Figure 6. Mobile IP
track users as they roam about, either at Layer2 between two APs on the same subnet, or at
Layer 3 when the user crosses a router boundary
between subnets.
The first issue is handled by vendor-specificinter-AP protocols, which vary in performance.
If the protocol is not efficient, there is a chance of packets being lost as the user roams from access point to access point. Eventually WECA and the IEEE are likely to create standards in this area.
The second issue is handled by Layer 3 roaming mechanisms. The most popular of these is Mobile IP (Figure 6), which is currently known as RFC 2002 in the Internet
Engineering Task Force (IETF). Mobile IP works by having an access point assigned as
the “home agent” for each user. Once a wireless station leaves the home area and enters a
new area, the new access point queries the station for its home agent. Once it has been
located, a packet forwarding is established automatically between the two access points to
ensure that the user’s IP address is preserved and that the user can transparently receive his
or her data. As Mobile IP is not finalized, vendors may provide their own protocols using
similar techniques to ensure that IP traffic follows a user across networks separated by a
router (e.g., across multiple buildings).
An incomplete but useful alternative to the Layer 3 roaming problem is to implement the Dynamic Host Configuration Protocol (DHCP) across the network. DHCP allows any user who shuts down or suspends their portable computer before crossing to a new network to automatically obtain a new IP address upon resuming or turning on their notebook.
Power Management
End-user wireless products are typically designed to work completely untethered, via
battery power. The 802.11b standard incorporates Power Saving Protocol to maximize the
battery life of products using wireless devices.
Safety
As with other wireless technologies, WLANs must meet stringent government and industry
standards for safety. There have been concerns raised across a number of wireless technology
industries regarding the health risks of wireless use. To date, scientific studies have been
unable to attribute adverse health effects to WLAN transmissions. In addition, the output
power of wireless LAN systems is limited by FCC regulations to under 100 mW, much less
than that of a mobile phone, and it is expected that any health effects related to radio transmisssions would be correlated to power and physical proximity to the transmitter.
Security
The WEP 40-bit encryption built into 802.11b WLANs should be sufficient for most applications. However, WLAN security needs to be integrated into an overall network
security strategy. In particular, a user may implement network layer encryption such as
IPSec across both wired and wireless portions of the network, eliminating the need to have
802.11 security in place. Or a customer may choose to have critical applications encrypt
their own data, thereby ensuring that all network data such as IP and MAC addresses are
encrypted along with the data payload.
Other access control techniques are available in addition to the 802.11 WEP authentication technique. For one, there is an identification value called an ESSID programmed into each access point to identify which subnet it is on. This can be used as an authentication check; if a station does not know this value, it is not allowed to associate with the access point. In addition, some vendors provide for a table of MAC addresses in an Access Control List to be included in the
access point, restricting access to clients whose MAC addresses are on the list. Clients can
thus be explicitly included (or excluded) at will.
Cost
Hardware costs include adding APs to the network infrastructure and WLAN adapter cards
to all wireless devices and computers. The number of APs depends on the coverage area,
number of users, and types of services needed. The coverage area of each access point extends
outward in a radius. Access point “zones” often overlap to ensure seamless coverage. Clearly, hardware costs will depend on such factors as performance requirements, coverage
requirements, and vendor product range at different data rates.
Beyond equipment costs, a customer must take into account installation and maintenance
expense, including the costs of poor product quality (help desk support costs, end user productivity). These costs can dwarf the initial equipment costs of a WLAN. Products
that are simple to install, use, and manage and that perform up to their specifications may be
worth significantly higher initial equipment investment. Features mentioned earlier, such
as power over Ethernet, bulk configuration of APs, and a rich set of management tools, will
lower the overall cost of a wireless LAN.
Conclusion
802.11 WLANs are already commonly used in several large vertical markets. The 802.11b
standard is the first standard to make WLANs usable in the general workplace by providing
robust and reliable 11 Mbps performance, five times faster than the original standard. The
new standard will also give WLAN customers the freedom to choose flexible, interoperable
solutions from multiple vendors, since it has been endorsed by most major networking and
personal computer vendors. Broad manufacturer acceptance and certifiable interoperability
means users can expect to see affordable, high-speed wireless solutions proliferate
throughout the large enterprise, small business, and home markets. This global wireless
LAN standard opens exciting new opportunities to expand the potential of network computing.
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