2G technology means that SlingSecure for establishing the secure connection uses a CSD data connection with a maximum of 9.6 Kbps of throughput channel.
Circuit Switched Data (CSD) is the original form of data transmission developed for the time division multiple access (TDMA)-based mobile phone systems like Global System for Mobile Communications (GSM). CSD uses a single radio time slot to deliver 9.6 kbit/s data transmission to the GSM Network and Switching Subsystem where it could be connected through the equivalent of a normal modem to the Public Switched Telephone Network (PSTN) allowing direct calls to any dial-up service.
In order for SlingSecure 2G to work properly, the SIM card operator should activate the CSD data channel for sending and receiving data calls (Mobile Originated/Mobile Terminated). Some operators limit such service to just postpaid/business contracts while refusing activation requests which involve prepay cards. When enabling CSD data channel, the operator usually assigns two additional numbers to the SIM card: the first number is reserved for voice calls (and it’s the one the SIM card gets issued with), the second number is reserved for data transmission, the third for faxes. Even if three numbers have been assigned to the SIM card being employed for encrypted calls, usually the main number can be used for all purposes (voice and crypto).
SlingSecure 2G uses the modem of the phone to establish the communication by means of the standard protocol v32 or v110; those two operation mode differ in handshake duration and information transmitted at the beginning of the connection. With v32 protocol, almost 20 additional seconds are required for the voice channel to get open, moreover some operators require that the call gets directly routed to the data number assigned to the SIM card. The v32 protocol is rather old and used only in few countries in the world.
On the other hand, with the v110 protocol (the same used in ISDN modems for Internet browsing) the connection is almost immediate. It usually doesn’t require to dial the data number, it is enough to dial the voice number of the SIM and the data call get automatically routed to the SIM.
SlingSecure 2G enables the user to set two additional modem settings: Transparent and Non Transparent. Those settings refer to the transport layer of the modem. Non transparent involves a data flow control and a retransmission of the data that arrive with wrong timing or corrupted. This retransmission causes an increase of the delay of the communication but less audio gaps during the communication. Transparent is usually preferred because in real time communication such as voice conversations it is a better choice that of having low delay and eventually lose some fractions of seconds when data are corrupted rather than hearing empty gaps lasting seconds.
Standard default settings of SlingSecure 2G is v110 and Transparent and this is the optimal setting for most European operators.
SlingSecure 2G can be used all over the word but some operators (mainly in Africa or South/Central America) do not implement the CSD data service in their networks because it is normally not used by normal customers and it is – from their low level point of view, not customer’s – more expensive than GPRS/EDGE connection because they have to reserve a full timeslot on the network as if it was a voice call instead of using a best effort politic like in GPRS/EDGE that usually have high delay.