Packet Data Systems
While Internet access via a mobile phone is hyped, there are already several data-only cellular systems. Though none offer the capacity needed for full-scale Web surfing, they are used for simple messaging and telemetry applications.
The most well known data systems are pagers, which have traditionally carried information in only one direction. The first were known as numeric, because they handled only the digits 0 through 9. They were intended to be used for phone numbers, the idea being that someone who was paged could find a payphone and call the number. Modern pagers are nearly all alphanumeric, meaning that in addition to numbers, they can display letters of the alphabet, and some include many different character sets.
Pagers are beginning to incorporate a return link, enabling them to be used for real interactive applications. These two-way systems are known collectively as narrowband PCS because they require less spectrum than the voice networks. For data, their great advantage is that they are always on, meaning users are not billed for time spent connected. Customers either pay a flat monthly rate or are billed per kilobyte.
With the exception of Metricom's Ricochet, none of these systems, summarized in Table 4.12, are able to provide dedicated capacity to a single user. The data rates are shared between all users in a cell, which can be very large, as they tend to operate at lower frequencies.
| System | Downstream | Upstream | Round-Trip Latency |
|---|---|---|---|
| POCSAG | 2.4 kbps per cell | 0 | N/A |
| ERMES | 6.4 kbps per cell | 0 | N/A |
| Flex | 6.4 kbps per cell | 0 | N/A |
| ReFlex | 9.6 kbps per cell | 6.4 kbps per cell | 5 seconds |
| CDPD | 19.2 kbps per cell | 9.6 kbps per cell | 2 second |
| Mobitex | 8.0 kbps per cell | 8.0 kbps per cell | 4 seconds |
| DataTAC | 19.2 kbps per cell | 19.2 kbps per cell | 6 seconds |
| Metricom Ricochet | 128 kbps per user | 128 kbps per user | 5 seconds |
Paging
With the growth of mobile phones and wireless data, paging often seems an endangered industry. The European Public Paging Association says that around three percent of all Europeans carry a pager, compared to more than ten times that number who have a cellphone.
Nevertheless, pager subscriber numbers are still growing in most countries, and the technology does have some advantages. Most important, pagers are cheap to own. Callers make a short call to a premium rate number, resulting in no cost to the person being paged. Pagers themselves are usually smaller and lighter than cellular devices. Coverage is also better, because paging systems tend to use lower frequencies, which allow a much larger cell size.
In America, paging has been given a new lease on life, thanks to two-way systems. These allow it to compete with the more advanced packet-switched systems, targeting both businesses and consumers with devices such as RIM's Blackberry that include tiny keyboards. Two-way systems are less developed, or absent entirely in other parts of the world, thanks to regulatory issues. They need paired spectrum, which the FCC was relatively quick to license. European countries were not, and many still have no band allocated to the pager uplink.
Many different paging systems have been developed. Among the most common are
POCSAG is named after the Post Office Code Standardization Advisory Group, a British project set up to design a paging standard in 1982. It was later adopted by ETSI and then by operators throughout the world, but has been replaced in many areas with more advanced systems.
ERMES (European Radio Message System) was defined by ETSI in 1992. Though developed for use in Europe, it was intended to be installed internationally and has partly succeeded. The ITU has adopted it as the recommended paging standard, and it has been deployed in parts of Asia. To promote international use, it allows paging in up to 32 different character sets. Its other innovation is multicast paging: a single message can be sent to everyone in a predefined group.
Flex was invented by Motorola and is the most popular paging system in America. Motorola has extended it to ReFlex, a two-way system designed for short text messages.
CDPD
Cellular Digital Packet Data (CDPD) is a simple packet-switched overlay to AMPS and D-AMPS cellular networks. It uses a single channel in each cell for data, with a maximum capacity of 19.2 kbps down and 9.6 kbps up, the same as D-AMPS+.
Though seemingly faster than regular D-AMPS, the fact that this capacity is shared between all users in a cell makes the throughput available to an individual user much lower. However, it is usually cheaper than circuit-switched D-AMPS, and is also more widely available. It can be used in virtually all major cities in the U.S. and Canada, and is the only way of sending data through an analog AMPS network.
CDPD terminals are usually inside PC cards designed for laptop computer users. Increasingly, they are also built in to AMPS or D-AMPS phones, which use the system for sending brief text messages.
Mobitex
This system was developed by Ericsson and intended for telemetry applications, such as monitoring the location of delivery trucks or remote meter reading. Its data rate of 8 kbps is fairly slow, and the time taken to process data creates a long round-trip delay. However, it does offer very good coverage, both in the U.S. and abroad.
Most Mobitex networks worldwide are operated by Bell South or by its subsidiary, RAM Mobile Data. They usually sell telemetry services directly to businesses, but in 1988 Palm Computing realized that it would make an ideal low-speed Internet access technology. The Palm VII and its immediate successors all use Mobitex, but they work only in the U.S. because every country's version of Mobitex uses different spectrum.
DataTAC
Better known under the brand name Ardis, DataTAC (Total Access Communications) was originally developed by Motorola and IBM. It differs from the other systems in that it is connection-oriented, meaning that a connection is set up whenever any data needs to be sent. This should make it more reliable, but also increases latency. Two versions exist and have been deployed in many countries worldwide.
MDC4800 (Mobile Data Communication) was the original standard, running at 4.8 kbps
RD-LAP (Radio Data Link Access Protocol) is backward compatible with MDC4800, but increases the speed to 19.2 kbps