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So the command returns the received signal strength(RSSI) and bit error rate(BER). I see that the RSSI is obtained even if the SIM card is not inserted in the GSM module. So what does RSSI signify?

AT+CSQ

+CSQ: 25,6

OK

Also how can we check if the network signals are available or not, when the SIM card is inserted in the GSM module?

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    \$\begingroup\$ The SIM card is only needed to log a subscriber into the network. The GSM data connection will work without the SIM card, how else can the phone send the data on the SIM card to the provider in order to login the user ? Also without a SIM card you can still make emergency calls (at least in my country). RSSI is what it says: an indication of the received signal strength. how can we check if the network signals are available or not By switching on the receiver perhaps ? \$\endgroup\$ – Bimpelrekkie Feb 5 '16 at 9:52
  • \$\begingroup\$ Thanks FakeMoustache, then how does the network coverage of the cellular network provider affect the GSM operation? \$\endgroup\$ – Sai Gautham Feb 5 '16 at 9:57
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    \$\begingroup\$ no coverage = no network. Why don't you read a textbook about wireless communications to get an idea of how these systems work ? \$\endgroup\$ – Bimpelrekkie Feb 5 '16 at 9:59
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According to this your Received signal strength is -63dBm. RSSI (=Received signal strength indication). As you say, you can receive RSSI without SIM (SIM IDs you in the network as a user so you can make calls. All the radio capabilities are in the phone, you can check the signal strength without the SIM).

For checking the available networks use:

AT+COPS=?
+COPS: [list of supported (,long alphanumeric,short alphanumeric,numeric[,])s][,,(list of supporteds),(list of supporteds)] +CME ERROR: 
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  • \$\begingroup\$ Yes, thank you this command servers the purpose of knowing the availability of network in particular area. \$\endgroup\$ – Sai Gautham Feb 5 '16 at 11:59

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