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Speaker: Yi-Jie Pan Advisor: Dr. Kai-Wei Ke 2013/11/25

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1 Speaker: Yi-Jie Pan Advisor: Dr. Kai-Wei Ke 2013/11/25



4 INTRODUCTION Wi-Fi Direct is a new technology defined by the Wi-Fi Alliance aimed at enhancing direct device to device communications in Wi-Fi. Given the wide adoption of Wi-Fi in many kinds of devices, a natural way for the technology to progress is to target device-to-device connectivity, i.e. without requiring the presence of an Access Point (AP), traditionally provided by other technologies. This is the purpose of the Wi-Fi Direct technology that has been recently developed by the Wi-Fi Alliance. Wi-Fi Direct 是 Wi-Fi 聯盟定義的一種增強 Wi-Fi 裝置間溝通能力的一種技術 由於 Wi-Fi 在許多不同類型的裝置上都有使用,一個很自然的想法就產生了,那就是裝置間的直接通訊,不需透過 AP 或是其他傳統機制,這就是 Wi-Fi Direct 想要達到的目的

5 INTRODUCTION Direct device to device connectivity was already possible in the original IEEE standard by means of the ad hoc mode of operation. Another relevant technology in the Wi-Fi device to device communications space is z, also known as Tunneled Direct Link Setup (TDLS). 這兩點是原先與裝置間通訊有關的技術 IEEE 的 standard 中就已經有定義一個 ad hoc mode 的 operation 用於 device to device 間的直接通訊 802.11z 內也定義一個 Wi-Fi device to device 的溝通機制,叫做 Tunneled Direct Link Setup,他可以讓裝置之直接通訊,但是必須先連到同一台AP

6 INTRODUCTION The Wi-Fi Direct technology takes a different approach to enhance device to device connectivity. Instead of leveraging the ad-hoc mode of operation, Wi-Fi Direct builds upon the successful IEEE infrastructure mode and lets devices negotiate who will take over the AP-like functionalities. Legacy Wi-Fi devices may seamlessly connect to Wi-Fi Direct devices. Wi-Fi Direct 不是使用上述者兩機制, Wi-Fi direct 是建立在 IEEE 的 infrastructure上,讓裝置之間能自己交涉決定誰要執行AP的功能,意思就是有一個裝置要扮演AP的腳色 透過這樣的方式讓傳統的 Wi-Fi 裝置也可以連線到 Wi-Fi Direct 的裝置上,因為對傳統裝置來說,他就是一個AP


In a typical Wi-Fi network, clients discover and associate to WLANs, which are created and announced by Access Points (APs). A device unambiguously behaves either as an AP or as a client, each of these roles involving a different set of functionality. 在傳統的 Wi-Fi network 中,client 要連到 WLAN 必須透過 AP,這種情況下每個裝置都會明確定義是 AP 還是 client,因為不同的腳色會涉及到不同的功能

A major novelty of Wi-Fi Direct is that these roles are specified as dynamic, and hence a Wi-Fi Direct device has to implement both the role of a client and the role of an AP(sometimes referred to as Soft-AP). These roles are therefore logical roles that could even be executed simultaneously by the same device, for instance by using different frequencies or time-sharing the channel through virtualization techniques. 而 Wi-Fi Direct 最主要的就是裝置的腳色是動態指定的,所以 Wi-Fi Direct 的裝置都必須 implement client 和 AP 兩部分,這種 AP 有時候就稱為 Soft-AP 因為這種腳色屬於一種邏輯腳色,所以一台裝置可同時扮演不同腳色,只要在 channel 上利用不同頻率或是 Time-Sharing 的機制就可以實現


11 ARCHITECTURE Wi-Fi Direct devices, formally known as P2P Devices, communicate by establishing P2P Groups. The device implementing AP-like functionality in the P2P Group is referred to as the P2P Group Owner (P2P GO), and devices acting as clients are known as P2P Clients. Wi-Fi Direct 裝置,其實就像是 P2P 的裝置,所以他們的連線就是建立 P2P 的群組 群組中扮演 AP 的那個裝置稱為 P2P Group Owner,其餘的 client 就稱為 P2P Client

12 ARCHITECTURE Given that these roles are not static, when two P2P devices discover each other they negotiate their roles (P2P Client and P2P GO) to establish a P2P Group. Once the P2P Group is established, other P2P Clients can join the group as in a traditional Wi-Fi network. 剛剛說過 Wi-Fi Direct 裝置扮演的腳色是動態決定的,所以當兩個 P2P 裝置發現對方時,他們必須做 negotiate 的動作,來決定誰要扮演甚麼腳色,決定完之後才建立群組 一旦 P2P 群組建立,其他 P2P 的 client 可以藉由傳統的 Wi-Fi 網路加入這個群組

13 ARCHITECTURE Legacy clients can also communicate with the P2P GO.
As long as they are not b-only devices and support the required security mechanisms. Legacy devices do not formally belong to the P2P Group and do not support the enhanced functionalities defined in Wi-Fi Direct, but they simply “see” the P2P GO as a traditional AP. 傳統裝置也可以連線到 P2P 的 Group Owner,但不能是 b-only 的裝置並且要 support 安全機制 但是傳統的裝置無法使用 Wi-Fi Direct 的一些加強性功能,因為在傳統裝置眼中,P2P 的 Group Owner 就只是個 AP 而已

14 ARCHITECTURE Like a traditional AP, a P2P GO announces itself through beacons, and has to support power saving services for its associated clients. The P2P GO is also required to run a Dynamic Host Configuration Protocol (DHCP) server to provide P2P Clients with IP addresses. Only the P2P GO is allowed to cross-connect the devices in its P2P Group to an external network 就像傳統 AP,P2P 的 Group Owner 需宣布他在哪裡,並且對所有連到他的 client 提供 Power Saving 的服務,然後他也必須 run DHCP server 提供 client IP address 只有 P2P Group Owner 可以同時連線到 P2P 群組與對外網路內的裝置

15 ARCHITECTURE Wi-Fi Direct does not allow transferring the role of P2P GO within a P2P Group. The P2P GO leaves the P2P Group then the group is torn down, and has to be re-established using some of the specified procedures. Wi-Fi Direct 不允許轉移一個群組中的 Group Owner,意思就是,當 Group Owner 離開群組時,這個群組就會被關閉了,還要繼續使用必須重新建立群組了


17 GROUP FORMATION Standard Autonomous Persistent 群組的建立分為三種 case

18 GROUP FORMATION Standard - Discover each other
Wi-Fi Direct devices usually start by performing a traditional Wi-Fi scan (active or passive). A P2P Device selects one of the so-called Social channels, namely channels 1, 6, or 11 in the 2.4 GHz band, as its Listen channel. It alternates between two states Search state - Probe Requests Listen state - Probe Responses 基本上 Wi-Fi Direct 建立群組比原本的 Wi-Fi 連線多了兩個步驟,一個是 Discover,一個是 Negotiate Wi-Fi Direct 會執行傳統的 Wi-Fi 掃描,掃描完後會執行一段 Discover algorithm 首先,P2P 裝置會先在 Social channels 中挑選一個當作 Listen channel 然後就會交替於兩個狀態 Serach State 和 Listen State Search State 會對所有的 Social channels 發送 Probe Request Listen State 接受到 Probe Request 後會回傳 Probe Responese 這兩個狀態的持續時間是隨機的,通常介於 100ms 到 300ms之間,並且會受到節能機制的影響 The amount of time that a P2P Device spends on each state is randomly distributed, typically between 100 ms and 300 ms

19 GROUP FORMATION Standard - Negotiate which device will act as P2P GO
This is implemented using a three-way handshake, namely GO Negotiation Request/Response/Confirmation The two devices agree on which device will act as P2P GO and on the channel where the group will operate, which can be in the 2.4 GHz or 5 GHz bands. 裝置間的 Negotiate 會藉由 three-way handshake 來實現,分別為 GO Negotiation Request/Response/Confirmation 兩裝置會決定誰要當 Group Owner 並決定要用 2.4G 還是 5G 的 channel

20 GROUP FORMATION Standard - Negotiate which device will act as P2P GO
P2P devices send a numerical parameter, the GO Intent value, within the three-way handshake, and the device declaring the highest value becomes the P2P GO. To prevent conflicts when two devices declare the same GO Intent, a tie-breaker bit is included in the GO Negotiation Request, which is randomly set every time a GO Negotiation Request is sent. 為了決定誰是 Group Owner,兩裝置會在 handshake 的時候發送一個 GO Intent Value,這個值高的為 Group Owner 當兩裝置發送的 GO Intent Value 一樣時,就要去看 tie-breaker bit,這個 bit 包含在 GO Negotiation 的 Request 內,是亂數決定的

A P2P Device may autonomously create a P2P Group, where it immediately becomes the P2P GO. Other devices can discover the established group using traditional scanning mechanisms. P2P 裝置可能會自主建立 P2P 群組,然後這個裝置會成為 Group Owner 其他的裝置就可以利用傳統搜尋機制找到這個群組

The Discovery phase is simplified in this case as the device establishing the group does not alternate between states. No GO Negotiation phase is required. 與 Standard 相比,Autonomous 在 Discovery 階段簡單許多,因為建立群組的時候少了狀態轉換的部分,且不需要花時間決定誰是 Group Owner

During the formation process, P2P devices can declare a group as persistent, by using a flag in the P2P Capabilities attribute pre-sent in Beacon frames, Probe Responses and GO negotiation frames. The devices forming the group store network credentials and the assigned P2P GO and Client roles for subsequent re-instantiations of the P2P group. 在建立群組過程中,P2P 裝置可宣布這個群組是 Persistent,這些裝置就會儲存一些資訊,在下次要建立群組時,就可以直接拿這些資訊快速決定裝置的腳色

If a P2P Device recognizes to have formed a persistent group with the corresponding peer in the past, any of the two P2P devices can use the Invitation Procedure (a two- way handshake) to quickly re-instantiate the group. 如果這些裝置發現要建立群組的對象是之前 Persistent 群組內的裝置,則 P2P 裝置在 Negotiation 的部分可以使用 Invitation Procedure 快速建立群組,這是一種 two-way handshake


26 POWER SAVING Opportunistic Power Save Notice of Absence

27 Opportunistic Power Save
The basic idea of Opportunistic Power Save is to the sleeping periods of P2P Clients. The P2P GO advertises a time window, denoted as CTWindow, within each Beacon and Probe Response frames. 基本的想法就是對 P2P Client 設置 sleeping periods P2P Group Owner 會設定一個 time window,叫做 CTWindow

28 Opportunistic Power Save
This window specifies the minimum amount of time after the reception of a Beacon during which the P2P GO will stay awake and therefore P2P Clients in power saving can send their frames. If after the CTWindow the P2P GO determines that all connected clients are in doze state, the P2P GO can enter sleep mode until the next Beacon is scheduled 這個 window 會定義 P2P Group Owner 醒著的最短時間,這時候 P2P Client 可以傳送資料 當過了 CTWindow 的時間後,Group Owner 會去確認是不是所有連線的 client 都處於 doze state,如果是的話 Group Owner 就會進入 sleep mode,直到下一次排程

29 Opportunistic Power Save
Announced a switch to that state by sending a frame with the Power Management (PM) bit set to 1 Already in the doze state during the previous beacon interval A P2P GO does not have the final decision on whether to switch to sleep mode or not, as this depends on the activity of the associated P2P Clients. 兩種情況 Group Owner 可以進入 sleep mode Client 的 Power Management bit 設為1 Client 已經處於 doze mode 的狀態 能不能進入 sleep mode 並不是由 Group Owner 最後決定,而是 depends on client 若 client 離開節能模式,意思就是 Power Management bit 設為0,這時候 Group Owner 就會被強迫喚醒,直到 client 回到節能模式

30 Notice of Absence The Notice of Absence (NoA) protocol allows a P2P GO to announce time intervals, referred to as absence periods. P2P Clients are not allowed to access the channel, regardless of whether they are in power save or in active mode. A P2P GO can autonomously decide to power down its radio to save energy. Notice of Absence protocol 允許 Group Owner 宣布 time 區間,這個區間是 absence periods 當時檢點處於 absence periods 時,不管 client 是在節能模式還是 active mode,都無法傳送資料 這種模式下 Group Owner 可以自主決定何時該節能

31 Notice of Absence A P2P GO defines a NoA schedule using four parameters Duration that specifies the length of each absence period Interval that specifies the time between consecutive absence periods Group Owner 會定義一個 NoA schedule 這個 schedule 有四個參數 Duration 定義 absence period 的長度 Interval 定義連續的 absence period 之間的間隔時間

32 Notice of Absence Start time that specifies the start time of the first absence period after the current Beacon frame Count that specifies how many absence periods will be scheduled during the current NoA schedule. A P2P GO can either cancel or update the current NoA schedule at any time by respectively omitting or modifying the signaling element. Start time 定義當前 beacon frame 結束後的第一次 absence period 啟動時間 Count 定義當前 Noa schedule 有幾次 absence period Group Owner 可以在任何時間更新 NoA schedule,Client 永遠會遵照最新的 NoA schedule



Two laptops 802.11a/b/g D-Link PCMCIA card with an Atheros chipset Linux mac80211/ath5k driver wpa_supplicant software open source implementation of Wi-Fi Direct

Discovery delay this being the time required for the two P2P devices to find each other Formation delay this being the time required to agree on the roles, establish a secure communication, and perform the DHCP exchange Discovery delay P2P 裝置發現彼此的時間 Formation delay 建立群組的時間,包含決定腳色,建立安全連線,完成 DHCP exchange

Y 軸是 Experimental 的 CDF 虛線是用 event-driven 模擬出來的結果 Discovery 每種程序的初始掃描時間最少都要三秒以上 Standard 與 Persistent 非常相似,因為他們使用了相同的 Discovery algorithm Formation 花費的時間成長大致相同,只有 persistent 因為從 three-way handshake 變成 two-way handshake 所以少了大約 0.5 秒

Beacon period to 100 ms NoA protocol Active policy Static policy Dynamic policy

NoA protocol - Active policy P2P GO remains always active This policy is optimal in terms of traffic performance, but should result in a worst case in terms of energy efficiency. NoA protocol - Static policy P2P GO advertises a fixed presence window of 25 ms right after each Beacon frame. Group Owner 會一直處於 active 中 這種策略下會有最佳的 performance,但是會有最差的 energy efficiency Group Owner 定義 absence period 為 25 ms

NoA protocol - Dynamic policy Based on the Adaptive Single Presence Period (ASPP) algorithm, which adjusts the presence window based on the estimated traffic activity (utilization) in the channel. 利用 ASPP 演算法,根據測量的 traffic activity 動態調整 absence period


42 CONCLUSIONS After a tremendous success whereby Wi-Fi has become a predominant way to access the Internet wirelessly, it is now embracing the challenge of becoming pervasive also in direct device to device communications. In this respect, the Wi-Fi Alliance has recently developed the Wi-Fi Direct technology that builds upon the Wi-Fi infrastructure mode to enable direct device to device connectivity. 現在 Wi-Fi 已經是一個連接到 Internet wirelessly 的主要方式,所以最主要的難題就是讓裝置間的直接通訊能夠普及 在這方面,Wi-Fi 聯盟開發出了建構在 Wi-Fi infrastructure 上的 Wi-Fi Direct 技術,讓裝置間的直接通訊能夠實現

43 CONCLUSIONS Regarding future research directions
The NoA protocol could also be re-used to virtualize the roles of P2P GO/Client over multiple concurrent P2P Groups. Concurrent operation together with the dynamic nature of the P2P GO/Client roles could be used to improve performance in dense environments, for instance by means of dynamic relays. If Wi-Fi Direct becomes a widespread technology as expected, it faces the challenge of improving coexistence and reducing interference with other unlicensed devices. 未來改進方向 希望 NoA protocol 可以再多個 P2P 群組被 re-used Group Owner 與 client 的並行操作,可以有效改善密集環境下的 performance 如果 Wi-Fi Direct 成為廣泛使用的技術,他將面臨與其他裝置間共存與干擾的問題


45 REFERENCES Camps-Mur, D. ; Garcia-Saavedra, A. ; Serrano, P. “Device-to-device communications with Wi-Fi Direct: overview and experimentation”, Wireless communications, IEEE , Page(s): 96 – 104, June 2013

46 Thanks for listening

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