Moving freely between WLAN access points
Lisa Phifer
01.19.2005
Rating: -4.58- (out of 5)
[TABLE]Users may not realize it, but 802.11 wireless stations frequently move from one wireless access point (AP) to another within the same Extended Service Set (ESS). In fact, this change in connectivity often occurs without physical movement. Anything that affects signal strength can cause a station to associate with another AP: a door opening, a cart passing through a nearby hallway, a wrist passing over a PC card. Depending upon your WLAN design, this process can be magically transparent -- or frustratingly disruptive.
802.11 association fundamentals
Every 802.11 AP transmits a steady stream of beacon frames. Beacons are used to advertise the AP's capabilities, including the AP's Service Set Identifier (SSID), supported data rates, use of short preambles, channel agility, modulation options like DSSS-OFDM, privacy requirements, and authentication and cipher suites.
Stations use this capability information to find APs with a desired SSID and compatible capabilities. (Alternatively, stations can send probe requests, causing nearby APs to return the same information in probe responses.) Once a station finds a suitable AP, it attempts to authenticate and then associate with it. If the association is successful, the station can transmit 802.11 data frames, through the AP, into the adjacent network.
An 802.11 association only provides data link (Layer 2) connectivity; it is the wireless equivalent of plugging a Cat5 Ethernet cable into a hub or switch. Network connectivity builds on top of this data link. After associating with an AP, most stations send a DHCP request to obtain an IP address from an upstream DHCP server. A station that cannot obtain an IP address will not be able to establish TCP sessions (e.g., check email, surf the web, download files), even though it is physically connected (associated with) the AP.
Thus, what end users think of as connectivity requires more than a successful 802.11 association -- it requires joining
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a subnet so that IP traffic can be routed through that subnet's gateway to the Internet or other network destinations.
Transparent 802.11 (re)association
802.11 stations constantly re-assess their surroundings by listening to AP beacons and/or sending probe requests to determine whether a better AP might now be available. The criteria that any given station uses to determine which is AP is "best" depends upon the product, but usually depends upon received signal strength.
For example, consider a Windows XP station configured to connect only to preferred network "MyCorpNet." Suppose that station hears beacons from five APs; only AP#1 and AP#2 advertise the SSID "MyCorpNet." Initially, received signal strength from AP#1 is 40, AP#2 is 12. The station will attempt to associate with AP#1. Later, if AP#1 drops to 20, and AP#2 climbs to 30, the station may disassociate with AP#1, then associate with AP#2.
This change in 802.11 link connectivity occurs automatically, without any user intervention. The user may see a brief "blip" in link status for the WLAN connection, reflecting the interval between disassociation from AP#1 and association with AP#2. In fact, this change often occurs so quickly the user does not even notice it.
In addition, when link status changes, Windows automatically renews the station's DHCP-assigned IP address. If both APs are in the same subnet, using the same upstream DHCP server, it is very likely that the station will keep the same IP address. Application sessions established from that IP address may even continue without disruption. That is, the station's network layer connectivity does NOT change, even though 802.11 connectivity has changed through (re)association with a different AP.
What can go wrong?
Careful readers will note that the above description incorporates several assumptions. If any of these implicit assumptions turn out to be wrong, this "transparent" operation may not be all that transparent.
Advanced WLAN speedbumps
Following the simple recommendations given above can promote more transparent station movement between APs in WLANs of modest size. However, the larger the WLAN, the more complex the network topology, yielding tougher challenges.
To learn more about inter-AP handoff and subnet roaming, I recommend visiting Bernard Aboba's excellent Web site. Related topics include the Wi-Fi Alliance's wireless ISP roaming (PDF) and the IEEE's new fast roaming task group (802.11r).
About the author: Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.
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