QUESTIONS?
Call us and we will help you with information on data transfer options and clarify the difference in Inmarsat services. Mobile Telesystems, Inc.
U.S. Government and Domestic Accounts
Q. "Since Inmarsat B service promises a maximum of 9600bps digital data transfer, can I send asynchronous data faster over an INMARSAT-A terminal from my regular modem equipped, Windows 9X computer and Dial Up Networking (DUN)?", is a question often asked of Mobile Telesystems.
A. "Yes, 19,200bps over the Inmarsat standard A is fairly easy to obtain on a regular basis with even 28.8Kbps about 15% of the time. This depends upon the equipment you use and the landline quality* at the other (POTS) end". These days, data is the number one priority for most INMARSAT users.
Recently MTI used two 28.8 Kbps external modems to test and evaluate over the INMARSAT-A system. The set-up consisted of:
The test objective was to derive the carrier rate (the rate at which the modems initially connect) and the throughput (the effective rate at which data is transferred). Many modems today utilize protocols, i.e., V.42bis, that include error correction and compression** so the effective throughput may be substantially higher than the carrier rate. Also, many modems allow the carrier rate to change up or down while connected depending on line quality.
The test consisted of twenty calls: ten calls placed through each of the two U.S. service providers, IDB and COMSAT. Half of the calls for each service provider were placed through the Atlantic East satellite and the other half were placed through the Atlantic West satellite. The carrier rates varied from 14,400 BPS to 21,600 BPS with 19,200 BPS being the most common. Once connected, 10 files of different types and sizes were transmitted and received to determine how the system would perform. It is interesting to note that even precompressed files, such as the ZIP files, were compressed an additional 8-14%. The following is a description of the files used, their size and the range of effective data rates (throughput) for each class of file. (The "throughput range" is directly correlated to the carrier rate range, 14.4 - 21.6 Kbps).
File Types, Size & Throughput Range
It is important to note that the INMARSAT-A system is very user friendly -- almost as simple as a regular telephone. Hence, this test was performed with a minimum of preparation; the components were simply connected together but no effort was made to optimize packet size, modify software, etc. With high-quality modems and the TCS-Ultralite, you too can achieve these data rates or higher.
Data Transfer Application Set-up:
*Note 1: Poor quality telephone lines on the landline side will limit the rate at which data can be transferred.
**Note 2: MNP5, a data compression standard, may slow data transfers of files previously compressed.
© Mobile Telesystem, Inc. 1998.