Mobility

 
Mobility
The mobility of wireless stations may be the most important feature of a WLAN. The chief motivation of
deploying a WLAN is to enable stations to move about freely from location to location either within a
specific WLAN or between different WLAN segments.
For compatibility purposes, the 802.11 MAC must appear to the upper layers of the network as a
standard 802 LAN. The 802.11 MAC layer is forced to handle station mobility in a fashion that is
transparent to the upper layers of the 802 LAN stack. This forces functionality into the 802.11 MAC layer
that is typically handled by upper layers in the OSI model.[15]
To understand this design restriction, it is first important to appreciate the difference between true mobility
and mere portability. Portability certainly results in a net productivity gain because users can access
information resources wherever it is convenient to do so. At the core, however, portability removes only
the physical barriers to connectivity. It is easy to carry a laptop between several locations, so people do.
However, portability does not change the ritual of connecting to networks at each new location. It is still
necessary to physically connect to the network and reestablish network connections, and network
connections cannot be used while the device is being moved.
Mobility removes further barriers, most of which are based on the logical network architecture. Network
connections stay active even while the device is in motion. This is critical for tasks requiring persistent,
long-lived connections, which may be found in database applications.
802.11 is implemented at the link layer and provides link-layer mobility. The Internet Protocol (IP) does
not allow this. 802.11 hosts can move within the last network freely, but IP, as it is currently deployed,
provides no way to move across subnet boundaries. To the IP-based hosts of the outside world, the
virtual private network (VPN)/access control boxes are the last-hop routers. To access an 802.11
wireless station with an IP address on the wireless network, it is possible to simply go through the IP
router to the target network regardless of whether a wireless station is connected to the first or third AP.
The target network is reachable through the last-hop router. As far as the outside world can tell, the
wireless station might as well be a workstation connected to an Ethernet.
A second requirement for mobility is that the IP address does not change when connecting to any of the
APs. New IP addresses interrupt open connections. If a wireless station connects to the first AP, it must
keep the same address when it connects to the third AP.
A corollary to the second requirement is that all the wireless stations must be on the same IP subnet. As
long as a station stays on the same IP subnet, it does not need to reinitialize its networking stack and can
keep its Transmission Control Protocol (TCP) connections open. If it leaves the subnet, though, it needs
to get a new IP address and reestablish any open connections. Multiple subnets are not forbidden, but if
you have different IP subnets, seamless mobility between subnets is not possible.
The single IP subnet backbone restriction is a reflection on the technology deployed within most
organizations. Mobile IP was standardized in late 1996 in RFC 2002, but it has yet to see widespread
deployment. Until Mobile IP can be deployed, network designers must live within the limitations of IP and
design networks based on fixed locations for IP addresses. A backbone network may be physically large,
but it is fundamentally constrained by the requirement that all APs connect directly to the backbone router
(and each other) at the link layer.[16]
[14]Plamen Nedeltchev, "WLANs and the 802.11 Standard," a white paper from Cisco Systems, www.
cisco.com/warp/public/784/packet/jul01/pdfs/whitepaper.pdf
[15]"Introduction to IEEE 802.11," a white paper from Intelligraphics, www.intelli-graphics.com/
articles/80211_article.html.
[16]Gast, 295-296.