id: how
layout: default.twig
title: How it Works
---
<p>
In the simplest configuration, EnvayaSMS only needs to be installed on one phone. 
Suppose that you have installed EnvayaSMS on an Android phone in Dar es Salaam, 
Tanzania, with phone number <code>+255987654321</code> (local number <code>0987654321</code>), 
with GSM service from Vodacom.
</p>

<p>
Suppose also that your web server is <code>example.com</code>, and its servers are located in a datacenter in London.
</p>

<p>
In Dar es Salaam, you would put the Android phone in an office with internet access and a Wi-Fi router,
and connect the phone to the Wi-Fi network. (Wi-Fi is not strictly necessary, since EnvayaSMS will also
work over the phone's mobile data connection, but Wi-Fi is likely to be more reliable and cheaper.)
</p>

<p>
Then your network would look like this:
<br />
<img src='/media/topology.png' width='690' height='284' />
</p>

<p>
    Typically your organization will need only one phone running EnvayaSMS in a 
    particular country. However, it is possible to use multiple phones to load-balance requests 
    for higher throughput, to offer different user services, or to serve different mobile networks.
</p>

<h3>Receiving incoming SMS and sending replies</h3>

<p>Here is an example of how EnvayaSMS would forward an incoming SMS to your server, and send a SMS back as a reply:</p>

<ol>
<li>User in Tanzania, using Nokia phone with phone number <code>0555551234</code>, sends SMS message to <code>0987654321</code></li>
<li>Your Android phone at <code>0987654321</code> receives SMS</li>
<li>EnvayaSMS sends SMS to <code>example.com</code> via HTTP</li>
<li>example.com processes the message and returns SMS reply in HTTP response</li>
<li>EnvayaSMS forwards SMS reply to <code>0555551234</code></li>
</ol>

<p>
Since users generally are charged only for outgoing messages in Tanzania, the user would be charged
for sending one local SMS message, and your phone would also be charged for sending one local SMS message.
</p>

<h3>Sending outgoing SMS notifications</h3>

<p>EnvayaSMS uses a different process to send SMS notifications that were not in response to an incoming message.</p>

<p>Assume now that <code>example.com</code> has multiple phones using EnvayaSMS in different countries, 
and it is configured to send all messages to phone numbers with a <code>255</code> prefix (Tanzania's country code) 
via the cell phone with number <code>+255987654321</code>.

<ol>
    <li><code>example.com</code> wants to send an SMS message to <code>+255555551234</code></li>
    <li><code>example.com</code> identifies that the recipient number has a <code>255</code>
        prefix, and chooses sender phone number <code>+255987654321</code></li>
    <li>The next time EnvayaSMS polls <code>example.com</code> for queued outgoing messages 
        for its phone number <code>+255987654321</code>, the server responds with 
        the message to send
    </li>
    <li>EnvayaSMS forwards the message to <code>0555551234</code></li>
    <li>EnvayaSMS notifies <code>example.com</code> via HTTP that message was sent</li>
    <li><code>example.com</code> removes the SMS message from its queue of outgoing messages.</li>
</ol>

<p>
In these examples, even though SMS messages are being communicated between Dar es Salaam and London,
all SMS messages are sent domestically and are not charged as international SMS messages.
</p>