NAL Research Corporation (TN2014-001-V1.4.5)
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C.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 D2. 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 D3 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 D2.
PSTN Dial-Up Connectivity.
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