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IntroductionTCP/IP stands for the 'Transport Control Protocol / Internet Protocol' suite. TCP/IP was created in 1983 to replace NCP. The advantage of TCP/IP is it's versitility. It can successfully switch packets of all shapes and sizes, and work across a varieties of networks. TCP/IP has become the backbone of the Internet and its composite LANs and WANs. As already stated, it is due to it's ability to switch packets from any computer systems, regardless of network peculiarities, operating system differences and other packet differences. The TCP/IP protocol suite refers to several separate protocols that computers use to transfer data across the Internet. Listed below are four of the most commonly used TCP/IP protocols, | |||||||||||||
| Components of TCP/IP
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Saturday, February 18, 2012
What is TCP/IP
Tuesday, February 14, 2012
Pulse Code Modulation (PCM)
Analog transmission is not particularly efficient. When the signal-to-noise ratio of an analog signal deteriorates due to attenuation, amplifying the signal also amplifies noise. Digital signals are more easily separated from noise and can be regenerated in their original state. The conversion of analogue signals to digital signals therefore eliminates the problems caused by attenuation. Pulse Code Modulation (PCM) is the simplest form of waveform coding. Waveform coding is used to encode analogue signals (for example speech) into a digital signal. The digital signal is subsequently used to reconstruct the analogue signal. The accuracy with which the analogue signal can be reproduced depends in part on the number of bits used to encode the original signal. Pulse code modulation is an extension of Pulse Amplitude Modulation (PAM), in which a sampled signal consists of a train of pulses where each pulse corresponds to the amplitude of the signal at the corresponding sampling time (the signal is modulated in amplitude). Each analogue sample value is quantised into a discrete value for representation as a digital code word. Pulse code modulation is the most frequently used analogue-to-digital conversion technique, and is defined in the ITU-T G.711 specification. The main parts of a conversion system are the encoder (the analogue-to-digital converter) and the decoder (the digital-to-analogue converter). The combined encoder/decoder is known as a codec. A PCM encoder performs three functions:
- sampling
- quantising
- encoding
The human voice uses frequencies between 100Hz and 10,000Hz, but it has been found that most of the energy in speech is between 300 Hertz and 3400 Hertz - a bandwidth of approximately 3100 Hertz. Before converting the signal from analog to digital, the unwanted frequency components of the signal are filtered out. This makes the task of converting the signal to digital form much easier, and results in an acceptable quality of signal reproduction for voice communication. From an equipment point of viev, because the manufacture of very precise filters would be expensive, a bandwidth of 4000 Hertz is generally used. This bandwidth limitation also helps to reduce aliasing - aliasing happens when the number of samples is insufficient to adequately represent the analog waveform (the same effect you can see on a computer screen when diagonal and curved lines are displayed as a series of zigzag horizontal and vertical lines).
Sampling
Sampling the analogue signal
Sampling is the process of reading the values of the filtered analogue signal at discrete time intervals (i.e. at a constant sampling frequency, called the sampling frequency). A scientist called Harry Nyquist discovered that the original analogue signal could be reconstructed if enough samples were taken. He found that if the sampling frequency is at least twice the highest frequency of the input analogue signal, the signal could be reconstructed using a low-pass filter at the destination.
Quantisation
Quantisation is the process of assigning a discrete value from a range of possible values to each sample obtained. The number of possible values will depend on the number of bits used to represent each sample. Quantisation can be achieved by either rounding the signal up or down to the neares available value, or truncating the signal to the nearest value which is lower than the actual sample. The process results in a stepped waveform resembling the source signal. The difference between the sample and the value assigned to it is known as the quantisation noise (or quantisation error).
Quantisation noise can be reduced by increasing the number of quantisation intervals, because the difference between the input signal amplitude and the quantization interval decreases as the number of quantization intervals increases. This would, however, increase the PCM bandwidth. Uniform quantisation uses equal quantisation levels throughout the entire range of an input analogue signal. The signal-to-noise ratio (SNR), including quantisation noise, is the most important factor affecting voice quality in uniform quantisation. The signal-to-noise ratio is measured in decibels (dB). The higher the signal-to-noise ratio, the better the voice quality. Quantisation noise reduces the signal-to-noise ratio of a signal, so an increase in quantisation noise degrades the quality of a voice signal. Low signals will have a small signal-to-noise ratio and high signals will have a large signal-to-noise ratio. Because most voice signals are relatively low, having better voice quality at higher signal levels is an inefficient way of digitising voice signals. Uniform quantisation was therefore replaced by a non-uniform quantisation process called companding (see below).
Narrowband speech is typically sampled 8000 times per second, and each sample must be quantised. If linear quantisation is used, 12 bits per sample are required, giving a bit rate of 96 kbits per second. This can be reduced using non-linear quantisation, in which 8 bits per sample is sufficient to provide speech quality almost indistinguishable from the original. This results in a bit rate of 64 kbits per second. Two non-linear PCM codecs were standardised in the 1960s - ยต-law (mu-law) coding was the standard developed in the United States, while A-law compression was used in Europe. These codecs are still widely used today.
Encoding
Encoding is the process of representing the sampled values as a binary number in the range 0 to n. The value of n is chosen as a power of 2, depending on the accuracy required. Increasing n reduces the step size between adjacent quantisation levels and hence reduces the quantisation noise. The down side of this is that the amount of digital data required to represent the analogue signal increases.
Stages in the analogue-to-digital conversion process
Source : - http://www.technologyuk.net
Monday, January 16, 2012
Modulation
Modulation means to vary or change. In wireless we first take a signal, say a telephone conversation, and then impress it on a constant radio wave called a carrier. Once done the voice signal varies or modulates this radio wave. The two go together over the air. A voice frequency in the audible or audio range, what we can hear, thus modulates or varies a constant frequency in the radio range, which we can't hear. That's an important point. Modulation makes voice band and radio band frequencies work together. Different modulation techniques, such as A.M., F.M., P.C.M. and so on, represent different ways to shape or form electromagnetic radio waves.
There are many reasons to modulate a signal in a particular way. Amplitude modulation, like that still used by Citizens' Band radios, produces a simple, robust wave that doesn't use much spectrum or radio bandwidth. It's plagued by noise though and requires high transmitting power. Frequency modulation, such as analog cell phones use, provides better sound but it needs more bandwidth to achieve that quality and is technically more complex to produce. And then there are modulation types just for transmitting digital information. GSM and IS-136 use these schemes.
Amplitude modulation means a carrier wave is modulated in proportion to the strength of a signal. The carrier rises and falls instantaneously with each high and low of the conversation. Check out the diagram below. See how the voice current produces an immediate and equivalent change in the carrier.
Low frequency commercial broadcast stations in the "A.M band" use amplitude modulation. Most C.B. or citizens band radios use it too. It's a simple, robust method to form a radio wave but it suffers from static and high battery power requirements, reasons enough that few personal communications devices use it.
Frequency Modulation
Frequency modulation confuses many people but it shouldn't. FM is not limited to the FM band. It is not frequency dependent, that is, it can be used at high or low frequencies. That's because it is a modulation technique, a way to shape a radio wave, not a service by itself. The word frequency in FM relates, instead, to the rate at which this method varies a carrier wave, not to any particular radio frequency it is used on. This will become more clear as we go on.
An FM signal quality is apparent by listening to the FM band: low distortion, little static, good voice quality and immunity from electrical and atmospheric interference. It's why television audio and analog cellular use it. FM also exhibits a capture effect, whereby the receiver seizes on the strongest signal and rejects any others. That's unlike A.M, with signals fading in and out. What's more, F.M. needs far less power to transmit a signal the same distance than A.M.
See the difference in the waveform on the right in the diagram below? You don't have the modulated carrier varying in amplitude, as with A.M., but in the number of cycles or rate. Although perhaps not obvious at first, the right hand side does differ from the left hand side.
This diagram above is courtesy of Douglas-Young's brilliant article on modulation. We have an unvarying carrier wave as we do with A.M. See that? But in F.M. the carrier wave is engineered to deliver a uniform output signal. When we impress upon the carrier a audio signal, such as a 440 hertz dial tone, things begin to happen.
Frequency modulation varies the carrier at a rate of 440 cycles per second, matching the original signal. This differs dramatically from A.M., where a wildly swinging sine wave would be produced instead. In F.M. a quick change in audio frequency results in a quick rate change to the carrier. Despite this seemingly complicated operating method, F.M. circuitry after sixty years is now well established, cheap, simple to make, and easily miniaturized.
The July, 1999 Popular Electronics outlined a simple F.M. transmitter kit. It used only one transistor, eleven other parts, and took up no more than a square inch or two. F.M. is now everywhere, developed largely by one man.
Time Out for History!
- On January 31, 1954 a 64 year old man wrote a letter to his wife, dressed for work, and walked out of his 13th floor apartment window, plunging to his death. Colonel Edwin Armstrong, the father of modern radio, and the creator of the first F.M. system, had committed suicide. A brilliant but sensitive man, Armstrong allowed the U.S. military to use his patents royalty-free for the duration of World War II. Before that he played a crucial role in communications during the First World War. He believed, rightly so, that F.M. was a revolutionary operating system and that it should replace A.M. equipment for broadcasting. Tired and despondent after fighting one lawsuit after another against RCA and others, his personal fortune spent on promoting and defending F.M., Armstrong finally gave up and killed himself. Every modern radio has circuits Armstrong designed. And you thought modulation was boring. . .
Frequency shift keying, an F.M. variation
Conventional cellular makes much use of frequency shift keying modulation to send signaling and control messages. It's old technology, in fact, the earliest modems were built with this technique, but FSK works well for what it does. To explain its title, FSK means sending data by slightly shifting frequencies. Simple. Keying, by the way, simply means forming or creating a signal. When you "key up" a microphone you create a signal. You turn on
Frequency shift keying uses the existing carrier wave, say, 879.990 MHz. The data rides 8kHz above and below that frequency. It's just like the earliest modems. 0s and 1s. 0s go on one frequency and 1s go on another. They alternate back and forth in rapid succession. FSK gives you only two states to send information. There's a low limit, then, how much and how quickly you can send information. There is a more efficient way.
Phase Modulation
Three ways exists to modulate a signal: by amplitude, frequency or phase. And although there are dozens of modulation techniques, under the most confusing names possible, all of them will fit into one of these categories. We've looked at amplitude modulation, which changes the carrier wave by signal strength, and frequency modulation, which converts the originating signal into cycles. Now we look at phase modulation, which changes the angle of the carrier wave. Phase modulation is strictly for digital working and is closely related to F.M. Phase in fact enjoys the same capture effect as F.M. First, a note.
A digital signal means an ongoing stream of bits, 0s and 1s, on and off pulses of electrical energy. Like those signals running around the inside your computer. Well, how do we transmit that staccato beat of electrical pulses? Very good. We put it on a carrier wave.
- A not to scale diagram of a digital signal
You might think you could send digital without a carrier wave, like the earliest wireless telegraphs but your results wouldn't be good. As Dwayne Rosenburgh, N3BJM, puts it, "Transmitting pure radio frequency energy, with no carrier, is like a spark gap transmitter. Very wideband, very inefficient, and with a limited data rate capacity." Ever had an AM radio on nearby by when you switched on a light dimmer? Blast! That's sort of how an old spark gap transmitter worked. Poorly. But new technology is bringing back an old idea.
Dwayne mentions that ultra wideband technology or UWB does now what old spark transmitters couldn't: transmit without a carrier wave. "UWB transmissions can use 'direct modulation' of the rf energy and transmit pulses without carrier modulation. Think of this as a spark gap transmitter that is controlled, with very low power, and spread across about 7 GHz of rf spectrum. Modern processing technology has been able to allow this type of communication using low rf power and efficient digital signal processing algorithms. Therefore, we can reduce the noise and inefficiencies associated with a spark gap transmitter and create UWB transmitters. Dwayne continues, "This ultra wideband technology exception aside, our radio technology is built on carrier waves. No matter how we transmit RF energy, there is always some type of 'carrier' involved."
Ever hear an A.M. radio station go silent for a minute or two? If they are off the air completely you'll hear static. But if they have simply lost audio for a while you'll hear a slight hum. That's the carrier wave.
Let's get back to phase modulation. How does P.M. represent those on and offs?, those 0s and 1s? By playing with angles.
A continuous wave produced to transmit analog or digital information. The many phases or angles of a sine way give rise to different ways of sending information.
Summary
Modulation and Cellular Radio
A.M. or Amplitude Modulation Types
Quadrature amplitude modulation (QAM): Used in Motorola's iDEN system. Some argue that QAM is a hybrid system, not belonging to A.M., but a cross between A.M. and phase modulation.
F.M. or Frequency Modulation Types
Normal F.M.: Used in all analog cellular radio systems, such as AMPS, TAC, ETACS, NMT 900 and so on.
Narrowband F.M: Was used in the now defunct NAMPS cellular system.
Frequency shift keying (FSK): Used in AMPS for control signaling.
Gaussian minimum shift keying (GMSK): Used by GSM systems.
P.M. or Phase Modulation Types
Quadrature phase shift keying (QPSK): IS-95 and the coming Universal Mobile Telephone System.
Differential quadrature phase shift keying (DQPSK): Used by IS-54, the first North American digital cellular system. I'm not sure if it is now incorporated into IS-136.
Pi/4 differential quadrature phase shift keying: IS-136, Japanese Handy Phone and the European TETRA systems.
Extended discussion on F.M. The word frequency in an F.M. discussion confuses many people. That's because this word is used in three separate but related contexts. Here are the subjects; I describe them separately and then discuss them together after that:
1. Frequency and the original audio signal. Usually our voice, this is the message we want to put on a carrier wave. This is what varies the carrier. The audio signal varies in two ways: strength or amplitude and in frequency. Let's concentrate on the audio. Being in the voice band, an audible frequency might range from, say, 300 to 3,000Hz. I used a dial tone at 440Khz as an example before. For our discussion, let's call this signal the voice frequency.
2. Frequency and the the carrier wave. Easily understood. A radio frequency. For an example, let us use 94.5MHz, something in the F.M. band. A carrier wave stays roughly on the same frequency, give or take, no matter if it is modulated by amplitude, frequency, or phase. This is the radio frequency.
3. Frequency and Frequency Modulation. Whereby our signal is put on or merged with the carrier wave. This is the modulation technique.
Okay, let's take a slight time-out. Look again at our friendly F.M. waveform diagram below. See the channel width an F.M. signal occupies? You have a median point, say 880 Mhz, represented here by 0. With an analog FM cellular signal the radio channel is 30Khz wide. That allows 15Khz of room or deviation above and below its assigned frequency. Following this so far?
"The amplitude (volume) of the input or audio signal (Beethoven's 5th, or whatever) produces the AMOUNT of deviation. More volume, more deviation from the original carrier frequency. The FREQUENCY of the input signal is contained in the RATE of deviation. The faster the signal deviates in frequency, the faster audio is output from the FM detector. Hmm. That makes sense - faster = frequency. Amount = amplitude."
Did you get that explanation from Mark van der Hoek? We have two steps here, amount of deviation and rate of deviation. Let's first discuss amount or amplitude or strength. Whatever.
1) Amount of deviation: The diagram doesn't portray signal strength and FM very well. At first glance the output signal looks uniform. Which it is. But we know that no conversation is of uniform strength. What happens, and I know this is difficult to understand, the carrier frequency moves slightly up up or down to reflect the audio signal strength. That is, the carrier frequency itself deviates slightly from the median line or 0 when modulated. So the radio channel width and the output signal remain uniform, it's just that the carrier frequency deviates within the channel assigned it. Got it?
2) Rate of deviation. Back to simple stuff. This is as depicted above, with the carrier modulated by the audio signal at a rate in lock step to the frequency of the original signal. A higher audio frequency? A quicker rate. A lower frequency signal? A lower rate.
As Mark says in summing all this up, "With F.M. the radio frequency does change, slightly, within a window defined by the information that is pressed onto the carrier wave. Both the amount of deviation and the rate of deviation carry information. The original audio signal varies both in amplitude and in frequency. The rate of deviation in the FM signal carries the frequency info, and the amount of deviation carries the amplitude of the original audio signal." Mark van der Hoek.
Monday, January 2, 2012
Types of Information System
For most businesses, there are a variety of requirements for information. Senior managers need information to help with their business planning. Middle management need more detailed information to help them monitor and control business activities. Employees with operational roles need information to help them carry out their duties.
As a result, businesses tend to have several "information systems" operating at the same time. This revision note highlights the main categories of information system and provides some examples to help you distinguish between them.
The main kinds of information systems in business are described briefly below:
| Information System | Description |
| Executive Support Systems | An Executive Support System ("ESS") is designed to help senior management make strategic decisions. It gathers, analyses and summarises the key internal and external information used in the business. A good way to think about an ESS is to imagine the senior management team in an aircraft cockpit - with the instrument panel showing them the status of all the key business activities. ESS typically involve lots of data analysis and modelling tools such as "what-if" analysis to help strategic decision-making. |
| Management Information Systems | A management information system ("MIS") is mainly concerned with internal sources of information. MIS usually take data from the transaction processing systems (see below) and summarise it into a series of management reports. MIS reports tend to be used by middle management and operational supervisors. |
| Decision-Support Systems | Decision-support systems ("DSS") are specifically designed to help management make decisions in situations where there is uncertainty about the possible outcomes of those decisions. DSS comprise tools and techniques to help gather relevant information and analyse the options and alternatives. DSS often involves use of complex spreadsheet and databases to create "what-if" models. |
| Knowledge Management Systems | Knowledge Management Systems ("KMS") exist to help businesses create and share information. These are typically used in a business where employees create new knowledge and expertise - which can then be shared by other people in the organisation to create further commercial opportunities. Good examples include firms of lawyers, accountants and management consultants. KMS are built around systems which allow efficient categorisation and distribution of knowledge. For example, the knowledge itself might be contained in word processing documents, spreadsheets, PowerPoint presentations. internet pages or whatever. To share the knowledge, a KMS would use group collaboration systems such as an intranet. |
| Transaction Processing Systems | As the name implies, Transaction Processing Systems ("TPS") are designed to process routine transactions efficiently and accurately. A business will have several (sometimes many) TPS; for example: - Billing systems to send invoices to customers |
| Office Automation Systems | Office Automation Systems are systems that try to improve the productivity of employees who need to process data and information. Perhaps the best example is the wide range of software systems that exist to improve the productivity of employees working in an office (e.g. Microsoft Office XP) or systems that allow employees to work from home or whilst on the move. |
source: http://tutor2u.net
Methods of Data Collection
Source : http://tutor2u.net
Collecting data can be a time-consuming, labour intensive process. So businesses are constantly looking for ways in which data capture and analysis can be automated. However, manual data collection is still common for many business processes. This revision note summarises the main kinds of data collection you need to be aware of.
The table below summarizes the main methods of data collection
| Method | Commentary |
| Manual Input Methods | |
| Keyboard | A very familiar input device. Typically used to input data into personal computer applications such as databases and spreadsheets |
| Touch-sensitive screens | Developed to allow computer monitors to be used as an input device. Selections are made by users touching areas of a screen. Sensors, built into the screen surround, detect what has been touched. These screens are increasingly used to help external customers input transactional data - e.g. buying transport tickets, paying for car parking or requesting information |
| Automated Input Methods | |
| Magnetic ink character recognition (MICR) | MICR involves the recognition by a mchine of specially-formatted characters printed in magnetic ink. This is an expensive method to set up and use - but it is accurate and fast. A good example is the use of magnetic ink characters on the bottom of each cheque in a cheque book |
| Optical mark reading (OMR) | Optical Mark Reading (OMR) uses paper based forms which users simply mark (using a dash) to answer a question. OMR needs no special equipment to mark a form other than a pen/pencil. Data can be processed very quickly and with very low error rates. An OMR scanner then processes the forms directly into the required database. An example you are probably familiar with is the National Lottery entry forms, or answer sheets for those dreaded multiple choice exam papers! |
| Optical character recognition (OCR) and scanners | OCR is the recognition of printed or written characters by software that processes information obtained by a scanner. Each page of text is converted to a digital using a scanner and OCR is then applied to this image to produce a text file. This involves complex image processing algorithms and rarely achieves 100% accuracy so manual proof reading is recommended. |
| Intelligent Character Recognition (ICR) | Intelligent Character Recognition (ICR) again uses paper based forms which respondees can enter handprinted text such as names, dates etc. as well as dash marks with no special equipment needed other than a pen/pencil. An ICR scanner then processes the forms, which are then verified and stored the required database. |
| Bar coding and EPOS | A very important kind of data collection method - in widespread use. Bar codes are made up of rectangular bars and spaces in varying widths. Read optically, these enable computer software to identify products and items automatically. Numbers or letters are represented by the width and position of each code's bars and spaces, forming a unique 'tag'. Bar codes are printed on individual labels, packaging or documents. When the coded item is handled, the bar code is scanned and the information gained is fed into a computer. Codes are also often used to track and count items. Businesses of all types and sizes use bar code systems. Best known are retailers using Electronic Point of Sale (EPOS) technology, familiar in supermarkets and many retail operations. Not only saving time at checkout, EPOS cuts management costs by providing an automatic record of what is selling and stock requirements. Customers receive an accurate record of prices and items purchased. Producers use bar coding for quick and accurate stock control, linking easily to customers. Distributors use bar codes as a crucial part of handling goods. Larger businesses and those with high security requirements can use bar codes for personnel identification and access records for sensitive areas. |
| EFTPOS | EFTPOS stands for Electronic Funds Transfer at Point Of Sale. You will find EFTPOS terminals at the till in certain shops. An EFTPOS terminal electronically prints out details of a plastic card transaction. The computer in the terminal gets authorisation for the payment amount (to make sure it's within the credit limit) and checks the card against a list of lost and stolen cards. |
| Magnetic stripe cards | A card (plastic or paper) with a magnetic strip of recording material on which the magnetic tracks of an identification card are recorded. Magnetic stripe cards are in widespread use as a way of controlling access (e.g. swipe cards for doors, ticket barriers) and confirming identity (e.g. use in bank and cash cards). |
| Smart cards | A smart card (sometime also called a "chip card") is a plastic card with an embedded microchip. it is widely expected that smart crads will eventually replace magnetic stripe cards in many applications. The smart chip provides significantly more memory than the magnetic stripe. The chip is also capable of processing information. The added memory and processing capabilities are what enable a smart card to offer more services and increased security. Some smart cards can also run multiple applications on one card, this reducing the number of cards required by any one person. One of the key functions of the smart card is its ability to act as a stored value card, such as Mondex and Visa cash. This enables the card to be used as electronic cash. Smart cards can also allow secure information storage, making them ideal as ID cards and security keys. |
| Voice recognition | A data collection technology that converts speech into text or interprets it as a sequence of computer commands. Voice recognition is most common in data entry and word processing environments, and fields where a user needs to interact with a computer without using their hands. |
| Web Data Capture | Web data capture use electronic forms on either on an Intranet or Internet. They are becoming increasingly popular and have the advantage of being accessible by any user having access to a computer. Users complete the questions online and the returned data is then imported in electronic format to the required database. |
Sunday, January 1, 2012
CDMA(EDVO) vs EDGE of GSM technology
In the past year, the US cell phone companies have been building out their wireless high-speed networks. The theory is these networks will keep us connected even when wireless hotspots aren't available by using your cell phone or a PC card. This is a great convenience for mobile professionals who need to access work files or the web. The carriers also have new services such as streaming videos that take advantage of the faster speeds.
These wireless broadband networks have speeds that are many times faster than typical cell phone connections, but the throughput rates can differ dramatically. One difference is the protocol your cell carrier uses. In the US, the predominant standards are CDMA and GSM. Each of these standards has their own high-speed protocol. In some regards, this is like DSL and cable access. Both offer high-speed access at different rates and each requires their own equipment.
As you may have discovered when trying to switch cell phone carriers, sometimes you need a new phone. Although your carrier may have this high-speed option, it doesn't mean it works on your current cell phone. Unlike your notebook, you can't buy hardware to add to your cell phone. Instead, you need to have a phone with the required hardware. The good news is even though the phones are more expensive, the carriers usually subsidize your purchase if you're a new customer. After all, they anticipate a nice revenue stream from data subscription plans. Sorry, but speed comes with a cost.
EVDO (CDMA)
In the CDMA world, the high-speed wireless protocol is called EvDO. Like many abbreviations, this one has two translations: Evolution Data Optimized and Evolution Data Only. The main national carriers include Verizon and Sprint although there are some regional ones.
Using our previous DSL and cable reference, you could say that EVDO is more like cable since it is the faster of the two protocols. While I've seen references that mention burst speeds up to 2Mbps, the typical download speed is often cited in the 300-700 kbps range. The upload speed, which is usually lower on most systems, is about 50-100kbps.
As with many new technologies, they don't arrive at everyone's doorstep at the same time. While Verizon and Sprint have been building their networks, EvDO coverage isn't everywhere. Typically, the carriers do a staged rollout that involves a testing and tweaking period.
One benefit to carriers bringing more consumers online is the phone manufacturers begin to offer more EvDO phones. The pickings are small, but growing. You'll also find that most carrier websites don't make it easy to find these phones. I still find it easier to search eBay for cell phones for specific features such as EvDO.
If you already have an EvDo-capable phone, you may need to do a PRL update to see the signals. If the phones don't detect EvDO, they are designed to work on the slower networks. As the carriers expand their networks, you should consider updating the PRL every couple of months to make sure you have the new cell tower locations.
Update: Feb 10,2007: Both Sprint and Verizon have been deploying EVDO Revision A or Rev A in various cities. This newer technology is roughly 5 to 6 times faster than EVDO.
EDGE (GSM)
Not to be overlooked, the GSM world has its own protocol and acronym EDGE. EDGE stands for Enhanced Data Rates for Global Evolution. Although it is much slower than EvDO, it does offer some advantages. EDGE coverage may be greater although it is difficult to tell from the vendor maps. The main EDGE carriers include Cingular and T-Mobile.
Another advantage is there are more EDGE compatible phones so in theory it's easier to find a phone with the features you like. As with the CDMA carriers, we had a hard time searching for phones by this feature. In some cases, the carrier's phone detail page failed to mention EDGE. Some cell phone manufacturers such as Nokia have provided their own page.
As with many network connections, it's difficult to tell if you'll get the maximum throughput of 384k for downloads. Carrier spec sheets usually cite 70-135kbps with bursts to 200kbps.
If you've been browsing web pages on a cell phone with a 14.4 data connection, you'll be pleased with either EDGE or EvDO. You'll find your download times are considerably less. You can get a useful traffic report while you're stuck in the jamsource:http://www.timeatlas.com