Draft Amendment to STANDARD FOR Information Technology -Telecommunications and Information Exchange Between Systems - LAN/: 802.11 R: Fast BSS Transition The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA 林靖倫 2005/11/24

前言 使用無線電話技術的移動用戶必須能夠從一個AP迅速斷開連接，並重新連接到另一個AP。這個切換過程中的延遲時間不應該超過50毫秒，因為這是人耳能夠感覺到的時間間隔。 目前802.11網路在漫遊時的平均延遲是幾百毫秒，這直接導致傳輸過程中的斷續，造成連接丟失和語音品質下降。

前言 在一個新的接入點上，只有經過切換以後移動設備才可得知是否能獲得足夠的QoS資源，這樣就不可能事先瞭解切換會否帶來滿意的應用性能。

解決方案 Client可以停留在現有通道，並使用當前的接入點與其他備選接入點通訊。這使Client資料流程中斷的可能性降到最小，但客戶無法探測到通過無線電與其他接入點通訊能力的任何細節。 Client也可以直接切換到另一個接入點的通道，這使客戶機可以確定通過無線電與其他接入點通訊的品質，但會造成與當前接入點通訊過程中的一定程度間斷。

802.11 r 概述 802.11r改善了移動的用戶端設備在AP之間移動時的切換過程。協議允許一個無線AP在實現切換之前，就建立起與新AP之間安全且具備QoS的狀態。 Client可以將現有的接入點作為通向其他接入點的管道，使由於通道改變所引起的通訊中斷最小化。

802.11 r 架構示意圖

研究與討論 一些專家認為來自802.11i和802.11k工作的技術將保證50ms範圍內的切換，足以使802.11r變得多餘。 來自802.11k的協議主要可被移動站用於發現在失去連接前應當向哪里移動。來自802.11i的PMK緩存技術加快安全連接的速度(20ms～30ms)。這些協議可能實現WLAN連接在AP之間的快速、安全、無縫的切換。

802.11 r：Fast BSS Transition

Limitation of Fast BSS Transition Applies only to the STA<->Access Point (AP) connection state. Within the same Extended Service Set (ESS), and will Not apply to the Independent Basic Service Set (IBSS)

Definition pairwise master key (PMK): The PMK may be derived from an Extensible Authentication Protocol (EAP) method or may be obtained directly from a pre-shared key (PSK). pairwise transient key (PTK): A value that is derived form the pairwise master key (PMK)

Definition (cont.) Fast Transition Information Element (FTIE): An Information Element for enabling fast transitions between APs, which carries resource reservation and security policy information. Transition Enabled Access Point (TAP): Fast BSS Transition Enabled Access Point. Transition Enabled Station (TSTA): The non-AP station capable of executing the fast BSS transition procedures, as defined in this standard.

Definition (cont.) Mobility Domain: A set of BSSs, within the same ESS, identified by a Mobility Domain Identifier that provide the following assurances to the STA: (1) Fast transition is possible between any two BSSs using either “over the DS” or “over the air” procedures. (2) Common key infrastructure is accessible at all BSSs. (3) APs in the same mobility domain advertise the same Mobility Domain Identifier.

Fast BSS Transition stages scanning 802.11 authentication re-association PTK derivation - four-way handshake QoS admission control

State description There are three state transitions involved when a STA transitions from its current AP to a new AP: Discovery: the station locates and decides to which AP it will transition. Resource establishment: the station may establish that the new AP will provide connection resources it needs to maintain active sessions. Transition: the station abandons the current AP and establishes a connection with a new AP.

Fast BSS Transitions 1)Base Fast BSS Transition: This mechanism is executed when a TSTA needs to transition to a target TAP and does Not require a reservation prior to its transition. 2) Pre-reservation Fast BSS Transition: This mechanism is executed when a TSTA needs assurances that the required security and QoS resources be available prior to a transition.

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