Device speeds for a wireless network

I am installing a wireless-n network. Although specifications refer to maximum speeds of 150Mbps and 300Mbps, I'm well aware that in my environment (with the signals having to travel through several walls), I will achieve nothing close to these speeds. So is there any advantage of having a 300 Mbps wireless network card, rather than a 150Mbps card? In other words, if you are achieving 40Mbps with a card capable of 150Mbps, would you achieve a faster speed with a card capable of 300Mbps?

A 300Mbps card will use two wireless channels (and will have two antennas), hence you should get double the throughput.

A card advertised as 150Mbps only achieves that on a 40MHz (double-width) channel.

Which is almost never appropriate if talking about the 2.4Ghz band. On a compatible 20Mhz width it would max out at 65 or 72Mbps. Even this is effectively a signalling rate and half-duplex and assuming nothing else on this channel. So your usable throughput will be significantly lower, and then reduced by passing through multiple walls.

I would go for a 300Mbps (or higher) both on network card (or USB Wi-Fi) and router / access point and shop around.

I normally opt for simultaneous dual band: 2.4GHz (b/g/n) and 5GHz (a/n) gear to give myself choices down the road but if multiple walls are unavoidable 5GHz will probably not be relevant and running network cable is the safe bet.

if you are achieving 40Mbps with a card capable of 150Mbps, would you achieve a faster speed with a card capable of 300Mbps?

Yes you would assuming all else is unchanged and your ISP or devise that you are transferring the data from can cope.

Roughly speaking throughput is 2/3 of the data rate (advertised connection speed of the network card). In reality this may be less due to non-perfect conditions, noise from other networks, noise from bounced signals (self interference) difference between signals received on each antenna etc.

Many thanks for the responses. I'm learning more, but this is giving rise to more questions. In particular, I'm now confused about dual channel equipment.

My new router (which I had thought had a single wireless channel) is the BIPAC 7800N - the configuration utility allows only a single channel to be specified, and there is no reference to it being a "dual channel" router. But it has 3 x 2 dbi detachable antennas, and elsewhere the literature refers to the antennas being "in a 3 x 2 configuration", and my old Edimax 801.11b/g NIC reports seeing 2 different channels (channel 10 and channel 12).

So my follow-up questions are:

On how many channels can the BIPAC 7800N broadcast?

Since the 7800N is not a dual channel router in the sense of broadcasting on both the 2.4Ghz and the 5Ghz bands, I assume that a dual channel NIC would be of no benefit. What type of wireless NIC would make best use of the BIPAC 7800N signal?

A 150 MBit/s card is more properly a single spatial stream card; 300 MBit/s needs two spatial streams, 450 needs 3 spatial streams, and the standard maxes out at 4 spatial streams for 600 MBit/s.

Spatial streams are a clever trick - by broadcasting specially coded signals on multiple antennae (one per spatial stream) and receiving on multiple antennae (at least one per spatial stream), you can use the fact that the path lengths vary to send multiple signals on the same channel and pick them up again at the receiver.

http://en.wikipedia.org/wiki/802.11n#Data_rates helps here. In general, although not always true (depends on noise environment), it's reasonable to guess that each extra spatial stream lets you climb 6 to 8 MCS indexes, with the most probable climb being 7 indexes (and no change to needed GI or permitted channel width); thus, if you're getting 39MBit/s from one spatial stream now (MCS 4, long GI, 20MHz channel), you expect 39 to 78MBit/s with an upgrade to 2 spatial streams, most likely to be a climb to .52MBit/s (MCS 11).

It's also worth noting that more spatial streams tends to mean a more robust network - because each spatial stream runs at a lower usable data rate to get a given speed, you are less likely to completely lose a block of data and need a retry to get through. This directly translates to longer range at a given speed; I have 2 spatial stream kit that can get a reliable 90MBit/s at a range where single stream kit struggles to sustain 40.5MBit/s.

Yes some of the terminology sounds similar but refers to different things.

Double channel width (dual channel) refer to using a 40MHz wide instead of 20MHz standard channel.
This would be like if a radio station was spread across more of the frequency dial when tuning in.

Dual band refers to operation in the 2.4GHz band or 5GHz band (or both simultaneously)
This would be like a radio station could transmit on MW or LW (or both simultaneously).

There is room for double channel widths on the 5Gz band because there are more legal channels.
Whereas on 2.4GHz band there are only 13 channels which overlap,
so using the analogy running 40MHz wide is like a radio station that takes up half of the dial even when configured in the least selfish way, and worse if not positioned at the "left or right edge of the dial".

With that in mind, the diagrams on List of WLAN channels should make more sense.

So, other things being equal it is better to have something that uses multiple antennas (spatial streams) to achieve higher throughput, rather than relying on double channel width, especially in the more common 2.4GHz band (b/g/n).

In other words MIMO does not require use of 40MHz wide channels in this band and 20MHz is more compatible (in the senses of both older devices and neighbouring wireless networks).

Edited by prlzx (Wed 07-Mar-12 10:58:40)

Many thanks for such a clear and detailed response. Things are much clearer now!