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WIRELESS WIDE AREA NETWORKS (WWAN)

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 have plans to move to this technology over next one to three years. Currently, due to physical layer constraints, wide-area networks typically feature low-speed wireless data transmission, on the order of 9.6 Kbps. However, with the emerging new protocols, much higher data transmission speed is supported.

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:

Table 1: Main Features of Cellular Technologies 1st  Generation Cellular Radio 2nd Generation GSM 2nd + Generation GPRS 3rd Generation 3G Analogue transmission. Digital transmission Digital transmission Digital transmission Mainly speech Mainly speech Mainly speech Speech and video Voice band data Digital data Increasing digital data Mainly digital data Circuit switched Circuit switched Increasingly packet switched Mainly packet switched Local systems Global roaming Global roaming Global roaming

 

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:

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 ------------   GSM TDMA CDMA Frequency band 890-960 MHz 824-894 MHz 824-894 MHz Allocated bandwidth 50 50 50 Access scheme TDMA TDMA CDMA Duplex method FDD FDD TDD Channel bandwidth 200 KHz 30 KHz 1250 KHz No. of voice/frequency. channels 8 / 16 3 / 6   Total traffic channels 1000 / 2000 2496 / 4992   Channel bit rate 270.833 Kbps 48.6 Kbps Vendor dependent Voice coding 22.8 Kbps 8 / 4.5 Kbps 8 Data rate 9.6 Kbps 9.6 Kbps 14.4 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:

Second-Plus Generation Technologies

The second-generation technologies provide data transfer rates only up to 14.4 Kbps. The high data speeds that are needed for video and graphic image transmission are not available on most of the today’s mobile phone systems. Such capabilities require a highly complex and robust technology platform that will not be available in most of the countries until few years from now. An interim step to the next generation technologies is second-plus generation or 2.5G technologies as shown in Figure 4. These technologies support data transfer rates of 57.6 Kbps and higher and offer subscribers access to the Internet at speeds that are comparable to a wire-line ISDN connection or even faster. These include HSCSD, GPRS and EDGE. An overview of these technologies is given next:

Figure 4: Evolution of 2G Networks to 2.5G and 3G

HSCSD (High Speed Circuit Switched Data):

HSCSD is a circuit-switched mobile data standard that gives a single user simultaneous access to multiple channels, up to four, at the same time. In comparison, GSM supports only one user per channel per time slot. Assuming a standard data transmission rate of 14.4 Kbps, using four timeslots with HSCSD allows theoretical speeds of up to 57.6 Kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel.

HSCSD does not disrupt voice service availability. In fact, HSCSD can be pre-empted by voice calls- such that HSCSD calls can be reduced to one channel if voice calls are seeking to occupy these channels. In networks where HSCSD is deployed, GPRS (discussed in next section) may only be assigned third priority, after voice as number one priority and HSCSD as number two. HSCSD is therefore more likely to be deployed in start up networks or those with plenty of spare capacity – since it is relatively inexpensive to deploy and can turn some spare channels into revenue streams. It is however easier to implement in mobile networks than GPRS because some GSM vendor solutions require only a software upgrade of base stations and no new hardware.

HSCSD is expensive for end users as they have to pay for multiple simultaneous calls. However, being a circuit-switched standard, HSCSD could be the best way of communicating with other circuit switched communications media such as the PSTN and ISDN.

GPRS (General Packet Radio Service):

GPRS is a new packet-based bearer that is being introduced on many GSM and TDMA mobile networks from the year 2001 onwards. It is a non-voice value added service that allows a subscriber to send and receive data in an end-to-end packet transfer mode, without using any network resources in circuit-switched mode. It also permits the user to receive voice calls simultaneously when sending or receiving data calls.

GPRS facilitates instant connections (no dial-up) whereby information can be sent or received immediately as the need arises. This is why GPRS users are sometimes referred to be as being “always connected”. A GPRS mobile device displays a mobile portal service all the time, but it is only activated, and the user is only charged, when information is being transmitted. The main feature of GPRS is that it reserves radio resources only when there is data to send and that these radio resources are shared by all Mobile Stations (Mess) in a cell. It handles data transfer rates from 14.4 Kbps, using just one TDMA slot, up to 115.2 Kbps, using all eight TDMA slots. This will allow it to handle all types of transmission from slow-speed short messages, to the higher speeds needed for browsing complex web pages with high graphics content.

GPRS fully enables a true “Mobile Internet” scenario by allowing integration between the existing Internet and the GPRS network, via interfaces to TCP/IP. Its network can be viewed as a sub-network of the Internet with GPRS capable mobile phones being viewed as mobile hosts. This means that each GPRS terminal can potentially have its own IP address and will be addressable as such. Any service that is used over the fixed Internet today – web browsing, file transfer, chat, email, telnet – will also be available over mobile network via GPRS. In addition, higher data rates will allow users to take part in video conferencing and interact with multimedia websites and similar applications as well.

Enhanced Data rates for Global Evolution (EDGE):

EDGE is a radio based high-speed mobile data standard that was first proposed to the European Telecommunications Standards Institute (ETSI) in 1997 as an evolution of GSM. In fact, it was formerly called GSM384. It is the result of a joint effort between TDMA industry association and the GSM Alliance to develop a common set of third generation wireless standards which supports high-speed modulation. EDGE allows mobile operators to offer 3G services without having to purchase a 3G license. It allows data transmission speeds from 48 Kbps, using just one timeslot, up to 384 Kbps, using all eight timeslots. It supports 800/900/ 1800/1900 MHz frequency bands. Although it reuses the GSM carrier bandwidth and timeslot structure, it is by no means restricted to use within GSM cellular systems. In fact, by enhancing the capability of existing GSM or TDMA systems, it facilitates an evolution of existing cellular systems towards third-generation capabilities.

Implementation of EDGE by network operators has been designed to be simple. Only one EDGE transceiver unit will need to be added to each cell. The new EDGE capable transceiver can also handle standard GSM traffic and will automatically switch to EDGE mode when needed. EDGE capable terminals will also be needed since the existing mobile phone or terminals do not support the new modulation techniques and will need to be upgraded to use EDGE network functionality. EDGE provides the most cost-effective means to provide IP-based multimedia services and applications within existing spectrum. The advantages of EDGE include rapid availability, the reuse of existing GSM and TDMA infrastructure, and support for gradual introduction. In addition, it allows the full advantages of GPRS to be explored, with fast connection set-up, higher bandwidth, and data rates as high as 384 Kbps.

Third Generation Technologies

Two shortcomings of the second generation bearer networks are low bandwidth and limited network capacity which negatively impact the user experience and the reliability of the service. Third generation or 3G technology is a new technological evolution that will offer far more bandwidth and greater data and voice call capacity than today's digital mobile networks allow. It is a next giant step in mobile technology development with its goal being full interoperability and inter-working of mobile systems. The idea behind 3G is to unify the disparate standards that today's second generation wireless networks use.

With 3G technology, portable bandwidth will rise to the level of wired broadband connections and the data transfer rates of up to 2 Mbps will be possible (128 Kbps in a car, 384 Kbps when a device is stationery or moving at pedestrian speed and 2 Mbps in fixed applications). When this speed is achieved, wireless technology will find a new audience that is interested in Internet browsing, wireless gaming, and listening to music. Current mobile networks are only designed for voice and text messaging, whereas 3G networks will allow faster and more complex data transmission such as streaming video and audio, video conferencing, satellite navigation and interactive application sharing. These networks will provide packet switched data access to the Internet with an end-to-end IP connection. This means that when the mobile phone is activated it is automatically connected to the Internet via a normal browser. Subscribers will then enjoy capabilities similar to today’s fixed-line Internet services with significant add-ons such as location-based and highly personalized services.

Third generation technology allows handsets to be left permanently connected to the network and use capacity only when they receive or transmit packages. Subscribers can thus pay for the volume of data transmitted, not how long they talk.

Although the technology behind 3G may seem complicated, the ways in which 3G will affect all of our lives are easy to imagine. Just imagine having a combined camera, computer, stereo, and radio included in your mobile phone. Rich-media information and entertainment will be at your fingertips whenever and wherever you want. Being able to do so much, the end user device is no longer just a mobile phone, and will be referred to as a terminal. 

Standards:

Standardization of third generation mobile communications began in the mid-1990s under supervision of the International Telecommunications Union (ITU). The goal was full interoperability and inter-working of mobile systems capable of providing value-added services. In 1998, the ITU called for Radio Transmission Technology (RTT) proposals for IMT-2000 (International Mobile Telecommunications-2000), the formal name for the third generation standard. Under the brand IMT-2000, it approved three standards to achieve this: W-CDMA, CDMA2000 and TD-SCDMA. W-CDMA (Wideband Code Division Multiple Access) was backed by the European Telecommunications Standards Institute (ETSI) and the GSM operators in Europe and elsewhere; while the CDMA2000 was backed by the North American CDMA community, led by the CDMA Development Group (CDG). The third standard won the support in the other parts of the world. Earlier, in January 1998, the W-CDMA standard was also incorporated by ETSI in the specification of

UMTS (Universal Mobile Telecommunications system) Terrestrial Radio Access; hence W-CDMA and UMTS are often used synonymously.

IMT-2000 is to ensure that these technologies can work in different networks, primarily in IP networks, but for the sake of backwards compatibility, in the GSM and the American ANSI networks as well. Most major network operators in Europe and Asia are committed to the W-CDMA standard for 3G mobile communications. Nevertheless, other standards are being implemented in other parts of the world. In North America and Asia Pacific, the next generation wireless network is going to be mainly based on CDMA2000 and China, the world's largest market for mobile communication, will be using TD-SCDMA standard for 3G networks.

Availability:

Upgrading from 2G to 3G requires significant capital investment. In the UK, for example, five 3G mobile licenses were auctioned off at a total of $35 billion with the expectation that it will cost each license-holder between $4 billion and $9 billion to build out their 3G network. For this reason carriers have been reluctant to upgrade their networks before they see a real demand for high-speed wireless data and many view 2.5G as more than just an interim solution as it delivers significant bandwidth improvements at greatly reduced cost.

Today, however, as major wireless service providers assess the high costs of deploying 3G services and the accompanying technical difficulties such as 3G handset and network infrastructure readiness, a few are already working on deployment of W-CDMA in Europe and Japan. Table 3 shows market snapshot and status of deployment of mobile Internet technologies in some of these countries. NTT DoCoMo in Japan has already released a third generation phone service FOMA (Freedom Of Mobile multimedia Access) in major urban areas of the country. FOMA receives data at 384 Kbps and transmit at 64 Kbps, and delivers everything from movie trailers and sports highlights to music, video clips and news feeds.

The Strategies Group predicts that there will be 9.5 million 3G mobile high-speed data subscribers by 2005 and UMTS Forum predicts that by 2010 data services will represent $300 billion or 66% of all worldwide 3G revenues.

Fourth Generation Technologies

As the major network operators have just started providing 3G services, some groups and companies have already started working on fourth-generation mobile-phone system. The 4G technology will take mobile communication another step up to integrate radio and television transmissions, and to consolidate world's phone standards into one high-speed technology.

There are two key elements which are required to deliver a legitimate 4G network. First is the ability to roam across different wireless network standards with the one device; and the second, and most obvious, is a higher level of bandwidth. Figures of 100 Mbps have been tossed around, but a more reasonable figure to expect is about 20 Mbps.

At present, there are two competing 4G standards: a joint effort by Hewlett Packard and Japan's NTT DoCoMo to create Moto-Media, and the Wireless World Research Forum specifications with the backing of some of the Europe's largest phone makers.

The questions engaging most observers at the moment are just how big is the 4G market going to be, and when can the industry be reasonably expected to invest in a new network. Some of the analysts have estimated that the 4G mobile systems will have 50 million subscribers by the end of year 2007, and it would account for 14 percent of total mobile data revenues. But most of the analyst estimate the technology to be ready around 2008-2010. Nokia and Samsung have already teamed up to create 4G wireless equipment, a move which demonstrates the support for the 4G mobile systems.

Comparison of Data Services

The demand for mobile data services is growing. Increased mobility has fuelled an expanding market for both consumers and the enterprises. A comparison of various data services for cellular networks is shown in Table 4. For consumers, second-plus and third generation networks will bring access to the Internet, with near wire-line speed and quality. 2.5G services will mostly be text-based with still images and short audio clips. Services will include web browsing, financial transactions, image downloads, e-mail and instant messaging. As networks migrate to 3G, these same services will be enriched with multimedia content including full audio and video clips.

Table 4: Comparison of Data Services for 2G, 2.5G and 3G Networks SERVICES 2ND GENERATION GSM 2ND + GENERATION GPRS 3RD GENERATION 3G Web browsing Short text screens 100KB web page takes approx. 30 sec to download 100KB web page takes approx. 2 sec to download File transfers No 500 KB document takes approx. 2mn to download 500 KB document takes approx 10 sec to download e-mail Short Message Service (SMS) Text-based with small attachments Full attachments Instant messaging SMS Text-based With audio/video clips VoIP (Voice over IP) No Limited Yes Streaming audio/video No Short clips Yes Access to corporate intranet Very limited Text-based Yes Access to corporate apps Very limited Text-based Yes

 

For enterprises, second-plus generation networks will allow access to corporate intranet and e-mail, business applications and databases, and increasing mobile sales and field employees’ productivity. In the future, 3G capabilities will enable even greater benefits from wireless business applications through VoIP (Voice over IP), rapid file transfer and video-conferencing.

PACKET SWITCHED MOBILE DATA NETWORKS

A packet switched mobile data network is a type of specialised mobile radio system which functions as a wireless wide-area data-only network for the mobile professionals. Like first generation cellular telephone systems, these packet radio systems use analogue radio technology. Unlike cellular systems, however, these networks offer connectionless support – subscribers do not maintain a dedicated, point-to-point connection to the destination station. Subscribers using the packet radio system are billed a monthly fee, plus a usage fee based on the amount of information (packets) transmitted through the system. The primary packet data services currently available in United States for mobile applications include ARDIS, MOBITEX, and a number of other services based on CDPD (Cellular Digital Packet Data) technology. A brief discussion of these services is given below:

ARDIS (Advanced Radio Data Information Services)

The ARDIS is a two-way radio service that is based on Motorola's RD-LAP technology. It was originally created and jointly owned by Motorola and IBM to serve IBM field technicians. However, later it was made available to the public. In 1998, it was acquired by the American Mobile Satellite Corporation. ARDIS support data transfer rate of 19.2 Kbps in urban areas, where it has 90% coverage of U.S business population (business population is considered the top 200-300 metro areas). Outside of those areas service can still be achieved, albeit at a lower 4.8 Kbps data rate. Due to overhead burdens associated with the radio channel protocol and error correction, subscriber data throughputs are actually much less than the raw data rate. Moreover, the network latency is fairly high. These limitations make the network unsuitable for most Internet and corporate intranet applications.

MOBITEX

MOBITEX protocol was originally developed by Swedish Telecom as a private mobile alarm system used by field personnel. However, later it evolved into a public mobile radio service. Commercial operation was introduced in Sweden in 1986 and, since then, a number of networks have been deployed in U.K, U.SA, Canada, Australia and Scandinavian countries. In United States, the MOBITEX was introduced by RAM Mobile Data which is now a wholly owned subsidiary of BellSouth Wireless Data.

MOBITEX covers about 93% of the U.S. business population, making it a serious contender for some applications. Originally, MOBITEX transmission rate was 4.8 Kbps but now it has been upgraded to 19.2 Kbps. However, similar to ARDIS, the subscriber data throughput is much less than the raw data rate due to data transmission overhead. Moreover, network latency is fairly high - often several seconds. For these reasons, it is suitable only for limited text messages, not graphics or file transfers.

CDPD (Cellular Digital Packet Data)

CDPD specification was developed by a consortium of eight U.S. cellular companies. It allows data transmission to be overlaid onto the existing analogue cellular channels. It provides two significant enhancements to the AMPS cellular system – increased total system capacity and specifications for implementing data. CDPD networks are operated by various carriers in United States, including AT&T Wireless, Ameritech, Bell Atlantic Mobile and GTE.

CDPD offers standard RSA encryption over the air-link, making it the network of choice for public safety agencies and point of sale cash transactions. It offers users a raw data rate of 19.2 Kbps. However, overhead for coding and channel management to handle frequency hopping will reduce actual throughput. The reliability of CDPD data speeds is also questionable, particularly in mobile situations. Network-induced latency can be high, often more than one second. These limitations have made CDPD most useful for specific vertical-market applications.

Credits and Acknowledgements:

http://www.iepsac.org/papers/p09c03.htm