15 replies to this topic

[Kosmos]

I am waiting to get my Time Capsule to replace my AirPort Extreme, and was wondering if it will be possible to move my AirPort Extreme to my media center and plug all my hardware into it (TiVo, XBox360) and do away with those USB Adapters. Is this something thats possible? It seems to me that the Time Capsule can act as a receiver, however I'd like to plug that one into my office for faster back-ups. Any thoughts?

[Graham]

Could you explain a bit more clearly what you're wanting to do? What computers / devices are involved?

[Kosmos]

Could you explain a bit more clearly what you're wanting to do? What computers / devices are involved?

Sure! Right now I have my Airport Extreme Base Station in my office with my Cable Router plugged into it. I just purchased a Time Capsule and will replace my Airport Extreme with it. Now I wonder if I can move the AirPort Extreme into my living room and set it under my Apple TV for example and plug my NON-Wireless items into the Airport Extreme will they be able to transmit info with to the network Time Capsule is creating.

Can the Airport Extreme act as a receiver or bridge on my network?

For instance my XBox360 is using a USB network adaprter to conect to the internet... and my TiVo is going the same thing (I'd like to get rid of those). Can I plug both devices into the AirPort Extreme using Ethernet cables and thus connect to the network my Time Capsule is creating?

I hope that helps, if its too frustrating feel free to let this one go. But I look forward to your thoughts on this.

Edited by Kosmos, 05 March 2008 - 01:05 PM.

[Graham]

Yes, you can do this, but you will be cutting the speed of your network in half when using WDS. But of course, if you're using any 802.11g devices on the N network, then it will be limited to g speeds anyway.

[Kosmos]

Yes, you can do this, but you will be cutting the speed of your network in half when using WDS. But of course, if you're using any 802.11g devices on the N network, then it will be limited to g speeds anyway.

Thank you so much for that information. Thats exactly what I needed to know. Obviously, I would want to avoid using WDS if I have other options, correct?
Thank you so much for the info, Graham!

[Graham]

If you want the extreme and time capsule to talk to each other without having an ethernet cable between them, you'll have to use WDS.

[ganbustein]

Yes, you can do this, but you will be cutting the speed of your network in half when using WDS. But of course, if you're using any 802.11g devices on the N network, then it will be limited to g speeds anyway.

He's not using the old AirPort as a wireless relay. The only wireless traffic it will see is upstream, toward the Time Capsule, so its bandwidth won't be cut in half. All the downstream devices are going to be connected with wire.

[Graham]

That's wrong. There is always traffic both ways on the network. There might not be much, but there needs to be traffic both ways. I don't have time to explain why, but just take my word for it, mmkay?

[ganbustein]

That's wrong. There is always traffic both ways on the network. There might not be much, but there needs to be traffic both ways. I don't have time to explain why, but just take my word for it, mmkay?

I understand about ACK packets, but they're present even if you don't have WDS in the picture.

The halving of bandwidth that one normally associates with WDS comes about because if a base station is relaying packets over the air, it has to spend air time receiving the packet and then spend the same air time again forwarding the packet. Each packet counts twice against the available bandwidth. This happens whether the packet is going upstream or downstream, and whether it's a full-length packet or an ACK packet.

But the base station is going to be acting as a switch. It'll keep track of which MAC addresses show up as source addresses on packets arriving on each port, and send packets directed at those MAC addresses out on the same port.

In particular, if it receives over the air a packet addressed to a MAC address that it knows is hanging off a particular 802.3 port, it's going to forward the packet on that port, and not over the air. There won't be the normal halving of radio bandwidth.

There will of course be a few bookkeeping packets to update the routing tables as a wireless device moves between base stations, but that's negligible.

[Graham]

But if you have an xbox on there, it won't just be sending ACK's. It will require data going both ways when you're playing a game online. So the halving in bandwidth will happen.

[ganbustein]

But if you have an xbox on there, it won't just be sending ACK's. It will require data going both ways when you're playing a game online. So the halving in bandwidth will happen.

It would do that even without WDS.

[Graham]

Please explain how then? Because if I'm wrong I'm really not going to get that degree in computer networking. The only way to get them to talk to each other is to use WDS or to have an ethernet cable between them. Otherwise they're just two separate networks.

[ganbustein]

Please explain how then? Because if I'm wrong I'm really not going to get that degree in computer networking. The only way to get them to talk to each other is to use WDS or to have an ethernet cable between them. Otherwise they're just two separate networks.

AirPort, like most radio-based protocols, is half-duplex. Thus, the rated bandwidth puts a cap on the SUM of the throughput in both directions. That would be true even if his xbox was airport capable and talking directly to a single base station.

It's commonly stated that WDS cuts your bandwidth in half, but that's because the remote base station has to use up half of its bandwidth talking to the master base station, and half talking to the actual device. It doesn't matter which way the traffic is flowing. The sum of the upstream data rate and the downstream data rate cannot exceed HALF of the rated wifi bandwidth.

Example: Suppose an airport-capable xbox. (I neither know nor care what an xbox is capable of. Substitute a different airport-capable device if you feel the need.) Suppose it needs to send one packet to a WAN host, and get one packet in reply. (Multiply the example to get as many packets as you want. Throw in TCP ACK packets into the mix or not, as you wish. This example will proceed with one packet in each direction.)

Network 1: An AirPort base station acting as a simple (non-WDS) wireless router. One packet leaves the xbox, goes over the air to router, from there into WAN-land. One packet comes back, through the router, from there over the air to the xbox. Total packets sent over the air: 2.

Network 2: Two AirPort base stations acting as a WDS: WAN connects to the master base station, that connects over the air to the remote base station, which talks over the air to the xbox. Xbox sends one packet over the air the remote, which forwards it over the air to the master, which sends it to the WAN. Reply comes from the WAN to the master base station, over the air to the remote, and then over the air again to the xbox. Total packets sent over the air: 4.

Network 3: Two AirPort base stations acting as a WDS, but this time the xbox connects by ethernet cable to the remote base station. One packet goes from the xbox over cable to the remote, from there by air to the master, and on to the WAN. Reply comes from WAN to master, over air to remote, over cable to xbox. Total packets sent over the air: 2.

Notice that network 1 (non-WDS) and network 3 (WDS) send the same number of packets over the air. The only slowdown is the potential halving due to the half-duplex nature of wifi.

But network 2 gets an additional halving due to the fact that the remote base station uses double the air time because it's retransmitting over air packets that it just received over the air. That's the halving that's usually attributed to WDS, but (and this was my point from the beginning) that halving only happens when the route through a base station uses-over-the air links on both sides of it. That does not always happen in a WDS, and can happen in non-WDS.

You can eliminate the effect either by running a cable between the master and remote base stations or by running a cable between the remote base and the terminal device. They both have the same effect: the remote base station only has to devote air time to any one packet once.


I would also like to remind everyone that the WAN is probably going to be the bottleneck in all this.

[Graham]

Ah ok, I was thinking you meant that you could get the two routers talking without using wds or an ethernet cable.

And good explanation. Now I think about it you're right. I've just never seen WDS being used like this, since it's usually easier to just use a wireless adapter than it is to piss about with WDS.

[Dolphbucs]

I vote this thread gets pinned. I've never heard the"half-bandwidth" thing explained better. Great information! Cudos guys ( I give Graham equal credit because he prompted the explanation ).

[karinlord]

I vote this thread gets pinned. I've never heard the"half-bandwidth" thing explained better. Great information! Cudos guys ( I give Graham equal credit because he prompted the explanation ).

My vote as well, to have this thread pinned. That was a great explanation. Thanks Ganbustein and Graham.