i want to send data over wireless medium in the range of 6 kms.
what is the best method to achieve this. Is this Wi-Fi or some other method.
Assuming you're talking line of sight, WiFi would be possible, with the kit needed costing around 300 euro or up. That is of course largely dependant on your requirements like throughput, influence of bad weather, budget...
Related
Two Quick questions:
Are the seconds value of the clock/time on all iPhones around the world the same? If not, are they close at least & how close?
Do the seconds value of the clock/time on all iPhones around the world change/increment at the exact same time?
Upon request, I'm editing this post and adding the purpose for asking such questions:
I'm trying to make a corporate app that can play a video on multiple iPhones around the world at the exact same time (or as close as possible, ideally the exact same moment). Could you please guide me on how to do this?
Much thanks in advance!
To answer your actual question per se,
Are the seconds value of the clock/time on all iPhones around the world the same?
The fact is, yes, 99.9999% of iPhone users simply use the "get time from a server" system which is of course built in to any phone now.
(Indeed, this simply applies to any Windows, Mac, Android, iOS, etc etc device.)
Yes, they are all "about the same".
You could rely on it being within a second or two, probably even closer.
You cannot rely on it being closer than that.
It seems that you
I'm trying to make a corporate app that can play a video on multiple iPhones around the world at the exact same time
Synchronization in general and streaming video (is it streaming?) is a well-explored (and rather technical) branch of software engineering. (For example a massive amount of game engineering, which is a huge field, relates to inside-the-frame synchronization.)
This is not something you can learn how to do in five minutes, and it requires a stack of device-cloud stuff. Go ahead and ask new questions about this, or just google to get started! Good luck.
Regarding using push notifications.
With the push notification you would send a time. You would not send a "command to play it".
So say right now it is (example) 09:13:28. You would pick a time in the future by a couple minutes. So let's say 09:14:30.
Then using a push notification you would send that information "09:14:30" (and a video file name) to everyone connected. (You'd be sending a "command" as it were, to play video X at 09:14:30.)
Then every device would in fact play the video at 09:14:30 (simply using the local clock, as asked in your question).
Be aware that sending push notifications is extremely sloppy and slow. It can take any amount of time from 5 seconds to a minute, AND quite often there are delays beyond that (ie ten minutes or the like).
I personally would not even bother starting to experiment with push notifications, for the project you describe.
These days, making apps is entirely about using device-cloud services, such as Firebase. Everything is about "OCC" - occasionally connected computing.
(So, you can't get a job "making apps" anymore - i.e. if you know how to move buttons around on an iPhone screen. You get a job because you can make a total, live, device-cloud system - indeed such as you are making.)
Indeed your example project is the perfect such "demo" project for learning about how to do modern apps.
Simply use Firebase to sync everything up.
You'll essentially put a piece of information on Firebase ("play video X at 09:14:30") and that information will be communicated fairly quickly/reliably to everyone connected.
For the particular task you describe, I personally would use PubNub which is faster than Firebase and basically made for game-like problems precisely like you describe.
http://pubnub.com
If you truly needed performance/reliability better than pubnub, you are really talking major engineering. So, the (buildings of) engineers who make live games at Nintendo, Warcraft etc, would tackle such an issue as "being even faster than PubNub".
So, the answer in brief!
The very short answer then to your question posed is:
Learn to use the various device-cloud services, which are at the heart of all apps today. (Knowing how to make "an Android or iOS app", as such, is of no consequence today.) For your particular problem, you'll want to use PubNub specifically, as it is built for precisely realtime problems such as this. (Firebase more leans towards "OCC" type data problems.)
Really that's it.
I am attempting to stream audio files from a server to iOS devices and play them completely synchronized. For example on my phone I might be 20 secs into a song and then my friend next to me should also be 20 secs into the song as well. I know this is not an easy problem to solve, but I am attempting to do so.
I can currently get them within one second of each other by calculating the difference in time between the devices and then have them sync up, however that is not good enough because the human ear can detect a major difference in a second and this is over WIFI.
My next approach is going to be to unicast the one file from the server and then have the all devices pick it up directly from the server and then implement some type of buffer system similar to netflix so that network connectivity would be a limiting factor. http://www.wowza.com/ is what I would use to help with that.
I know this can be done, because http://lysn.in/ is does it with their app and I want to be able to do something similar.
Any other recommendations after I try my unicast option?
Would implementing firebase help solve a lot of the heavy lifting problems?
(1) In answer to ONE of your questions (the final one):
Firebase is not "realtime" in "that sense" -- PubNub is probably (almost certainly) the fastest "realtime" messaging for and between apps/browser/etc.
But they don't mean real-time in the sense of real-time, say, as race game engineers mean it or indeed in your use-case.
So firebase is not relevant to you here and won't help.
(2) Regarding your second general question: "how to synchronise time on two or more devices, given that we have communications delays."
Now, this is a really well-travelled problem in computer science.
It would be pointless outlining it here, because it is fully explained here http://www.ntp.org/ntpfaq/NTP-s-algo.htm if you click on "How is time synchronised"?
So in fact, to get a good time base on both machines, you should use that! Have both machines really accurately set a time to NTP using the existing (perfected for decades) NTP synchronisation.
(So for example https://stackoverflow.com/a/6744978/294884 )
In fact are you doing this?
It's possible that doing that will solve all your problems; then just agree to start at a certain exact time.
Hope it helps!
I would recommend against using the data movement to synchronize the playback. This should be straightforward to do with a buffer and a periodic "sync" signal that is sent at a period of < 1/2 the buffer size. Worst case this should generate a small blip on devices that get ahead or behind relative to the sync signal.
This is a little off the beaten path. I've got a DLink DWL-G520 card I'm using under OpenBSD and it works fine. What I want to do is be able to access the radio part of it. Why? I want to use it in a radio telescope. It's a 2.4 GHz receiver with an external antenna connector. I want to connect some coax, some amplifiers, and an old TV dish and point the dish at the sky. It has an RSSI signal and variable RF gain (which it adjusts, from what I can find) so all I'd need to do is record those over time while pointed at a certain spot in the sky. I don't need to control the frequency really since most natural events are broadband.
I'm poking through the OpenBSD ath driver following nested structs but I don't want any of the normal network stuff, which is most of what the driver does. dmesg identifies it as an AR5212 which according to the Atheros PDF is always paired with an AR2112 radio. Is there any easier way than wading through PCI stuff to see what my options are? I need to turn the transmitter off so it doesn't fry my amps too. Trying to find low level documentation is about impossible from what I've seen. Ultimately I'd like to have this work with other WiFi cards too, but I'll start with this one. I've got a Cistron with an external antenna connector also.
Alan, ab1jx
When I was writing a simple server for a simple client <> server multiplayer game, I thought of the following text-based protocol using a translation library. Basically, each command had a certain meaning, eg:
1 = character starts turning right
2 = character starts turning left
3 = character stops turning
4 = character starts moving forward
5 = character stops moving
6 = character teleports to x, y
So, the client would simply broadcast the following to inform that the player is now moving forward and turning right:
4
1
Or, to teleport to 100x200:
6#100#200
Where # is the parameter delimiter.
The socket connection would be connected to the player identifier, so that no identifier has to be broadcasted with the protocol to know what player the message belongs to.
Of course all data would be validated server side, but that is a different subject.
Now, this seems pretty efficient to me, only 2 bytes to inform the server that I am moving forward and turning right.
However, most "professional" code snippets I saw seemed to be sending objects or xml commands. This seems to require a lot more server resources to me, doesn't it?
Is my unexperienced logic of why my text based protocol would be efficient flawed? Or what is the recommended protocol for real-time action multiplayer games?
I want to setup a protocol that is as efficient as possible, because I do not want to use multiple clusters/servers to cover excessive amounts of bandwidth for my 2D multiplayer game, and to safe synchronization problems and hassle.
However, most "professional" code
snippets I saw seemed to be sending
objects or xml commands. This seems to
require a lot more server resources to
me, doesn't it?
Is my unexperienced logic of why my
text based protocol would be efficient
flawed? Or what is the recommended
protocol for real-time action
multiplayer games?
Plain text is more expensive to send than a binary format containing the same information. For example, if you only send 1 byte, you can only send 10 different commands, digits 0 to 9. A binary format can send as many different commands as there are different values you can fit into a byte, ie. 256.
As such, although you are thinking of objects as being large, in actual fact they are almost always smaller than the plain text representation of that same object. Often they are as small as is possible without compression (and you can always add compression anyway).
The benefits of a plain text format are that they are easy to debug and understand. Unfortunately you lose those benefits if you put your own encoding in there (eg. reducing commands down to single digits instead of readable names). The downside is that the format is bigger, and that you have to write your own parser. XML formats eliminate the second problem, but they can't compete with a binary format for pure efficiency.
You are probably overthinking this issue at this stage, however. If you're only sending information about events such as the commands you mention above, bandwidth will not be a concern. It's broadcasting information about the game state that can get expensive - but even that can be mitigated by being careful who you send it to, and how frequently. I would recommend working with whatever format is easiest for now, as this will be the least of your problems. Just make sure that your code is always in a state where you can change the message writing and reading routines later if you need.
You need to be aware of the latency involved in sending your data. "Start turning"/"stop turning" will be less effective if the time between the receipt of those packets is different than the time between sending them.
I can't speak for all games, but when I've worked on this sort of code we'd send orientation and position information across the wire. That way the receiver could do smoothing and extrapolation (figure out where the object should be "now" based on data that I have that is already known to be old). Different games will want to send different data, but generally speaking you will need to figure out how to make the receiver's display of the data match the sender's, so you'll need to send data that is resilient in the face of networking problems.
Also, many games use UDP for this sort of data transfer instead of TCP. UDP is unreliable, so you may not get all of your packets. That means that "stop moving now" or "start moving now" may not be received in pairs. When coding on top of UDP then it's even more important to send "this is the state right now" every so often so that clients get ample opportunity to sync up.
The common way is to use a binary format, not text, not xml. So with only one byte you can represent one of 256 different commands.
Also use UDP and not TCP. The game will be a lot more responsive with UDP in case of packet loss. In case of packet loss you can still extrapolate the movements. With each packet send a packet number so that the server knows when the command was sent.
I highly recommend that you download the Quake source code where you can learn more about network programming in modern multiplayer games. It's really easy to read and understand.
edit:
I almost forgot..
Google's Protocol Buffers can be of great help when sending complex data structures.
I thought I would give my two cents and provide a practical application to what is being referred to as Binary Serialization. The concept is actually incredibly simple, yet only seems complicated on the outside.
You can actually send XMLs and have a server that processes the data within the XML to different functions within the server itself. You can also just send the server a single number that is stored within the server as a variable. After that, it can process the rest of the data and choose the correct course of actions.
As an example, some rough code:
private const MOVE_RIGHT:int = 0;
private const MOVE_LEFT:int = 1;
private const MOVE_UP:int = 2;
private const MOVE_DOWN:int = 3;
function processData(e:event.data)
{
switch (e)
{
case MOVE_RIGHT:
//move the clients player to the right
case MOVE_LEFT:
//move the clients player to the left
case MOVE_UP:
//move the clients player to the up
case MOVE_DOWN:
//move the clients player to the down
}
}
This would be a very simple example, and would need to be modified but as you can see you merely just store the variables encoded with whole numbers that you transmit in strings of numbers. You can parse these and create headers of information to organize them into different sections of data that needs to be transmitted.
Also, it is better to do a UDP setup for games because just missing a packet should NOT halter the gaming experience, but instead should be able to handle it client-side AND server-side.
What should be the best way to recognize that a number is connected to a fax without actually sending faxes around ?
I suppose that a short phone call can be made: the goal is to determine if a number that is declared as a fax line is really a fax line, working and available.
If you can make a phone call, many faxes create a "Fax Identification tone".
This signal may be sent by the Terminating FAX machine anywhere between 1.8 to 2.5 seconds AFTER answering the call. The CED signal consists of a 2100 Hz tone that is from 2.6 to 4 seconds in duration. The CED tone is useful for disabling any echo cancellers on the line.
The CED also incorporates a "silent" interval following the 2100 Hz tone. This interval lasts from 55 to 95 (75 +/- 20) mS. Following this interval, the Terminating FAX machine will initiate the Pre-Message Identification procedures by transmitting a 300 BPS "Line Turn-around" preamble.
Some older faxes do not support that though, no idea how they can be triggered (i.e. by you sending a tone first, but that could be really annoying if you have a phone instead of a fax number)
I understand your question as that you have a list of fax-numbers in your customer database and want to verify that those numbers still are valid.
Then you could use TAPI to programatically call those numbers and check if its a fax that answering, no need to actually send any fax, just connect and ask the device (fax) what capabilities it has.
Here are Microsofts information about their TAPI
An easier way could be to have a fax modem on the com-port and using ATI-commands to call the device and send ATX3D and see if the device answers with ATA. (Or something similar, it was ages ago I programmed modems..)
It's essentially impossible.
They make line-sharing switch devices. For example, visit http://www.faxswitch.com/ to see their offerings.
The line appears like a voice line until you send a fax recognition tone. Then the line switches to a fax machine, if one exists.
So, a single number can be voice and fax.
A quick and dirty way of testing if a number is a fax number would be to google (using some API) for "Fax [number]".
e.g.
http://www.google.co.uk/search?q="Fax+01422+329262"
Not in New Zealand at least.
Here, Fax-Numbers do not differ from normal numbers in terms of namespacing.
A fax number may be only 1 digit ( the rightmost ) different from their normal line, or even a company may not have distinct FAX/Phone lines, and they just share a line and use tone detection to initiate fax protocol instead.
Also, you don't need to register with anybody to have fax services work on your phone line, so there's no index you can look up to see if its a fax or not. The only way to know is initiate a call, and see if you get a fax response, and even then, if the Fax happens to be turned off, you'll get a false negative.
I think your question may turn out to result in hunting for phantoms.
Oh, and for additional fun, you may have desktops with their dialup modem plugged into the wall merely emulating a fax device :). If you don't want to include these as "fax machines". you will be most likely out-of-luck.
First, there isn't anything inherent in the number that identifies it as a fax line. Even the phone company doesn't know - it's just a device attached to the wall jack, by the customer.
At some point you just have to trust the user. However, try the following ideas as well:
On each fax, publish a number (preferably toll-free) where the recipient can call to cancel. In some localities this might be a requirement anyway - "unsolicited" faxes are considered abusive in much the same way as spam since they can entail both material and bandwidth costs for the recipient. This will protect you in the eventuality that someone subscribes a number that doesn't belong to them.
Add some kind of error threshold to your software that will halt fax attempts once n attempts fail, and flag the number as inoperative. If you have some other way to contact the user, you could notify them of this event. The same principle works for e-mail addresses - I've received snail-mail notifications from companies when I changed e-mail addresses and forgot to notify them; once the e-mail had bounced a few times, they sent a courtesy letter to remind me to update it.
Many old fax lines are set to automatically pick up even on a voice call, so you could just call and check. On the other hand, it won't work on a lot of new ones, so while you could tell that one is a fax line, you couldn't tell it wasn't.
I'm sure a fax is a modem, so if you connected to it - using AT codes maybe (how 1990's!), you could determine from the response codes that there was a fax there - but you still have to make the call, make the negotiation etc - just dont send a page.