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wireless is a great way to connect, the implementation issues
can be daunting. A look at the key challenges for wireless
deployments. by Graeme K. Le Roux
major PC vendors have been pushing "wireless network
infrastructure" in the market place. The impetus for
this push has been a mixture of new technology in the form
of 802.11b adapters being available in quantity and the
usual need for a "new" feature to market (read:
Various low speed wireless systems have been around for
the best part of ten years while several proprietary systems,
mostly application-specific types, pre-date them by several
The major difference, from a design standpoint, between
working with systems which incorporate wired components
and exclusively wired systems, is the switching architecture.
Unlike their wired brethrens, wireless networks are usually
bridged rather than switched.
Wireless systems are often deployed because they allow users
to roam about. If a user is working on a laptop in one place,
then a wireless connection is logically similar to a simple
10Base-T Ethernet link, where the NIC in the laptop has
a message authentication code (MAC) layer address, and the
bridge has a port to which that NIC is connected. Also,
the bridge maintains a table which tells it which port the
NIC is connected to.
But what happens if the user picks up his laptop and walks
to the other side of the building? The NIC still has the
same MAC layer address, but it is not connected to the same
bridge port, in most cases it is not even on the same bridge.
Obviously there has to be some way of handing off the NIC
from one bridge to another.
In an 802.11b environment, each bridge has a discrete network
ID (also known as a Radio Group, and not to be confused
with an IP Net ID) which it shares with all the NICs it
is communicating with. A hand off from one bridge to another
is based on the signal to noise ratio (SNR) as measured
by the roaming NIC. The SNR is measured on the basis of
beacon signals transmitted by each bridge in the LAN as
These SNAP frames also contain information about the bridges'
network IDs, and so on. When a roaming NIC needs to initiate
a hand off, it first finds a new bridge then signals its
current bridge requesting a hand off to the new bridge.
The bridges then contact each other via their common backbone
connection using SNAP frames and update their tables. Finally,
the NIC changes its network ID to match that of the new
bridge. Actually, the NIC's MAC layer ID has not changed,
just the ID associated with the bridge it is communicating
Of course, the physical location of the NIC may change quite
frequently and this can be a problem in an IP-based network-since
IP implicitly assumes that IP addresses are geographically
fixed. Specifically, an IP address is composed of a host
ID and a net ID. Routes which are used to forward data are
sets of instructions for getting from one net ID to another.
For example, if a user whose laptop is configured with an
IP address of 10.0.1.2 mask 255.255.255.0 (that is, the
host 2 on sub-net 10.0.1.0) moves to a wireless bridge which
is connected to IP sub-net 10.0.2.0, it will not receive
To allow communication, we have to change the laptop's IP
address when we move between access points which are attached
to different IP networks. Consider a user with a laptop
containing a wireless NIC in a multi-storey office block
in which each floor is a different IP sub-net and each floor
has four wireless bridges (referred to more commonly as
access points). The user can happily roam anywhere within
any single floor of the building with a given IP address,
but as soon as they move to another floor they will not
be able to communicate unless they change their IP address.
Since having users manually and routinely fiddle with their
IP address is every network administrator's nightmare we
have to have a way of assigning IP addresses automatically.
This can be done with dynamic host configuration protocol
(DHCP), but there are limitations.
DHCP leases IP addresses from a pool. With it, clients in
the scenario described above have to request a new address
after checking and finding that the one they have is invalid.
Unfortunately, this sort of check only happens when a lease
times out, or when the client's IP stack is initialized.
Depending upon how the operating system on the user's laptop
implements the IP stack, this may happen when the laptop
comes out of a "suspend" or "sleep"
mode and will certainly happen when it is rebooted. In practice,
this means that users may have to suspend their laptops,
or reboot whenever they change floors.
For laptop users, a reboot would be inconvenient. However,
putting it to sleep may be viable. Besides, it is unlikely
that people will be using their laptops in a lift or while
walking up or down the stairs. There is one catch though:
you can't assume that your wireless network user will be
using a laptop-it might be a PDA or in future some sort
of mobile VoIP phone. Suspension may not be an option in
such cases. For that, you'll need mobile IP.
IP (RFC 2002 and updates) is a proposed standard which assigns
each mobile host to a "home agent"basically
a router which is on the IP network where the mobile host
is notionally based.
Take the building example above. If the "home agent"
is on the second floor, then routers on the other floors
are the "foreign agents". The home agent keeps
track of the mobile host's current location and in conjunction
with the foreign agents, tunnel data to the mobile host.
This implies an overhead on the routers and the wired network
backbone, but it should not be too much of a problem since
the wired system can always be provisioned beforehand to
cope with it. Actually, until the full standard specifications
of mobile IP are available (only pre-standard ones are around
at present), it is still too early to speculate on what
extra overheads mobile IP will create.
What are some good strategies to adopt when building wireless
networks? A simple way is to employ switches and bridges
at internal nodes, and routers at the edge of your system.
However, if you have a multi-storey implementation, having
routers on numerous floors can be expensive. A more cost
effective solution would be to interconnect all the wireless
bridges via a system of full-duplex 100Base-Tx Ethernet
Note that while having several 802.11b bridges per floor
creates overlapping signal areas, this does not introduce
loop paths in the network because each 802.11b bridge has
a different wireless network ID and thus are logically connected
via the wired backbone and not a wireless link.
But wireless backbones can be created if desired. Wireless
bridges with more than one port can be configured such that
each port has a different network ID. By configuring one
port on each of multiple ports with the same network ID,
you can create point-to-point links that can be employed
as a wireless backbone instead of using a wired system.
While this is low capacity compared to either UTP or fibre,
wireless backbones compare favorably to short haul leased
line services and eliminate the need for cable. This can
be useful if you need to connect say, solar-powered information
kiosks scattered in a large public area.
In fact, for a small system you need not even consider the
use of bridges and wired back bones. A small ad-hoc wireless
LAN can be created using 802.11b simply by bringing a number
of wireless NIC equipped hosts within range of each other
and setting each of them to have the same wireless network
Hey, this means that you can hold the company board meeting
in the local park. Or better still, in a half-decent pub.
K. Le Roux is the director of Morsedawn (Australia),
a company which specializes in network design and consultancy
and writes for Network Computing-Asian Edition. He can be
reached at email@example.com. Send your feedback
pros and cons
technology is generally intended to be used in locations
where it is too expensive or impossible to deploy a wired
solution. An example is a warehouse environment where terminal
devices are mounted on mobile platforms such as fork lifts.
Other examples include exhibition spaces where the cost
of re-wiring for each exhibition would be too expensive,
and heritage buildings where drilling of holes for cables
is not permitted.
Wireless technology in the form of 802.11b can be used as
a cheap point-to-point link for campus environments, usually
by adding an external high gain, uni-directional antenna.
The biggest downer to wireless is its lack of speed. A wireless
network is necessarily half-duplex, which means that at
best, an 802.11b system will provide performance equivalent
to a wired half-duplex 10Base-T system. If you are running
full-steam with wired networks, don't expect to move the
same applications to wireless without experiencing stutter.
Wireless systems also necessarily complicate the design
of large networks, and wireless network adapters are more
expensive compared to wired ones (although cabling costs
are reduced). Whether this reduction in cabling costs counters
the added cost of the adapter depends on the specific situation
in which you are thinking of deploying wireless.
802.11b NICs use direct sequence spread spectrum technology
which minimizes interference problems. Most NICs also support
some form of encryption. The aim is "wire equivalent"
security. That means roughly as good as you could expect
in the average office with a UTP Ethernet connection. If
you don't encrypt traffic on your wired Ethernet, you don't
need more than the security available in most 802.11b NICs.
If you do, then you can use the same encryption as you use
in your wired LAN over 802.11b.
802.11b equipment basics
vendors build 802.11b adapters as PC cards which can be
directly inserted into laptops or their access points (bridges).
The same cards can also be inserted into adapters to provide
ISA and PCI bus support.
Data rates in all wireless systems depend on the distance
between the client and the access point and any obstacles
in between. Typically, vendors use the 802.11b HR standard
which delivers 11 Mbps over a maximum range of between 25
and 125 m with a standard antenna. Such cards can fall back
to 5.5, 2 or 1 Mbps to interoperate with older systems or
to cover longer distancesanything up to roughly 500
m with a standard antenna in open country.
Most vendors also either supply or provide for a number
of alternative antennas, usually intended for attachment
to an access point, which can allow for coverage over distances
of between 1 and about 25 km.
Most access points support 10/100Base-T Ethernet via UTP.
Some units are also available with a port for, or a built-in,
WAN link. This usually ranges from a V.90 modem to most
kinds of retail broadband connection (cable, xDSL, etc.)
Most access points are shipped with diagnostic software
which will, in conjunction with a laptop and NIC, allow
you to "qualify" a site-that is, check how many
access points you will need and where you will have to put
them. This is a basic and necessary part of the planning
process. Installers worth their salt will qualify a site
prior to giving a quote on supply and installation of equipment.
Some vendors also provide powered Ethernet options. This
is simply a way of providing an access point with DC power
via its UTP back bone connection. This makes it much easier
to install an access point in an awkward place such as inside
the false ceiling in the middle of a room far away from
a wall socket.
For more on the use of 802.11b in LAN and networking environments,
visit the following Web sites:
www.WirelessEthernet.com (see under "member companies")