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February 20, 2008

The Ethernet Distance Issue

TMCnet Web Editor


 
How does one solve the issues of installing wireless AP’s easily? The issues are: Power for the AP, Ethernet Data for the AP, and being able to combine the two over distance.
 
Most all wireless installers have the same issue of requiring a new power source provisioned for their remote equipment. This is traditionally solved by having an electrician run separate circuits to the remote area and can be costly if not altogether prohibitive. Several installers have turned to using inexpensive Power over Ethernet (POE) units, since Ethernet cabling still has to be run to the Wireless Access Points anyway.
 
The issue now becomes POE at distance over cabling. Voltage drops over distance using wires. At 200M, you lose maybe 4 or 6 volts. At 48VDC, that leaves you with 42 volts. If you drop to around 15VDC, your Trango (News - Alert) may not come up. You can determine the amount of voltage drop over a distance by finding the resistance of the cable for a specific temperature from the cable manufacturer or electrical wholesaler.
 
As a rule of thumb you will loose 6V per every 100 feet – 200 feet of wiring, depending on the gauge.  Lower power also means more amperage, which means more heat and perhaps issues with the 24 gauge wires in CAT5. You may need to use 18 gauge wiring. For anything greater than 400 feet, use a 48V POE.
 
Now the second issue of Ethernet distance limits for CAT3 - CAT6 poses a challenge. Ethernet signals can reach a maximum distance of 328 feet or 100 metter safely without degradation (packet collisions, crosstalk, etc) according to the IEEE Specs. In practical reality you can squeeze more distance out typically depending on lots of interference factors.
 
One solution to this distance issue and support POE at the same time is to use an Ethernet extender. It was designed to carry Ethernet data over the standard 2-wire pairs (pins 1/2 and 3/6) and POE power over the spare pairs (pins 4/5 and 7/8). So they co-exist.
 
Another trick is to reduce the Ethernet line speed to 10Mb Full Duplex. This reduces the risk of packet collisions at distance.  If you have to, use a low cost 10Mb switch to front end your equipment at the head end. 
 
Brian Solomon is a Web Editor for TMCnet, covering news in the IP communications, call center and customer relationship management industries. To see more of his articles, please visit Brian Solomon’s columnist page
 
Don't forget to check out TMCnet’s White Paper Library, which provides a selection of in-depth information on relevant topics affecting the IP Communications industry. The library offers white papers, case studies and other documents which are free to registered users.
 

 
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