I'm not going to speak about adding an antenna or adding a repeater to increase your signal. This topics are trivial and well covered (just see here for instance). Instead, we will look at different optimizations for signal gain, that you probably were not aware of.
In this topic, I will mostly address the problem you might have when dealing with a medium distance access point (like your neighbour for example).
In this topic, I will mostly address the problem you might have when dealing with a medium distance access point (like your neighbour for example).
Antenna related
1) The antenna is probably the first tool you will need to have a descent signal. Choosing a good antenna does depend on the place where you live. For example, if you live in a very dense city, it makes no sense to use a 26dbi parabolic dish. Instead, you will prefer a Yagi or a panel antenna. Looking at the radiation pattern of the antenna will help you understanding what is the best for you. Moreover, if you have a high gain antenna, you will face up the hidden terminal problem. So don't fall in the trap of "the biggest the best". Of course, it will be a good idea to put your antenna outside, ie on your balcony or on the window edge.
2) Avoid using too many connectors. Indeed, every time you use a connector (or a pigtail), a large part of the signal is lost. Prefer only one wire, directly connected from your access point(AP)/card to the antenna.
3) Use very small cables. I'm always amazed when I see people on ebay selling Yagi antenna with 30 meters of coax cable. A good low-loss cable has a loss of around 0.4db/meter, it means that if you have 30 meters of cable, you are loosing 12dbi. So what's the point of having a 19dbi antenna then ?
2) Avoid using too many connectors. Indeed, every time you use a connector (or a pigtail), a large part of the signal is lost. Prefer only one wire, directly connected from your access point(AP)/card to the antenna.
3) Use very small cables. I'm always amazed when I see people on ebay selling Yagi antenna with 30 meters of coax cable. A good low-loss cable has a loss of around 0.4db/meter, it means that if you have 30 meters of cable, you are loosing 12dbi. So what's the point of having a 19dbi antenna then ?
4) The polarization of the antenna is something people usually don't care. However, a "mis-polarized" antenna can have tremendous damages on your signal If you don't know what "polarization" is, just think about your polarized sunglasses. Basically, the glasses let only pass certain type of waves. In wireless it's the same: if someone is emitting a vertically (linearly) polarized wave and you set up your antenna horizontally, you will miss the signal.
If you still don't understand what this "polarization" is, the best way to proceed is by "trial and error": turn your antenna all the way around until your get the maximal strength.
The following antenna has an horizontal polarization (source: Wikipedia), for some obvious reasons. If you take another parabolic antenna and you turn it to have a vertical polarization, you won't be able to get any signal.
If you still don't understand what this "polarization" is, the best way to proceed is by "trial and error": turn your antenna all the way around until your get the maximal strength.
The following antenna has an horizontal polarization (source: Wikipedia), for some obvious reasons. If you take another parabolic antenna and you turn it to have a vertical polarization, you won't be able to get any signal.
However, waves might change polarization when they hit objects, so don't go too crazy with that. Usually, omnidirectional antennas are low profile antennas and tempts to have a circular polarization. Based on my observation, I would say that the vertical polarization is predominant, though.Note that it doesn't mean because your are pointing to the AP that you will get the maximum strength (this is only true for parabolic antennas). In the future, we will see how to measure the signal in a pretty accurate manner.
802.11 & hardware related
5) If you can change the AP channel, try to take one that is not over-used in your neighbourhood. You can figure this out by counting the number of access points that are on the same channel, by using kismet for example. This makes sense, since data is braodcasted, the more people will use the same channel (even with different APs), the more collisions there will be. Note that the channels go by group of 3, like on this figure. Thus, channel 1 and 3 lie on the same frequency band, but channel 1 and 5 don't. Usually, people don't change their channel, and leave the default one (usually 6 or 1), so switching to channel 14 seems to be a good plan. Of course, if you can go 5GHz, go 5GHz, there will be even less people on there.
6) Many times, changing channel does not help, because they are all busy and a lot of people are using them. In this case, enabling RTS/CTS (Request to Send / Clear to Send) will definitely help. Basically, when a client wants to send a packet, it first sends a RTS to make sure nobody is using the channel. Then, the AP replies with a CTS to confirm that it (the client) can send its packet.
RTS/CTS is usually disabled (ie set at a very high value). If the channel is very busy, you'll need to decrease the RTS/CTS threshold, at 500b for example, meaning that a RTS will be sent only for packets >= 500b. You'll increase the overhead, but ultimately you'll transmit more packets (and avoid the hidden node problem).
RTS/CTS is usually disabled (ie set at a very high value). If the channel is very busy, you'll need to decrease the RTS/CTS threshold, at 500b for example, meaning that a RTS will be sent only for packets >= 500b. You'll increase the overhead, but ultimately you'll transmit more packets (and avoid the hidden node problem).
To understand better why the RTS/CTS is needed, let's take this example: suppose that you are in a restaurant. If it's crowded, there will be a lot of people speaking at the same time (ie channel "busy"). By analogy - if you assume that the restaurant is the channel you are using and the people speaking are the clients using this channel - when using RTS/CTS mechanism, we take care that no other people are speaking. It's like saying "Now I'm speaking, please shut up (RTS)" to everyone in the restaurant, and the person to whom you are speaking would reply (once everybody is silent) "OK, I let you speak now, go ahead (CTS)". Of course, if you are alone in the restaurant, there is no need to say that. Well, don't do it in real life, it might be weird...
7) We have seen that when a channel is busy, we can enable the RTS/CTS mechanism. However, sometimes the link is BAD, meaning there is a lot of white noise on the line. Basically, you might have a good signal, but a lot of noise, decreasing your signal/noise ration (SNR). The trick here is to transmit smaller packets. Suppose that you are sending BIG packets and the noise destroys a packet with a certain probability, you will loose more data (and hence need to retransmit) when these packets are big rather than small. Well, this doesn't work linearly like that, but you got the idea. It means that if the packet is smaller, there will be less to retransmit. This setting can be changed through the fragmentation threshold.
That's interesting to notice that your microwave oven is running at around 2GHz and Wifi is operating at 2.4GHz. Microwave ovens are studied to maximize the absorption by water (to warm it), and it happens to be at around 2GHz. What does it mean ? It means that when it's rainy, the signal is lost in... the rain, or in other words, the Wifi is somehow warming the rain. Thus, humidity acts as a wireless barrier.
8) Have you ever looked at a wireless chip datasheet ? Here is a snippet.
7) We have seen that when a channel is busy, we can enable the RTS/CTS mechanism. However, sometimes the link is BAD, meaning there is a lot of white noise on the line. Basically, you might have a good signal, but a lot of noise, decreasing your signal/noise ration (SNR). The trick here is to transmit smaller packets. Suppose that you are sending BIG packets and the noise destroys a packet with a certain probability, you will loose more data (and hence need to retransmit) when these packets are big rather than small. Well, this doesn't work linearly like that, but you got the idea. It means that if the packet is smaller, there will be less to retransmit. This setting can be changed through the fragmentation threshold.
That's interesting to notice that your microwave oven is running at around 2GHz and Wifi is operating at 2.4GHz. Microwave ovens are studied to maximize the absorption by water (to warm it), and it happens to be at around 2GHz. What does it mean ? It means that when it's rainy, the signal is lost in... the rain, or in other words, the Wifi is somehow warming the rain. Thus, humidity acts as a wireless barrier.
8) Have you ever looked at a wireless chip datasheet ? Here is a snippet.
What ca we see from the datasheet ?
- The output power is maximized when the data rate is low.
- The sensibility is maximized when the data rate is low (ie 1Mbps).
It makes sens to set the AP between 1->6Mbps, doesn't it ? We will increase the sensibility (ie we will "receive better"), and also increase the output power (ie we will "better send"). However, if you decrease *YOUR* bit rate, you will reserve the channel longer and thus, other people will see a drop down in their maximal bitrate.
9) Decrease the power of your AP: if you are living in a very tiny place and you have access to the tx power (you are using dd-wrt for example), you can decrease it. Indeed, you will radiate less and thus you will decrease the problems you might have with the neighbourhood (with RTS/CTS for example, avoiding the exposed terminal problem).
9.2) Finally, when dealing with multiple user connected to an AP, we can hack the wireless driver to gain priority. Basically, there exist techniques to reserve the channel more than usual (ie greedy user) and this will probably be a topic I'll discuss on this blog.
We are the end of our wifi-tips. I hope your learned something new and feel free to leave your impressions/comments.
9) Decrease the power of your AP: if you are living in a very tiny place and you have access to the tx power (you are using dd-wrt for example), you can decrease it. Indeed, you will radiate less and thus you will decrease the problems you might have with the neighbourhood (with RTS/CTS for example, avoiding the exposed terminal problem).
9.2) Finally, when dealing with multiple user connected to an AP, we can hack the wireless driver to gain priority. Basically, there exist techniques to reserve the channel more than usual (ie greedy user) and this will probably be a topic I'll discuss on this blog.
We are the end of our wifi-tips. I hope your learned something new and feel free to leave your impressions/comments.
Genuinely informative and useful blog entry. Thanks :-)
ReplyDeleteAnon.
Point 9 is really something anyone can do right now. Great stuff! Will try it when I get back home. Probably a less occupied channel will help too.
ReplyDeleteThanks