Monday, 23 March 2015

Practical Number 2

Practical Number 2

Network Cabling connection for computer Lab
1. Cable Length Matters
The EIA/TIA standard says a 100 metres is the maximum cable length. THIS INCLUDES PATCH LEADS.  If you are using 10 metre patch leads at each end your horizontal cable run should be only 80 metres. If you have 20 metre patch leads then you can only have 60 metres in the wall.
2. Long Patch Leads Can be a Bad Idea
You can go longer – it might work. It’s more likely to work if you have good quality cable and connectors and a well executed cabling system since the electrical signal will degrade less in that environment. Cheap installs are more likely to have problems.
That’s why long patch leads can be a bad idea in some places in your office work just fine. And in other places they don’t. A 150 metre cable run probably won’t work, or worse, won’t work reliably.
Physical Considerations
3. Overloaded Cable Trays
Cable trays should not be overloaded. Suspended cable trays are mounted to something – either ceiling mounts or support from a rack underneath. If it’s too heavy, they can fall off the wall/ceiling etc. On to expensive things. That break.
4. Leave Space to Remove Unused Cables
Too many cables is not only a safety problem, but leads to poor operational practices when people stop removing cables from trays because it’s too hard or fear of disturbing cables.
5. Crushed Cables in overloaded cable trays.
Cables at the bottom of cable tray can be crushed by weight of cables. This especially applies to Cat6 cabling which is thicker and heavier than previous copper cabling. Cables which have been crushed will degrade signal propagation.
6. Hanging Cables will Stretch
Hanging cables within the rack means that means that gravity will induce physical stress on the copper core which will stretch and distort over time. A longer patch lead is heavier and will cause more shear stress on the cable. This will create signal degradation over time and leads to intermittent failures over time. Patch management isn’t just to keep your rack tidy, it has a mechanical purpose too.
7. Don’t pull too hard
Pulling cables can damage them by stressing the copper core. Stressing the copper core can cause stretching and thinning of the copper wire which affects the signal performance. In extreme cases it will cause unwinding of the twists in the sheath. That’s why standards specify 110Newtons as maximum draw force to be applied during installation. The cabling manufacturer should ensure that their cables are able to withstand this stress (cheap cabling is often not tested or designed, it’s just “made”).
Your cabling installer should use a force meter when pulling cable to Category 6 standards to ensure that pulling strength does not exceed to the stated maximum.
8. Electrical Conduits can’t be used for data cabling
Electrical conduits and termination boxes don’t work for Category 6 cabling because the right angle bend radii are smaller than the permitted bend radius for Cat 6 copper cable. Bend damage increases changes of cable kinking, copper core stress and leads to signal degradation.
Cheap Cables
9. Less Copper
Cheap cable has lower than standard diameter copper cores which causes signal loss and results in imperfect signal propagation. When non-standard copper cores are mated to standard cores it creates more interference because impedance mismatch causing signal reflections due to impedance mismatch.
Narrow diameter cores are also physically weaker and are more likely to snap or stretch further over time thus making them more likely to fail.
10. Cheap Patch Leads
Don’t use cheap patch cords. Again, cheaper cables often don’t quite match the standard and can cause signal degradation. Other possibilities to reduce manufacturing costs include:
·         non pure copper cores,
·         poor quality control over twist ratios,
·         poor packaging
·         sub standard sheathing
These things can all result in damaged copper cables.

Degrading Signals

11. Neat Cabling Causes Signal Interference

Neat cabling creates cross talk by electromagnetic induction and therefore signal degradation by virtue of induction. Cabling should be untidy, loosely bundled, and randomly mixed to avoid signal induction.
This is not common practice, and, to be fair, is an extreme approach since the signal leakage from your copper plant should be minimal. But technically, it’s correct that induction might occur.

12. Keep Away from Power Cables

When copper cables run parallel to electrical cables, they will act like transformers or inductors and induce 50/60Hz currents and noise spikes from the electrical cable. Put as much distance between data and power cables as possible.

13. Keep Away from Electrical Interference Sources

There are many sources of 50/60hz interference and you should also consider them. Don’t use flouro lights in your data center, use LEDs or some other low power lighting that will use less power and generate less interference. Keep motors in air conditioning room completely away from data cabling. Of course, your coolers should be on different power infrastructure to reduce power ripples.

14. Keep Your Cable Dry

Moisture changes the dielectric coefficient in copper and significantly impacts the signal performance. Keep your cable dry during the installation process. Using copper cable in conduiots or trenche sbetween buildings should include special considerations such as waterproof conduits and capped ends to prevent moisture creep.

15. Cables need to be cool

Cables, especially cables with PoE, can overheat in large bundles. This change in thermal property changes the electrical performance and impact the signal propagation.

16. Star Pass is Fail at Installation Time

A “STAR PASS” is not good enough. As shown in earlier points, your cable plant WILL degrade over time. When a cable tester shows the cable has barely passed (often called a Star Pass by cabling installers) the signal performance check it means that your cable will work today but eventually it will fall below specifications. In other words, a star pass is failed test and starred for attention (not praise). Make sure that your cabling contract demands rectification of “star pass” cable tests.

Fibre Optic Cables

17. OTDR testing is not enough

Fibre Optic cabling must be tested with an OTDR and Power Meter. OTDR testing is easily fudged and is really just shining a torch as a simple light test. What you really want to know is that the acceptable amount of light power is being lost in the installed and terminated cable run.

18. Fibre Optic Loss is Power Sum

The length of fibre cabling is less important than quality connectors and proper terminations. Each connector and splice causes a small amount of signal loss. Therefore, power levels are the important test factor for fibre optic cables.

19. Dust Caps have a Purpose

Dust caps on fibre optics connectors are used to prevent dust buildup inside the connector. A single mode fibre is 9nm wide and about the same size as dust. The laser signal can be seriously attenuated and thus reduce the run length or cause signal problems.

20. Don’t Kink Fibre Cables

Fibre optic patch leads are flexible but the fibre core can break, or worse, can fracture. This causes weak laser signal by creating power loss in the cable. Weak signals may not be decoded by the laser reciver. Be nice to your fibre cable.