NETWORK HUB

A network hub or repeater hub is a device for connecting multiple twisted pair or fiber optic Ethernet devices together and making them act as a single network segment. Hubs work at the physical layer (layer 1) of the OSI model. The device is a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or AUI connector to allow connection to legacy 10BASE2 or 10BASE5 network segments. The availability of low-priced network switches has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications.

Technical information

A network hub is a fairly unsophisticated broadcast device. Hubs do not manage any of the traffic that comes through them, and any packet entering any port is broadcast out on all other ports. Since every packet is being sent out through all other ports, packet collisions result—which greatly impedes the smooth flow of traffic.

The need for hosts to be able to detect collisions limits the number of hubs and the total size of a network built using hubs (a network built using switches does not have these limitations). For 10 Mbit/s networks, up to 5 segments (4 hubs) are allowed between any two end stations. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 hubs) between any two end stations, and even that is only allowed if the hubs are of the low delay variety. Some hubs have special (and generally manufacturer specific) stack ports allowing them to be combined in a way that allows more hubs than simple chaining through Ethernet cables, but even so, a large Fast Ethernet network is likely to require switches to avoid the chaining limits of hubs.

Most hubs detect typical problems, such as excessive collisions and jabbering on individual ports, and partition the port, disconnecting it from the shared medium. Thus, hub-based Ethernet is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2), where a misbehaving device can adversely affect the entire collision domain. Even if not partitioned automatically, a hub makes troubleshooting easier because status lights can indicate the possible problem source or, as a last resort, devices can be disconnected from a hub one at a time much more easily than a coaxial cable. They also remove the need to troubleshoot faults on a huge cable with multiple taps.

Hubs are classified as Layer 1 (Physical Layer)devices in the OSI model. At the physical layer, hubs support little in the way of sophisticated networking. Hubs do not read any of the data passing through them and are not aware of their source or destination. Essentially, a hub simply receives incoming packets, regenerates the electrical signal, and broadcasts these packets out to all other devices on the network.

Uses

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Historically, the main reason for purchasing hubs rather than switches was their price. This has largely been eliminated by reductions in the price of switches, but hubs can still be useful in special circumstances:

For inserting a protocol analyzer into a network connection, a hub is an alternative to a network tap or port mirroring.

Some computer clusters require each member computer to receive all of the traffic going to the cluster. A hub will do this naturally; using a switch requires special configuration.

When a switch is accessible for end users to make connections, for example, in a conference room, an inexperienced or careless user (or saboteur) can bring down the network by connecting two ports together, causing a loop. This can be prevented by using a hub, where a loop will break other users on the hub, but not the rest of the network. (It can also be prevented by buying switches that can detect and deal with loops, for example by implementing the Spanning Tree Protocol.)

A hub with a 10BASE2 port can be used to connect devices that only support 10BASE2 to a modern network. The same goes for linking in an old thicknet network segment using an AUI port on a hub (individual devices that were intended for thicknet can be linked to modern Ethernet by using an AUI-10BASE-T transceiver).

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Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), which use CDMA as an underlying channel access method.

One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a bandwidth of different frequencies. This concept is called multiplexing. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes, which enhances privacy and makes cloning difficult.

The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide roaming.

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The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.

Steps in CDMA Modulation

CDMA is a spread spectrum multiple access technique. A spread spectrum technique is one which spreads the bandwidth of the data uniformly for the same transmitted power. Spreading code is a pseudo-random code which has a narrow Ambiguity function unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logically XOR (exclusive OR) added with the faster code. The figure shows how spread spectrum signal is generated. The data signal with pulse duration of Tb is XOR added with the code signal with pulse duration of Tc. (Note: bandwidth is proportional to 1 / T where T = bit time) Therefore, the bandwidth of the data signal is 1 / Tb and the bandwidth of the spread spectrum signal is 1 / Tc. Since Tc is much smaller than Tb, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal. The ratio Tb / Tc

is called spreading factor or processing gain and determines to certain extent the upper limit of total number of users supported simultaneously by a base station

Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference.

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In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).

GSM (Global System for Mobile Communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile telephone systems in the world. The GSM Association, its promoting industry trade organization of mobile phone carriers and manufacturers, estimates that 80% of the global mobile market uses the standard. GSM is used by over 3 billion people across more than 212 countries and territories. Its ubiquity enables international roaming arrangements between mobile phone operators, providing subscribers the use of their phones in many parts of the world. GSM differs from its predecessor technologies in that both signaling and speech channels are digital, and thus GSM is considered a second generation (2G) mobile phone system. This also facilitates the wide-spread implementation of data communication applications into the system. Enhanced Data Rates for GSM Evolution (GSM EDGE) is a 3G version of the protocol.

The ubiquity of implementation of the GSM standard has been an advantage to both consumers, who may benefit from the ability to roam and switch carriers without replacing phones, and also to network operators, who can choose equipment from many GSM equipment vendors. GSM also pioneered low-cost implementation of the short message service (SMS), also called text messaging, which has since been supported on other mobile phone standards as well. The standard includes a worldwide emergency telephone number feature (112).

Newer versions of the standard were backward-compatible with the original GSM system. For example, Release '97 of the standard added packet data capabilities by means of General Packet Radio Service (GPRS).

Cellular radio network

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.

Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell, where the cell radius

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could be double or even more, depending on the antenna system, the type of terrain and the timing advance.

Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors; for example, in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from any nearby cell.

The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent-channel interference).

GSM carrier frequencies

GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G). Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. Most 3G GSM EDGE networks in Europe operate in the 2100 MHz frequency band.

The rarer 400 and 450 MHz frequency bands are assigned in some countries where these frequencies were previously used for first-generation systems.

GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and 935–960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used.

In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses 880–915 MHz (uplink) and 925–960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

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Voice codecs

GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 6.5 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (6.5 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

GSM was further enhanced in 1997 with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.

Network structure

The structure of a GSM network

The network behind the GSM seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.

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The Base Station Subsystem (the base stations and their controllers). the Network and Switching Subsystem (the part of the network most similar to a

fixed network). This is sometimes also just called the core network. The GPRS Core Network (the optional part which allows packet based Internet

connections). The Operations support system (OSS) for maintenance of the network..

GSM service security

GSM was designed with a moderate level of service security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional Universal Subscriber Identity Module (USIM), that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.

GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a ciphertext-only attack, and in February 2008, Pico Computing, Inc revealed its ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow table attack.[14] The system supports multiple algorithms so operators may replace that cipher with a stronger one.

On 28 December 2009 German computer engineer Karsten Nohl announced that he had cracked the A5/1 cipher.[15] According to Nohl, he developed a number of rainbow tables (static values which reduce the time needed to carry out an attack) and have found new sources for known plaintext attacks. He also said that it is possible to build "a full GSM interceptor ... from open source components" but that they had not done so because of legal concerns.

In 2010, threatpost.com reported that "A group of cryptographers has developed a new attack that has broken Kasumi, the encryption algorithm used to secure traffic on 3G GSM wireless networks. The technique enables them to recover a full key by using a tactic known as a related-key attack, but experts say it is not the end of the world for Kasumi. Kasumi is the name for the A5/3 algorithm, used to secure most 3G GSM EDGE traffic.

Although security issues remain for GSM newer standards and algorithms may address this. New attacks are growing in the wild which take advantage of poor security implementations, architecture and development for smart phone applications. Some wiretapping and eavesdropping techniques hijack the audio input and output providing an opportunity for a 3rd party to listen in to the conversation. Although this threat is

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mitigated by the fact the attack has to come in the form of a Trojan, malware or a virus and might be detected by security software.

Emerging Wireless TechnologiesA look into the future of wireless communications – beyond 3GAlthough the new, third generation (3G) wireless technology has not yet been implemented, leading companies in the industry are already laying the groundwork for what some are calling fourth generation (4G) technology. The first generation (1G) and second generation (2G) of mobile telephony were intended primarily for voice transmission. The third generation of mobile telephony (3G) will serve both voice and data applications. There really is no clear definition of what 4G will be. It is generally accepted that 4G will be a super-enhanced version of 3G – i.e., an entirely packet switched network with all digital network elements and extremely high available bandwidth. For the most part, it is believed that 4G will bring true multimedia capabilities such as high-speed data access and video conferencing to the handset. It is also envisioned that 4G systems will be deployed with software defined radios, allowing the equipment to be upgraded to new protocols and services via software upgrades. 4G also holds the promise of worldwide roaming using a single handheld device. Wireless Generations At-a-GlanceAs with all technology progressions, the “next” upgrades must be in planning and development phases while its predecessors are being deployed. This statement holds true with all mobile telecommunications to date. It seems that it will also hold true for the next generations of wireless networks. The original analog cellular systems are considered the first generation of mobile telephony (1G). In the early 1980s, 1G systems were deployed. At the same time, the cellular industry began developing the second generation of mobile telephony (2G). The difference between 1G and 2G is in thesignaling techniques used: 1G used analog signaling, 2G used digital signaling. As experience shows, the lead-time for mobile phone systems development is about 10 years. It was not until the early to mid 1990s that 2G was deployed. Primary thinking and conceptdevelopment on 3G generally began around 1991 as 2G systems just started to roll out. Since the general model of 10 years to develop a new mobile system is being followed, that timeline would suggest 4G should be operational some time around 2011. 4G would build on the second phase of 3G, when all networks are expected to embrace Internet protocol (IP) technology. During the last year, companies such as Ericsson, Motorola, Lucent, Nortel and Qualcomm came up with "3G-plus" concepts that would push performance of approved, though still emerging, standards beyond current ones.Interoperability and the Evolution of Network ArchitecturesOne of the most challenging issues facing deployment of 4G technologies is how to make the network architectures compatible with each other. New signaling techniques are being designed specifically to enhance today's second generation (2G) networks, deliver unprecedented functionality for 3G, and successfully drive the Fourth Generation (4G) of wireless, thus delivering immediate and long-term benefits to carriers. With the architecture of each generation of wireless devices addressed in the development of advanced technologies, carriers can easily evolve their systems without additional network modifications, significantly reducing costs and implementation time. Currently, different wireless technologies (e.g., GSM, CDMA, and TDMA1) are used throughout

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the world for the 2G, 2.5G, and eventually 3G networks. There are two approaches being used to develop 4G access techniques: 3xRTT (currently 1xRTT for 2.5 and 3G) and Wideband CDMA (W-CDMA). These disparate access techniques currently do not interoperate. This issue may be solved with software defined radios. LinkAir Communications is developing a new access technology called large-area-synchronized code-division multiple access (LAS-CDMA). LASCDMA will be compatible with all current and future standards, and there is a relatively easy transition from existing systems to LAS-CDMA (using software defined radios). LinkAir emphasizes that LASCDMA will accommodate all the advanced technologies planned for 4G and that LASCDMA will further enhance either 3xRTT or W-CDMA system’s performance and capacity.Internet Speeds2.5G is the interim solution for current 2G networks to have 3G functionality. 2.5G networks are being designed such that a smooth transition (software upgrade) to 3G can be realized. 2.5G networks currently offer true data speeds up to 28kbps. In comparison, the theoretical speed of 3G can be up to 2 Mbps, i.e., approximately 200 times faster thanprevious 2G networks. This added speed and throughput will make it possible to run applications such as streaming video clips. It is anticipated that 4G speeds could be as high as 100 Mbps. Thus, 4G will represent another quantum leap in mobile Internet speeds and picture quality. Ericsson confirms that 4G could bring connection speeds of up to 50 times faster than 3G networks and could offer three-dimensional visual experiences for the first time. The following graph represents what has been the typical progression of wireless communications:

Quality of Service ChallengesIn wireless networks, Quality of Service (QOS) refers to the measure of the performance for a system reflecting its transmission quality and service availability (e.g., 4G is

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expected to have at least a reliability of 99.99%). Supporting QOS in 4G networks will be a major challenge. When considering QOS, the major hurdles to overcome in 4G include: varying rate channel characteristics, bandwidth allocations, fault tolerance levels, and handoff support among heterogeneous wireless networks. Fortunately, QOS support can occur at the packet, transaction, circuit, and network levels. QOS will be able to be tweaked at these different operating levels, making the network more flexible and possibly more tolerant to QOS issues. Varying rate channel characteristics refers to the fact that 4G applications will have varying bandwidth and transition rate requirements. In order to provide solid network access to support the anticipated 4G applications, the 4G networks must be designed with both flexibility and scalability. Varying rate channel characteristics must be considered to effectively meet user deand and ensure efficient network management. Spectrum is a finite resource. In current wireless systems, frequency licensing and efficient spectrum management are key issues. In 4G systems, bandwidth allocations may still be a concern. Another concern is interoperability between the signaling techniques that are planned to be used in 4G (e.g., 3xRTT, WCDMA). In comparison with current 2G and 2.5G networks, 4G will have more fault tolerance capabilities built-in to avoid unnecessary network failure, poor coverage,and dropped calls. 4G technology promises to enhance QOS by the use of better diagnostic techniques and alarms tools. 4G will have better support of roaming and handoffs across heterogeneous networks. Users, even in today’s wireless market, demand service transparency and roaming. 4G may support interoperability between disparate network technologies by using techniques such as LAS-CDMA signaling. Other solutions such as software defined radios could also support roaming across disparate network technologies in 4G systems. These major challenges to QOS in 4G networks are currently being studied and solutions are being developed. Developers believe that QOS in 4G will rival that of any current 2G or 2.5G network. It is anticipated that the QOS in 4G networks will closely approximate the QOS requirements in the wireline environment (99.999% reliability).4G Applications and Their Benefits to Public SafetyOne of the most notable advanced applications for 4G systems is locationbased services. 4G location applications would be based on visualized, virtual navigation schemes that would support a remote database containing graphical representations of streets, buildings, and other physical characteristics of a large metropolitan area. This database could be accessed by a subscriber in a moving vehicle equipped with the appropriate wireless device, which would provide the platform on which would appear a virtual representation of the environment ahead. For example, one would be able to see the internal layout of a building during an emergency rescue. This type of application is sometimes referred to as "Telegeoprocessing", which is a combination of Geographical Information Systems (GIS) and Global Positioning Systems (GPS) working in concert over a high-capacity wireless mobile system. Telegeoprocessing over 4G networks will make it possible for the public safety community to have wireless operational functionality and specialized applications for everyday operations, as well as for crisis management. The emergence of next generation wireless technologies will enhance the effectiveness of the existing methods used by public safety. 3G technologies and beyond could possibly bring the following new features to public safety:

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1. Virtual navigation: As described, a remote database contains the graphical representation of streets, buildings, and physical characteristics of a large metropolis. Blocks of this database are transmitted in rapid sequence to a vehicle, where a rendering program permits the occupants to visualize the environment ahead. They may also "virtually" see the internal layout of buildings to plan an emergency rescue, or to plan to engage hostile elements hidden in the building.

2. Tele-medicine: A paramedic assisting a victim of a traffic accident in a remote location could access medical records (e.g., x-rays) and establish a video conference so that a remotely based surgeon could provide “on-scene” assistance. In such a circumstance, the paramedic could relay the victim's vital information (recorded locally) back to the hospital in real time, for review by the surgeon.

3. Crisis-management applications: These arise, for example, as a result of natural disasters where the entire communications infrastructure is in disarray. In such circumstances, restoring communications quickly is essential. With wideband wireless mobile communications, both limited and complete communications capabilities, including Internet and video services, could be set up in a matter of hours. In comparison, it may take days or even weeks to re-establish communications capabilities when a wireline network is rendered inoperable.

Limitations of 4GAlthough the concept of 4G communications shows much promise, there are still limitations that must be addressed.One major limitation is operating area. Although 2G networks are becoming more ubiquitous, there are still many areas not served. Rural areas and many buildings in metropolitan areas are not being served well by existing wireless networks. This limitation of today’s networks will carry over into future generations of wireless systems. The hype that is being created by 3G networks is giving the general public unrealistic expectations of always on, always available, anywhere, anytime communications. The public must realize that although high-speed data communications will be delivered, it will not be equivalent to the wired Internet – at leastnot at first. If measures are not taken now to correct perception issues, when 3G and later4G services are deployed, there may be a great deal of disappointment associated with the deployment of the technology, and perceptions could become negative. If this were to happen, neither 3G nor 4G may realize its full potential. Another limitation is cost. The equipment required to implement a nextgeneration network is still very expensive. Carriers and providers have to plan carefully to make sure that expenses are kept realistic.One technique currently being implemented in Asian networks is a Pay-Per-Use model of services. This model will be difficult to implement in the United States, where the public is used to a service-for-free model (e.g., the Internet).

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