Asymmetrical line. Asymmetry ohmic and capacitive
Unbalanced stripline transmission line
An asymmetric strip transmission line or microstrip line (Fig. 12.3, 12.4, a) is a strip line in which the conductor (1) is separated from the general metallization (3) by a dielectric layer (2). Such a line is easily manufactured using modern technological processes, has small dimensions, low cost in serial production, and high reliability. The distribution of the lines of strength of the electric and magnetic fields is shown in Fig. 12.4 , b. Despite the obvious simplicity of the design, an accurate analysis of the characteristics of a microstrip line with an inhomogeneous dielectric medium is rather difficult. Line characteristics are calculated, as a rule, assuming the propagation of a quasi T-wave. Strictly speaking, a mixed wave with noticeable dispersion propagates in the line, which causes a change in its parameters with frequency. Accurate determination of frequency-dependent parameters is possible when solving a boundary value problem by numerical methods on a computer.
Rice. 12.3. Unbalanced strip transmission line design
Rice. 12.4. The design of an asymmetric strip transmission line (a) and the distribution of lines of electric and magnetic field strength (b).
For NPL, the calculation of wave impedance and other parameters is a more difficult task than for SPL. The main difference is that the NPL is an open structure, and the construction of its rigorous theory turned out to be associated with the solution of a number of complex problems in the mathematical theory of diffraction and computational electrodynamics. At the same time, various approximate results have proved to be very useful for a number of applications. One such approach involves the use of the so-called Oliner model. This model is based on comparing the characteristic impedances of a real line having a relative dielectric constant of the substrate material ε r and a uniformly filled waveguide with magnetic side walls. Moreover, the filling of this waveguide is ε eff- effective relative dielectric constant different from ε r .The quantity ε eff determines the equality of phase velocities in both lines. Effective width W eff The NPL for the Oliner model is determined from the equality of the wave impedances of the original line and the model.
A number of approximate relations are obtained for determining the wave impedance Z IN and effective relative permittivity ε eff in the quasi-static approximation. So, the wave resistance Z IN can be calculated with low error (± 1%) for 1 ε r16 and geometric dimensions in the area.
For wide conductors ()
and for narrow conductors ()
, (12.8)
where parameter ε eff is equal to:
Losses in MSL are usually divided into losses in the substrate dielectric, in metallic line elements, and in radiation into the surrounding space due to surface and spatial types of waves. To calculate the losses in the metal and dielectric of the substrate, rather simple calculated relations are known. Radiation losses are usually associated with the presence of various kinds of inhomogeneities in the PLP. So, it can be a line break, or its bend; hole in the center conductor; located next to another line (in this case, they speak of connected PLPs).
The damping factor due to dielectric losses is determined by the following formulas:
; [dB / m] (12.11)
where 
, where is the frequency [GHz].
When considering the finite thickness of the conductor, instead of the ratio W/ D it is necessary to substitute the value W * / D:
, (12.12)
. (12.13)
Addiction Z IN on the ratio for different values ε r(curve 1 corresponds to ε r = 2.2; curve 2 - ε r = 4.0; curve 3 - ε r = 6.0; curve 4 - ε r = 9.6) can be shown by the curves shown in Fig. 12.5. Analysis of these curves shows that the quantity Z IN in the MPL decreases with increasing W, ε r and with decreasing substrate thickness D.
Calculations show that for the values of the MPL parameters W= 1 mm, D= 1 mm, made on the basis of polycor with ε r = 9.6, its characteristic impedance is approximately 50 ohms.
A more rigorous analysis shows that not a pure T-wave propagates in the MSL, therefore the characteristic impedance and effective dielectric constant depend on the operating frequency. This relationship is called variance. In the calculated ratios presented above, when taking into account the variance, it is necessary to replace with.
Rice. 12.5. Dependence of the magnitude of the wave resistance on the design parameters and dimensions.
Based on the generalization of numerous experimental data, the following empirical formula was obtained, which makes it possible to take into account the dependence on frequency:
, (12.14)
, (12.15)
where f- operating frequency [dimension in GHz], dimension W and D in commensurate quantities.
The accuracy of calculations according to formulas (12.14) and (12.15) is not worse than 2% at and mm.
Attenuation coefficient m in metal is determined by the following approximate formulas:
(12.17)
where, a is the conductivity of the material used for the manufacture of microstrip line conductors, is the conductivity of copper.
(12.18)
where ; 
;
;
;
.
In fig. 12.6 shows the dependences of the attenuation coefficient of the microstrip transmission line on the frequency for the values of the parameters r = 9,6, D = l mm, = 75 Ohm (curve 1) and = 50 Ohm (curve 2.) It can be seen that with increasing frequency, the attenuation coefficient increases according to the law f. With an increase in the wave impedance, the losses also increase with the equality of all other parameters. Real microstrip circuits are housed in a shielding case. In this case, the idealized concept of conducting boundaries located at an infinite distance from the strip turns out to be inaccurate in a number of cases. However, it is considered that if the shielding body is located at a distance greater than 10 W, then the parameters of such a transmission line can be determined using the formulas presented above for lines without shielding.
In real microstrip lines, the attenuation increases due to the roughness of the substrate, the finite thickness of the adhesive sublayer between the conductor and the substrate, as well as due to a number of other factors not considered above.
Rice. 12.6. Dependence of attenuation of a microstrip transmission line on frequency.
where the value f cr expressed in GHz and D - in mm.
In the mode of continuous oscillations, the losses in the microstrip line, as well as the intensity of heat removal from the substrate, determine the dielectric strength. Approximate values of the limiting average power for a line with a sapphire substrate are 80 - 100W , and the limiting impulse power (with a signal duty cycle of more than 50) is several kilowatts.
From the above, it is clear that the electrical characteristics of a microstrip line are determined by its geometric dimensions. A decrease in the thickness of the substrate provides: low radiation losses, a decrease in the probability of excitation of surface waves, an increase in the mounting density. However, other things being equal, to maintain a constant wave resistance, it is necessary to reduce W, which, in turn, leads to an increase in conductor losses. In addition, for small values of the parameters D and W the required process tolerances to achieve satisfactory electrical performance can be difficult to achieve. A compromise decision when choosing D is the accepted number of standard values of the substrate thickness for microstrip lines: D = 0.25; 0.5; 1 mm.
Let us dwell on the definition of one more geometrical size of the microstrip line - the conductor thickness. The current in the conductor of the microstrip line flows mainly along the side of the conductor facing the substrate and is concentrated in a layer, the thickness of which is approximately equal to the thickness of the skin layer. To ensure low losses in the conductor, it is necessary that the thickness of the conductor and the grounded plate is approximately 3-5 skin thicknesses.
The most reliable unbalanced links are made using coaxial cable, but they are expensive. Another disadvantage of single-ended lines is the high level of noise present in the common conductor. These disadvantages are practically absent in symmetrical communication lines.
Balanced lines are two conductors isolated from a common conductor. Both at the input and at the output, the balanced line is loaded on the characteristic resistance, and the load is connected symmetrically with respect to the common conductor.
Usually balanced lines are made in the form of a twisted pair (see Fig. 114), the characteristic (characteristic) impedance of which is usually about 130 ohms.
Fig. 114. Symmetrical communication line.
A balanced line has increased noise immunity due to the fact that both line conductors are connected to the common conductor of the circuit through the same resistance. To organize the normal operation of the line, it is necessary to transmit the signal in both conductors of the line in antiphase, which means that if the signal is at a high level at the input of one conductor of the line, then at the input of the other conductor the signal must have a low level.
This can be done using two inverters when transmitting and, accordingly, an RS flip-flop when receiving (Fig. 115).
Fig. 115. Symmetrical communication line with TTL elements.
Logic elements used as transmitters must have an increased load capacity, for example 155LA6 or transistor stages based on the 155LP7 microcircuit (Fig. 116).
Fig. 116. Transmitter on the 155LP7 microcircuit.
In the figure, the following designations are adopted: D - data input, C - synchronization input, A - communication line input. Since for the normal operation of a symmetrical communication line, the signals must be supplied to the line conductors in a paraphase code, in the left circuit the transistors are connected by emitter followers, and the inversion is carried out by the lower element 2I-NOT. In the right circuit, one transistor is switched on according to the emitter follower circuit (there is no inversion), and the other is switched on by a switch (inversion is present). For matching, resistors equal to half of the characteristic impedance are used as loads in both circuits.
As receivers of symmetrical communication lines, it is necessary to use devices designed for paraphase presentation of information and with hysteresis at the input.
Lecture 35.
Digital-to-analog and analog-to-digital converters.
Electronic devices designed to change the form of representation of variable values. There are analog and digital forms of information presentation. The analog form of representation is that any variable is represented by a continuously changing quantity. An example would be electrical voltage or current in any electrical circuit. Indeed, the current in an electrical circuit can take on a value determined by the parameters of the circuit, but the number of these values is infinitely large. The digital form of representation consists in the fact that the value of a variable is represented by a multi-digit number of the positional number system. In this case, the number of values of the variable is determined by the error in the representation of the variable. So if a variable is represented by a four-digit decimal integer, then the representation error is a low-order unit, and the number of variable values is 10,000.
In addition to the transmission parameters, the influence parameters also have a huge influence on the electrical characteristics of balanced cables.
INFLUENCE PARAMETERS
The main method for reducing such influences is the twisting of the cores of the copper pair. The most stringent requirements in this regard are imposed in structured cabling systems (SCS) with a wide range of operating frequencies: the absence of twisting of conductors is allowed at a distance of no more than 1/2 inch from the junction point of two cable segments.
A measure of crosstalk assessment is Near End Crosstalk (NEXT) and Far End Crosstalk (FEXT). These parameters allow you to evaluate the suitability of pairs of balanced cables for high-speed data transmission. Cross-talk attenuation NEXT and FEXT can be expressed in terms of the logarithm of the ratio of the power of the generator P 1 supplying the influencing circuit to the interference power P 2 in the affected circuit, i.e., as 10lg (P 1 / P 2) dB or as the difference in levels in the indicated points p 1 - p 2.
It is worth recalling that the signal level or interference at an arbitrary point X of the communication line is estimated as px = 10lg (P x / 1mW) dB. Here, P x is the signal power at point X. Sometimes the notation dBm is used instead of dB to emphasize the fact that the signal power of 1 mW is selected as the reference power. The abbreviation dB will be used below.
The value of NEXT is estimated by the difference between the signal levels at the output of the transmitter of one pair and the interference created by it at the input of the receiver of another, measured at the same point, i.e., NEXT = p 10 - p 20.
The NEXT parameter is decisive in the single-cable mode of the communication line, when signals of opposite transmission directions are transported over pairs of one cable. It also plays a key role in cases where echo cancellation is used to separate signals from opposite directions transmitted over the same pair. As you know, the spectra of signals of opposite transmission directions completely (for example, for HDSL) or partially (for ADSL) coincide. Earlier in the domestic technical literature, the designation A 0 was used for the NEXT parameter.
The FEXT value is estimated by the difference between the signal levels at the output of the transmitter of one pair and the interference it created at the input of the receiver of the other. However, unlike NEXT, in FEXT measurement, the transmitter of the affected pair and the receiver of the affected pair are located at opposite points of the transmission line.
FEXT is a defining parameter in a two-cable mode of operation of a communication line, when signals from opposite directions of transmission are transported through pairs of different cables. It is of key importance when FDM is used to separate signals from opposite directions carried over the same pair (for example, in ADSL or VDSL systems). Then the spectra of signals of opposite transmission directions do not overlap, and there is no transient effect at the near end. Previously, the FEXT parameter was usually referred to as A L.
All other things being equal, the FEXT value is significantly higher than NEXT, since in the first case the influencing signal undergoes attenuation in the communication line, and in the second it directly affects the affected pair.
The NEXT parameter with increasing line length L first decreases and then stabilizes: starting from a certain length, interference currents from remote areas come so weak that they practically do not affect the NEXT value. The situation is different in the case of the addition of the currents of mutual influences at the far end - with an increase in the length of the line, all its sections introduce the same noise values. Crosstalk decreases with increasing frequency, with NEXT decreasing with frequency at a rate of 15 dB per decade, and FEXT at a rate of 20 dB per decade. The lower steepness of the frequency dependence of FEXT is explained by the fact that with increasing frequency, the attenuation of transient interfering currents, arriving at the near end from remote sections of the line, increases.
In addition to the considered parameters NEXT and FEXT, in the practice of evaluating structured cabling systems, two new ones are widely used - ACR and ELFEXT, on which we will dwell in more detail.
The Attenuation to Crosstalk Ratio (ACR) is equivalent to the signal-to-noise ratio for near-end crosstalk NEXT, that is, it serves as an estimate at the receiver input for the attenuated signal line and for near-end crosstalk interference. ACR is quantified as a logarithmic measure of the difference between NEXT and the attenuation of the cable. If, for example, the ACR value is 10 dB, this means that the NEXT interference power at the receiver input will be 10 times less than the wanted signal power, i.e. the signal-to-noise ratio will be 10.
Let the communication system operate in a single-cable mode, and the signal levels at the outputs of the transmitters at points A and B are the same and equal to 0 dB. If the line attenuation at frequency f is denoted by a k, then with the crosstalk NEXT at the same frequency, the levels of signal p c and crosstalk p p at the input of receiver A will be, respectively, a k and NEXT.
Then ACR = p с - p p = NEXT - a k.
The practical meaning of the ACR parameter becomes clearer if the frequency characteristics of the attenuation of balanced pair (a), crosstalk (NEXT) and parameter (ACR) are presented on one graph. The frequency at which the values of attenuation and NEXT are the same (in this case it is equal to 100 MHz) determines the upper limit of the operating frequency range. At frequencies above the cutoff, the NEXT interference power exceeds the signal power.
Equal Level Far End Crosstalk (ELFEXT) has the same physical meaning as ACR. The only difference between them is that ACR is associated with NEXT, while ELFEXT is associated with FEXT. The ELFEXT parameter becomes critical for cases when several transmitters of the same system transmit in one direction over pairs located in one cable.
In this case ELFEXT = FEXT - a k.
It should be noted that earlier in the domestic technical literature for the ELFEXT parameter, which was called protection from transient influence at the far end, the designation A z was used.
In addition to the ACR and FEXT parameters, two additional parameters are used - PS-ACR (Power Sum ACR) and PS-ELFEXT (Power Sum ELFEXT), taking into account the total effect of all other cable pairs on this pair.
ASYMMETRY OF A LINE
Asymmetry is both a transmission parameter, since it is determined by the parameters of a pair and affects its throughput, and an influence parameter, since it affects the transitions between other pairs.
Each balanced line must be balanced with respect to the ground in a certain way. Depending on the current - direct or alternating - two types of asymmetry are distinguished.
DC asymmetry is estimated by the relative value of the difference in the resistances of the cores of the symmetrical line and should not exceed 1%. The presence of a resistive imbalance in the line, equal to the difference in the resistances of its cores measured at alternating current, can be interpreted as the inclusion of an additional low-pass filter with the resistance of the longitudinal arm dR into it. In addition to the resistive component, the longitudinal imbalance of the line generally contains a capacitive component; it can arise, for example, due to accidental crossing of conductors of different pairs at the points of connection of cables. This component can be interpreted as the transverse capacitance of the additional low-pass filter mentioned above.
The degree of AC longitudinal unbalance is measured by the Longitudinal Conversion Loss (LCL). The reasons for the longitudinal imbalance of the twisted pair conductors can be loose contact at the junction of the cable conductors (twisting or soldering points, distribution cabinets, etc.). The problem of longitudinal unbalance cannot be considered solved, even if the longitudinal asymmetry of the pair in question is normalized. This fact is a necessary, but not yet sufficient condition for solving the problem of longitudinal asymmetry in a particular cable. The condition of sufficiency requires a mandatory check of all pairs of the bundle or twist for compliance with the norms of longitudinal asymmetry. The fact is that any imbalance, even in an inoperative pair, is a source of interference for all operating pairs, resulting in a decrease in their throughput.
Signal transmission over communication lines.
Of particular importance are electrical circuits, through which signals are transmitted both between the inputs and outputs of microcircuits on a printed circuit board, and between various computer devices located on different boards and in different cases.
Such electrical circuits will be called communication lines. Most communication lines are unbalanced.
Figure 105 shows the types of asymmetric communication lines: a - single conductor, b - twisted pair, c - coaxial cable
Fig. 105. Unbalanced communication lines.
Single conductor - A common communication line widely used on printed circuit boards, the output of the transmitter and the input of the receiver are connected by a single conductor, and the circuit is electrically closed through the common conductor of the printed circuit board. The advantage of a single-wire communication line is simplicity, and the disadvantage is a large amount of interference arising in the common conductor of the printed circuit board and affecting the transmitted signal.
Twisted pair - two insulated conductors twisted together, one of them connects the transmitter and receiver of signals, and the second is used to close the electrical circuit. When using a twisted pair within a printed circuit board, the noise immunity of information transmission is significantly increased, but the cost of this design is higher than that of a single conductor.
Coaxial cable is a special design consisting of a center conductor in an insulating sheath, on top of which is a cylindrical shield conductor.
It makes sense to consider the effect of signal reflection if the communication line operates as a long line, and this is determined by the fulfillment of the condition
Where is the propagation time of the signal through the communication line, is the duration of the pulse signal.
When this inequality is fulfilled, the reflected signals from the ends of the line do not affect the pulse shape, i.e. such a line does not make sense to consider as a long line. Taking into account that the speed of propagation of signals in the connecting lines is about 25 cm / ns, and the duration of the edges of the series formed at the outputs of TTL elements from 2 to 20 ns, it is possible to determine the length of the connecting conductors for which the indicated inequality is fulfilled. Data on TTL series are given in Table 16.
Table 16
If we assume that is the output resistance of the signal source, is the characteristic impedance of the communication line, is the load resistance connected to the line output, then the voltage at the line input (at point A) can be determined by the formula, where is the output voltage of the transmitter element. In the process of transmitting signals along a long line, there is a reflection of signals from the ends of the communication line and inhomogeneities along its length. The reflection coefficient at the line input (at point A) can be estimated by the relation
and at the output of the line (at point B) -
The magnitude of the reflected wave is defined as the product of the magnitude of the incident wave and the reflection coefficient.
Let us consider by example the influence of reflection on the quality of signal transmission over a communication line between two logical elements with the following parameters:,,, logical element - the transmitter changes the output state from zero to one with a voltage level of 4V. Reflection coefficients will take the values and.
When switching an element at the input of the line (at point A), we have
This signal arrives at the end of the line and is reflected, at the end of the line (at point B) we will have, and the product is a reflected wave that comes to the beginning of the line and is reflected again. In this case, at the input of the line, we obtain
The calculation results in the form of graphs are shown in Figure 106.
As can be seen from the graph, the signal at the input and output of the line is a smoothly increasing voltage, the form of which only leads to a signal delay in time. However, with other resistance ratios, the waveform undergoes more serious changes that can lead to malfunctioning. Let's consider the operation of the line with:, the rest of the parameters are the same as in the previous example. Reflection coefficients will take the values and.
Fig. 106. Graph of voltage changes at the ends
The worst ratio will be when the reflection coefficients at both ends of the line are single and with different signs, a complete loss of information is possible.
Fig. 107. Schedule of signal transmission over the communication line.
Such distortions of signals when they are transmitted over long lines lead to a decrease in the reliability of the operation of the entire computing device. To reduce distortion with long lines, it is necessary to match them with signal transmitters and receivers.
Digital communication with a subscriber and digital modems
For most of the years of the past century, the connection of a subscriber's telephone to a telephone exchange (or "local section of the communication line", "last mile") was carried out with a copper wire ("twisted pair", twisted pair), hidden in underground collectors or stretched through the air.
For a long time, the used bandwidth did not exceed 3 kHz, which was limited by analog terminals. However, twisted pair is inherently capable of much higher bandwidths, and can carry video or broadband data over short distances. New technologies (ISDN and ADSL) have been developed to provide better performance within the existing infrastructure.
In addition, in the 1990s. cable TV companies have invested heavily in alternative home connections. Both twisted pair technologies and fiber optic and coaxial cables were used here. In most cases, these cable networks have been installed to provide television coverage. However, their communication capabilities and high bandwidth can also be used to provide other forms of digital services.
The Integrated Services Digital Network (ISDN) could be regarded as the best kept secret of the computer networking world for too long. ISDN has long been hidden from users of telephone networks (public switched telephone network - PSTN), since it only provides communication between telephone exchanges, and the subscriber was still connected to the exchange via an analog channel.
ISDN was originally available in two versions:
Basic Rate (ISDN - BRI), also known as ISDN-2. BRI is intended for the home user or small business, it consists of two "B channels" (64 kbps) for data transmission and one covert "D channel" (16 kbps) for control information. Two
64Kbps channels can be used alone or linked together to form a 128Kbps channel;
Primary Rate (Primary Rate ISDN - PRI) or ISDN-30. PRI consists of 30 "B-channels" (a minimum of six can be set) of 64 kbps, plus a 64-kbps "D-channel" for control data. B-channels can be aggregated into a single 1.92 Mbps channel.
In late 1998, British Telecomm (BT) made its first serious attempt to bring ISDN technology to the home user with the announcement of the BT Highway service. If a customer subscribes to one of these services, the existing telephone line is retained, but the old main connector is replaced by the Trunk module. It has four connectors, two analog and two ISDN, and can support up to three conversations at a time. The subscriber retains the old analog telephone number and receives two additional ones, one for the second analog port and one for the ISDN lines. The two main differences between home and business services are that the latter supports Multiple Subscriber Numbering (MSN), whereby different devices connected to the same ISDN line can have different phone numbers. as well as a new data service (ISDNConnect) or an always-on slow connection that uses the ISDN signaling channel.
At the same time Internet-onepaTop BT, BT Internet, announced support for 128 Kbps, allowing users to use two ISDN lines as one high-bandwidth one.
xDSL is the group name for a variety of Digital Subscriber Line (DSL) technologies designed to offer telephone companies a way into the cable TV business. This is not a new idea - Bell Communications Research Inc developed the first digital subscriber line back in 1987 to provide video-on-demand and interactive television over wired communications. At that time, the spread of such technologies was difficult due to the lack of standards for the entire industry.
XDSL technologies offer upstream (download) speeds up to 52 Mbps and outgoing (offload) speeds from 64 Kbps to 2 Mbps (and more) and have a number of modifications:
Asymmetric Line (ADSL);
Single Line (SDSL);
Very high data rate (HDSL).
Practice shows that ADSL (Asymmetric
Digital subscriber line) are the most promising for domestic use.
ADSL. ADSL is similar to ISDN — both require that landline telephone lines are free and can only be used within a limited distance from the local telephone company. In most cases, ADSL can operate over twisted pair connections without disrupting existing telephone connections, which means that local telephone companies do not have to run special lines to provide ADSL service.
ADSL takes advantage of the fact that since voice communication does not take up the full bandwidth available for standard twisted pair cable, it is possible to provide high-speed data transmission at the same time. To this end, ADSL splits the maximum 1 MHz wired bandwidth into 4 kHz channels, of which one channel is used for the plain old telephone system (POTS) - voice, facsimile and analog modem data. The other 256 available channels are used for parallel digital communication. Communication is asymmetric: 192 channels of 4 kHz are used for incoming information and only 64 for outgoing.
ADSL can be thought of as converting a serial line of digital data to a parallel line, thus increasing the bandwidth. The modulation technique is known as Discrete Multitone (DMT), and encoding and decoding are performed respectively, in the same manner as with a conventional modem.
The earlier system, called Carrierless Amplitude Phase (CAP), was able to use the entire bandwidth above 4 kHz as a single transmission channel and had that
Rice. 3.9. Network connected via ADSL modem: / - telephone input; 2 - analog output; 3 - digital output
The property is that it is close to the Quadrature Amplitude Modulation (QAM) technique used by high-speed modems at speeds over 9.6 Kbps, and is also cheaper to implement. However, DMT - a more reliable, complex and flexible technology - has proven to be more suitable for a universally accepted standard.
When the service began to be commercially available, the only equipment that ADSL subscribers had to use was a dedicated modem. The device has three connections - telephone input (Fig. 3.9, /); standard telephone socket RJ11 for servicing an analog telephone (Fig. 3.9, 2) and a twisted pair Ethernet connector, which connects the ADSL modem to a PC (Fig. 3.9, 3).
On the user side, the ADSL modem collects high frequency digital data and broadcasts it for transmission to a PC or network. On the telephone service side, a Digital Subscriber Line Access Multiplexer (DSLAM) connects an ADSL user to the high-speed Internet by aggregating incoming ADSL lines into a single voice or data connection. Telephone signals are routed to the switched telephone network, and digital signals to the Internet via a high-speed backbone (fiberglass, asynchronous data transmission, or digital subscriber line).
Currently, there are various designs of ADSL modems. Some connect to a PC via a USB port, others via an Ethernet cable. Most devices allow
Share your Internet connection between multiple PCs. The integrated modem / router supports PC networking, some include an integrated firewall to provide different levels of protection against unauthorized access.
192 channels at 4 kHz provide a maximum bandwidth of 8 Mbps. The fact that ADSL services are limited by the 2 Mbps limit is due to artificial bandwidth constraints and the fact that actual levels of performance depend on a number of external conditions. These include wiring length, number of sensor wires, dangling pairs, and mutual interference. Signal attenuation increases with line length and frequency, and decreases with increasing wire diameter. A "dangling pair" is an open wire pair that runs parallel to the main wire pair, for example, each unused telephone jack is a dangling pair.
If you ignore the influence of dangling pairs, ADSL performance can be represented as it appears in Table. 3.11.
In 1999, following proposals from Intel, Microsoft, Compaq and other equipment manufacturers, a specification was developed that was adopted by the International Telecommunication Union (ITU) as a universal industry ADSL standard known as G.922.2 or G.lite. The standard assumes that users can make regular voice phone calls simultaneously with digital data transmission. There are some restrictions on the speed - 1.5 Mbps for receiving data and 400 Kbps for transmitting.
ADSL2. In July 2002, the International Telecommunication Union finalized two new asymmetric digital subscriber line standards, defined as G992.3 and G992.4 for asymmetric digital subscriber line (hereinafter referred to as ADSL2).
The new standard has been designed to improve the speed and range of an asymmetric digital subscriber line, achieving better performance on long lines in narrowband interference environments. The speed of ADSL2 for incoming and outgoing information streams reaches 12 and 1 Mbps, respectively, depending on the communication distance and other circumstances.
The increase in efficiency was achieved due to the following factors:
Improved modulation technology - a combination of four-dimensional trellis modulation (16 states) and 1-bit quadrature amplitude modulation (QAM), which provides, in particular, increased immunity to interference from AM broadcasting;
The use of a variable number of service bits (which in ADSL constantly occupy a 32 Kbps bandwidth) - from 4 to 32 Kbps;
More efficient coding (based on the Reed-Solomon code).
ADSL2 +. In January 2003, the ITU introduces the G992.5 (ADSL2 +) standard - a recommendation that doubles the downstream bandwidth, thus increasing data rates on telephone lines shorter than about 1.5 km.
While the ADSL2 standards define the downstream bandwidth of 1.1 MHz and 552 kHz, respectively, ADSL2 + increases this frequency to 2.2 MHz. The result is a significant increase in downstream data rates on shorter telephone lines.
ADSL2 + also helps reduce mutual interference. This can be especially useful if the asymmetrical digital subscriber line wires from both the central office and the remote terminal are in the same bundle when they are routed to the subscribers' homes. Mutual interference can significantly harm the data transfer rates on the line.
ADSL2 + can remedy this problem by using frequencies below 1.1 MHz from the central station to the remote terminal and frequencies between 1.1 and 2.2 MHz from the remote terminal to the subscriber station. This will eliminate most of the crosstalk between services and conserve line data rates from the central office.
Other xDSL technologies (Table 3.12)
RADSL. In 2001, the Rate Adaptive Digital Subscriber Line (RADSL) specification was introduced, which corrects the transmission rate according to the length and quality of the local line. Previously, subscribers had to be located within 3.5 km of the local telephone exchange in order to be able to connect to ADSL. For RADSL, the range has been extended to 5.5 km, and the noise tolerances have increased from 41 to 55 dB.
Table 3.12 Characteristics of xDSL Technologies
|
HDSL. HDSL technology is symmetrical, which means that the same bandwidth is provided for the output and input data streams. It uses wiring with 2-3 or more twisted pairs in the cable. Although the typical range (3 km) is lower than for ADSL, carrier signal repeaters can be installed to extend the connection by 1 - 1.5 km.
SDSL. The technology is similar to HDSL, but with two exceptions: a single wire pair is used and the maximum length is limited to 3 km.
VDSL. It is the fastest digital subscriber line technology. The speed of the input stream is 13-52 Mbit / s, and the output stream is 1.6-2.3 Mbit / s over a single wired pair. However, the maximum communication distance is only 300-1500 m and ADSL and VDSL equipment are not compatible, although similar compression algorithms and modulation technologies are used.
Cable modems. Cable modems offer the prospect of fast Internet access using existing cable TV broadband networks. The technology is more suitable for home rather than office applications, as residential areas are usually more wired.
Typical devices, made, for example, by vendors such as Bay Networks or Motorola, are plug-ins that connect to a client PC via Ethernet, USB, or FireWire. In most cases, the user's cable modem is assigned a single IP address, but additional IP addresses can be supplied for multiple computers, or multiple PCs can share a single IP address using a proxy server. The cable modem uses one or two 6 MHz TV channels.
Because the cable TV network has a bus topology, each cable modem in the neighborhood shares access to a single coaxial cable backbone (Figure 3.10).
The function of the cable modem is to modulate and demodulate the signal into the data stream; but the similarity with analog modems ends there. Cable modems also include a tuner (to separate the data signal from the rest of the broadcast stream); network adapter components
Rice. 3.10. Communication systems using cable modems
terra, bridges and routers (to connect to multiple PCs); network management software (so that the cable provider can control operations) and encryption devices (so that the data stream is not interrupted and sent to the recipient by mistake).
Cable has a number of practical disadvantages compared to xDSL - not all homes are equipped with cable TV (and some never will); in addition, for many users who are connected, it is still more likely that a PC is located near a telephone jack than near a television or cable gland. However, for many home users, cable offers the prospect of fast Internet access at an affordable cost. Speeds up to 30 Mbps are theoretically possible. In practice, cable companies set the upstream speed at 512KB / s and the downstream rate at 128KB / s.
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