The Institute of Electrical and Electronics Engineers, Inc.

1. 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

2. 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。

3. 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。

4. 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。

5. 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。

6. 802.11 r 架構示意圖

7. 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。

8. 802.11 r：Fast BSS Transition

9. 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)

10. 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)

11. 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.

12. 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.

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

14. 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.

15. 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.

20. Thank you