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Cellular Networking Perspectives

Wireless Data


There are a dizzying array of wireless data standards and technologies available to choose from. While wireless data has been much slower to catch on than wireless voice services, they are slowly growing in acceptance, along with the speeds they provide and their availability.

More details on wireless data are found in several back issues of Cellular Networking Perspectives.

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Modem Data

Modems transmit data from a serial line over an analog voice facility (e.g. analog cellular radio channel) as a series of tones. They work best over analog channels because digital coding and compressing of audio damages or destroys the modem tones. Analog cellular channels (30 kHz) using the MNP–10 or ETC protocols can transmit at around 9.6-19.2 kbps. However, the modem at the other end also has to have similar capabilities. Because of this problem, some wireless carriers installed modem pools using pairs of back-to-back cellular and standard modems.

An important form of wireless data using modems is TTY. This slow bit-rate (about 45 bps). This works fine over analog cellular, but digital cellular (TDMA, GSM and CDMA) are having to be adapted to support this.

Digital Circuit-Switched Data

Digital circuit-switched data attempts to replicate the modem experience with TDMA, GSM or CDMA digital cellular or PCS. The problem is that modem tones cannot be reliably transmitted through a voice coder. Removing the voice coder requires a new protocol (of which some have been developed) and a modem pool is not an option. But, this is different than an analog modem pool, because only a single modem is required as the switch receives the data in a digital format. A rough estimate of the data capacity of digital cellular is to look at the voice coder bit rates. Usually, this is about the amount of bandwidth available for data. TDMA uses 8 kbps voice coders, and up to three time slots can be aggregated (for a price). GSM uses 13 kbps voice coders and up to 8 time slots can be aggregated (but this is usually done only for GPRS, which is a packet data standard). CDMA uses 8 or 13 kbps voice coders, but is more flexible in the amount of bandwidth that can be assigned to an individual customer.

‘Analog’ Control Channel Data

Some clever engineers have figured out ways to use the ‘analog’ control channel (it is actually a digital channel, set up to service analog cellular systems) to transmit low bit-rate data. This channel only runs at about 1 kbps, and has to be shared with a large number of voice users. Aeris and Cellemetry are the prime users of this service. By faking a voice transaction, they can cause a small amount of data (4-16 bytes) to be sent to a central computer (which emulates an HLR). The advantages of this are high mobility (e.g. for asset tracking applications) and low capital costs, because the infrastructure is generally in place. These systems are largely used for industrial purposes, although some consumer applications exist, such as alarm monitoring systems.

‘Analog’ Packet Data - CDPD

CDPD uses an analog voice channel to send digital packet data directly from a phone to an IP network. It provides about 19 kbps for each channel, but this must be shared by multiple users. The strength of this technology is that the cost is kept low because it reuses much of the existing cellular infrastructure, but it takes channels away from voice users. Originally, it was planned that CDPD would transmit data when voice channels were idle, thus not consuming any capacity, but this proved to be too difficult to manage. CDPD systems service over 50% of the US population and are found in several other countries, including Canada. CDPD has experienced some new life as a bearer protocol for WAP, eliminating many of the delays experienced when circuit-switched data is used.

Data-Only Systems

There are two major public data-only wireless systems available in the US - Motient and Mobitex. According to Mobitex, their system covered 93% of the US population in 2000, and provides coverage in Canada through a relationship with Rogers Wireless (Cantel). Motient (according to Mobitex) had coverage of 79% of the US population at the same time. By comparison, CDPD only covered about 55% of the US population.

These data systems are similar in performance to CDPD, giving a shared bandwidth (per cellsite) in the 9600-19200 bps range.

Digital Cellular/PCS Packet Data

The next big game in town is 3G wireless data. This implies speeds of 144 kbps for mobile terminals and 2 Mbps for stationary devices. Here, the world is divided into two camps - GSM/W-CDMA versus cdma2000/1xEV.

The GSM/W-CDMA strategy is to move first to GPRS, which allows use of multiple timeslots within a GSM channel (composed of 8 timeslots). Theoretically, this should allow speeds up to 115 kbps, but early devices are more in the 20 kbps range. W-CDMA will provide higher capacity, but it is too early to tell what realistic values are.

cdmaOne provided second generation data rates of 14.4 kbps circuit data and up to 115 kbps packet data in theory. IS-2000/cdma2000 is being more widely implemented for data services. It is claimed to provide 144 kbps in its 1X mode. Future plans are for 1XEV-DO (a data-only system) that will provide 2Mbps from the cellsite and 144kbps from the mobile. Yet another generation, known as 1xEV-DV (including voice services) is being designed to support about 2 Mbps in both directions.

Note that it is hard to validate these speed claims. Technology is changing almost as fast as the marketing hype. Furthermore, carriers may decide that high speed data is not as profitable as lower speed data and voice services. Note that with voice coders running at 8 kbps, someone running at 800 kbps is taking approximately 100 times the resources. Are they going to pay 100 times the per-minute rate for voices services? Even if higher speed data is implemented, packet data channels are shared resources. Combined with overhead from multiple protocol layers, throughput may be limited to much less than the theoretical maximum.

iMode

A Japanese specification for providing internet-like content to wireless devices. Uses cHTML for data encoding, unlike WAP, which uses WML. Both protocols plan to migrate to xHTML which should accommodate advances made by both protocols.

WAP (Wireless Application Protocol)

WAP is an application protocol designed to bring web-like services to wireless devices with extremely limited input and output capabilities. It uses a variant of HTML coding that, among other things, includes a binary compression scheme to make transmission of web pages more efficient. Its biggest limitation is probably the fact that wireless devices with a numeric keypad and tiny, low resolution screen simply do not make great web-surfing devices. However, no matter what its detractors say, it was a big advance in data, moving attention away from merely moving bits and bytes to actually supporting real-life applications for consumers and businesses. The specification was developed by the WAP Forum.

Wireless LAN Protocols

Wireless LAN protocols have a somewhat easier job with wireless data. Terminals are usually stationary and systems are not expected to cover a wide area. Most of the standards use unlicensed spectrum, so anybody can set up one of these networks. IEEE 802.11b (‘WiFi’) is definitely the best known and most used standard here, allowing transmission at Ethernet speeds (10 Mbps), with higher speeds planned for the future (802.11a and 802.11g). HomeRF is a competitor, but it seems to be treading on similar territory, and has perhaps missed the window of opportunity. Bluetooth is not truly a wireless LAN standard, but a PAN (Personal Area Network) standard. It provides a 1 Mbps channel to connect up to 8 devices together. Rather than aiming at connecting computers and printers (such as 802.11 is usually used for), Bluetooth is more oriented at personal cable replacement, perhaps connecting a phone, mouse, keyboard and computer together. RF technology is also often used for wireless networks. It provides good speeds, but is limited by the need to maintain line-of-sight between communicating devices.

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© – Copyright Mon, May 14, 2007: Cellular Networking Perspectives Ltd.