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If SNAC is unsupported, use the Ocean Regions Number that the aircraft (based on 
its projected flight path) is probably logged on to. If the call fails to connect to the 
aircraft, re-try the call using an alternate Ocean Region Number. 

Figure 2–5 illustrates the Mobile-to-Mobile dialing sequence.

This section provides a basic overview of the requirements for making data connections 
using the HSD transceiver. Refer to the HSD Transceiver User’s Guide (MN-1110-10048) 
for detailed instructions on creating and configuring HSD dial-up networking connections.

(1) Mobile ISDN versus MPDS

Deciding which service type to use depends how you intend on using your data 
connections. 

For surfing the Web (with high turnover of pages and graphics), video conferencing, 
video streaming, and large file transfers, Mobile ISDN is faster and more efficient. 
However, service charges for Mobile ISDN are time-based (based on the length of 
your total connection time). 

MPDS provides a cost-effective solution for user applications that involve intermittent 
interaction or transmission of data, such as e-mail, database queries, and connectivity 
to IP/LAN and intranets.

MPDS also has an “always on” advantage over Mobile ISDN. Service charges for 
MPDS are based on the number of data bits transmitted over the system—not on the 
length of your connection time.

(2) Connecting to Inmarsat Mobile ISDN Service

The transmission of data over satellite networks differs from land-based ISDN 
applications. Factors inherent in satellite communications (signal fading, latency, and 
transmission blockages) can cause data transit delays and bit errors.