Siera Wireless MP555, Руководство по установке программного обеспечения

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Sending Data to your Network Application
Rev 1.2 43
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server on your network with a
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<destport>
AT!MPMTCONF
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Jan.05
Figure 6-1: An MP 555 GPS connected to a network server. In this scenario,
AT!MPMTCONF would have been used to configure the modem to send data
to the IP address 67.43.212.8. Since there are no firewalls, the network server
simply sends data to the IP address 192.218.36.5.
If the MP 555 GPS has a public IP address, the host application
can simply check the IP address on the incoming data packets
to determine the IP address of the modem. However, if NAT
(Network Address Translation) is in use, the IP address
ass gned to the modem is like y to be a private IP address. This
may present problems, as discussed in the next section.
The problem created by Network Address
Translation
Note: A similar problem may
occur if NAT s in use on your
network. In this case, the host
application may reside on a
server that has a private IP
address. The MP 555 GPS must
be configured to send data to a
public IP address, and the server
must be conf gured to recognize
the MTP data and route it appro-
priately. (The UDP port number
might be used to route the data.
This is configured on the modem
using the
parameter
of the AT command,
Many service providers use NAT (Network Address Trans­
lation) which effectively creates a firewall in front of the
CDMA network. Inside the CDMA network, the service
providers assign private IP addresses to registered devices,
since public IP addresses are not needed to route data within
the CDMA network. (This saves the service provider the
expense of having a large pool of public IP addresses.) A
public IP address is substituted for the private IP address
when data packets pass through the gateway between the
CDMA network and the Internet. This public IP address might
be used on data transmissions from many different devices.
The problem this creates is that the network server is only
aware of the substituted public IP address. If the server replies
to this address, the CDMA network cannot identify the device
for which the data was intended.
Assume for example, an MP 555 GPS is registered on a CDMA
network. On the CDMA network the modem is assigned a
private IP address of 10.95.236.1. When the modem sends MTP
data to the network server, a public IP address of 24.7.6.30 is
substituted at the gateway to the Internet. When the network
server receives the data, the host application is only aware of
the public IP address, 24.7.6.30. However, if the network server
sends data back to the IP address 24.7.6.30, when the data