Mobile IP Enables WLAN Roaming

A traditional Internet protocol (IP) address plan calls for a single subnet
for the entire wireless LAN (WLAN). This is simple to
implement, and it provide no barriers to effective user roaming. Some companies,
however, want to implement multiple IP subnets across a common WLAN in order
to make network management
easier, facilitate location-based
, and decrease the spread of broadcast packets throughout the network.

For example, a company may want to deliver specific information to users based
on their location in a specific building. By designating different subnets throughout
the WLAN, the location of the user can be found and content delivered to the
user based on where they are. The location of the user in an airport, for instance,
could deliver a map of the applicable concourse or terminal, indicating flight
information and the location of coffee shops and ticket counters. The system
could also deliver advertisements from concessions located within the general

With multiple subnets, however, mobile users must be able to seamlessly roam
from one subnet to another while traversing a facility. WLAN access points do
a great job of supporting roaming at Layer 2 . Users
automatically associate and reassociate with different access points as they move through a facility. As users roam across subnets, though,
there must be a mechanism at Layer 3 to ensure that the user device configured
with a specific IP address can continue communications with applications.

Mobile IP , offered by some access
point vendors
, solves this problem by allowing the mobile user to use two
IP addresses. One address, the home address, is static. The second address,
the "care-of" address, changes at each new point of network
attachment and can be thought of as the mobile user’s position-specific address.

The home address enables the mobile node to continually receive data relative
to its home network, through the use of a network node called the home agent.
Whenever the user is not attached to the home network, the home agent receives
all the packets sent to the mobile user and arranges to deliver them to the
mobile user’s current point of attachment, which is its care-of address.

Whenever a Mobile IP user moves, it registers its new care-of address
with its home agent. This makes it possible for the home agent to keep up with
the whereabouts of the mobile user. The home agent then sends any packets it
receives for that user to the applicable care-of address.

In order to implement Mobile IP, you need two major components: a Mobile IP
server and Mobile IP client software. The Mobile IP server will fully implement
the Mobile IP home agent functionality, providing mobility management for the
mobile users. The Mobile IP server can generally also keep track of where, when,
and how long users utilize the roaming service. That data can then provide the
basis for accounting and billing purposes.

The requirement for client-side software makes Mobile IP impractical for some
applications. For example, public networks demand open connectivity for users,
which makes it difficult to deploy solutions that require client software. The
task of installing software on user devices before enabling roaming is too cumbersome.

Another problem with Mobile IP is that it is somewhat vendor specific. To ensure
interoperability among multi-vendor Mobile IP clients and servers, definitely
do some up front testing.

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 workshops
on deploying WLANs.

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

802.11 Planet Conference

I think the B-52’s said it best: “Roam where you want to.”

Join us at the 802.11 Planet Conference
& Expo
, June 25 – 27, 2003 at the World Trade Center Boston in Boston, MA.

You can make sure your users are dancing to that same tune by going to our panel called

Building A Viable Wi-Fi Roaming Infrastructure.

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