Wiring WLAN Access Points

When deploying a wireless LAN, you can easily forget about the wired side of
the network. The access point wiring, sometimes referred to as a distribution
system, is very important because it is what connects the access points into
the corporate network. In addition to providing access to resources on the network,
this wiring also provides the means for roaming.

802.3 Basics

The IEEE 802.3 standard
specifies the Ethernet
protocol, which includes a medium access
method that’s similar to 802.11. All users on the network must take turns using
the medium with Ethernet hubs. An Ethernet switch, however, enables multiple
collision domains that can allow simultaneous transmission among users to improve
performance. For larger networks beyond the size of a home or small office application,
be sure to use switches for optimum performance.

Of course the major difference between 802.11 and 802.3/Ethernet is that the
latter uses twisted pair wiring, coaxial cable, or optical fiber for interconnecting
network devices. The use of coaxial cables in older LANs was fairly common about
ten years ago, but today most companies use twisted pair and sometimes fiber
for moving major amounts of data.

The Electronic Industries Association and
Telecommunications Industry Association
(EIA/TIA) specifies Category 5 (referred to as "Cat 5") twisted pair
wiring, the most popular of all twisted pair cables in use today with Ethernet.
Cat 5 consists of four unshielded twisted pairs of 24 gauge wires that support
Ethernet signals over 100 meters (or about 328 feet) of cabling. You can use
Ethernet repeaters to increase this range if necessary, which is one method
of reaching an access point that’s beyond a hundred meters from a communications
closet.

There are also other variations of twisted pair wiring. Enhanced Cat 5 (referred
to as "Cat5e") makes use of all 4 pairs of wires to support short
range Gigabit Ethernet (1000 Mbps) connectivity. It is also backward-compatible
with regular Cat 5. Cat 6 and Cat 7 cable are now available, bringing more bandwidth
and range to copper-based Gigabit Ethernet networks. Cat 7 cable features individually
shielded twisted pairs of wires, making it ideal for installation in locations
there is a high potential for electro-magnetic interference.

10Base-T

10Base-T is one of the 802.3 physical layers and specifies data rates of 10Mbps.
A typical 10Base-T cable uses two of the four pairs within a Cat 5 cable for
sending and receiving data. Each end of the cable includes RJ-45 connectors
that are a little larger than the common RJ-11 telephone connector used within
North America.

The advantage of having extra pairs of wires open is for supporting other uses,
such as power-over-Ethernet
(PoE). This is a mechanism in which a module injects DC current into the Cat
5 cable, enabling you to supply electrical power to the access point from the
communications closet. PoE eliminates the need for having an electrician install
new electrical outlets at every access point location. For larger networks,
definitely consider the use of PoE.

If you are going to run a strictly 802.11b network, 10Base-T cable will do
fine. Even though 802.11b boasts
data rates of up to 11Mbps, the actual throughput offered (about 4 to 5 Mbps)
fits well within the capacity of 10Mbps Ethernet.

100Base-T

Another 802.3 physical layer, 100Base-T, supports data rates of 100Mbps. Similar
to 10Base-T Ethernet, 100-Base-T uses twisted pair wiring, with the following
options:

  • 100Base-TX uses two pairs of Cat 5 twisted-pair wires.
  • 100Base-T4 uses four pairs of older, lower quality (Cat
    3) twisted-pair wires.

In most cases, you’ll be using the 100-Base-TX cabling. As with 10Base-T, you
can use the unused pairs of wires for PoE. 100-Base-T4 was popular when needing
to support 100Mbps data rates over the older Cat 3 cabling, which was prominent
during the early 1990s.

If you’re deploying 802.11a or
802.11g
access points, strongly consider using 100Base-T. 802.11a and 802.11g consistently
deliver actual throughput over 10Mbps, making 10Base-T a possible bottleneck
to performance.

Optical Fiber

Optical fiber cable is more expensive than twisted pair, but it can be cost-effective
due to support of gigabit speeds and a range of up to two kilometers. Instead
of using the traditional electrical-signal-over-copper wire approach, optical
fiber cable uses pulses of light over tiny strips of glass or plastic.

This makes optical fiber cable very resistant to electro-magnetic interference,
and very valuable in situations where electronic emissions are a concern. In
addition, it’s nearly impossible to passively monitor the transmission of data
through optical fiber cable, making it more secure than twisted pair wiring.

With wireless LANs, optical fiber is a possible solution to reach an access
point located beyond a hundred meters from a communications closet. This requires
the use of a relatively expensive pair of transceivers, though, which transform
electrical signals into light (and vice versa).

One issue when dealing with optical fiber cable, however, is the difficulties
in splicing cables, mainly because of the need to work with glass or plastic
materials that requires precise alignment. You need special tools and training
to make effective optical fiber cables. As a result, purchase precut fiber cables
to avoid problems that are difficult to troubleshoot.

Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs and
offers computer-based
training
focusing on wireless LANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

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