The man in his eagerness to communicate with each other, this has led him to seek various ways. Starting from the signs or gestures of prehistoric men to obtain a spoken language. Later, he needed to be able to expand his communication in his geographical environment using smoke signals, flashes of mirrors, flag signals, all with a common purpose and that covered and still cover the needs of the time. With the passage of time and technological advance came innovations, the Morse CODE, through a telegraph by means of cables, allowed communications over long distances and in seconds.
Later some way was devised to be able to transmit the voice over great distances and thus the telephone was born. Later radio communications appeared, then the transmission of images through television.
In the 60s, the computer age began and with it the idea of interconnecting several computers with each other, which became a reality in the 70s. In this last point, the main emphasis will be placed on communication between computers, their interfaces, techniques and means.
Objectives and main parts
The main objective of communications is to exchange information between two entities.
The source: this is the device that generates the data to be transmitted.
The transmitter: normally the generated data is not transmitted as it is generated. The transmitter transforms and encodes the information producing electromagnetic signals to be transmitted through some transmission system.
The transmission system: which can be a cable from a simple transmission line to a complex network that connects the source with the destination.
The receiver: which accepts the signal from the transmission system and converts it in such a way that it can be handled by the destination device
The destination: which takes the data from the receiver.
Data transmission
The transmission of data between a sender and a receiver is always carried out through a transmission medium. They can be classified as guided and unguided. In both cases, communication is done with electromagnetic waves. In guided media, such as twisted pairs, coaxial cables, and optical fibers, waves are transmitted by confining them along the physical path. In contrast, unguided means provide a way to transmit electromagnetic waves but without channeling them, such as propagation through air, sea, or vacuum.
Analog and Digital Signals
Consideration must be given to the nature of the data, how it is physically propagated, and what processing or adjustments are needed along the way to ensure that the data received is intelligible. For all of these considerations, the crucial point is whether they are digital or analog entities. In communications, these two terms are frequently used to characterize the following three concepts:
Data: entity that carries information. The signals are electrical or electromagnetic encodings
Signaling: it is the act of propagating the signal through a suitable medium
Transmission: is the communication of data, through the propagation and processing of signals.
Data
Analog data can take values in a certain continuous interval.
Digital data takes discrete values.
An analog data can be the voice, which is a value of intensity that varies continuously. And a digital data would be a text.
We take the voice as an example of analog data. In the acoustic spectrum of the human voice, components in frequency between 20 Hz and 20 KHz can be found. Although most of the energy of the voice is found in the low frequencies, experiments have shown that frequencies below 600-700 Hz contribute little to the intelligibility of the voice in the human ear.
The most typical example of digital data is text strings or characters. While texts are more suitable for humans, they generally cannot be easily transmitted or stored (in character form) in processing or communication systems. Such systems are designed for binary data. This is why a good number of codes have been designed by which the characters are represented by bit sequence. Perhaps the first best-known example is Morse code and is currently ASCII code.
Signals
In a communications system, data is propagated from one point to another using electrical signals. An analog signal is an electromagnetic wave that varies continuously. A digital signal is a sequence of voltage pulses that can be transmitted through a cable; For example, a constant positive voltage level can represent a binary 1 and a constant negative voltage level can represent 0.
Examples
In the case of voice, data can be directly represented by an electromagnetic signal that occupies the same spectrum.
Transmission
Both analog and digital signals can be transmitted through suitable transmission media.
The pasticualities of how these signals are treated will depend on the specific transmission system. The following table specifies these details in greater detail:
Data and signals
| Analog data | Digital data | |
| Analog signal | There are two alternatives: (1) the signal occupies the same spectrum as the analog data; (2) analog data is encoded occupying a different portion of the spectrum. | Analog data is encoded using a codec to generate a bit string |
| Digital signal | Digital data is encoded using a modem to generate an analog signal | There are two alternatives: (1) the signal consists of two voltage levels representing two biannual values (2) the digital data is encoded to produce a digital signal with the desired properties |
Signals processing
| Analog transmission | Digital broadcast | |
| Analog signal | It spreads through amplifiers; it is treated equally if the signal is used to represent analog or digital data | The analog signal is supposed to represent digital data. The signal propagates through repeaters; In each repeater, two digital data are obtained from the input signal and used to regenerate a new analog output signal. |
| Digital signal | Not used | The digital signal represents a string of ones or zeros, which can represent digital data or can be the result of encoding analog data. The signal propagates through |
Choosing the best transmission method:
Both analog and digital signals can be transmitted through suitable transmission media. Analog transmission is a way of transmitting analog signals regardless of their content; The signals can represent analog data (for example, voice) or digital data (for example, binary data that passes through a modem). In all cases, the signal will weaken with distance and the use of amplifiers will be necessary to achieve greater distances.
In analogue transmission, when cascade amplifiers are used, the signal is increasingly distorted. For analog data certain small distortions may be allowed as the data remains intelligible. The opposite occurs for digital transmission, since the amplifiers would introduce noise and these would become errors.
In digital transmission, on the contrary, it is dependent on the content of the signal. A digital signal can only be transmitted over a limited distance, since attenuation and other negatives can introduce errors into the transmitted data. In this case, repeaters are used, which regenerate the pattern of zeros and ones and retransmit it.
The current choice is for digital technology as the most reliable transmission medium, contrary to the various investments made in analog communication. Gradually the former is taking over users and companies. Let's give some reasons why this trend to digital technology.
- Digital Technology: Improvements in Large Scale Integration (LSI) and Very Large Scale (VLSI) technologies have been a decrease in both size and cost of digital processor techniques. On the contrary, analog technology has not undergone a similar change. Data integrity: When repeaters are used instead of amplifiers, noise and other negative effects are not cumulative. This implies that using digital technology it is possible to transmit data while preserving its integrity over longer distances using even lower quality lines. Capacity utilization:the laying of broadband transmission lines has become feasible for media, such as via satellite and fiber optics. For the efficient use of all this bandwidth, a high degree of multiplexing is required. This is done more easily and at less cost with digital techniques (time division) than with analog techniques (frequency division). Security and privacy: encryption techniques can be applied to digital or analog data that has previously been digitized. Integration: With analog and digital data processing, all signals can be treated in a similar way. Thus allowing the integration of voice, video and data using the same infrastructure.
Transmission disturbances
In any communication system it must be accepted that the received signal will differ from the transmitted signal due to various adversities and mishaps of the transmission.
The most significant disturbances are:
- Attenuation and Attenuation Distortion Delay Distortion Noise
Attenuation
The signal energy decays with distance in any transmission medium. Three considerations can be made regarding attenuation.
First, the received signal must have enough energy for the electronic circuitry in the receiver to detect and interpret the signal properly. Second, to be received without error, the signal must retain a level sufficiently higher than the noise. Third, attenuation is an increasing function of frequency.
In summary; The first two problems are solved by controlling the signal energy, repeaters or amplifiers are used for them.
The third problem is especially relevant in the case of analog signals, since the attenuation varies as a function of frequency, the received signal is distorted, thus reducing intelligibility.
Delay distortion
This is a phenomenon particular to guided media. It is caused by the fact that the speed of propagation of the signal in the medium varies with frequency.
For a band-limited signal, the speed tends to be higher near the center frequency and decrease as you approach the ends of the band. This is called delay distortion, since the received signal is distorted due to the variable delay suffered by its components.
Noise
In any transmitted data, the received signal will consist of the transmitted signal modified by the distortions introduced by the transmission system, in addition to the unwanted signals that are inserted between the emitter and the receiver. The latter are called noise
The noise can be classified according to its origin in:
- Thermal noise:
It is due to thermal agitation of the electrons inside the conductor. It is `present in all electronic devices and transmission media, as its name suggests it is a function at temperature.
- Intermodulation noise:
When the signals of different frequencies share the same transmission medium, an intermodulation noise can be produced. This is to generate signals at frequencies that are sum or difference of the two original frequencies or multiples of these. This could be due to malfunctioning of the systems or the use of excessive signal energy.
- Crosstalk:
it is an unwanted coupling between the lines that carry the signals. This can occur due to electrical coupling between close pair cables. Crosstalk is the same or the same order of magnitude as thermal noise
- Impulsive noise:
the other previous noises are predictable and of constant magnitude, on the contrary the impulsive noise is not continuous and is constituted by pulses or irregular peaks of short duration and of relatively large amplitude. They can be generated by a variety of causes, such as external electromagnetic disturbances caused by atmospheric storms, or failures and defects in communication systems. Impulsive noise is one of the main causes of data loss in digital communication.
- Channel capacity:
A wide variety of negative effects were seen that distort or corrupt the signal. Only for digital data the question that arises is to what extent these defects affect the speed at which it can be transmitted. That is why the channel capacity is discussed here, which is the rate at which data can be transmitted on a data communication channel or path.
There are four main concepts related to channel capacity, which are
- The speed of the data; which is the speed expressed in bits per second (bps), at which the data can be transmitted. Bandwidth; is the bandwidth of the transmitted signal that will be limited by the transmitter and by the nature of the transmission medium; is measured in cycles per second or hertz noise; the average noise level through the transmission path The error rate; is the reason for errors, where an error is considered when a 1 is received having transmitted a 0 and vice versa.
Transmission media
Here are the 3 most commonly used guided media and an introduction to what is the unguided medium.
The transmission medium is the physical path between the transmitter and the receiver. They are classified as guided and unguided. In both cases, transmission is carried out with electromagnetic waves. In guided media the waves are confined to a solid medium, such as a twisted pair. The atmosphere or outer space are examples of unguided media, which provide a means of transmitting signals but without confining them; This type of transmission is called wireless.
The characteristics and quality of the transmission are determined both by the type of signal and by the characteristics of the medium. In the case of guided media, the media is most important in determining transmission limitations.
In unguided media, the bandwidth of the signal emitted by the antenna is more important than the medium itself in determining transmission characteristics.
Guided transmission media
In guided transmission media, transmission capacity, in terms of transmission speed or bandwidth, depends dramatically on distance and whether the medium is used for a point-to-point link or for a multipoint link, such as a LAN.
The three most commonly used guided media for data transmission are twisted pair, coaxial cable, and fiber optics.
Twisted pair
- Physical description:
The twisted pair consists of two copper cables embedded in an insulator crossed in a spiral shape. Each pair of cables constitutes only one communication link. Typically, bundles are used in which multiple pairs are encapsulated by a protective wrap. The use of braiding tends to reduce electromagnetic interference (crosstalk) between adjacent pairs within the same envelope.
- Applications:
For both digital and analog signals, twisted pair is currently the most widely used. In digital applications the braided APR is the most widely used, especially for connections to a digital switch, also for the connection of local area networks within buildings. The typical speed is 10Mbps. Although currently that speed is already exceeded widely.
- Transmission characteristics:
Twisted pair cables can be used to transmit both analog and digital signals. For analog signals amplifiers are needed every 5 or 6 km and for digital signals it is every 2 or 3 km.
Twisted pair have a strong attenuation dependence on frequency. Its main characteristics are its great susceptibility to interference and noise.
Variants
Unshielded and shielded twisted pairs
The twisted pair has two variants: the shielded and the unshielded.
Unshielded twisted pair (UTP) is the common means of telephony. Yel unshielded (STP "Shielded Twisted Pair") is the one used for computer network connections
UTP type 3 and 5
In 1991, the EIA (“Electronic Industries Association”) published the EIA-568 standard called “Commercial Building Telecommunications Cabling Standard”, which defines the use of unscreened twisted pairs of telephone quality and shielded pairs as means for transmission applications of data in buildings.
Three types of UTP cables are considered in the EIA-568-A standard.
- Type 3: Consists of cables and associated hardware, designed for frequencies up to 16 MHz Type 4: Consists of cables and associated hardware, designed for frequencies up to 20 MHz Type 5: Consists of cables and associated hardware, designed for frequencies up to 100 MHz..
Coaxial cable
Physical description
Like twisted pair, it has two conductors but is constructed differently so that it can operate over a wider range of frequencies. It consists of an outer cylindrical conductor that surrounds a conductor wire. The inner conductor is maintained along the axial axis by a series of regularly spaced insulating rings with a solid dielectric material. The outer conductor is covered with a protective cover or sleeve.
Applications
Due to its versatility, its most important applications are:
- TV distribution Long distance telephony Connection with peripherals at short distance Local area networks
- Transmission characteristics
Fiber optic
Physical description
Fiber optics is a flexible and extremely fine medium capable of conducting energy of an optical nature.
A fiber optic cable is cylindrical in shape and consists of three concentric sections: the core, the sheath, and the sheath. The core is the innermost section, and is made up of one or more glass or plastic fibers. Each fiber is surrounded by its own cladding, which is nothing but its other crystal with different optical properties than the core.. The outermost layer that surrounds one or more coatings is the roof.
Applications
The differential characteristics of optical fiber compared to coaxial cable and twisted pair.
- Greater bandwidth: The bandwidth, and therefore the transmission speed, in the fibers is enormous. Experiments have shown that transmission rates of 2 Gbps can be achieved for tens of kilometers away. Smaller size and weight: they are appreciably thinner than coaxial cable or embedded twisted pairs. The reduction in size leads in turn to a reduction in weight that reduces the infrastructure. Less attenuation: it is significantly less in optical fibers than in coaxial cables and twisted pairs, and it is also constant in a wide frequency range. Greater separation between repeaters: the fewer repeaters there are, the lower the cost and in turn the fewer sources of error.
Characteristic of transmission
Light from the source enters the core. Lightning strikes at surface angles are reflected and spread within the core of the fiber, while for other angles, lightning strikes are absorbed by the coating material. There are two types of transmission: multimode and singlemode.
Wireless transmission
In unguided media, both transmission and reception is carried out using antennas. There are basically two types of settings for wireless transmissions: directional and omnidirectional. In the first, the transmitting antenna emits electromagnetic energy concentrating them in a beam; therefore the transmitting and receiving antenna must be perfectly aligned. In the omnidirectional case, the antenna radiation pattern is scattered, emitting in all directions, and the signal can be received by several antennas.
Physical interface
For two devices connected by a transmission medium to exchange data, a high degree of cooperation is necessary. Typically, the data is transmitted bit by bit through the medium; the timing of these bits must be common between the receiver and the sender. There are two common techniques for timing control: asynchronous and synchronous transmission.
Synchronous and asynchronous transmission
Asynchronous transmission
It consists of avoiding the problem of timing by sending uninterrupted strings of bits that are not very long. Instead the data is transmitted by sending it character by character, where each character is 5-8 bits long. The timing or synchronization must be maintained during character emission, since the receiver has the opportunity to resynchronize at the beginning of each character.
The beginning of each character is indicated by a start bit that corresponds to the binary value 0. The character is then transmitted, starting with the least significant bit, which will have from five to eight bits. For example, in ASCII characters, the first bit transmitted is labeled b1. Typically this is followed by a parity bit, which will correspond to the most significant bit. The parity bit is determined at the emitter such that the number of ones within the character, including the parity bit, is even or odd. This bit is also used for error detection. The last element is stop, which corresponds to a binary 1.
Asynchronous transmission is simple and inexpensive, although it requires 2 or 3 extra bits for each character. For example, in an 8-bit code, if 1 stop bit is used, out of every 10 bits, 2 will not contain information since they will be dedicated to synchronization; therefore the supplementary bits reach 20%.
Synchronous transmission
Here a bit block is transmitted as a stationary string without using start or stop codes. To prevent desynchronization between the sender and receiver, your clocks must be synchronized in some way. One possibility may be to provide the clock signal through a separate line.
One end (the receiver or the transmitter) will regularly send out a short pulse. The other end will use this signal as a clock. This technique works well at short distances, not long.
The other alternative is to include the synchronization information in the data signal itself.
In synchronous transmission, an additional level of synchronization is also required so that the receiver can determine where the beginning and the end of each data block are. To accomplish this, each block begins with a preamble bit pattern and generally ends with an ending bit pattern.
Line settings
Two of the configurations that distinguish the different link configurations are the topology and whether the link is "half-duplex" or "full-duplex."
Topology
The term topology refers to the physical arrangement of the stations in the transmission medium.
Full Duplex and Semi-Duplex
The exchange of data on a transmission line can be classified as "full-duplex" "semi-duplex". In semi-duplex transmission each time only one of the two point-point link stations can transmit. This is comparable to a single lane bridge with two-way traffic.
In full-duplex transmission the two stations can simultaneously send and receive data. This mode is referred to as simultaneous in two directions and is comparable to a two-lane bridge with traffic in both directions.
In digital signaling, where a guided medium is required, full-duplex transmission normally requires two separate paths (eg two twisted pairs), whereas semi-duplex transmission normally requires one. For analog signaling it will depend on the frequency: if a station transmits and receives at the same frequency, for wireless transmission it must be operated in semi-duplex mode, although for guided media it can be operated in full-duplex using two different transmission lines.
Interfaces
Most of the devices used for data processing have a limited data transmission capacity. The devices considered, usually terminals and computers, are generally called DTE (“data terminal equipment”). The DTE uses the transmission medium through the DCE (“data circuit-termianting equipment”). An example of the latter is a modem. On the one hand, the DCE is responsible for transmitting and receiving bits, one by one, through the transmission or network medium. On the other hand, the DCE must interact with the DTE. Each DTE-DCE pair must be designed to work cooperatively. The interface specification has four important characteristics:
- Mechanical Electrical Functional Procedural.
The mechanical characteristics are related to the physical connection between the DTE and the DCE. Typically, the signal and control exchange circuits are grouped together into a cable with a connector, male or female, at each end. The DTE and DCE must have different gender connectors at each end of the cable.
The electrical characteristics are related to the voltage levels and their timing. Both DTE and DCE must use the same code (for example NRZ-L), must use the same voltage levels, and must use the same duration for signal elements.
The functional characteristics specify the sequence of events that must occur in the transmission of the data, based on the functional characteristics of the interface.
There are several standardizations for the interface. Here are two of the most significant: V.24EIA-232-E and the ISDN physical interface.
V.24 / EIA-232-E
The most widely used interface is the one specified in the ITU-T V.24 standard. In fact, this standard specifies only the functional and procedural aspects of the interface; V.24 refers to other standards for electrical and mechanical aspects. In the United States there is the corresponding specification that covers the four aspects mentioned: EIA-232. The correspondence is:
- Mechanical: ISO 2110 Electrical: V.28 Functional: V.24 Procedural: V.24
This interface is used to connect DTE modem devices over quality telephone lines for use in public analog telecommunication systems.
Mechanical specifications
For the EIA-232-E a 25 metal contact connector is used, distributed in a specific way as defined by ISO 2110. This connector is the terminator of the cable that goes from the DTE (terminal) to the DCE.
Electric specifications
Digital signage is used on all exchange circuits. The electrical values will be interpreted as binary or as control signals, depending on the function of the exchange circuit. This normalization specifies that, with respect to a common ground reference, a voltage more negative than 3 volts is interpreted as a binary 1, while a voltage greater than 3 volts is interpreted as a binary 0. This corresponds to the NRZ-L code. The interface is used at a rate of less than 20 kbps for a distance of less than 15 meters.
Functional specifications
The circuits are grouped into data, control, timing, and ground circuits. There is a circuit in each direction, so full-duplex operation is allowed. What's more, there are two secondary data circuits that are useful when the device operates in semi-duplex.
There are fifteen control circuits. The first 10, related to the transmission of data on the primary channel. For asynchronous transmission, six of these circuits are used. In addition to these six circuits, three other control circuits are used in synchronous transmission.
The signal quality detector circuit (“Signal Quality Detector”) is turned on by the DCE to indicate that the quality of the input signal through the line has deteriorated above a certain threshold. The data signal rate detector selection circuits are used to change speed; Both the DTE and DCE can begin the modification.
The last group of signals is related to the verification of the connection between the DTE and the DCE. These circuits allow the DTE to have the DCE test the connection. These circuits are useful only if the modem or DTE in question allows a control loop.
Loop control is a useful tool for fault diagnosis. The local loop checks the operation of the local interface as well as the local DCE. With remote tests, the operation of the transmission channel and the remote DCE can be checked.
The timing signals provide the clock pulses in synchronous transmission. When the DCE sends data through the Data Receive circuit, it simultaneously sends transitions from 0 to 1 or 1 to 0 through the Receiver Timing circuit with transitions in the middle of each signal element of the Receive Circuit. Data. When the DTE sends synchronous data, both the DTE and the DCE can provide the timing pulses.
Procedural Specifications.
The characteristics of the procedure define the sequence of how the different circuits of a given application are used.
For example: there are two devices connected at a very short distance, these are called private line modems. They accept DTE signals and convert them to analog signals and transmit them at a short distance through a medium and transmit them at a short distance through a medium. At the other end of the line is another limited-distance modem that accepts digital input signals, converts them to digital, and transfers them to the remote terminal or computer. It is assumed that the exchange of information is in both directions. In this application only exchange circuits are needed:
- The ground signal (102) Data transmission (103) Data reception (104 Send request (105) Ready to send (106) DCE ready (107) Received signal detector (109)
When the modem (DCE) is turned on and ready to go, it activates the DCE Ready line (applying a constant negative voltage). When the DTE is ready to send data, it will activate the activated line to prepare. The modem responds, when ready, activating the Ready to Send circuit. If the transmission is semi-duplex, the Request to Send circuit, in turn, inhibits the receive mode. The DTE can now transmit data through the Data Transmission line. When data is received from the remote modem, the local modem activates the Received Signal Detector line to indicate that the remote modem is transmitting, and also transfers the data through the Data Receive line.
The ISDN physical interface
The wide variety of functions provided by the V.24 / EIA-232-E are carried out through the use of a large number of exchange circuits. This is an expensive solution. An alternative would be to use fewer circuits incorporating more control logic between the DTE and DCE interfaces. The ISDN is a completely digital network that will replace the analog telecommunications and public telephony networks that currently exist.
Physical connection
In ISDN terminology, a physical connection is established between the terminal equipment (TE) and the network terminating equipment (NT). For the study to be carried out here, these terms correspond fairly roughly to DTE and DCE respectively. The physical connection, defined in ISO 8877, specifies that the NT and TE cables have two corresponding connectors, each with 8 contacts.
Two contacts are used to transmit data in each of the two addresses. The contact terminals are used to connect the circuits between the NT and the TE by twisted pairs. Because the circuits do not have specific functional specifications, the receiving and transmitting circuits are used to transmit data and control signals. The control transmission is transmitted using messages.
Electric specifications
The ISDN electrical specification states that balanced transmission is used, the signals are transmitted using two conductors for example a twisted pair. The signals are transmitted as a current that goes through one of the conductors and returns through the other, thus forming a closed circuit. In the case of digital signals, this technique is called differential signaling, since the binary values depend on the direction of the voltage differences between the two conductors. Unbalanced transmission is used on older interfaces like EIA-232, distances are generally short.
The balanced mode is more tolerant, and produces less noise than the unbalanced mode. Ideally, interference on a balanced line will affect both conductors equally and therefore will not affect voltage differences. Because unbalanced transmission does not have these advantages, its use is normally restricted to coaxial cables.
The format used in encoding the data on the ISDN interface depends on the data ratio. In the basic link (192 kbps) the standard specifies the use of pseudoternary coding. The binary ones are represented by the absence of voltage, and the binary zero is represented by a negative or positive pulse of 750mV. On the primary link, there are two possibilities: if you choose a data rate equal to 1,544 Mbps, you will use alternate mark inversion (AMI) encoding with B8ZS and if you choose a speed equal to 2,049 Mbps AMI encoding is used with HDB3.
Data encryption
Analog transmission is based on a continuous constant frequency signal called a carrier. The carrier frequency is chosen to be compatible with the characteristics of the medium to be used. The data can be transmitted by modulating the carrier signal, where modulation means the process of encoding the data generated by the source into the frequency signal fc.
All modulation techniques involve modifying one or more of the three fundamental parameters of the carrier:
- Amplitude Frequency Phase
The input signal is called the modulating signal.
Digital data, Digital signals
A digital signal is a sequence of discrete and discontinuous pulses, where each pulse is an element of the signal.
If all the signal elements have the same algebraic sign, that is, if they are all positive or negative, it is said to be the "unipolar" signal. In a "polar" signal, on the other hand, one logic state will be represented by a positive voltage level and the other by a negative level. The data rate of a signal is the transmission rate, expressed in bits per second, at which the data transmits. The "modulation ratio," by contrast, is the rate or rate at which the signal level changes per second.
| Finished | Units | Definition |
| Data elements | bits | A one or zero binary |
| Data reason | Bits per second (bps) | Reason at which data elements are transmitted |
| Signal element | Digital: a pulse of constant amplitude voltage | That part of the signal that occupies the shortest interval corresponding to a signaling code |
| Signaling ratio or modulation rate | Number of signal elements per second (baud) | Reason to which signal elements are transmitted |
An important factor that can be used to improve system performance is the coding scheme itself. This is simply the correspondence that is established between the bits of the data with the elements of the signal.
We consider the following procedures for their evaluation and comparison:
Definition of digital signal encoding formats
| No return to zero (NRZ-L) | 0 = high level
1 = low level |
| No return to zero reversed (NRZI) | 0 = no transition at beginning of interval (one bit at a time)
1 = transition to beginning of interval |
| Bipolar -AMI | 0 = no signal
1 = positive or negative level, alternating |
| Pseudoternary | 0 = positive or negative level, alternating
1 = transition from low to high in mid-range |
| Manchester | 0 = transition from high to low in mid-range
1 = transition from low to high in mid-range |
| Differential Manchester | There is always a transition in the middle of the interval
0 = transition to beginning of interval 1 = no transition at beginning of interval |
| B8ZS | Same as bipolar-AMI, except that any string of eight zeros is replaced by a string that has two code violations |
| HDB3 | Same as Bipolar-AMI, except that any string of four zeros is replaced by a string that contains a code violation |
- Signal spectrum: The absence of a signal at high frequencies means that less bandwidth is needed for its transmission. Furthermore, the absence of DC component is also a desirable feature. If the signal is continuous, its transmission requires the existence of a direct physical connection; If the signal does not contain a continuous component, it can be transmitted using coupled transformers. Synchronization: it is necessary to determine the beginning and end of each bit. It is done by providing synchronization through the transmitted signal itself. Error detection: These techniques are the responsibility of a layer above the signaling level, called data link control. It is useful to have an error detection capability built into the encoding scheme at the physical layer. Immunity to noise and interference: Some codes exhibit superior behavior than others in the presence of noise. This is measured in terms of bit error rate. Cost and complexity: the higher the ratio of signal elements to a given transmission rate, the higher the cost.
Non-return to zero (NRZ, “Nonreturn to Zero”)
The most frequent and easiest way to transmit digital signals is by using a different voltage level for each of the bits
NRZ is generally used to generate or interpret binary data on terminals and other devices.
A variant of the NRZ is called NRZI ("Nonreturn to Zero, invert on ones"). Like NRZ-L, the NRZI keeps the voltage level constant for a bit. Data is encoded by the presence or absence of a signal transition at the beginning of the bit duration interval, a 1 is encoded by the transition (low to high or high to low) at the beginning of the bit interval, while a zero is represented by the absence of the transition.
NRZI is an example of differential coding. In differential encoding, instead of determining the absolute value, the signal is decoded by comparing the polarity of the adjacent signal elements. An advantage of this scheme is that in the presence of noise it may be safer to detect a transition rather than to compare a value with a threshold. Another advantage is that in a complicated transition system, it is not difficult to lose the polarity of the signal. For example, on a twisted pair line, if the cables are accidentally reversed, all 1s and 0s on the NRZ-L will reverse.
NRZ codes are the easiest to implement and are also characterized by making efficient use of bandwidth.
The main limitation of NRZ signals is the presence of a continuous component and the absence of synchronization capability. For example, a long chain of ones or zeros in a NRZ-L scheme or a chain of zeros in NRZ-I will be coded as a constant voltage level over a long time interval. In these situations, any fluctuation between the transmitter and receiver timings will result in a loss of synchronization between the two.
Multilevel Binary
Coding techniques called multilevel binary address some of the shortcomings mentioned for NRZ codes.
In the case of the bipolar-AMI scheme, a binary 0 is represented by absence of signal and the binary 1 is represented as a negative or positive pulse. The pulses corresponding to 1 must have an alternating polarity. The advantages of this scheme are: there will be no synchronization problems in the event that there is a long chain of 1. Every 1 forces a transition, so the receiver can be synchronized in that transition. A long string of 0 is still a problem. There are no continuous components. Furthermore, the bandwidth of the resulting signal is much less than that corresponding to NRZ.
The same comments apply for pseudoternary codes. In this case bit 1 is represented by the absence of a signal, and 0 by pulses of alternating polarity.
One of the problems not yet resolved is the degree of synchronization of these codes.
Biphase
Alternative techniques are Biphasic, which overcome the limitations found in the NRZ codes. Two of these techniques, called Manchester and Differential, are frequently used.
In Manchester code, there is always a transition in the middle of the bit duration interval. This transition in the middle of the bit serves as a synchronization procedure as the data is transmitted: a transition from low to high represents 1, and a transition from high to low represents 0. In Manchester Differential, the transition to Half the span is used only to provide synchronization. The encoding of a 0 is represented by the presence of a transition at the beginning of the bit interval and a 1 is represented by the absence of a transition.
All Biphasic techniques force at least one transition for each bit and can have up to two in the same period. Therefore, the maximum modulation speed is twice that of the NRZ; this means that the necessary bandwidth is greater.
The advantages of two-phase schemes are:
- Synchronization: Due to the transition that always occurs during the duration interval corresponding to a bit, the receiver can synchronize using this transition.It has no continuous component Error detection: errors can be detected if an absence of the expected transition is detected in the middle of the interval
Ups and Downs Techniques
The acceptance that two-phase schemes have achieved in LAN networks at relatively high speeds (up to 10 Mbps), is not transferable to long-distance networks.
The main reason for this lies in the fact that a high speed of signal elements compared to the data rate is required in two-phase.
Another alternative approach is to use some "ups and downs" procedure or technique. The idea is simple: replace the bit sequences that give rise to constant voltage levels with other sequences that provide enough number of transitions up and down so that the receiver clock can be kept synchronized. In the receiver, the replaced sequence must be identified and will have the same length as the original.
The objectives of these techniques are:
- Avoid DC component Avoid long sequences that correspond to zero voltage signals. Do not reduce data rate. Ability to detect errors
A coding scheme used in North America is called B8ZS ("Bipolar with 8-Zeros Substitution"), and is based on a bipolar AMI. The drawback to AMI codes is that a long sequence of zeros can lead to a loss of synchronization. To avoid this problem, an encoding is performed according to the following rules:
- If an octet appears with all zeros and the last tension value before said octet was positive, encode said octet as 000 + -0- + If an octet appears with all zeros and the last tension value before said octet was negative, encode that octet as 000- + 0 + -
This procedure forces two code violations of the AMI code, which is highly unlikely to have been caused by noise or other transmission defects. The receiver will identify that pattern and conveniently interpret it as an all-zero byte.
A coding scheme used in Europe and Japan is called HDB3 ("High Density Bipolar-3 Zeros"). It is also based on AMI encoding. In this scheme, strings of four zeros are replaced by strings containing one or two pulses. In this case, the fourth zero is replaced by a signal state not allowed in the code, this procedure is called code violation.
Substitution rules in HCB3
| Bipolar pulse numbers (ones) since last replacement | ||
| Previous pulse polarity | Odd | Pair |
| - | 000- | +00+ |
| + | 000+ | -00- |
Digital data, Analog signals
The best-known case of digital data transmission over the telephone network. This network was designed to receive, switch and transmit analog signals in the voice range between 300 and 3400 Hz.
Coding techniques
Modulation has been mentioned to affect one or more of the characteristic parameters of the carrier signal: amplitude, frequency and phase.
There are three basic coding or modulation techniques that transform digital data into analog signals:
- Amplitude shift (ASK, “Amplitudes-shift keying”) Frequency shift (FSK, “Frequency-shift keying”) Phase shift (PSK, “Phase-shift keying”)
In ASK, the two binary values are represented by two different amplitudes of the carrier. It is usual that one of the amplitudes is zero; that is, one of the binary digits is represented by the presence of the carrier constant amplitude, and the other by the absence of the carrier. The resulting signal is therefore
S (t) = {A cos (2πfct) 1 binary
{0 0 binary
In which the carrier is A cos (2πfct). ASK is sensitive to sudden changes in gain, plus it is an ineffective modulation technique. ON phone quality lines, ASK is typically used at 1200 bps. It is used for the transmission of digital data in optical fibers.
In FSK, the two binary values are represented by two different frequencies close to the carrier frequency. The resulting signal is
s (t) = {A cos (2πf1t) 1 binary
{A cos (2πf2t) 0 binary
Where frequently f1 and f2 correspond to displacements of equal magnitude but in opposite directions of the carrier frequency.
FSK is less error sensitive than ASK. ON phone quality lines, it is typically used at speeds of up to 1200 bps. It is also used in radio transmission at higher frequencies (from 3 to 30 MHz).
In the PSK scheme, the phase of the carrier signal is shifted to represent digital data with them. In this system, a binary 0 is represented by transmitting a signal with the same phase as the signal from the previously sent signal. While a 1 is represented by the transmission of a signal with the same phase, it is in phase opposition with respect to the preceding signal. This technique is known as differential PSK, since the phase shift is relative to the phase corresponding to the last transmitted symbol, rather than being relative to some constant reference value. The resulting signal is
s (t) = {A cos (2πfc1t + π) 1 binary
{A cos (2πfct) 0 binary
Analog data, Digital signals
Pulse Coding Modulation
Delta modulation
Spread Spectrum
Bibliography consulted
- Communications and Computer Networks, 6th edition, William Stallings Computer Networks, 3rd edition, Andrew S. Tanenbaum Cisco Networking Course, first semester, Chapters 4 and 10 How to do an monograph, monographs.com
A continuous signal is one in which the intensity of the signal varies smoothly over time, with no jumps or discontinuities.
A discrete signal is one in which the intensity remains constant for a certain time interval, after which it changes to another constant value
Large Scale Integration: technology used in electronics for the construction of electronic digital integrates.
Very Large Scale Integration - More modern and lower cost technology than LSI.
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