IEEE 802.11r
Encyclopedia
IEEE 802.11r-2008 or fast BSS transition (FT) is an amendment to the IEEE 802.11
standard to permit continuous connectivity aboard wireless devices in motion, with fast and secure handoffs from one base station to another managed in a seamless manner. It was published on July 15, 2008.
, is widely used for wireless communications. Many deployed implementations have effective ranges of only a few hundred meters, so to maintain communications devices in motion that use it will need to handoff from one access point to another. In an automotive environment, this could easily result in a handoff every five to ten seconds.
Handoffs are already supported under the preexisting standard. The fundamental architecture for handoffs is identical for 802.11 with and without 802.11r: the mobile device is entirely in charge of deciding when to hand off and to which access point it wishes to hand off. In the early days of 802.11, handoff was a much simpler task for the mobile device. Only four messages were required for the device to establish a connection with a new access point (five if you count the optional "I'm leaving" message the client could send to the old access point). However, as additional features were added to the standard, including 802.11i with 802.1X authentication and 802.11e or WMM
with admission control requests, the number of messages required went up dramatically. During the time these additional messages are being exchanged, the mobile device's traffic, including that from voice calls, cannot proceed, and the user will hear loss approaching that of seconds. Generally, the highest amount of delay or loss that the edge network should introduce into a voice call is 50 msec.
802.11r was launched to attempt to undo the added burden that security and quality of service added to the handoff process, and restore it back to the original four-message exchange. In this way, handoff problems are not eliminated, but at least are returned to the status quo ante.
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.
but defined in 802.11e) to occur in parallel.
The key negotiation protocol in 802.11i specifies that, for 802.1X-based authentication, the client is required to renegotiate its key with the RADIUS
or other authentication server supporting Extensible Authentication Protocol
(EAP) on every handoff, a time consuming process. The solution is to allow for the part of the key derived from the server to be cached in the wireless network, so that a reasonable number of future connections can be based on the cached key, avoiding the 802.1X process. A feature known as Opportunistic Key Caching exists today, based on 802.11i, to perform the same task. 802.11r differs from OKC by fully specifying the key hierarchy.
At this point in an 802.1X BSS, the AP and Station have a connection, but are not allowed to exchange data frames, as they have not established a key.
A fast BSS transition performs the same operations except for the 802.1X negotiation, but piggybacks the PTK and QoS admission control exchanges with the 802.11 Authentication and Reassociation messages.
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
standard to permit continuous connectivity aboard wireless devices in motion, with fast and secure handoffs from one base station to another managed in a seamless manner. It was published on July 15, 2008.
Rationale for the amendment
802.11, commonly referred to as Wi-FiWi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
, is widely used for wireless communications. Many deployed implementations have effective ranges of only a few hundred meters, so to maintain communications devices in motion that use it will need to handoff from one access point to another. In an automotive environment, this could easily result in a handoff every five to ten seconds.
Handoffs are already supported under the preexisting standard. The fundamental architecture for handoffs is identical for 802.11 with and without 802.11r: the mobile device is entirely in charge of deciding when to hand off and to which access point it wishes to hand off. In the early days of 802.11, handoff was a much simpler task for the mobile device. Only four messages were required for the device to establish a connection with a new access point (five if you count the optional "I'm leaving" message the client could send to the old access point). However, as additional features were added to the standard, including 802.11i with 802.1X authentication and 802.11e or WMM
Wireless Multimedia Extensions
Wireless Multimedia Extensions , also known as Wi-Fi Multimedia , is a Wi-Fi Alliance interoperability certification, based on the IEEE 802.11e standard. It provides basic Quality of service features to IEEE 802.11 networks. WMM prioritizes traffic according to four Access Categories - voice,...
with admission control requests, the number of messages required went up dramatically. During the time these additional messages are being exchanged, the mobile device's traffic, including that from voice calls, cannot proceed, and the user will hear loss approaching that of seconds. Generally, the highest amount of delay or loss that the edge network should introduce into a voice call is 50 msec.
802.11r was launched to attempt to undo the added burden that security and quality of service added to the handoff process, and restore it back to the original four-message exchange. In this way, handoff problems are not eliminated, but at least are returned to the status quo ante.
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.
Fast BSS Transition
IEEE 802.11r specifies fast Basic Service Set (BSS) transitions between access points by redefining the security key negotiation protocol, allowing both the negotiation and requests for wireless resources (similar to RSVPRSVP
RSVP may refer to:* RSVP , request for responsesEtymologyFrom the French RSVP, répondez s'il vous plaît, meaning “reply please” or "please respond".-Music:* "R.S.V.P." , 1985...
but defined in 802.11e) to occur in parallel.
The key negotiation protocol in 802.11i specifies that, for 802.1X-based authentication, the client is required to renegotiate its key with the RADIUS
RADIUS
Remote Authentication Dial In User Service is a networking protocol that provides centralized Authentication, Authorization, and Accounting management for computers to connect and use a network service...
or other authentication server supporting Extensible Authentication Protocol
Extensible Authentication Protocol
Extensible Authentication Protocol, or EAP, is an authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined in RFC 3748, which made RFC 2284 obsolete, and was updated by RFC 5247....
(EAP) on every handoff, a time consuming process. The solution is to allow for the part of the key derived from the server to be cached in the wireless network, so that a reasonable number of future connections can be based on the cached key, avoiding the 802.1X process. A feature known as Opportunistic Key Caching exists today, based on 802.11i, to perform the same task. 802.11r differs from OKC by fully specifying the key hierarchy.
Protocol Operation
The non-802.11r BSS transition goes through six stages:- Scanning – active or passive for other APs in the area.
- Exchanging 802.11 Authentication messages (first from the client, then from the AP) with the target access point.
- Exchanging Reassociation messages to establish connection at target AP.
At this point in an 802.1X BSS, the AP and Station have a connection, but are not allowed to exchange data frames, as they have not established a key.
- 802.1X master key (PMK) negotiation
- Key (PTK) derivation – 802.11i 4-way handshake of session keys, creating a unique encryption key for the association based on the master key established from the previous step.
- QoS admission control to re-establish QoS streams
A fast BSS transition performs the same operations except for the 802.1X negotiation, but piggybacks the PTK and QoS admission control exchanges with the 802.11 Authentication and Reassociation messages.
See also
- Unlicensed Mobile AccessGeneric Access NetworkGeneric Access Network or GAN is a telecommunication system that extends mobile voice, data and IP Multimedia Subsystem/Session Initiation Protocol applications over IP networks...
- IEEE 802.11sIEEE 802.11sIEEE 802.11s is an IEEE 802.11 amendment for mesh networking, defining how wireless devices can interconnect to create a WLAN mesh network, which may be used for static topologies and ad-hoc networks....
- Mesh networking - IEEE 802.11uIEEE 802.11uIEEE 802.11u-2011 is an amendment to the IEEE 802.11-2007 standard to add features that improve interworking with external networks.802.11 is a family of IEEE technical standards for mobile communication devices such as laptop computers or multi-mode phones to join a wireless local area network ...
- Cellular interworking - Inter-Access Point ProtocolInter-Access Point ProtocolIEEE 802.11F or Inter-Access Point Protocol is a recommendation that describes an optional extension to IEEE 802.11 that provides wireless access point communications among multivendor systems. 802.11 is a set of IEEE standards that govern wireless networking transmission methods...