| Analog and Digital Comparison | NTC/362 Fundamentals in Networking | EDWARD BROWN | Jose Giralt | 3/18/2013 |

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A signal is simply the transmission of data from one place to another place. In our day to day life we deal with various signals constantly like signals from music, power lines, telephones, and cellular devices. Analog and digital are two kinds of signals which are used for the transmission of information from source to destination. Usually the information to be transmitted from one place to another is either audio or video. This information signal is then transformed into those signals which can be transmitted via different channels.
For the analog format, the data is transformed into electrical pulses with varying amplitude while for the digital format; the data is transformed into binary format representing two amplitudes. We have various such equipment like analog or digital phones, fax machines, modems, clocks, watches etc. Analog technology is the older one and has been used for decades. It is cheap too but the problem with analog signals is that there is a limitation on the size of the data that can be transmitted at any given point of time.
With the advent of digital technology many improvements and new techniques have been introduced. Now days almost every appliance or equipment is based on digital technology. In this, the transmitter translate the data into binary form and the receiver re assemble and produces the original information signal. Digital signal as compared to analog gives us more options as it can be easily computed and manipulated by software. Also digital signals are frequently used in telecommunication. Instead of having various advantages over analog signal, digital signal lag behind in quality. Since in digital devices there is translating and reassembling of data because of this reason its quality is not that good. But with new technologies we can possibly remove errors and noise in the digital signal. But digital is quite expensive as compared to analog and working is going on globally to reduce its price.
Digital to Analog conversions involve a conversion process with DAC. This devise takes the binary code converting it to the analog signal. Digital to analog signals can compromise the quality of the signal, if it is not actively monitored. Analog lines, which are also known as POTS lines usually support standard phone lines, fax machines or alarm lines, keeping them separate from the digital PBX equipment. An example of digital to an analog signal would be the use of a modem for Internet use that converts computer digital signals using POTS lines.

Advantages and Disadvantages of Common Modulation Techniques

Amplitude Modulation

Analog signals have four modulation techniques. The most simple of the four is Amplitude Modulation (AM). AM transmits data by varying the intensity of the waves. The most common use for AM is transmitting radio stations. AM is very simple by design that makes it the least costly method of transmitting data. The main disadvantage with AM is how easily the transmission can contain interference. AM signals also use more energy than other types of modulation methods.

Frequency Modulation

Frequency Modulation (FM) is also most commonly used in broadcasting radio stations. FM transmits data by using multiple frequencies of the signal as opposed to the varying the intensity wave. Using multiple frequencies makes FM transmissions less susceptible to line interference. Disadvantages of FM are the amount of bandwidth required. It is much higher than that of other methods. ANALOG SIGNALS
Analog signals are continuous electrical signals that vary with time as shown in figure. In other words an Analog signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e. analogous to another time varying signal. It produces small fluctuations in the signal which are very useful and this lacks in digital signals. The resolution of analog signals is infinite. In real world, an analog signal is subject to noise and a finite slew rate. Therefore, both analog and digital systems are subject to limitations in resolution and bandwidth. As analog systems become more complex, effects such as non-linearity and noise ultimately degrade analog resolution to such an extent that the performance of digital systems may surpass it.
Telephone voice signal is analog. The variations in the analog signal are due to changing intensity in voice. At the receiving end, the signal is reproduced in the same proportion. Advantages of analog signals
The main advantage of analog signals is their fine definition which has the potential for an infinite amount of signal resolution. In comparison to digital signals, analog signals have higher density. Analog signals are easier and simpler to process as compared to complex digital signals. An analog signal can be processed directly by analog components, though some processes aren't available except in digital form. Analog signals are relatively easy to create and carry from place to place. Disadvantages of analog signals
The major disadvantage of analog signaling is that any system has noise, any random unwanted variation. As the signal is copied and re-copied, or transmitted over long distances, these random variations interfere with our signal and distort it. Electrically, these losses due to interference of noise can be diminished by shielding, good connections, and several cable types such as coaxial or twisted pair.
In the figure the blue lines represents the voltage capacity of signal to travel clearly and for this it must lie between minimum value of X and maximum value of Y.
Here the noise had distorted our signal. Noise is unwanted variations in the signal which deteriorates the quality of signal. The signal had crossed the limits of X and Y and lost its authentication. Noise is sometimes called "distortion" or "clipping". We can say that Noise creates loss in authentic information and produces distortion. If the pattern of our original signal is slightly altered by unwanted noise or distortion, the output will not be identical to the input. This is impossible to recover, because when we amplify the signal to recover attenuated parts of the signal amplifies the noise (distortion/interference) as well. Although the resolution of analog signals is higher than digital signals but the difference can be overshadowed by the noise in the signal. DIGITAL SIGNALS
Digital signals are the one which are non-continuous, change in individual steps. They consist of pulses or digits with discrete levels or values. The value of each pulse is constant, but there is an abrupt change from one digit to the next. Digital signals have two amplitude levels called nodes. The value of which are specified as one of two possibilities such as 1 or 0, HIGH or LOW, TRUE or FALSE and so on. In reality, the values are anywhere within specific ranges and we define values within a given range. In the Digital Revolution, the usage of digital signals has increased significantly. Many modern media devices, especially the ones that connect with computers use digital signals to represent signals that were traditionally represented as continuous-time signals; cell phones, music and video players, personal video recorders, and digital cameras are examples. In most applications, digital signals are represented as binary numbers, so their precision of quantization is measured in bits. Suppose, for example, that we wish to measure a signal to two significant decimal digits. Since seven bits, or binary digits, can record 128 discrete values , those seven bits are more than sufficient to express a range of one hundred values.
A. Transmission of digital signals
There are two ways to transmit digital data between one or several devices or communication participants, either parallel or serial transmission. Bit-parallel transmission:
In bit parallel transmission all the bits of information signal are transmitted at once at the same time. Bit-serial transmission
Serial transmission is a good for long distances. In this case, only one signal line transmits the bits one after the other. As a result, the transmission of information takes more time, which is nevertheless acceptable because, on the other hand, the installation effort and the costs are considerably reduced. Advantages of digital signal over analog signal
Digital signals consist of patterns of bits of information. These patterns can be generated in many ways, each producing a specific code. Modem digital computers store and process all kinds of information as binary patterns. All the pictures, text, sound and video stored in this computer are held and manipulated as patterns of binary values. Here the signal acquire two basic forms i.e. ON (high or 1) and OFF (low or 0). If we compare with analog these digital signals are more uniform. Here, we see the main advantage of digital over analog. Since the signal is very uniform, noise has not severely altered its shape or amplitude. The digital signal shows a far less change to the actual waveform than the previous analog signal. They are both shown below for a close comparison. We can say that main advantage of digital signals over analog signals is that the precise signal level of the digital signal is not vital. This means that digital signals are fairly immune to the imperfections of real electronic systems which tend to spoil analog signals. As a result, digital CD's are much more robust than analog LP's.
Codes are often used in the transmission of information. These codes can be used either as a means of keeping the information secret or as a means of breaking the information into pieces that are manageable by the technology used to transmit the code.
Thus we can say that though digital technology is expensive as compared to that of analog but because of various advantages and more options it has made analog technology redundant.
Disadvantage of digital signals
The one main drawback of digital communication is that they require greater bandwidth as compared to analog communication for the transmission of same signal.
Quadrature Amplitude Modulation (QAM)
Quadrature Amplitude Modulation or QAM is a form of modulation which is widely used for modulating data signals onto a carrier used for radio communications. It is widely used because it offers advantages over other forms of data modulation such as PSK, although many forms of data modulation operate alongside each other. Quadrature amplitude modulation, QAM may exist in what may be termed either analogue or digital formats. The analogue versions of QAM are typically used to allow multiple analogue signals to be carried on a single carrier. For example it is used in PAL and NTSC television systems. Digital formats of QAM are often referred to as "Quantized QAM" and they are being increasingly used for data communications often within radio communications systems. Radio communications systems ranging from cellular technology through wireless systems including WiMAX, and Wi-Fi 802.11 use a variety of forms of QAM
ADSL
ADSL is an asymmetrical service deployed over one twisted-pair. With ADSL, the majority of bandwidth is devoted to the downstream direction, from the network to the user, with a small return path that is generally sufficient to enable telephony or simple commands. ADSL is limited to a distance of about 3.5 miles (5.5 km) from the exchange point. With ADSL, the greater the distance, the lower the data rate; the shorter the distance, the better the throughput. New developments allow the distance to be extended because remote terminals can be placed closer to the customer. There are two main ADSL standards: ADSL and ADSL2. The vast majority of the ADSL that is currently deployed and available is ADSL. ADSL supports up to 7Mbps downstream and up to 800Kbps upstream. This type of bandwidth is sufficient to provide good Web surfing, to carry a low grade of entertainment video, and to conduct upstream activities that don’t command a great deal of bandwidth. However, ADSL is not sufficient for things such as digital TV or interactive services. For these activities, ADSL2, which was ratified in 2002, is preferred. ADSL2 supports up to 8Mbps downstream and up to 1Mbps upstream. An additional enhancement, known as ADSL2+, can support up to 24Mbps downstream and up to 1Mbps upstream.
Wi-Fi
Wi-Fi (also spelled Wifi or WiFi) is a popular technology that allows an electronic device to exchange data wirelessly (using radio waves) over a computer network, including high-speed Internet connections. The Wi-Fi Alliance defines Wi-Fi as any "wireless local area network (WLAN) products that are based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards". However, since most modern WLANs are based on these standards, the term "Wi-Fi" is used in general English as a synonym for "WLAN".
The Synchronous Optical Network (SONET)
The Synchronous Optical Network (SONET) is a standard for optical transport that was formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI) which sets standards for telecommunications and other industries in the United States. The standard was originally initiated by Bellcore labs in 1984 on behalf of the Regional Bell Operating Companies (RBOC’s) for the following key purposes: The compatibility of equipment by all vendors who manufacture to a certain standard.
Synchronous networking
Enhanced operations, administration, maintenance and provisioning (OAM&P)
Efficient add/drop multiplexing (ADM)
Standards-based survival rings
Transport of new services, such as Asynchronous Transfer Mode (ATM)
North American Digital hierarchy (T x)
The North American Digital hierarchy starts off with a basic Digital Signal level of 64 KBPS (DS0). Thereafter, all facility types are usually referred to as "T x", where "x" is the Digital Signal level within the hierarchy. Up to the DS3 rate, these signals are usually delivered from the provider on Twisted-Pair or Coaxial cables. North American T1 service providers often refer to the signal interface between the User and the Network as "DS-1" signals. In the case of User to User interfaces, the term "DSX-1" is used to describe those DS1 signals at the "cross-connect" point. CONCLUSION
In conclusion, the benefits of using a digital system over analog is clear. Digital signals are easier to transmit and offer less room for errors to occur, vary less distortion in our original signal. This leads to accurate data transmission that in turn leads to faster transmission rates and better productivity.
REFRENCES
Telecommunications Essentials. The Complete Global Source, Second Edition Lillian Goleniewski , Kitty Wilson Jarrett http://en.wikipedia.org/wiki/Analog_signal http://en.wikipedia.org/wiki/Digital
http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-qam-quadrature-amplitude-modulation-tutorial.php