Designing the business network: Cabling schemes

Why do you need a cabling scheme? Why not simply lay the cables on the floor?

Safety and reliability is the main justification here: You don't want your family/employees tripping over trailing cables: they may hurt themselves and will almost certainly damage the cables. Damaged cables are a principal cause of mysterious networking faults and can be extremely hard to trace...

The single hub-and-spoke approach

If you are implementing a small home or very small business installation, then you will most probably want to use a simple hub-and-spoke design, with a central Internet Access switch/router and cabled or wireless spokes. This approach can be used relatively easily for up to two adjacent floors: the cables run in the lower floor ceiling void or in trunking at floor level of the upper floor and feed up to the upper floor and down to the lower.

The multiple hub and spoke approach

More than two floors are better addressed by a multiple hub-and-spoke approach, where each floor (or pair of floors) is covered by a separate hub with its own spokes. The separate hubs can then be linked by a single cable.

Unfortunately, many buildings do not lend themselves to easy cabling. Lifting floorboards and drilling through walls and floors is disruptive and damaging. Hence, most people use wireless. The oddities of wireless will be discussed in a separate article.

Ad-hoc or flood wired?

Wired installations generally fall into two types: ad-hoc, where cables are run as needed, and flood-wired, where cables are run to all the likely locations.

Flood wiring has a higher initial cost and greater initial disruption but is far more versatile. Adhoc may have lower initial cost and less disruption, but the cost and disruption is repeated whenever a new cable needs to be run due to expansion of the network or rearragement of the house/office environment. Thus most businesses will opt for a flood-wired installation to be done while the office is being set up, converted or refurbished.

Flood-wiring schemes

It may appear that a flood-wiring scheme is simple to implement, but a number of things should be borne in mind:

• You will need to identify a central location to house your switch/router. Although a central location is not essential, it is sensible as it minimizes cable lengths.
• If your office is on more than one floor of a building, then the location needs easy access to all floors.

To minimize disruption, most businesses use skirting or dado rail trunking to distribute data network cables. These often contain mains power distribution and telephone cabling as well.

• Skirting trunking is popular but can be slightly obtrusive where it has to cross doorways and corridors.
• Dado trunking is more obvious, but can often be routed above doors and windows and across corridors at ceiling level. You can either put in 'drops' where conduit feeds down the wall from the trunking to sockets at desk level, or, perhaps more commonly, have the power, phone and data sockets mounted on the dado with cables forming the drop. The choice is yours. The former is more visually appealing and may be slightly more reliable but the latter is more versatile.

There are a number of 'rules' (actually, strong recommendations) to bear in mind when flood wiring:

1. Put in more network cables and points than you think you'll need. They are cheap (at this stage) and will avoid the need for additional wiring later or the inconvenience of scattering small portable hubs/switches around the office to connect additional devices.
2. Never drape loose cables across access ways or where they can be damaged by furniture. Wheeled office chairs are death to networking cables (and I have seen some extremely dangerous mains extension damage caused by people repeatedly rolling office chairs across a mains lead!). If you must have desk islands that don't touch a wall at any point, then you can:
   
   • Run trunking across the ceiling and drop down onto the desk island (often seen in supermarket check-outs)
   • Have suitable sockets inset into sunken boxes in the floor: this is nice if you own the building, but it is expensive, not very versatile (if you move the desks, you can't easily move the sockets) and may upset the landlord!
   • Run cables across the gangway inside a special shaped floor-level cable protector. However, remember that people can still trip over cable protectors (especially if they're not well maintained) and that they can be hard to wheel trolleys over.
3. Keep cables away from heating pipes, radiators and other hot equipment. Steam pipes will melt PVC network cables in seconds! If your cables need to be in contact with heated surfaces, use PTFE-sheathed cable or cover PVC cables with a loose-fitting heat-resistant sheath. Use of trunking obviates such problems.
4. Keep network cables away from powerful sources of interference. Most networked cables are un-screened and can easily pick up interference, especially pulse interference from electromagnets, arc welders and radio transmitters. If proximity to such equipment is essential, you might want to use screened twisted pair cable, but this is much more expensive and will need special jack boards to terminate and earth the screens properly.
5. Keep networking devices such a switch/routers, away from heat and interference sources.
6. If you put your networking devices in a cabinet or cupboard, make sure the ventilation is adequate. Most commercial-grade equipment will fail if the ambient temperature exceeds about 70°C. Similarly, cold will affect the devices too, as will damp, mice and other environmental hazards...
7. Most flood-wiring schemes result in all the spoke wires being returned to a central location and terminated on jack panels. The hub devices are normally mounted in or near the same rack. Make sure that you provide adequate power, security, access and lighting. Label all jack points so you know which spoke they relate to.

So I've installed my flood-wiring scheme: How do I use it?

To use a flood-wired installation:

• Plug the device you want to connect into the nearest network socket, remembering the 'rules' about trailing cables, heat sources etc. Note the ID of the socket you've chosen.
• Go to the network cubby and fit a patch lead between the jack socket that relates to the socket you've used and a spare port on the switch-router.
• Switch on your device and configure it (see the notes on designing your network and DHCP).

What if there are no spare ports on the switch/router?

Then you didn't design your network properly! But there is a fix:

• You buy an Ethernet switch-hub and install it in your cubby (this is why you need spare power points in the cubby!)
• Unplug a patch lead that services a spoke device (NOT the one to your main file server or to another hub!) from the switch-router
• Plug a new patch lead into the port you've just freed up on the switch-router. Plug the other end into the switch-hub. Normally you can use any port, but sometimes you must use a particular one designated for up-links, and sometimes there's a switch you have to set to the 'up-link' position
• Plug the patch lead you unplugged into one of the switch-hub ports.
• Plug a new patch lead from the jack socket you've connected your new end-user device into to a spare port on the new switch-hub.

You're done, and you now have more spare ports on the new switch-hub that you can use in the future...

Some flood-wiring 'gotchas'

There are a few things you need to look out for when flood-wiring a building.

• The maximum length of any one piece of Cat-5 UTP cable is about 90 meters. Sometimes longer lengths will work, but usually strange faults will appear that will be hard to trace. This limit is a design feature of Ethernet, where there is a maximum design distance of 100 meters between devices
• If you use a multi-hub design, make sure that you run a few spare cables between the hubs. Why?
  
  • Firstly, it gives you some spares in case the cable interconnecting your hubs fails.
  • Secondly, if you run out of hub ports on one floor but have spares on another, you can link them between floors using one of these cables, but remember that 100 meter maximum cable length between two devices.
  • Thirdly, if you later decide to upgrade to a resilient design, you can use a spare cable to provide the additional link.