications

In the world of local area networks (lans), where unshielded twisted-pair (utp) cable seems to be installed everywhere, running video signals over the lan involves using some specialized equipment that cabling-plant managers probably don`t have to consider when overseeing a voice and data network.

Users who want to send data and video through a lan must use specialized connection devices.

Patrick McLaughlin

In the world of local area networks (lans), where unshielded twisted-pair (utp) cable seems to be installed everywhere, running video signals over the lan involves using some specialized equipment that cabling-plant managers probably don`t have to consider when overseeing a voice and data network.

"In the world of networking, utp cable dominates," says Eric Lawrence, manager of network technologies at Berk-Tek, an Alcatel company (New Holland, PA). "However, both baseband and broadband video applications are designed to run over coaxial cable. Most components in a video-transmission system look for a coaxial connection. So you need something to convert the 75-ohm, unbalanced coaxial cable to the 100-ohm, balanced utp cable." That something, he says, is typically the balun.

The balun derives its name from exactly that function--converting between balanced ("bal") and unbalanced ("un") signals. Some of the first baluns were used when the networking world transitioned from coaxial or twinaxial cabling to utp cabling.

"In the past, a lot of networks ran on twinaxial cable," says Mack Morgan, who works in technical support at Unicom Electric (City of Industry, CA). "The purpose of baluns is to connect to legacy systems such as an AS-400 system," he continues. "Our baluns allow users to connect a current utp system to a legacy ibm-type system."

Video transmission

Balun-makers contend that when it comes to transmitting video signals over a utp cabling system, their equipment is one of the few sureties. "There are so many sources of video transmission, it would be difficult to list them all," says Marci Mearns, sales director at Energy Transformation Systems Inc. (ETS--Newark, CA). "One that everyone is familiar with is the vcr," she says, "and other common sources include videoconferencing equipment and security cameras."

Berk-Tek`s Lawrence says that when the term "video" is used with relation to lans today, typically people are talking about analog as opposed to digital-video transmission. "A few years ago, it took many megabytes to get picture quality with digital video," he says. "But today, with the use of compression technology that has recently been developed, you can transmit digital video. For example, videoconferencing can be done digitally with the help of compression equipment."

Baseband and broadband

When dealing with analog as opposed to digital, the type of video--baseband or broadband--has implications on the ease or difficulty of transmission, according to Lawrence. "An example of baseband video is closed-circuit television, while cable TV is an example of multichannel or broadband video.

"Cabling makes the biggest difference on multichannel broadband video," he says. "Attenuation is an important factor, because you have loss throughout the cable run, as well as two video baluns, each of which contributes some loss. When the image appears snowy, that`s an indication that there is not enough power in the video signal."

He also points out that the dreaded characteristic return loss can affect video transmission. "The second most important factor is return loss, which results in the ghosting of images."

Still, the balun can be a valuable piece of the system, especially when working with a utp cabling plant. "If you have an unused pair in your utp cable, a balun can allow you to use that pair to bring video to the desktop," says ets`s Mearns. "That`s what`s being done at City Hall in San Francisco, and a similar approach is becoming popular in schools."

Media converters

Considering that running video over a utp cabling system typically involves the conversion from coaxial to utp cable--a media conversion--one might expect media converters, not baluns, to play a significant role. Mearns explains, "For many people, it`s a matter of terminology. I find that people who have implemented ibm systems use and accept the term `balun,` probably because a transition from an ibm system to a utp system requires balancing. But in a strict sense, `media converter` really means the same thing. So a balun is, in fact, a media converter in that it allows you to change from one medium to another and also balance the two forms of media."

ETS makes media converters as well as baluns, and Mearns says the term "media converter" doesn`t necessarily do justice to the device`s functions. "The industry by and large has accepted the term `media converter,` but often these devices also serve as protocol converters. They change the medium and the protocol in a lan."

For More Information...

"Coaxial cable," Cabling Installation & Maintenance 1999 Buyer`s Guide issue, page 8

"Unshielded twisted-pair cable," Cabling Installation & Maintenance 1999 Buyer`s Guide issue, page 15

"Baluns," Cabling Installation & Maintenance 1999 Buyer`s Guide issue, page 126

"Media converters," Cabling Installation & Maintenance 1999 Buyer`s Guide issue, page 128

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