由于Ad Hoc網絡無中心、動態(tài)拓撲等特點，它需要各個節(jié)點都具有路由轉發(fā)功能。本文開發(fā)的Ad Hoc網絡節(jié)點通過在ARM平臺上移植路由協(xié)議而實現(xiàn)了數(shù)據(jù)包轉發(fā)功能。
圖1 節(jié)點轉發(fā)
以圖1所示的網絡進行功能示意，其中A、B、D是普通的移動電腦終端，C是本文開發(fā)的ARM平臺。A和D是通信的端點，D不在A的一跳范圍之內(A的覆蓋范圍如圖中虛線所示)。假設開始時B成為了A和D的中繼節(jié)點，完成A、D之間的數(shù)據(jù)轉發(fā)功能。當B節(jié)點出現(xiàn)故障時，C能自動代替B成為新的中繼節(jié)點，維持A、D之間的通信。
該ARM平臺除了路由協(xié)議以外，同時完成了ftp、iptables等工具的移植，還可以繼續(xù)增添語音、視頻等服務。
硬件平臺
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title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" 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title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡" title="無線網卡">無線網卡收到數(shù)據(jù)包后交給上層處理，需要發(fā)出的數(shù)據(jù)包也由處理器控制無線網卡來發(fā)出。當然SDRAM、閃存、電源這些模塊也是系統(tǒng)不可缺少的。
本文采用ARM920T為內核的三星處理器S3C2410A。S3C2410A是32位低功耗RISC處理器，同時支持Thumb 16位壓縮指令集，其工作頻率為203MHz。S3C2410A有292個管腳，集成了許多片上功能，例如以太網控制器、UART控制器、可編程I/O口及中斷控制器等。
考慮到接口體積，該平臺選用USB接口的華碩WL-167g無線網卡，提供無線通信功能。
硬件平臺設計結構如圖2所示。
圖2 硬件平臺結構圖
在硬件調試中一個應該注意的問題就是S3C2410A的nWait引腳在不使用時應接上拉電阻，否則系統(tǒng)在啟動模式時將不能正常啟動。
軟件平臺
vivi是韓國MIZI公司開發(fā)的Bootloader，適用于ARM9處理器，其作用是初始化硬件設備、建立內存空間的映射圖，從而將系統(tǒng)的軟硬件環(huán)境設置成一個合適的狀態(tài)，以便為運行操作系統(tǒng)準備正確的環(huán)境。引導程序執(zhí)行完后會將控制權交給內核(zImage)，內核是操作系統(tǒng)的核心，內核需要的各種配置文件、數(shù)據(jù)及上層軟件都存放在根文件系統(tǒng)之中。整個軟件平臺的結構如圖3所示。
圖3 軟件平臺示意圖
內核鏡像的生成
這里不贅述內核生成方法，需要注意此過程容易遇到三類錯誤：一是編譯出錯，應該檢查庫文件是否齊全，另外gcc版本太高也會導致編譯錯誤，重新安裝低版本gcc后即可解決;二是內核配置應將Default kernel command string設置為noinitrd root=/dev/mtdblock3 console=ttySAC0,115200 init=/linuxrc，“console=ttySAC0，115200”使內核啟動期間的信息全部輸出到串口0上，波特率為115200，“mtdblock3”代表第4個Nand閃存分區(qū)，該分區(qū)為根文件系統(tǒng)，init是指定啟動腳本，“init=/linuxrc”表示啟動初始化文件位置;三是應該在drivers/mtd/nand/s3c2410.c中，設置NAND_ECC_SOFT = NAND_ECC_NONE，這樣就關閉了ECC校驗。因為內核是通過vivi寫到Nand閃存中的，vivi使用的軟件ECC算法與內核中校驗算法不同。
無線網卡驅動的移植
本文使用華碩USB無線網卡WL-167g，其網卡驅動是rt73。
移植步驟如下：
a、下載RT73_Linux_STA_Drv1.0.4.0.tar.gz，解壓后生成Module和WPA_Supplicant兩個目錄，將目錄Module中的所有文件都拷貝到內核源碼包drivers/usb/net/rt73下，修改Makefile如下編譯選項：
KDIR := path/linux-2.6.18
path為內核源碼包所在路徑。
b、由于要在內核源碼包里進行交叉編譯，所以修改linux-2.6.18 /drivers/usb/net/Kconfig，加入以下內容：
config RT73
tristate“support for rt73 wireless usb device”
depends on USB && NET && USB_USBNET
c、修改drivers/usb/net/Makefile，加入rt73的編譯項：
obj-$(CONFIG_RT73)+= rt73
d、對內核重新進行配置，
將配置界面中新增的“support for rt73 wireless usb device”選為模塊。
e、make modules
在drivers/usb/net/rt73目錄下生成rt73.ko驅動文件，將rt73.ko放到根文件系統(tǒng)中，再燒寫到ARM板上無線網卡即可正常工作。
f、無線網卡的配置
無線網卡有managed、Ad-hoc兩種模式。managed模式稱為基礎設施模式，又稱接入點模式;Ad-hoc模式稱為點對點模式或無中心模式，用來在無線網卡之間進行一跳通信。Ad Hoc網絡就是在Ad-hoc模式基礎上通過網絡協(xié)議使得該網絡支持多跳通信，因此該模式的使用和性能對Ad Hoc網絡的影響至關重要，以下是Ad-hoc模式配置方法，rausb0表示無線網卡：
1)、ifconfig rausb0 10.0.1.1 up
設置節(jié)點IP為10.0.1.1。
2)、iwpriv rausb0 set AdhocOfdm=2
設置rausb0為11g only模式，即54M速率模式，這是802.11g所能支持的最高速率。
3)、iwconfig rausb0 channel 3
設置信道為3。
4)、iwconfig rausb0 mode ad-hoc essid bcnl
設置網卡模式為ad-hoc，essid為“bcnl”。
上述配置過程中使用的“iw”開頭的命令都是無線工具集中的命令，其源碼包是wireless_tools.29.tar.gz，經過交叉編譯后即可使用。
AODV路由協(xié)議的移植
本文使用的代碼是aodv-uu-0.9.5.tar.gz。AODV分為兩個部分，一個是內核態(tài)模塊kaodv.ko，一個是用戶態(tài)模塊aodvd。AODV主要部分工作在用戶態(tài)，用于維護內核路由表。
AODV需要內核支持，在內核配置時要選上netfilter選項。先編譯內核態(tài)模塊kaodv.ko，步驟如下：
a、將aodv-uu-0.9.5/lnx目錄的內容拷到內核源碼包linux-2.6.18/net/ipv4/kaodv目錄下，修改linux-2.6.18/net/ipv4/Kconfig，添加如下內容：
config KAODV_UU
tristate "support for aodv-uu adhoc routing protocol"
b、在linux-2.6.18/net/ipv4/Makefile末尾添加如下的編譯選項：
obj-$(CONFIG_KAODV_UU) += kaodv/
c、make menuconfig，找到如下的目錄項：
Networking --->
[*] Networking support Networking options --->
support for aodv-uu adhoc routing protocol選為模塊編譯。
d、make modules將生成kaodv.ko。以下介紹用戶態(tài)aodvd編譯：
將aodv-uu-0.9.5/Makefile做適當修改，使其在交叉編譯的環(huán)境下只編譯用戶態(tài)部分。
將得到的kaodv.ko和aodvd拷貝到根文件系統(tǒng)中，再燒寫到ARM板上。
insmod kaodv.ko
./ aodvd
這樣aodv協(xié)議就運行起來了。在協(xié)議移植中有兩點需要注意：一是Makefile中ARM_CCFLAGS=-mbig-endian應該注釋掉，否則運行時會產生大小端混亂的問題;二是將kaodv.ko和aodvd分開編譯，因為同時編譯時總是提示編譯器缺少文件，甚至用其他交叉編譯器依然不能解決問題，而這些錯誤大多是編譯內核模塊產生的。
測試和結論
為簡單起見，本測試使用兩個筆記本和一個ARM平臺組建成Ad Hoc網絡，如圖4所示，A節(jié)點IP為10.0.1.1，B節(jié)點IP為10.0.1.2，C節(jié)點IP為10.0.1.3(經測試將ARM平臺做通信端，A或C做中繼，網絡同樣可以正常運行)。
圖4 測試拓撲示意圖
為了簡單地實現(xiàn)C節(jié)點不在A節(jié)點一跳范圍內，可以使用iptables實現(xiàn)過濾。
在A節(jié)點執(zhí)行：
iptables –A INPUT –p ALL –m mac –mac-source C.mac –j DROP
在C節(jié)點執(zhí)行：
iptables –A INPUT –p ALL –m mac –mac-source A.mac –j DROP
以上的C.mac、A.mac是C和A的實際mac地址，這樣A節(jié)點拒絕C發(fā)給它的數(shù)據(jù)包，C節(jié)點也拒絕A發(fā)給它的數(shù)據(jù)包，保證二者一跳不可達。
在C節(jié)點ping -R 10.0.1.1，鏈路不通，說明過濾成功，B沒有進行數(shù)據(jù)包轉發(fā)。
在各節(jié)點上運行AODV后，在C節(jié)點ping -R 10.0.1.1，結果如圖5所示。
圖5 測試結果
可見，B正確地進行了數(shù)據(jù)包轉發(fā)，AODV協(xié)議正常運行，ARM平臺成功運行。
在A節(jié)點執(zhí)行l(wèi)ftp 10.0.1.3，并下載普通文件。這樣數(shù)據(jù)包由中間的ARM平臺B轉發(fā)，下載完成后，查看C節(jié)點的日志文件/var/log/vsftpd.log，發(fā)現(xiàn)上傳和下載的速率基本相同，有將近700kBps的速率，達到5.4Mbps的速率。粗略估算聚合物電池供電能使ARM平臺穩(wěn)定運行8小時。
通過以上測試，搭建一個Ad Hoc網絡的ARM平臺的目標已經達到。通過對有多個ARM平臺的Ad Hoc網絡進行測試，發(fā)現(xiàn)當某節(jié)點感知無線信號很弱時，無線網卡的essid存在自動變化的情況。