A Primer on Wireless Network Essentials by lonyoo


									                           A Primer on Wireless Network Essentials
                                 J. Eric Smith, MCSE, CNE, CCNP, CISSP

                  A Primer on Wireless Network Essentials
So you've finally done it: you've committed your organization to deploying an 802.11-based
wireless network. If you're like most people, this decision was not arrived at lightly, with
security and usability concerns weighing heavily against the needs of the more mobile,
flexible workforce of today. But now it's in the budget, and you're on the hook to deliver.
Only one question remains: exactly which 802.11 standard will you deploy?

The fact that there are multiple types of 802.11 networks can come as a rude surprise to the
wirelessly uninitiated. Unlike the tried and true Ethernet with its well-known simplicity and
ubiquity, wireless networks have followed a more tortuous, divergent path to their current
incarnations. The result is a dizzying array of letters following the 802.11 moniker, such as
802.11a or 802.11g. Each one denotes a specific subset of the 802.11 standard, and
understanding what each letter brings to your network makes the difference between
deploying a marginally-capable wireless infrastructure and one that will serve your
organization for years to come.

Key factors to understand when designing a wireless networking solution are as follows:

   1.      Frequency band - This is the portion of the electromagnetic spectrum where the
           device operates. For networks in the United States, this spectrum is governed by
           the Federal Communications Commission (FCC). The various 802.11 standards
           all operate in what is called the "unlicensed" frequency spectrum, a spectrum that
           covers portions of the 900-megahertz band, the 2.4-gigahertz band, and the 5-
           gigahertz band. These bands are called unlicensed because anyone is allowed to
           transmit within them without the need for an FCC license. While this makes for a
           fantastically diverse array of products, it also means that anyone can potentially
           interfere with your 802.11 network. Also, certain frequencies penetrate structure
           better than others. Other frequencies are better at high data rates, while still
           others are easily interfered with by cordless phones, microwave ovens, and baby
           monitors. Proper spectrum selection is crucial in order to get the best range and
           least interference for your intended application.

           Frequency bands are divided into discrete slices known as "channels," with each
           channel being assigned a number. An important wireless concept to understand is
           how certain channels overlap other channels, restricting usage to far fewer
           channels than you might think. Access points operating on the same channel will
           interfere with one another and prevent proper operation of either one.
                           A Primer on Wireless Network Essentials
                                  J. Eric Smith, MCSE, CNE, CCNP, CISSP

   2.      Data rate - Often expressed in terms of megabits (mbit) per second, quoted data
           rates should be taken with a large grain -- or perhaps a small boulder -- of salt.
           Data rates shown by most vendors are the theoretical maximums achieved in
           laboratory conditions -- something you'll never find in a real-world environment.
           Most effective data rates are less than half the quoted rates -- an important thing
           to consider if your intended application is bandwidth-heavy. Data rates also
           decay rapidly as devices move further from the access point, making proper
           coverage planning essential.

   3.      Density - Unlike contemporary Ethernet switches, wireless bandwidth is shared
           among all devices associated with the same access point. This puts a reasonable
           maximum on the number of systems that can be used in a given area covered by a
           single access point. If your intended usage is a high-density environment such as
           a call center, selection of the proper 802.11 technology is crucial for success.

   4.      Range - Range depends heavily on three things: transmit power, the
           aforementioned frequency band, and the structure within which you deploy the
           wireless network. Of these three items, only one -- the frequency band -- is
           within your control. The FCC controls the maximum amount of transmit power,
           but careful selection of the proper antenna and access point will allow you to
           make the most of it. As noted above, certain frequencies are better at going
           through structure than others. Proper band selection is thus crucial to getting the
           range you need in order to get the coverage you want. But with greater range
           comes a greater possibility of interference from other radio sources, making
           proper access point placement the deciding factor between success and failure.

   5.      Features - The unique security issues surrounding wireless networks have
           spawned a wide array of features in contemporary wireless products. Enterprise-
           class devices will often feature support for a wide variety of encryption protocols
           such as WPA and WPA2, authentication mechanisms such as 802.1x, and even
           exotic features such as the ability to triangulate any wireless device down to an
           area of one meter -- your very own in-house GPS system, if you like. With the
           base features of 802.11 products rapidly reaching commodity status, many
           vendors are offering distinctive feature sets to differentiate themselves from their

With these criteria in mind, let's visit some of the common 802.11-based systems you're
likely considering right now. We’ll also detail the pros and cons of each.
                           A Primer on Wireless Network Essentials
                                  J. Eric Smith, MCSE, CNE, CCNP, CISSP

This is the base wireless standard that debuted ca. 1999. It featured a 2-megabit (mbit) data
rate and operated in the 900-megahertz (MHz) frequency band. Being the first iteration of a
standard, it also had vendor interoperability problems, as different vendors "interpreted" the
802.11 specification slightly differently. The 900MHz frequency band also became quite
cluttered with first-generation analog cordless phones, baby monitors, and garage-door
openers which were notorious for their inability to play well with wireless networks.

This standard has essentially been abandoned today, its data rate being far too anemic for
most uses. No vendors are actively pushing any products that are restricted to the original
802.11 specification.

At a Glance:
       Frequency band:                 900Mhz
       Quoted Data Rate:               2 megabits
       Max Range (indoor):             50-150 feet
       Max Range (indoor):             100-300 feet
       Interference sources:           First-generation cordless phones, garage door
                                       openers, baby monitors.
       Pros:                           900MHz frequency band was pretty good at
                                       penetrating building structure.
       Cons:                           Essentially a dead technology.

Following closely after the original 802.11 -- and skipping over the 802.11a standard which
was still in the works -- 802.11b also debuted ca. 1999. The IEEE committee learned from
the mistakes of the preceding standard and made this one much more rigid. The result was
the first real wireless standard that offered inter-vendor compatibility. With buyers no longer
being tied to a single vendor solution, the wireless market exploded. To date, there are more
802.11b devices deployed than any other, making it the de facto standard you're likely to find
anywhere you go.

802.11b operates in the 2.4-gigahertz (GHz) frequency band and has a quoted data rate of
11mbit. Its structure penetration is only slightly inferior to the 900MHz frequency band used
in the original 802.11 specification. Although the frequency band is divided into 14
channels, North American users are restricted to channels 1 through 11 (Japanese users can
use the extra channels). However, due to the way the channels are allocated, only channels 1,
                            A Primer on Wireless Network Essentials
                                   J. Eric Smith, MCSE, CNE, CCNP, CISSP

6, and 11 can actually be used in proximity to one another. As noted above, planning
channel allocation goes hand in hand with planning coverage and density. Too many access
points in too small of a space will result in overlapping channels, killing all access points in
the area. This puts a practical upper limit to how many people can reasonably use any
wireless network in a small space.

At a Glance:
       Frequency band:                  2.4Ghz
       Quoted Data Rate:                11 megabits
       Max Range (indoor):              50-150 feet
       Max Range (outdoor):             100-300 feet
       Interference sources:            Second-generation analog cordless phones, digital
                                        cordless phones, microwave ovens, Bluetooth
                                        devices, other 802.11b wireless networks on same
       Pros:                            It's everywhere you go. If a wireless network
                                        doesn't support anything else in the world, it will
                                        still support 802.11b.
       Cons:                            Low data rate limits usage to low-bandwidth
                                        applications. Ubiquity of wireless networks
                                        means interference is only going to get worse as
                                        time goes by.

Proving again that engineers can't count (or at least can't sequence their alphabets properly),
the 802.11a standard came out after the 802.11b standard. In almost every respect, 802.11a
is superior to 802.11b, and that has largely been its downfall: 802.11a engineers sought to
correct the many shortcomings of 802.11b, and in the process they created a standard so
advanced it was completely incompatible with 802.11b.

It didn't help 802.11a that 802.11b had been a smashing success, either. 802.11a uses the
5GHz frequency band and thus requires a completely different kind of antenna than the
2.4GHz 802.11b devices. In the computing world, economies of scale trump all, and 802.11a
devices were debuting at prices three or four times higher than the existing 802.11b devices.
And since 802.11a's 5GHz frequency band is less effective at penetrating structure, range
decreases by as much as 50% compared with 802.11b devices. To date, 802.11a has sold
very poorly. It is virtually non-existent in the consumer market and is sometimes difficult to
find even in the professional market.
                            A Primer on Wireless Network Essentials
                                  J. Eric Smith, MCSE, CNE, CCNP, CISSP

But this isn't to say 802.11a isn't a fantastic wireless variant. By breaking with 802.11b,
engineers were able to wring a 54mbit data rate out of the new standard, nearly five times
faster than 802.11b. Furthermore, 802.11a was endowed with a much larger number of
available channels, reducing the chances of interference from other 802.11a devices. And
moving to the 5GHz frequency band also meant moving away from all the potential
interference sources currently plaguing the 2.4GHz frequency band. These are not
insubstantial advantages, especially in environments where low interference -- and thus
higher reliability -- is paramount.

At a Glance:
       Frequency band:                 5Ghz
       Quoted Data Rate:               54 megabits
       Max Range (indoor):             30-60 feet
       Max Range (outdoor):            100-200 feet
       Interference sources:           Other 802.11a devices, fourth-generation digital
                                       cordless phones.
       Pros:                           It's tough to find anything that will interfere with
                                       it, and the data rate is high enough for most users
                                       to forget they're not attached to a wire.
       Cons:                           Gear costs are generally much higher than for
                                       other 802.11 variants due to lower sales volumes.
                                       This is magnified even more by the need for more
                                       access points due to the lesser range of 802.11a.

Shortly after the apparent failure of 802.11a, engineers decided to do something a little less
radical. Picking and choosing the best aspects of 802.11a and 802.11b, they came up with
802.11g. This variant is fully backwards compatible with all 802.11b devices -- an
advantage that gives it an immediate installed usage base of millions of devices. 802.11g
devices are currently the best selling products.

In order to remain compatible with 802.11b, 802.11g sticks with the 2.4GHz frequency band.
But by using the more advanced encoding mechanism of 802.11a, the data rate goes up to
54mbit. But with backwards compatibility comes some unwanted baggage.

802.11g shares 802.11b's limitation to only 11 channels, out of which only three are usable.
Worse, any 802.11b device operating near any 802.11g device will degrade the 802.11g
device's data rate somewhat. And since 802.11g also operates in the 2.4GHz frequency band,
it is susceptible to all the same interference sources as 802.11b.
                            A Primer on Wireless Network Essentials
                                   J. Eric Smith, MCSE, CNE, CCNP, CISSP

At a Glance:
       Frequency band:                  2.4Ghz
       Quoted Data Rate:                54 megabits
       Max Range (indoor):              50-100 feet
       Max Range (outdoor):             100-300 feet
       Interference sources:            Second-generation analog cordless phones, digital
                                        cordless phones, microwave ovens, Bluetooth
                                        devices, other 802.11b/g wireless networks on
                                        same channel.
        Pros:                           It's backwards compatible with 802.11b, meaning
                                        you can almost always find something to attach
                                        to. It's nearly five times faster than 802.11b and
                                        yet generally costs the same. Likely to soon
                                        displace 802.11b in terms of number of deployed
        Cons:                           Ubiquity of 2.4GHz wireless networks and other
                                        devices means interference is only going to get
                                        worse as time goes by.

As you might have guessed by now, any wireless network is designed around a series of
compromises. There is no such thing as a single, perfect, one-size-fits-all design, which
means each of the following criteria must be answered and weighed against each other. They
are: coverage, density, and intended usage.

Coverage Of the three, coverage is the easiest in practice to achieve by itself. One must
merely saturate one’s building with signal in order to achieve signal coverage.
Unfortunately, such a deployment is almost certainly not going to work well because it
neglects the second criteria, namely density. Also, coverage can be considered a two-edged
sword, as adding more access points leads to greater signal leakage outside the areas you
control as well as offering more opportunities for something to interfere with your network.

Density is derived from the number of devices you wish to be on a wireless network in a
given area divided by the maximum number of devices a single access point will support. As
a general rule of thumb, most experts will say that you want no more than 20 users per access
point, although this figure is heavily dependent upon how those users are using the network.
If you consider that the average 802.11b or 802.11g network will only allow three access
points to be in close proximity to one another, you'll quickly find that it's difficult to support
                            A Primer on Wireless Network Essentials
                                   J. Eric Smith, MCSE, CNE, CCNP, CISSP

more than 60 wireless users in a confined space. For office environments this is rarely a
problem, but lecture halls, theaters, and even large conference rooms can be exceptionally
difficult to accommodate without a proper design.

Intended usage is the last criterion but perhaps one of the most important ones. Infrequent,
low-priority wireless usage -- for Internet browsing by guests, for instance -- is much less
demanding than a bandwidth- and latency-sensitive service like Voice-over-IP (VoIP).
Whereas the former usage might support fifty users on one access point quite easily, the latter
might have difficulty supporting three or four simultaneous calls with one data user. This
criterion is also the one most likely to change over the life of the wireless infrastructure. This
means that if any criterion requires in-depth planning for the future, this is it.

In summation, the proper wireless network will have coverage where you want it, but only
where you want it and not spilling out into the parking lot for hackers to prey upon. It will
accommodate the density figures you require, but not be too dense or your network will
actually interfere with itself. Lastly, it will provide you the ability to do what you want it to
do with room for future growth, but will not be so over-the-top that it will bust your budget.
To say that this is a fine balancing act is an understatement.

While 802.11a, b, and g may be the letters most frequently used in wireless discussions, there
is quite literally an entire alphabet of lesser-known variants and extensions to 802.11. While
some of these are esoteric or submerged within 802.11 itself, others are going to be become
as well known as -- or perhaps better known than -- their contemporary brethren over the
next year. Knowledge of them and how they will affect the wireless landscape is essential to
planning a wireless network today. They are:

802.11i - This currently-ratified standard is perhaps better known by its acronym WPA2, or
"Wi-Fi Protected Access II." This is the latest encryption standard to be offered on wireless
networks and it works with 802.11a, b, or g. The prior wireless encryption standard was the
widely-disparaged Wired Equivalency Protocol (WEP). WEP gave wireless in general a bad
name in the security world because it has been laughably easy to circumvent for several
years. 802.11i, however, is based upon an encryption standard certified by the U.S.
government for use in agencies where secrecy is paramount. The encryption cipher is known
as the Advanced Encryption Standard (AES) and is widely considered to be one of the
strongest in the world. No known vulnerability exists in it. Any solution you are considering
should support this standard. If it doesn't, you should be looking elsewhere.
                            A Primer on Wireless Network Essentials
                                   J. Eric Smith, MCSE, CNE, CCNP, CISSP

802.11n - The letter "n" might as well stand for "next big thing" given its sweeping
implications. 802.11n seeks to succeed where 802.11a failed, namely at producing a
radically-advanced 802.11 variant that is still somewhat backwards compatible with existing
wireless networks. The goals of 802.11n are to increase the indoor range from tens to
hundreds of feet while simultaneously increasing the data rate. Exactly what the data rate
will be is still a matter of conjecture because the standard has yet to be ratified, but the IEEE
committee has publicly stated that it has a target rate of 100 megabits of usable -- not
theoretical -- bandwidth. If this target is met, it would represent a five-fold increase over the
real-world data rates achievable with either 802.11a or 802.11g devices. This increase,
coupled with an amazing increase in range, is sure to make 802.11n the technology that will
replace all existing 802.11 networks when it becomes available in late 2006 or early 2007.

802.11r - Also known as "fast roaming," 802.11r seeks to address a problem between
wireless networks and VoIP usage. Wireless devices, being wireless, have a tendency to be
quite mobile. On a wireless network, this means the device will eventually leave the
coverage area of one access point and enter the coverage area of another, much as a driver on
a cell phone will "roam" from one cell tower to another while moving down the highway.
When this roaming happens, the wireless device must re-establish its existing session with
the new access point, a process than can take upwards of 100 milliseconds. While this
presents no problem for a data-only user, voice users will suffer from interruptions or even
dropped calls. 802.11r, once ratified, will standardize a handoff mechanism between access
points, allowing roaming voice users to experience uninterrupted calls.

802.11e – This is another extension to 802.11 aimed at fixing some of the shortcomings
associated with wireless voice calls. Currently, all types of traffic on a wireless network are
treated equally. While a data-only user surfing the Internet will never notice a delay of a few
hundred milliseconds, a voice caller will experience a significant interruption similar to a bad
cell phone call. One way around this would be to have the network assign a higher priority
to voice traffic over data traffic, a technology known as Quality of Service (QoS). Thus,
when data traffic threatens to impede voice traffic, the voice traffic will automatically be
given a higher priority. Wired Ethernet networks have had this technology for several years
now. 802.11e will implement wireless QoS in much the same way. The lack of QoS is
considered to be the last remaining obstacle to large-scale wireless VoIP deployments. You
should expect an explosion in availability -- and demand -- of wireless VoIP products shortly
after 802.11e is ratified.
                           A Primer on Wireless Network Essentials
                                 J. Eric Smith, MCSE, CNE, CCNP, CISSP


Of all the technology infrastructure market segments, none is currently moving faster than
wireless. Whereas wireless networks were barely visible five years ago, today it is difficult
to find areas that aren't covered. The challenges of achieving adequate coverage have given
way to the challenges of securing the network and accommodating an ever-growing list of
required mission-critical uses. Tomorrow's applications will only bring additional challenges
of greater complexity requiring even greater reliability. Current experience as well as an in-
depth knowledge of the entire wireless market is essential to a successful deployment today.

If you are considering deploying a wireless network for your organization -- or if you've
already committed to doing so and are exploring the various solutions -- the value of an
experienced wireless consultant cannot be underestimated. Wireless networking is not
merely networking without wires; it combines every challenge of wired networking with a
whole new crop of design and implementation difficulties. Your organization's competitive
edge as well as its security is on the line. Choose wisely.

About the Author:
Eric has over 13 years experience in all facets of information technology management,
                          planning and implementation. He has directed the planning,
                          development, and implementation of a variety of e-commerce and
                          networking applications, managed LAN/WAN internetworking,
                          router configuration, and installation and configuration of
                          switches and hubs and is a certified Cisco/Airespace network
                          engineer. His experience includes the full array of Microsoft
                          products, multiple current operating systems and hardware
                          platforms, and planning, implementing and managing intranets,
                          extranets, and firewall/VPN solutions. He is a frequent contributor
                          to several online technology newsletters and an experienced
                          information technology trainer and speaker and is the Director of
                          Information Technology at EDI, Ltd.

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