Network Topology - Balancing A Cable based Tree Network
Various Network Topologies
The term Network Topology refers to the physical layout of the components of the network. There are a few different methods to lay out a network that are commonly used. They include; A Bus, A Star, A Tree and a Ring.
This article will focus on the Tree Network which is a mix of a star and a bus usually. The reason I am writing this is to clear up an issue I ran into when the cable installer came to troubleshoot the internet service at a new address.
A Bus Topology
A Bus network consists of a cable run that has multiple “spurs” coming off of the backbone and feeding the devices on the network. Each spur comes off the backbone by means of a tap. The tap has three terminals two of which are straight through connections with no noticeable loss and a tap which has attenuation built in. Taps come in various attenuation rates.
A Star Topology
A Star Network consists of a central hub with the devices connected to it like the spokes of a wheel. Each device communicates directly through the hub. Many times the hub will be both a gateway and a router but can be a simple hub feeding each device with no filtering at all as well.
A Tree Topology
The Tree Topology
The Tree Network is a combination of the Bus and the Star. It is the most common topology used in residential networks especially cable TV and Internet. In a Tree the devices connect to a hub or multiple hubs some of which feed from another hub and or the backbone if one is used. In cable TV/Internet, the drop from the utility pole ending at the point of entry is called the bus. This would include the modem if that is where the cable ends. There would be splitters coming off the bus and distributing the network to the various cable boxes and or modem.
In this case, the cable was already installed in the residence I moved to, and was quite a mess with old splitters and un-terminated cable. At some point a contractor came out and replaced the splitters and disconnected a long run of cable that was not in use. The network was still complicated, but was functional.
When I moved to this location, I connected the long run of cable that had been left off and replaced the connectors on that cable. I was not able to get a strong enough signal to allow internet service to my modem. A technician came out and determined that a booster was necessary, and installed one at the end of the long run, so that the entire boost was focused in one modem and TV box.
The problem was, that the signal was far too strong for the TV box and needed to be dropped using a tap. My contention is that the booster should have been placed at the point where the drop entered the home thereby boosting the entire network and making the tap unnecessary.
The Proposed Network
This network covers a main home and two remote buildings that are 100 and 180 feet away respectively.
The following two diagrams show the network as it exists and the proposed fix I made. In the first diagram, you can see that the booster was placed in the remote building (Dotted Green Box) right before the tap and the modem/TV Box. This makes the boost apply only to those two devices and is mostly wasted energy.
I proposed that the booster be placed on leg four of the splitter in the main House (Dotted Orange Box) so that the 180 + feet of cable going to the two remote buildings got the boost. The main house has adequate signal and does not require a boost. This is called "Balancing the Network."
Network Drawing 1
Network Drawing 2
Let’s assume that the signal coming from the cable supplier is +30DB as it enters the home. It first goes through a female to female connector with a ground tap. This has no appreciable loss. Next it hits a two way splitter and has a loss of -3.5DB on each leg. Then one leg hit another two way splitter at the main home. This means the Home modem is getting 23DB of signal and the TV box is getting 23 DB.
The other leg of the first splitter hits a four way splitter that feeds the other TV boxes in the main home and the remote buildings. At the output of the four way splitter the signal is now 16DB as each leg has a -7DB loss. At 16 DB we are still good for TV and for a modem.
The fourth leg of the four way splitter goes to another two way splitter at the remote building 1 and then to the remote building two. So remote building 1 gets 12.5 DB and remote building 2 gets 9 DB which is too low for reliable use.
Now at building 2, there is a booster at +15 DB and a tap. So after these added devices, the modem gets 24 DB and the TV/box get 20.5 DB. This is fine for building 2 but building one is marginal.
My proposal puts the boost in the main house boosting the run to the remote buildings. In this case, building 1 would get 27.5 DB and building 2 would get 24 DB. This would make the b2 modem receiving 20.5 DB and the TV box 20.5 DB. This way both remote buildings get good strong signal.
This is not rocket science, common cable company installers, get your acts together!
© 2012 Mark G Weller
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