Structured Network Cabling Primer (Part 2 of 3)

Structured Network Cabling Primer

Part 2 of 3

When working with cloud-based applications, such as Google products, its helpful to know networking basics to communicate effectively with technicians who provide the structure that make cloud use possible. In Part 1 of this three-part networking structure series examined the Office Environment and the components of the network that are typically found there.  In Part 2, reviews the hidden wiring (horizontal runs and vertical risers) and the components of a typical wiring closet.

Environments and components of a typical network

Wiring Closets

The next major network components encountered, when moving from your device across the network, are "horizontal cables" or "runs," followed by where "runs" terminate in wiring closets.

Each floor of a building typically has a wiring closet.  This room contains several pieces of network equipment, often including a small rack or communications equipment enclosure, one or more "patch panels," and one or more hubs or switches.


The purpose of a wiring closet and the network equipment in the wiring closet is to connect "horizontal cables" (or "runs") that lead from Work Area Outlets (WAOs) and "vertical cables" (or "risers") that lead to equipment rooms.

Patch panels are used to provide a mechanism for terminating horizontal runs and vertical risers so that they can be connected using patch cables.  By using patch panels for cable termination, changing how a work area outlet is connected to the network takes seconds and involves only unplugging a patch cable from one port and plugging it into another.

A patch panel

"Horizontal cables" and "vertical cables" are not named because of their orientation with respect to the horizon.  Horizontal cables run along a single floor and connect wiring closets with Work Area Outlets.  Vertical cables run between floors and connect wiring closets with equipment rooms.

Vertical cabling (that goes between floors) requires a jacket (the outer part of the cable) that has a specific resistance to flame; this is called "riser-rated cable."  Horizontal cabling (that does not leave the floor) generally does not require this increased flame resistance.  The exception is when a cable is laid inside part of the air handling (HVAC) components (e.g., plenums / ducts); cable that goes through HVAC components -- regardless of whether it leaves the floor (vertical) or not (horizontal) -- must have an even higher resistance to flame and release very little smoke when on fire.  This is called "plenum-rated cable."

Each rating of cabling can substitute for lesser rated cabling.  So, for example, you can use plenum- or riser- rated cabling for horizontal runs (where only general-use cabling is required); you can use plenum-rated cabling for vertical risers (where only riser-rated cabling is required).  You can not, however, use general-use cabling for risers or in HVAC components; similarly, you can not use riser-rated cabling in HVAC components.

Why not use plenum-rated cabling everywhere?  Plenum-rated cabling is generally more expensive than riser-rated cable; riser-rated cable is generally more expensive than general-use cable.

The rating of the cable (e.g., plenum, riser, general-use, etc.) doesn't affect the speed, performance, or quality of the conductors.  That is, plenum-rated Cate5e cable has the exact same performance characteristics as general-use Cate5e cable.  The rating of the cable only refers to the jacket and how that jacket reacts to flames.  So, even though Cat6 has the potential to move more data faster than Cat3 cable, you still need to use riser- or plenum-rated for vertical cabling.

Using a cable that is not rated for it's application (e.g., using general-use cabling in a riser) is a violation of the National Electric Code (NEC) and places the building and it's inhabitants in danger in the event of a fire.

Note -- local codes and ordinances may supersede the NEC; be sure to check with your local inspector or fire marshal before laying cable.

Moving from the office to the wiring closet (via horizontal runs), cables need to be terminated.  The most common and most flexible approach is to terminate the horizontal runs at a Patch Panel.  A Patch Panel is a device that has a bunch of network jacks on one side and connectors that the horizontal cables attach to on the other side.  These connectors are usually "Insulation Displacement Connectors" (IDC); when the individual conductors are pressed into the correct place (e.g., being "punched down"), small metal blades cut through the insulation and make contact with the individual conductors.

How do you know what conductors go where?  Well, each conductor's insulation is color-coded.  The IDCs on the back of the patch panel are usually color coded as well.  The colors include Blue, Blue and White stripe, Orange, Orange and White stripe, Green, Green and White stripe, Brown, and Brown and White stripe.

Just to complicate things, there are different standards for which conductors go where.  For networking, the two predominant standards are TIA/EIA T568A and TIA/EIA T568B.  The T568A standard is used mostly in Europe and United States Federal Government installations; the T568B standard is used mostly in older installations in the United States.  The T568B standard is the most common standard in the United States; however, the T568A standard is the currently recommended standard.

T568A and T568B Wiring Diagrams

The standard selected doesn't matter, as long its consistently used as the same standard everywhere on your network.  Yet another standard is the USOC standard; it's predominantly used in voice (e.g., telephone) communications.  USOC and the T568A standards are the same for the innermost two pairs; the outermost two pairs are different between each of T568A, T568B, and USOC.

The important thing to remember is that patch panels need to be wired the same way as the Work Area Outlets (WAOs).  Pick a single standard for all data networking and stick to it throughout the entire network.

After horizontal runs are punched down to patch panel(s), the next step is to connect patch panel(s) to hubs / switches in the wiring closet.  For this, use standard patch cables, just like the types used to connect office devices to their Work Area Outlets.

Hubs and switches are used to allow devices on the same network segment (e.g., subnet) to communicate with each other.  For example, if you have two devices in one office that are on the same network segment, data from one would go through a patch cable to a WAO, then through a horizontal run to a patch panel, then through another patch cable, and finally to a hub or a switch.  The hub or switch then sends that data to another port on the hub or switch; this port is connected via yet another patch cable to a different port on the patch panel, then through another horizontal run that leads back to the WAO, and finally through another patch cable to the other device.

So, what's the difference between a hub and a switch?  When data comes in through a port on a hub, it is distributed out to every other port on the hub; when data comes in through a port on a switch, the switch figures out which port that data should go to and sends it to only that port.  Switches are generally smarter, usually higher performance, and typically more expensive than hubs.

What happens if one device wants to communicate with another device on a different network segment?  That's where routers come in -- they route data between network segments.

The final part of the wiring closet is vertical (riser) cabling that connects wiring closets with equipment rooms.  Vertical (riser) cabling is usually connected to ports on the patch panel; patch cables then connect those ports on the patch panel to ports on the hub or switch.

Now, we've covered the office and wiring closet environments.  The next piece of the puzzle -- to be addressed in Part 3 -- will be the equipment rooms.


-- Wes Dean, a Google Apps Certified Deployment Specialist and a Google Apps Trusted Tester, is Principal of KDA Web Technologies, a Google Apps Authorized Reseller. To learn how Wes and KDA Web Technologies can help you, go to http://www.kdaweb.com/.