Date: Thu, 27 May 1999 18:39:04 -0400 (EDT) From: wpaul@ctr.columbia.edu (Bill Paul) To: raj@cisco.com (Richard Johnson) Cc: freebsd-mobile@freebsd.org, karp@eecs.harvard.edu Subject: Re: wi driver and WaveLAN IEEE 802.11 Turbo cards Message-ID: <199905272239.SAA07328@startide.ctr.columbia.edu> In-Reply-To: <199905272024.NAA13414@kitab.cisco.com> from "Richard Johnson" at May 27, 99 01:24:44 pm
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Of all the gin joints in all the towns in all the world, Richard Johnson had to walk into mine and say: > All of the wireless ethernet systems I've seen have a central box > (switch or bridge) which communicates with the PCMCIA cards. For my > home use I would only have two PCMCIA cards (two laptops) using it and > connected to an inhouse ethernet. I'm wondering if I could purchase > simply three PCMCIA cards, one for each of two laptops, and one for an > extra FreeBSD system. Then use the FreeBSD system as a router between > the inhouse wired ethernet and the wireless ethernet? The real > question is whether the central box does something other than simply > bridging packets between the two networks. Does it do some part of > the protocol which the PCMCIA cards can't do? Can a system with one > PCMCIA card receive packets from multiple other PCMCIA cards directly, > or do you have to always set it up as a star using the specialized > central box? Translation: "I don't actually know how the IEEE 802.11 protocol works, please explain it to me." With 802.11, wireless stations can either talk directly to one another (ad-hoc mode) or have all their traffic relayed through an access point (BSS mode). The combination of the access point and the wireless hosts is refered to as a basic service set. The access point actually does two things: in addition to relaying all of the traffic between the wireless stations in the service set, it also connects to a wired network which lets the wireless stations communicate with wired counterparts. This is *NOT* the same as what the older WaveLAN cards and the older WavePOINT bridges do. Of course, you didn't specify if you were talking about the old legacy WaveLAN cards or the new 802.11 cards, so I'm going to assume that you meant the 802.11 and if that's not what you meant, to bad. A bunch of WaveLAN/IEEE NICs all configured for ad-hoc mode (which is the default with the wi driver) will behave exactly like a bunch of ethernet interfaces all wired to the same segment. Consequently, you can put a WaveLAN/IEEE interface into a desktop or server host with an ethernet interface, set it up for IP forwarding and make a router out of it. This is not how a WavePOINT II works though. The 802.11 protocol specifies a mechanism for wireless end stations to associate with an access point by exchanging beacons. End stations will only communicate with other end stations that have become associated with the same access point (i.e. that are part of the service set). The important thing to realize is that with the WaveLAN/IEEE cards, this part of the protocol is handled internally by the firmware. When you set the WaveLAN/IEEE card for BSS mode, it will listen for and generate beacons all on its own. It will not actually let the host send or receive data until it becomes associated with an access point. By contrast, in ad-hoc mode, the station can send and receive data right away, and the association process is never performed. This could turn out to be a problem for us since one of the things the guys in the lab wanted to do was to have a series of workstations pretend to be access points so that they could experiment with having wireless mobile stations handing off from one access point to another. This was possible with the older WaveLAN cards because the hardware was different: there was no firmware, and the host software could generate its own beacons. With the new cards, I think the host is only allowed to send 802.11 data frames: I don't think it's possible to generate control or management frames for the purpose of transmitting your own hand-crafted beacons. In BSS mode anyway, it looks like the host can't do anything until the card has located an access point. It may be possible to send a beacon when the card is set to ad-hoc mode, but we haven't had a chance to try that yet (we were hoping to get some info from somebody at Lucent to confirm one way of the other if what we want to do is possible, but so far there's been no word). -Bill -- ============================================================================= -Bill Paul (212) 854-6020 | System Manager, Master of Unix-Fu Work: wpaul@ctr.columbia.edu | Center for Telecommunications Research Home: wpaul@skynet.ctr.columbia.edu | Columbia University, New York City ============================================================================= "Mulder, toads just fell from the sky!" "I guess their parachutes didn't open." ============================================================================= To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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