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Satellites are playing an increasing role in establishing still another layer of worldwide voice communications. Two of the most widely heralded services are the Iridium and Teledesic low earth orbital (LEO) systems. These systems offer worldwide telephone service through the use of a small handheld telephone similar to those used for cellular systems. Iridium is owned by a consortium of international companies, one of which is Motorola, Inc. Teledesic is owned by McCaw Communications and Microsoft Corp. (Redmond, WA).
Inmarsat now offers voice services through a worldwide consortium of 65 member nations. Special briefcase-size terminals are used to communicate with the satellite. Typical terminal costs range between $18,000 to $22,000 and connect- time costs are approximately $5.00 per minute. A new service that is planned, Inmarsat-P, will compete directly with the LEO systems. Although details of the Inmarsat-P service are still in the making, terminal and initiation costs are expected to be in the range of $1,500, with connect costs of about $1 per minute.
These satellite-based voice systems provide the capability to support both voice and data communications in any remote area of the world.
A variety of services are available to support wireless data communications. Wireless services like cellular digital packet data (CDPD), enhanced special mobile radio (ESMR), Ardis Mobile Data, and RAM Mobile Data Inc. support slightly different needs, although there is some overlap. For this reason, users should not look for a single vendor to supply an all-encompassing wireless service solution. In fact, it is less costly to consider a mix of voice, paging, and data services.
ESMR and CDPD offer competitive data communications services. For example, the Nextel interconnect option on the Motorola Integrated Radio System (MIRS)-based network costs $40 per month for the first 256 minutes plus $0.50 for each additional minute. This assumes that the subscriber also is a dispatch subscriber at about $25 per month for access.
A MIRS Motorola Lingo mobile handset is required to access service on a MIRS system and is priced around $1,000. In comparison, cellular telephones can cost up to $350. Cellular subscribers start out at $14.95 for monthly access plus about $0.45 or more per minute for airtime.
Although there are still many smaller special mobile radio (SMR) operators across the US that will continue to offer dispatch and interconnect services in second-tier markets, major players such as Nextel, Dial Page, and other members of the MIRS-related roaming consortium are likely to maintain their interconnect rates in competition with cellular service providers.
Implementing cellular digital packet data networks often requires a number of systems applications modifications. For example, a special CDPD modem is required at either end for the transmission of data from one point to a host computers communications port. This device must be established separately from the other host communications ports and should be installed by the cellular service provider. This task includes assigning the device with an IP address and configuring it for access to the cellular network.
CDPD Costs. Communicating over a wireless network is more costly than using the public network for a number of reasons, not the least of which is the cost for airtime. For example, regular transmission control protocol/Internet protocol (TCP/IP) applications generate a lot of extraneous traffic that can drive up the cost for transmission on a network that is usage sensitive. A hardware fix is available to alleviate this type of network condition. For example, products are available to monitor data flow as a means of reducing the number of acknowledgments being sent.
Potential Performance Problems. Another problem that must be taken into account is packet delay. This condition can result in dropped connections or unnecessary retransmissions and is caused by network congestion. Although cellular networks are still relatively lightly loaded, network congestion becomes a problem as greater penetration develops in the wireless market and CDPD networks become crowded In addition, under some traffic circumstances, it is possible for packets to be dropped, therefore delivery of packets cannot be guaranteed. Noisy lines and poor radio coverage can also present the same types of problems as a congested network.
Under some traffic conditions, duplicate packets can be introduced through retransmission facilities, If the packet acknowledgment is lost, the packets source will time out and retransmit a second or duplicate packet. Packets can also be thrown out of order when the data path is subjected to delay from rerouting events. These are just a few of the transmission characteristics that must be countered when a CDPD network is used for data transmission. Users should carefully review their applications and develop the measures that may be required to safeguard their data transmissions.
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