Technical:P2P Radio

From Openmoko

(Difference between revisions)
Jump to: navigation, search
Line 1: Line 1:
 
This page attempts to correct some 'obvious' facts about peer-peer radio networking.
 
This page attempts to correct some 'obvious' facts about peer-peer radio networking.
 
 
 
Radio waves are not like the internet.
 
Radio waves are not like the internet.
 
 
To make a computer analogy, they are like having everyone within several kilometers on one unswitched ethernet network.
 
To make a computer analogy, they are like having everyone within several kilometers on one unswitched ethernet network.
 
 
Consider a thousand people in a smallish room.
 
Consider a thousand people in a smallish room.
 
 
If they are all silent, and speak only on an agreed schedule, everyone can easily hear everyone else, without raising their voice. (transmission power)
 
If they are all silent, and speak only on an agreed schedule, everyone can easily hear everyone else, without raising their voice. (transmission power)
 
 
However, this only gives so much bandwidth (words/minute) before the channel becomes saturated.
 
However, this only gives so much bandwidth (words/minute) before the channel becomes saturated.
 
 
GSM works exactly this way. Everyone has their turn to speak. The cellphone tower hands out the timeslots, and to extend the analogy a bit - separates people into different rooms using pico and microcells, so that you can go off into a 'corner' and not interfere with the room as a whole.
 
GSM works exactly this way. Everyone has their turn to speak. The cellphone tower hands out the timeslots, and to extend the analogy a bit - separates people into different rooms using pico and microcells, so that you can go off into a 'corner' and not interfere with the room as a whole.
 
 
If you don't have an agreed schedule, any two people in the room conversing will mean that you can't really hear someone if they are further away than the two people talking.
 
If you don't have an agreed schedule, any two people in the room conversing will mean that you can't really hear someone if they are further away than the two people talking.
  
Line 21: Line 13:
 
This is when communication with people other than your neighbours becomes impossible.
 
This is when communication with people other than your neighbours becomes impossible.
 
Also, you can't meaningfully pass messages to other people - without a drastic crash in bandwidth, as every member of the network between you and the person you want to message is also passing messages for dozens of others.
 
Also, you can't meaningfully pass messages to other people - without a drastic crash in bandwidth, as every member of the network between you and the person you want to message is also passing messages for dozens of others.
 
  
 
If you are on average 20 nodes away from the destination, then on average, (neglecting routing problems) you, and all the nodes you route through, are sharing their bandwidth with 20 other users.
 
If you are on average 20 nodes away from the destination, then on average, (neglecting routing problems) you, and all the nodes you route through, are sharing their bandwidth with 20 other users.
Line 43: Line 34:
 
They get best (momentary) gain from the system by simply turning their radio power up all the way, and ignoring any traffic by stamping on it.
 
They get best (momentary) gain from the system by simply turning their radio power up all the way, and ignoring any traffic by stamping on it.
 
[[Category:Technical  ]]
 
[[Category:Technical  ]]
[[Category:Documentation]]
 
[[Category:Reorg:MiscTechnical]]
 

Revision as of 04:31, 27 March 2008

This page attempts to correct some 'obvious' facts about peer-peer radio networking. Radio waves are not like the internet. To make a computer analogy, they are like having everyone within several kilometers on one unswitched ethernet network. Consider a thousand people in a smallish room. If they are all silent, and speak only on an agreed schedule, everyone can easily hear everyone else, without raising their voice. (transmission power) However, this only gives so much bandwidth (words/minute) before the channel becomes saturated. GSM works exactly this way. Everyone has their turn to speak. The cellphone tower hands out the timeslots, and to extend the analogy a bit - separates people into different rooms using pico and microcells, so that you can go off into a 'corner' and not interfere with the room as a whole. If you don't have an agreed schedule, any two people in the room conversing will mean that you can't really hear someone if they are further away than the two people talking.

This is a reduction in range due to interference.

If everyone talks at once, you can only hear your neighbours. This is when communication with people other than your neighbours becomes impossible. Also, you can't meaningfully pass messages to other people - without a drastic crash in bandwidth, as every member of the network between you and the person you want to message is also passing messages for dozens of others.

If you are on average 20 nodes away from the destination, then on average, (neglecting routing problems) you, and all the nodes you route through, are sharing their bandwidth with 20 other users.

Then there is the problem that it isn't as good as this.

Because users are not evenly spread, there are 'hot' links, which are links between 'islands' of relatively isolated users. This means that a vastly disproportionate amount of traffic goes to them, jamming their link.

There are similar problems with uplinks to the 'real' internet for long-term traffic.

Then you run into problems with interference.

Even neglecting users who are abusing the network, the noise floor goes up significantly, because everyone is 'shouting' at once.

Once you get a few people that decide that streaming video from their webcams to their office is a fun app, they utterly screw the people using the same frequency in the same range.

The problem _is_ the sociopaths. With an open protocol, and open devices, and shared radio frequency it is simply impossible to stop them interfering with other users.

They get best (momentary) gain from the system by simply turning their radio power up all the way, and ignoring any traffic by stamping on it.

Personal tools

This page attempts to correct some 'obvious' facts about peer-peer radio networking.


Radio waves are not like the internet.

To make a computer analogy, they are like having everyone within several kilometers on one unswitched ethernet network.

Consider a thousand people in a smallish room.

If they are all silent, and speak only on an agreed schedule, everyone can easily hear everyone else, without raising their voice. (transmission power)

However, this only gives so much bandwidth (words/minute) before the channel becomes saturated.

GSM works exactly this way. Everyone has their turn to speak. The cellphone tower hands out the timeslots, and to extend the analogy a bit - separates people into different rooms using pico and microcells, so that you can go off into a 'corner' and not interfere with the room as a whole.

If you don't have an agreed schedule, any two people in the room conversing will mean that you can't really hear someone if they are further away than the two people talking.

This is a reduction in range due to interference.

If everyone talks at once, you can only hear your neighbours. This is when communication with people other than your neighbours becomes impossible. Also, you can't meaningfully pass messages to other people - without a drastic crash in bandwidth, as every member of the network between you and the person you want to message is also passing messages for dozens of others.


If you are on average 20 nodes away from the destination, then on average, (neglecting routing problems) you, and all the nodes you route through, are sharing their bandwidth with 20 other users.

Then there is the problem that it isn't as good as this.

Because users are not evenly spread, there are 'hot' links, which are links between 'islands' of relatively isolated users. This means that a vastly disproportionate amount of traffic goes to them, jamming their link.

There are similar problems with uplinks to the 'real' internet for long-term traffic.

Then you run into problems with interference.

Even neglecting users who are abusing the network, the noise floor goes up significantly, because everyone is 'shouting' at once.

Once you get a few people that decide that streaming video from their webcams to their office is a fun app, they utterly screw the people using the same frequency in the same range.

The problem _is_ the sociopaths. With an open protocol, and open devices, and shared radio frequency it is simply impossible to stop them interfering with other users.

They get best (momentary) gain from the system by simply turning their radio power up all the way, and ignoring any traffic by stamping on it.