IEEE 802.11r

802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, and 802.11g versions to provide wireless connectivity in the home, office and some commercial establishments.

Fast BSS Transitions

802.11r is the unapproved IEEE 802.11 standard that specifies fast BSS ("Basic Service Set") transitions. This will permit connectivity aboard vehicles in motion, with fast handoffs from one base station to another managed in a seamless manner. Handoffs are supported under the "a", "b" and "g" implementations, but only for data (using IEEE 802.11f or Inter-Access Point Protocol commonly known in the wireless circles as IAPP). The handover delay is too long to support applications like voice and video.

The primary application currently envisioned for the 802.11r standard is VOIP ("voice over IP", or Internet-based telephony) via mobile phones designed to work with wireless Internet networks, instead of (or in addition to) standard cellular networks.

These wireless enabled mobile phones/PDAs must be able to rapidly disassociate from one access point and connect to another. The delay that occurs during handoff cannot exceed about 50 msec, the interval that is detectable by the human ear. However, current roaming delays in 802.11 networks average in the hundreds of milliseconds. This can lead to transmission "hiccups," loss of connectivity and degradation of voice quality. Faster handoffs are essential for 802.11-based voice to become widely deployed.

Another problem with current 802.11 wireless gear is that a mobile device cannot know if necessary QoS resources are available at a new access point until after a transition. Thus, it is not possible to know whether a transition will lead to satisfactory application performance.

802.11r refines the transition process of a mobile client as it moves between access points. The protocol allows a wireless client to establish a security and QoS state at a new access point before making a transition, which leads to minimal connectivity loss and application disruption. The overall changes to the protocol do not introduce any new security vulnerabilities. This preserves the behavior of current stations and access points.

When approved, 802.11r will govern the way roaming mobile clients communicate with candidate access points, establish security associations and reserve QoS resources. Under 802.11r, clients can use the current access point as a conduit to other access points, allowing clients to minimize disruptions caused by changing channels. Until that time, however, enterprises will need to use proprietary hardware from vendors such as SpectraLink to get fast roaming for applications such as VoIP. It should be noted that some wireless VoIP systems use the insecure WEP (Wired Equivalent Privacy) encryption to keep authentication time under the 20-millisecond threshold mark. More secure encryption types such as TKIP or AES-CCM (CCMP) involve handshakes that happen after association, and they can typically take 30-40 milliseconds, potentially disrupting the call.

Motivation

*Transitions between 802.11-1999 BSSes are hard enough task
*Many additions to 802.11 are making transitions much harder
*Need to minimize the time required to move between BSSes
*VoIP is primary application target
*For 802.21: what 802.11r creates might be used by 802.21802.21 might have to use 802.11r facilities for roaming between 802.11 domains. (Similar to using 802.11u facilities for roaming between 802.11 and external domains.)

Protocol Operation

Access Points and Stations are both given the optional ability Fast Transition

The BSS â€" BSS transition goes through five stages:
*Scanning â€" active or passive for other APs in the area
*Authentication with a (one or more) target AP
*Re-association to establish connection at target APAt this point, the AP and Station have a connection, but still can't do anything.
*Key (PTK) derivation â€" 4-way handshake of session keys based on 802.1x authentication
*QoS admission control to re-establish QoS streams

Proposals

Following is the shortlisting of all proposals that were suggested for IEEE 802.11r implementation.
*Fast BSS-Transition Tunnel
*TAP (Transition Acceleration Protocol)
*Fast Roaming Using Multiple Concurrent Associations
*Motorola TGr Fast Handover Proposal
*PEKM (Post-EAP Key Management Protocol)
*Proposal for Fast Inter-BBS Transitions
*AP Scanning
*Just-In-Time 2 Phase Association

Status

Currently, the standard body TGr is combining the two remaining proposals, TAP (Transition Acceleration Protocol) and Just-In-Time 2 Phase Association, into refining the accepted proposal into a draft which will go to a letter ballot in November 2005.

See also

*handover
*Unlicensed Mobile Access
*IEEE 802.11u Cellular interworking
*IEEE 802.11f Inter-Access Point Protocol

External references

*IEEE P802 Status of Project IEEE 802.11r

*802.11r strengthens wireless voice Pat Calhoun and Bob O'Hara, Network World, August 22, 2005
*WLAN hurdles 3: Roaming and interoperability

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