由于Ad Hoc网络无中心、动态拓扑等特点,它需要各个节点都具有路由转发功能。本文开发的Ad Hoc网络节点通过在ARM平台上移植路由协议而实现了数据包转发功能。
图1 节点转发
以图1所示的网络进行功能示意,其中A、B、D是普通的移动电脑终端,C是本文开发的ARM平台。A和D是通信的端点,D不在A的一跳范围之内(A的覆盖范围如图中虚线所示)。假设开始时B成为了A和D的中继节点,完成A、D之间的数据转发功能。当B节点出现故障时,C能自动代替B成为新的中继节点,维持A、D之间的通信。
该ARM平台除了路由协议以外,同时完成了ftp、iptables等工具的移植,还可以继续增添语音、视频等服务。
硬件平台
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title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡" title="无线网卡">无线网卡收到数据包后交给上层处理,需要发出的数据包也由处理器控制无线网卡来发出。当然SDRAM、闪存、电源这些模块也是系统不可缺少的。
本文采用ARM920T为内核的三星处理器S3C2410A。S3C2410A是32位低功耗RISC处理器,同时支持Thumb 16位压缩指令集,其工作频率为203MHz。S3C2410A有292个管脚,集成了许多片上功能,例如以太网控制器、UART控制器、可编程I/O口及中断控制器等。
考虑到接口体积,该平台选用USB接口的华硕WL-167g无线网卡,提供无线通信功能。
硬件平台设计结构如图2所示。
图2 硬件平台结构图
在硬件调试中一个应该注意的问题就是S3C2410A的nWait引脚在不使用时应接上拉电阻,否则系统在启动模式时将不能正常启动。
软件平台
vivi是韩国MIZI公司开发的Bootloader,适用于ARM9处理器,其作用是初始化硬件设备、建立内存空间的映射图,从而将系统的软硬件环境设置成一个合适的状态,以便为运行操作系统准备正确的环境。引导程序执行完后会将控制权交给内核(zImage),内核是操作系统的核心,内核需要的各种配置文件、数据及上层软件都存放在根文件系统之中。整个软件平台的结构如图3所示。
图3 软件平台示意图
内核镜像的生成
这里不赘述内核生成方法,需要注意此过程容易遇到三类错误:一是编译出错,应该检查库文件是否齐全,另外gcc版本太高也会导致编译错误,重新安装低版本gcc后即可解决;二是内核配置应将Default kernel command string设置为noinitrd root=/dev/mtdblock3 console=ttySAC0,115200 init=/linuxrc,“console=ttySAC0,115200”使内核启动期间的信息全部输出到串口0上,波特率为115200,“mtdblock3”代表第4个Nand闪存分区,该分区为根文件系统,init是指定启动脚本,“init=/linuxrc”表示启动初始化文件位置;三是应该在drivers/mtd/nand/s3c2410.c中,设置NAND_ECC_SOFT = NAND_ECC_NONE,这样就关闭了ECC校验。因为内核是通过vivi写到Nand闪存中的,vivi使用的软件ECC算法与内核中校验算法不同。
无线网卡驱动的移植
本文使用华硕USB无线网卡WL-167g,其网卡驱动是rt73。
移植步骤如下:
a、下载RT73_Linux_STA_Drv1.0.4.0.tar.gz,解压后生成Module和WPA_Supplicant两个目录,将目录Module中的所有文件都拷贝到内核源码包drivers/usb/net/rt73下,修改Makefile如下编译选项:
KDIR := path/linux-2.6.18
path为内核源码包所在路径。
b、由于要在内核源码包里进行交叉编译,所以修改linux-2.6.18 /drivers/usb/net/Kconfig,加入以下内容:
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的编译项:
obj-$(CONFIG_RT73)+= rt73
d、对内核重新进行配置,
将配置界面中新增的“support for rt73 wireless usb device”选为模块。
e、make modules
在drivers/usb/net/rt73目录下生成rt73.ko驱动文件,将rt73.ko放到根文件系统中,再烧写到ARM板上无线网卡即可正常工作。
f、无线网卡的配置
无线网卡有managed、Ad-hoc两种模式。managed模式称为基础设施模式,又称接入点模式;Ad-hoc模式称为点对点模式或无中心模式,用来在无线网卡之间进行一跳通信。Ad Hoc网络就是在Ad-hoc模式基础上通过网络协议使得该网络支持多跳通信,因此该模式的使用和性能对Ad Hoc网络的影响至关重要,以下是Ad-hoc模式配置方法,rausb0表示无线网卡:
1)、ifconfig rausb0 10.0.1.1 up
设置节点IP为10.0.1.1。
2)、iwpriv rausb0 set AdhocOfdm=2
设置rausb0为11g only模式,即54M速率模式,这是802.11g所能支持的最高速率。
3)、iwconfig rausb0 channel 3
设置信道为3。
4)、iwconfig rausb0 mode ad-hoc essid bcnl
设置网卡模式为ad-hoc,essid为“bcnl”。
上述配置过程中使用的“iw”开头的命令都是无线工具集中的命令,其源码包是wireless_tools.29.tar.gz,经过交叉编译后即可使用。
AODV路由协议的移植
本文使用的代码是aodv-uu-0.9.5.tar.gz。AODV分为两个部分,一个是内核态模块kaodv.ko,一个是用户态模块aodvd。AODV主要部分工作在用户态,用于维护内核路由表。
AODV需要内核支持,在内核配置时要选上netfilter选项。先编译内核态模块kaodv.ko,步骤如下:
a、将aodv-uu-0.9.5/lnx目录的内容拷到内核源码包linux-2.6.18/net/ipv4/kaodv目录下,修改linux-2.6.18/net/ipv4/Kconfig,添加如下内容:
config KAODV_UU
tristate "support for aodv-uu adhoc routing protocol"
b、在linux-2.6.18/net/ipv4/Makefile末尾添加如下的编译选项:
obj-$(CONFIG_KAODV_UU) += kaodv/
c、make menuconfig,找到如下的目录项:
Networking --->
[*] Networking support Networking options --->
support for aodv-uu adhoc routing protocol选为模块编译。
d、make modules将生成kaodv.ko。以下介绍用户态aodvd编译:
将aodv-uu-0.9.5/Makefile做适当修改,使其在交叉编译的环境下只编译用户态部分。
将得到的kaodv.ko和aodvd拷贝到根文件系统中,再烧写到ARM板上。
insmod kaodv.ko
./ aodvd
这样aodv协议就运行起来了。在协议移植中有两点需要注意:一是Makefile中ARM_CCFLAGS=-mbig-endian应该注释掉,否则运行时会产生大小端混乱的问题;二是将kaodv.ko和aodvd分开编译,因为同时编译时总是提示编译器缺少文件,甚至用其他交叉编译器依然不能解决问题,而这些错误大多是编译内核模块产生的。
测试和结论
为简单起见,本测试使用两个笔记本和一个ARM平台组建成Ad Hoc网络,如图4所示,A节点IP为10.0.1.1,B节点IP为10.0.1.2,C节点IP为10.0.1.3(经测试将ARM平台做通信端,A或C做中继,网络同样可以正常运行)。
图4 测试拓扑示意图
为了简单地实现C节点不在A节点一跳范围内,可以使用iptables实现过滤。
在A节点执行:
iptables –A INPUT –p ALL –m mac –mac-source C.mac –j DROP
在C节点执行:
iptables –A INPUT –p ALL –m mac –mac-source A.mac –j DROP
以上的C.mac、A.mac是C和A的实际mac地址,这样A节点拒绝C发给它的数据包,C节点也拒绝A发给它的数据包,保证二者一跳不可达。
在C节点ping -R 10.0.1.1,链路不通,说明过滤成功,B没有进行数据包转发。
在各节点上运行AODV后,在C节点ping -R 10.0.1.1,结果如图5所示。
图5 测试结果
可见,B正确地进行了数据包转发,AODV协议正常运行,ARM平台成功运行。
在A节点执行lftp 10.0.1.3,并下载普通文件。这样数据包由中间的ARM平台B转发,下载完成后,查看C节点的日志文件/var/log/vsftpd.log,发现上传和下载的速率基本相同,有将近700kBps的速率,达到5.4Mbps的速率。粗略估算聚合物电池供电能使ARM平台稳定运行8小时。
通过以上测试,搭建一个Ad Hoc网络的ARM平台的目标已经达到。通过对有多个ARM平台的Ad Hoc网络进行测试,发现当某节点感知无线信号很弱时,无线网卡的essid存在自动变化的情况。