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F.3 Dial-Up Data Service
Dial-up data service provides connectivity through the Iridium satellite network to another Iridium
modem, to the public switch telephone network (PSTN), to the Defense Switch Network (DSN), to a remote
LAN (e.g., a corporate network) or to an Internet Service Provider (ISP) at a nominal data rate of 2.4
kilobits per second (Kbps). The connection time involving user authentication and handshaking (or modem
training) can range from 15 to 30 seconds. For an Iridium-to-Iridium call, dial-up data service offers an
additional option known as data after voice or DAV. Similar to a voice call, a DAV call is routed directly from
one Iridium modem to another Iridium modem without going through the gateway.
Many desktop and laptop computers are equipped with either an internal or external modem to perform
dial-up data applications across the landline telephone network (PSTN). On these computers, terminal
emulator software or a dial-up networking connection can be configured to a specific modem with a phone
number to dial, user identification and password. The modem can then be used to call another computer, a
remote LAN or an Internet service provider as shown in Figure F2. The handshaking and protocols are
established between the modems independent of the landline.
The Iridium dial-up data service functions in much the same way as the PSTN dial-up connectivity. From
the perspective of a computer, the Iridium modem is just another external modem. The only difference is
that the dialed telephone number must conform to the international dialing pattern used by Iridium. When a
data call is placed, the Iridium modem actually dials and initiates a connection with the Iridium gateway
through the Iridium satellite constellation. Since the Iridium modem is requesting to establish a data
connection, the switch at the gateway routes the call through another modem. The modem at the Iridium
gateway then dials into and connects to another modem at the other end. Figure F3 illustrates how an
Iridium dial-up data service call is routed. The handshaking and protocols established between the modems
independent of the Iridium network.
For those ISU-to-ISU dial-up calls where data transmission delay is critical such as the application of
TCP/IP protocol, DAV should be considered in the design. This option eliminates the Iridium gateway once
Figure F2. PSTN Dial-Up Connectivity.
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