Someone pointed out that Iridium had a data rate of "2,400baud".
That's because its a voice oriented system. Just like cellular. That's why CDPD is so slow- its using a voice channel for data, and you only get 2,400 baud of voice data per phone. The entire cellular network is built based on this assumption- a voice channel costs a fixed amount of bandwidth (because everyone is using cellphones designed to use only this fixed encoding scheme)-- so ever cell of the network has a fixed number of channels and every region has a fixed number of network connections to connect to the cells. The total number of calls can never go up in a cellphone senario.
However, with internet traffic, bandwidth is dynamic. And not only that, it is a-symetrical. Therefore, if you start letting people buy CDPD modems and connect to the net, you end up with too many people browsing at the same time and the cell site starts dropping voice calls. This is why the cellular networks will never support flat rate CDPD-- gotta keep the prices high ($65+the fine print) or keep the data rate fixed (as Go America does with its "flat rate" CDPD plans) or you might start loosing channels that could be used for voice (Pay by the minute!) traffic.
However, the Metricom network was designed for mobile data and will always be technically supperior to CDPD. Here's some of the ways:
1) Density can be easily changed. If there is a lot of use in a given region, additional poletops can be added cheaply and easily. For cellular, they have to put up new towers and that's a big hassle.
2) Bandwidth is cheap: All Metricom has to do is add more Wired Access points to a region and the network automatically routes data to them. So, if useage goes up, they can add bandwidth over time (and since the usage increase precedes the new bandwidth the cost of a nother T1 is already paid for by the additional customers). For Cellular, they have to predict bandwidth in advance and regulate it closely. Additionally, the cellsites are connected to the POTS network, not to the net, so cellular based data systems have a limited pipe to the net for an entire region (another reason its glacially slow.)
3) Intenret access requires 19.2k. You cannot do a net connection (reliably) over a 2400 baud modem. The reason is its so slow that connectiosn time out-- software expects a response from a host over the net in a given amount of time, otherwise it assumes the host is down. Metricom's network will always give you a good data rate. Soon it will be amazingly fast.
4) Coverage will always grow faster. Because of the lower costs and easier regulatory situation, Metricom can wire a new area much faster than a cellular provider. Granted, cellular providers are already there- they've had 20 years to build their network... but Metricom wires a region in an extremely quick fashion. With only $100million investment, they could have better coverage than CDPD by the end of this year. (Contrast that to the many billions paid to build the cellular network in those regions.) Unfortunatly, Metricom doesn't have $100 million right now to spend on this.
5) Transmission power requirements are a lot lighter than cellular becuase it can handle dropped bits a lot easier. Cellular is a switched system and has to stay "connected" all the time. Therefore metricom devices are lighter and can stay connected a lot longer on a given battery.
Dragonfly |
