{"id":796,"date":"2015-02-11T10:04:45","date_gmt":"2015-02-11T08:04:45","guid":{"rendered":"https:\/\/excentiscom.webhosting.be\/?p=796"},"modified":"2022-06-24T08:29:44","modified_gmt":"2022-06-24T06:29:44","slug":"the-operation-of-wi-fi-bridges","status":"publish","type":"post","link":"https:\/\/www.excentis.com\/blog\/the-operation-of-wi-fi-bridges\/","title":{"rendered":"The operation of Wi-Fi bridges"},"content":{"rendered":"

Wi-Fi has become the preferred technology for in-home connectivity, but sometimes it\u2019s needed to connect a wired device (e.g. a printer) to a Wi-Fi network using a Wi-Fi bridge. In a wired world bridges are used very often and typically don\u2019t cause much problems. In the Wi-Fi world the situation is slightly different, read on and find out why!<\/p>\n

The operation of Wi-Fi<\/h2>\n

The Wi-Fi environment is a challenging environment, due to the unpredictable and very variable quality of the wireless link. Therefore it was decided to implement, on the wireless link itself, a mechanism to make sure packets that were lost in the transmission are retransmitted. As a consequence every packet that is transmitted on the wireless link needs clear identifiers which wireless device transmitted the packet and which wireless device is the intended receiver.<\/p>\n

Wi-Fi was also invented at a time that every bit that was transmitted, counted. Over low bit-rate links, any extra bit not only causes the effective throughput to be smaller, but also introduces additional latency.<\/p>\n

As a consequence of the above two reasons, standard Wi-Fi uses three MAC addresses on the wireless link, as illustrated in the figure below. One of the MAC addresses contains the BSSID, this one identifies the Wi-Fi network. The example below shows the addresses in use when a broadcast packets (e.g. an ARP request) is sent by the device with MAC address A.<\/p>\n

\"\"<\/p>\n

Let\u2019s now consider the situation that a Wi-Fi client is bridging packets. This situation is illustrated below.<\/p>\n

\n
\"\"<\/div>\n
On the wireless link MAC address A is not in the packet anymore, as a consequence the receiver will reply to the MAC address of the bridge and, unless the bridge does some tricks, the bridge does not know where to send packet 5. As a consequence, communications between A and B will not be possible.<\/div>\n
\n

What are the solutions?<\/h2>\n

NATting at the Wi-Fi bridge<\/h3>\n

A possible solution, implemented by some devices, is shown below. The Wi-Fi bridge implements a NAT-function which means that the Wi-Fi bridge replaces the source IP address of A by its own IP address. By storing information of the outgoing packets (e.g. protocol and port numbers, ICMP identifier) it can map the response to the request on the Wi-Fi link back to the IP address of the original outgoing request.<\/p>\n

\"\"<\/p>\n

Of course, NATting can introduce other problems, as explained in our\u00a0blog post about NAT<\/a>.<\/p>\n

MAC address spoofing<\/h3>\n

Another solution that is used is shown in the figure below:<\/p>\n

\n
\"\"<\/div>\n
\n

In this case the Wi-Fi bridge sets up a new association on the Wi-Fi link for each attached MAC address. The Wi-Fi bridge spoofs the MAC address. In this scenario each device behind the Wi-Fi bridge will have its own Wi-Fi association. An implementation of this is typically limited by the number of MAC addresses it can spoof. If security on the Wi-Fi link is enabled (WPA\/WPA2), the Wi-Fi bridge also needs to perform the 4-way security handshake for each newly detected MAC address, which results in even more overhead.<\/p>\n

In this solution it is important to realize that the AP will actually detect multiple MAC addresses on the Wi-Fi link, one for each device behind the Wi-Fi bridge.<\/p>\n

Wireless Distribution System (WDS)<\/h3>\n

The cleanest solution is of course to use 4 MAC addresses on the link between Wi-Fi bridge and Wi-Fi AP. This method of operation is used in the WDS (Wireless Distribution System). Although the WDS architecture is mentioned in the standard Wi-Fi specification, its use cases are not described and it cannot be used in general since it requires support of the Wi-Fi AP.<\/p>\n

Conclusion<\/h2>\n

Since the original design of Wi-Fi did not take into account the use case of a Wi-Fi bridge, using a Wi-Fi bridge can be troublesome. Different solutions exist to cope with this limitation and we explained them in this blog post.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Wi-Fi has become the preferred technology for in-home connectivity, but sometimes it\u2019s needed to connect a wired device (e.g. a printer) to a Wi-Fi network using a Wi-Fi bridge. In a wired world bridges are used very often and typically don\u2019t cause much problems. In the Wi-Fi world the situation is slightly different, read on …<\/p>\n","protected":false},"author":4,"featured_media":797,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"content-type":"","inline_featured_image":false,"_genesis_block_theme_hide_title":false,"footnotes":""},"categories":[71],"tags":[],"acf":[],"yoast_head":"\nThe operation of Wi-Fi bridges - Excentis<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.excentis.com\/blog\/the-operation-of-wi-fi-bridges\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The operation of Wi-Fi bridges - Excentis\" \/>\n<meta property=\"og:description\" content=\"Wi-Fi has become the preferred technology for in-home connectivity, but sometimes it\u2019s needed to connect a wired device (e.g. a printer) to a Wi-Fi network using a Wi-Fi bridge. 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