This design has two purposes.

One access level provides remote programming of the module over the air, using access to the Application Programming Interface (API). This architecture will be covered in detail in another document.
The second purpose of this architecture is to define the IP addresses of the servers that will be used in conjunction with the Wakeup/Keep Alive parameters.
The command format for defining the Friends is as follows:

AT$FRIEND=
friend#: (1-10) Indicates which friend to add or replace.
server-indication: (0-1) This flag indicates whether the specified friend will work as a server or not. 1=Server
IP: “(0-255).(0-255)-(0-255).(0-255)”

Up to 10 Friendly IP addresses can be defined. Each of these addresses will be able to access the modem. Each Friend can be identified as a server. An additional use of this Friends list is to allow which IP addresses can execute commands over the air using the Enfora API.
The command to define the Wakeup/Keep Alive architecture is as follows:

1 = Send messages starting from the receipt of the new IP every<retry-period> minutes.

2 = Send an acknowledgment message using the at$acktm command once the new IP is received every<retry-period> minutes.

<retry-period>
The number of minutes the Keep-alive request is sent to the server. Zero indicates no retries.

• When this command is used, an event from group 0 will be generated in the module's event table.
• The parameter
The acknowledgment and retry messages are used in conjunction with Wakeup/Keep Alive.
The command format is as follows:

AT$ACKTM:
<retry-period><IP-reselect>

<max-retry>
The maximum number of wakeup messages the modem will send to each server once it receives the new IP address, or once the Keep-Alive period has ended.
Zero indicates that no messages should be sent.

<retry-período>
The number of seconds between successive Wakeup message retries.
Zero indicates no retries.

<IP-reselect>
1. IP reselection disabled.
2. If an acknowledgment message has not been received on all defined Friends servers and the number of retries has ended, it is assumed that there is a communication problem and a new IP reselection process will begin.

Upon receiving the first or a new IP address, the modem will check its Friends list (starting with the first and going up to the tenth) to determine if any servers exist. If a server is found, a Wakeup message will be sent at the interval defined in the $ACKTM command until the specified retry limit is reached or an acknowledgment message is received. This process will be repeated for each IP address in the Friends list until the list is exhausted or an acknowledgment is received from a server in the list. An acknowledgment from the server is not required.
If Keep-Alive is enabled, wakeup messages will be repeated at the interval defined in the $WAKEUP command after the previous wakeup sequence has finished. The Keep-Alive request will be sent regardless of when the previous message sequence was acknowledged.
The IP reselection option is an automated feature that allows the modem to obtain a new IP address from the network if communication cannot be established. The modem will send messages to all IP addresses in the Friends list that are defined as servers. If the modem does not receive an acknowledgment message, as defined in the Enfora API, from any of the server's logs, it will obtain a new IP address from the network.
An additional parameter, consisting of a 20-character field, allows the device to be uniquely identified within its system. This parameter will be included in the UDP packet payload when the Wakeup/Keep-alive data is transmitted.
Modem identification is performed as follows:

AT$MDMID =
<modem-identificador>
A 20-character field is used to identify the modem.
Figure 2 illustrates the use of Friends with a single server.
In this example, the Enabler-IIIG is configured to send Wakeup messages to IP address 168.121.100.1 every 20 seconds for up to 10 times, or until an acknowledgment is received by the Enabler-IIIG from the server. The Enabler-IIIG will send a Keep-alive message to the server every 60 minutes. In this example, the IP reselection period is disabled.
Figure 3 illustrates the use of Friends with multiple servers.
In this example, the Enabler-IIIG is configured to send the Wakeup packet to the first server in its Friends list. The packet will be sent to this first server every 20 seconds for up to 10 times, or until the server responds with an acknowledgment. If no acknowledgment is received after 10 attempts, the modem will initiate the Wakeup process with the next server in the list. This process continues (one server at a time) until the Friends list is exhausted or until the first acknowledgment is received from a server. In this example, 10 Wakeup packets will be sent during the first 200 seconds to address 168.121.100.1. Then, another 10 packets will be sent to address 168.121.100.2 during the second 200 seconds. Finally, 10 Wakeup packets will be sent to address 168.121.100.3 during the third 200-second period. If the modem does not receive an acknowledgment during these 600 seconds (10 minutes), the modem will stop sending Wakeup packets until the next Keep-alive period ends. In this example, the Keep-alive period is 60 minutes.
The IP address reselection option is enabled. If the modem does not receive an acknowledgment message from any of the servers at the end of the wake-up period, it will obtain a new IP address from the network.

More information or a quote

José Adarve (M2M Applications Engineer, Venco Electrónica) and Angel Gutiérrez (M2M Product Manager, Venco Electrónica).