WIRELESS WIDE AREA NETWORKS
For wireless wide-area networks there are mainly two available technologies: data transmission over cellular networks, whether analogue or digital, and data transmission over mobile data networks. The main difference between these technologies is the data transport mode. Cellular networks, being primarily voice oriented, generally utilise circuit switching technology; whereas mobile data networks employ packet switching technology. The new cellular network technologies support packet switching and many major wireless voice carriers. Orange, Vodafone, T-Mobile, etc, all now offer one or more of the following higher speed data connections; GPRS, 3G and HSDPA (HSDPA (High Speed Downlink Packet Access, also known as 3.5G, 3G+, or mobile Broadband)
CELLULAR VOICE AND DATA NETWORKS
Cellular standards fall into three categories; first generation analogue cellular systems, and second and third generation digital cellular systems. An interim technology, usually known as second generation-plus supports high speed data communication over today’s digital cellular systems. Table 1 shows main feature of these technologies. A brief discussion and of these technologies and a comparison of data services follows:
First Generation Technologies
First-generation mobile communications systems, sometimes referred as 1G, were basic analogue radio systems that established the first cellular radio infrastructure. The biggest problem with this system for cellular providers is the lack of capacity to handle the sheer number of users that demand voice service. The analogue cellular networks use circuit switched connections for data transport; however, the radio link performance for data is considered marginal due to the limitations imposed by the analogue nature of the technology. Radio channel dynamics such as dropouts, signal fades, and multi-paths, which can be tolerated during a voice connection, can be disastrous to a mobile data subscriber. Subscriber data rates of 2400 bits/s or less can be sustained using standard modems with some adaptation for connection to the cellular network.
In general, the analogue cellular infrastructure systems are not an efficient means of sending data due to limited available capacities, limitations of data recovery, low security, and the high cost of use for many applications. Some of the widely used standards include the following:
*** I welcome suggestions of better descriptions and explanations regarding this technology
Table 1: Main Features of Cellular Technologies
2nd + Generation
HSDPA, 3.5G, Mobile Broadband
Speech and video
High Bandwith Data
Voice band data
Increasing digital data
Mainly digital data
Increasingly packet switched
Mainly packet switched
Advanced Mobile Phone system (AMPS):
The AMPS was the first standardized cellular service in the world and was released for commercial use in 1983 in USA. The system uses 800 MHz to 900 MHz frequency band and the 30 KHz channel bandwidth. This is the most widely used analogue cellular standard.
Narrow-band Advanced Mobile system (N-AMPS):
This system operates in 800 MHz range and provides three times greater capacity than AMPS by using 10 KHz channel bandwidths instead of the standard 30 KHz channel bandwidths used in the AMPS system.
Nordic Mobile Telephone (NMT):
This system was in use throughout the Nordic countries. The system has two variants based on the frequency of allocation. NMT450 operates on 450 MHz, while NMT900 operates on 900 MHz.
Total Access Communications systems (TACS):
This system was based in the U.K and has several variants. The most popular are J-TACS (similar to AMPS) and E-TACS (Expanded TACS).
Second Generation Technologies
Second-generation mobile communications systems, sometimes referred as 2G, are currently predominant in the wireless communication industry. These use digital technology to provide many advantages for both the voice- and data-based mobile professional. These include increased system capacity, increased security against casual eavesdropping, superior cell hand-off, and better recovery of radio signal under different conditions. In addition to speech, these support services such as fax, short messaging, and roaming of mobile end-stations.
Table 2: Technical Summary of Second Generation Technologies
---- EUROPE -----
------------ UNITED STATES ------------
No. of voice/frequency. channels
8 / 16
3 / 6
Total traffic channels
1000 / 2000
2496 / 4992
Channel bit rate
8 / 4.5 Kbps
The second-generation technologies use circuit switched connections for data transport and provide data transmission rate of 9.6 to 14.4 Kbps. These implement a high level of flow control and error correction and provide reliable data transfer. With second-generation systems, multiple users can share a single cellular channel, thus reducing congestion and providing access for more users. These use the multiple access methods and provide extensive coverage with a proven and reliable communications infrastructure. The existing standards in use worldwide include the following:
GSM (Global system for Mobile Communications):
This was the first European digital open standard and is in commercial use in 1992. It was developed to establish cellular compatibility throughout Europe. Its success has spread to all parts of the world and by the year 2000, there were over 250 million subscribers worldwide. It is based on a combination of TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) techniques and operates at 900 MHz and 1800 MHz frequency bands in many parts of the Europe and Asia, and uses 1900 MHz in North America. Today, it provides an error-free Internet access at 9600 bps to the subscribers. Some analysts suggest that due to a single dominant network standard, GSM, Europe is 18 months ahead of the US wireless market.
TDMA (Time Division Multiple Access):
TDMA refers to products developed using the IS-136 specification for advanced digital wireless services. It was the first U.S. digital standard and was started in 1993. It is a natural evolution of analogue AMPS networks and, therefore, was previously known as D-AMPS (Digital AMPS). It is the most widely used wireless technology in the USA, and as of year-end 2000, there were about 61 million TDMA subscribers worldwide, with an estimated 31 million subscribers in the North America.
TDMA technology provides a 3 to 1 gain in capacity over analogue technology by dividing a single radio frequency channel into a series of timeslots. Each user is assigned a set of timeslots during which they are allowed to broadcast. This technique is better at handling heavy traffic than others, since there is a hard upper limit on the amount of bandwidth that a particular user will utilize, but this is its weakness as well. Cells that do not have a large number of users will have underutilized bandwidth. Similarly, there is a much harder limit on the total number of users that can be supported within a cell.
CDMA (Code Division Multiple Access):
This system, known as IS-95, was adopted by the Telecommunications Industry Association (TIA) in 1993. It uses the same frequency bands as AMPS and supports AMPS operation, employing spread-spectrum technology and a special coding scheme. In this technique the call is spread over a series of frequencies based on a sequence of jumps that are semi random in nature. The spread spectrum approach minimizes signal loss within any particular frequency band, as well as providing security for the communications. The handset and the base station agree on the sequence ahead of time, which gives the base station the capability to minimize collisions within a cell. It is characterized by high capacity and small cell radius.
CDMA provides outstanding voice and call quality, fewer dropped calls, improved security and privacy, greater capacity, reduced background noise and interference, and possibility of simultaneous voice and data calls. Designed with about 4.4 trillion codes, CDMA virtually eliminates cloning and other types of fraud. Globally, commercial CDMA networks serve tens of millions of subscribers. Table 2 provides a comparison of the main features of the second generation cellular technologies:
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Mobile Networks 2