Lesson Objectives :

STUDENT HANDOUT

Module :Industrial CommunicationCode : SRD 35103

Topic 2Fundamentals of Data Communication

2.1What is Data Communications

Data Communications can be defined very simply as the transmission of data between two points. It is one of many functions that must be performed for the processing of information. With the explosive growth-in computer applications, data connectivity now spans to every

department of the organization.

As services continue to expand across these networks, the need to transparently transport whatever type of information is requested will become the guiding requirement of the network. The transparency between the network and the requested services made possible by layered architectures and well-defined standards is a vital break-through in enhancing modem information systems. To assure effective network services, data communication managers will increasingly be asked to integrate transport connections that span a broad range of technical complexity and employ a diversity of protocols.The distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single IC chip, to as much as several feet along the backplane of the main circuit board. Over such small distances, digital data may be transmitted as direct, two-level electrical signals over simple copper conductors. Except for the fastest computers, circuit designers are not very concerned about the shape of the conductor or the analog characteristics of signal transmission.

Frequently, however, data must be sent beyond the local circuitry that constitutes a computer. In many cases, the distances involved may be enormous. Unfortunately, as the distance between the source of a message and its destination increases, accurate transmission becomes increasingly difficult. This results from the electrical distortion of signals traveling through long conductors, and from noise added to the signal as it propagates through a transmission medium. Although some precautions must be taken for data exchange within a computer, the biggest problems occur when data is transferred to devices outside the computer's circuitry. In this case, distortion and noise can become so severe that information is lost.

Data Communications concerns the transmission of digital messages to devices external to the message source. "External" devices are generally thought of as being independently powered circuitry that exists beyond the chassis of a computer or other digital message source. As a rule, the maximum permissible transmission rate of a message is directly proportional to signal power, and inversely proportional to channel noise. It is the aim of any communications system to provide the highest possible transmission rate at the lowest possible power and with the least possible noise.

2.2Communication Model

2.3Bits and Bytes

a) Bits Computers happen to operate using the base-2 number system, also known as the binary number system (just like the base-10 number system is known as the decimal number system). The reason computers use the base-2 system is because it makes it a lot easier to implement them with current electronic technology. You could wire up and build computers that operate in base-10, but they would be fiendishly expensive right now. On the other hand, base-2 computers are relatively cheap. So computers use binary numbers, and therefore use binary digits in place of decimal digits. The word bit is a shortening of the words "Binary digIT." Whereas decimal digits have 10 possible values ranging from 0 to 9, bits have only two possible values: 0 and 1. Therefore, a binary number is composed of only 0s and 1s, like this: 1011. How do you figure out what the value of the binary number 1011 is? You do it in the same way we did it above for 6357, but you use a base of 2 instead of a base of 10. So: (1 * 2^3) + (0 * 2^2) + (1 * 2^1) + (1 * 2^0) = 8 + 0 + 2 + 1 = 11

b) Bytes

Bits are rarely seen alone in computers. They are almost always bundled together into 8-bit collections, and these collections are called bytes. Why are there 8 bits in a byte? A similar question is, "Why are there 12 eggs in a dozen?" The 8-bit byte is something that people settled on through trial and error over the past 50 years. With 8 bits in a byte, you can represent 256 values ranging from 0 to 255, as shown here: 0 = 00000000

1 = 00000001

2 = 00000010

...

255 = 11111111

When you start talking about lots of bytes, you get into prefixes like kilo, mega and giga, as in kilobyte, megabyte and gigabyte (also shortened to K, M and G, as in Kbytes, Mbytes and Gbytes or KB, MB and GB). The following table shows the multipliers: NameAbbr.Size

KiloK2^10 = 1,024

MegaM2^20 = 1,048,576

GigaG2^30 = 1,073,741,824

TeraT2^40 = 1,099,511,627,776

PetaP2^50 = 1,125,899,906,842,624

ExaE2^60 = 1,152,921,504,606,846,976

ZettaZ2^70 = 1,180,591,620,717,411,303,424

YottaY2^80 = 1,208,925,819,614,629,174,706,176

2.4Representation codesBytes are frequently used to hold individual characters in a text document. In the ASCII character set, each binary value between 0 and 127 is given a specific character. Most computers extend the ASCII character set to use the full range of 256 characters available in a byte. The upper 128 characters handle special things like accented characters from common foreign languages. You can see the 127 standard ASCII codes below. Computers store text documents, both on disk and in memory, using these codes. For example, if you use Notepad in Windows 95/98 to create a text file containing the words, "Four score and seven years ago," Notepad would use 1 byte of memory per character (including 1 byte for each space character between the words -- ASCII character 32). When Notepad stores the sentence in a file on disk, the file will also contain 1 byte per character and per space. Try this experiment: Open up a new file in Notepad and insert the sentence, "Four score and seven years ago" in it. Save the file to disk under the name getty.txt. Then use the explorer and look at the size of the file. You will find that the file has a size of 30 bytes on disk: 1 byte for each character. If you add another word to the end of the sentence and re-save it, the file size will jump to the appropriate number of bytes. Each character consumes a byte.

If you were to look at the file as a computer looks at it, you would find that each byte contains not a letter but a number -- the number is the ASCII code corresponding to the character (see below). So on disk, the numbers for the file look like this: F o u r a n d s e v e n

70 111 117 114 32 97 110 100 32 115 101 118 101 110

By looking in the ASCII table, you can see a one-to-one correspondence between each character and the ASCII code used. Note the use of 32 for a space -- 32 is the ASCII code for a space. We could expand these decimal numbers out to binary numbers (so 32 = 00100000) if we wanted to be technically correct -- that is how the computer really deals with things.

The ASCII Character Set Characters sent through a serial interface generally follow the ASCII (American Standard Code for Information Interchange) character standard. This standard relates binary codes to printable characters and control codes. Fully 25 percent of the ASCII character set represents nonprintable control codes, such as carriage return (CR) and line feed (LF). Most modern character-oriented peripheral equipment abides by the ASCII standard, and thus may be used interchangeably with different computers.

THE ASCII TABLE :

SignalingAnalogue and Digital Signals

Continuous signals are analog. Analog data can take any value in a range.

Discrete signals are digital. Digital data can have only a limited number of values (e.g.: 1, 0)

Digital communication facilities are often preferred over analog communication facilities because of the following digital transmission characteristics:

Stability

Quality

Speed

Capacity/GrowthAlthough a digital signal's error loss from noise and interference is much less than that of analog signals, analog signals are not as affected from attenuation over comparable distances.

Signal Attenuation and Noise

Any signal carried on a transmission media will be affected by attenuation and noise.

Attenuation

The decrease in amplitude of signal (current, voltage, power) during its transmission from one point to the next. To allow for attenuation, a limit is set on the length of cable that can be used to ensure that the receiver will properly detect and interpret the attenuated signal. Signal attenuation increases as function of frequency. The higher the frequency the greater the attenuation.

Cross-Talk Noise

Usually caused by capacitive or inductive coupling between adjacent channels. You

hear the signal from the other channel. This can be a problem in high-speed digitalcircuits- Proper shielding and cable separation can reduce cross-talk noise.

White NoiseSometimes called Gaussian noise, this results from the transmission of extraneous Signals over lines. It sounds like the ocean, at a distance.

DCE-DTE Interconnections

Data. Terminal Equipment (DTE)

is the source/destination of the data transmission. These devices are usually terminals, personal computers, printers, or mainframe computers.

Data Communications Equipment (DCE)

provides the connection to the data communications facility and transmits the message. This device is usually a modem or digital service unit.2.5Serial and Parallel Communication

i) Asynchronous Serial Communication

Most PC serial devices such as mice, keyboards and modems are asynchronous. Asynchronous communication requires nothing more than a transmitter, a receiver and a wire. It is thus the simplest of serial communication protocols, and the least expensive to implement. As the name implies, asynchronous communication is performed between two (or more) devices which operate on independent clocks. Therefore, even if the two clocks agree for a time, there is no guarantee that they will continue to agree over extended periods, and thus there is no guarantee that when point A begins transmitting, point B will begin receiving, or that Point B will continue to sample at the rate Point A transmits. See the figure below for an illustration of what happens when transmission clocks differ significantly.

To combat this timing problem, asynchronous communication requires additional bits to be added around actual data in order to maintain signal integrity. Asynchronously transmitted data is preceded with a start bit which indicates to the receiver that a word (a chunk of data broken up into individual bits) is about to begin. To avoid confusion with other bits, the start bit is twice the size of any other bit in the transmission. The end of a word is followed by a stop bit, which tells the receiver that the word has come to an end, that it should begin looking for the next start bit, and that any bits it receives before getting the start bit should be ignored. To ensure data integrity, a parity bit is often added between the last bit of data and the stop bit. The parity bit makes sure that the data received is composed of the same number of bits in the same order in which they were sent. Use the link below to view a portrayal of how asynchronous communication works.Asynchronous Serial Communication Model

In asynchronous communication, data is preceded with a start bit which indicates to the receiver that a word (a chunk of data broken up into individual bits) is about to begin. To avoid confusion with other bits, the start bit is twice the size of any other bit in the transmission. The end of a word is followed by a stop bit, which tells the receiver that the word has come to an end, that it should begin looking for the next start bit, and that any bits it receives before getting the start bit should be ignored. To insure data integrity, a parity bit is often added between the last bit of data and the stop bit. The parity bit makes sure that the data received is composed of the same number of bits in the same order in which they were sent. See the diagram in Figure 11 for a portrayal of how asynchronous communication works.

Upgraded UARTs For Increased Performance

At the heart of every asynchronous serial system is the Universal Asynchronous Receiver/Transmitter or UART. The UART is responsible for implementing the asynchronous communication process described above as both a transmitter and a receiver (both encoding and decoding data frames). The UART not only controls the transfer of data, but the speed at which communication takes place. However, the first UARTs could only handle one byte of information at a time, which meant that the computer needed to immediately process any transmission or risk losing data as the next byte of information pushed its way onto the UART. Not only does this makes for unreliable and slow communication, it can slow down the entire system.

Improved UARTs, such as the 16750 UARTs, increase communication speed and lower system overhead by offering 64-byte FIFOs (first in first out buffers). With the 64-byte FIFO buffer, the UART can store enough information that the data stream need not be suspended while the computer is busy. This is particularly helpful in heavy multitasking operating systems such as Windows 95/98/Me/NT/2000/XP and OS/2.

ii) Synchronous Serial Communication

Coordinated SpeedAs its name implies, synchronous communication takes place between a transmitter and a receiver operating on synchronized clocks. In a synchronous system, the communication partners have a short conversation before data exchange begins. In this conversation, they align their clocks and agree upon the parameters of the data transfer, including the time interval between bits of data. Any data that falls outside these parameters will be assumed to be either in error or a placeholder used to maintain synchronization. (Synchronous lines must remain constantly active in order to maintain synchronization, thus the need for placeholders between valid data.) Once each side knows what to expect of the other, and knows how to indicate to the other whether what was expected was received, then communication of any length can commence.

The theory behind asynchronous and synchronous communication is essentially the same: Point B needs to know when a transmission from Point A begins, when it ends, and if it was processed correctly. However, the difference lies in how the transmission is broken down. Think of the difference in terms of a friendly chat. With asynchronous communication you would need to stop after every word to make sure the listener understood your meaning, and knew that you were about to speak the next word. With synchronous communication, you would establish with your listener that you were speaking English, that you will be speaking words at measured intervals, and that you would utter a complete sentence, or paragraph, or extended soliloquy, before pausing to confirm understanding. Further, you would establish with your listener beforehand that any extraneous noises you make during the speech or between speeches (coughing, burping, hiccupping) should be ignored. Clearly the second approach is much faster, even though initializing communication may take slightly longer. In fact, by replacing the start, stop and parity bits around individual words with start, stop and control (processing instructions and error checking) sequences around large continuous data blocks, synchronous communication is about 30% faster than asynchronous communication, before any other factors are considered.

Clock Synchronization

In order to initiate a successful synchronous communication link, several distinct pieces of hardware must be configured around a common clock. This configuration must take two data lines into account, the transmission line (the line it uses to send data) and the reception line (the line it uses to receive data). It is essential not only that all devices in the system be synchronized with each other, but also that each individual device have its transmission and reception lines synchronized as well.

There are three clocking methods by which to achieve synchronization: internal, external, and recovered clocking. All three methods derive the clock signal for the reception line from the incoming data. The clock signal for the transmission line will always be generated by the devices internal oscillator, but the phase reference used by the internal oscillator differs for each of the clocking methods. When internal clocking is used, the transmit clock is phase locked to the device's own internal oscillator. For external clocking, the transmit clock is phase locked to the phase of the oscillator belonging to another device in the network. For recovered clocking, the transmit clock phase is locked to the clock derived from the incoming data.

In general, the DCE device (such as a modem) uses internal clocking, while the DTE device (such as a PC) uses external clocking and synchronizes around the DCE device. In cases where DTE-DTE or DCE-DCE connections are necessary, one device must be configured atypically, or a device such as a modem-eliminator or tail-circuit buffer must be placed between the two. However, in large networks with multiple devices this is not always possible. One solution for such networks is to have all devices synchronize around a single modem's clock source. However, this solution has the tendency to result in clock drift, and thus can potentially corrupt data. The other solution is to use recovered clocking so that a modem can derive the clock from data on its reception line then send that information out on its transmit line to be used by the next modem in line, etc.

Data BuffersThough synchronous communication enables transmission of large amounts of data at high speed, it puts in place extensive control and error-checking mechanisms to prevent data corruption. However, in full-duplex networks using bit-oriented protocols, the transmitter is most likely sending frame B before it knows if frame A was received successfully. (This is not as much of a problem in slower byte-oriented protocols where data flows in only one direction at a time.) To maintain the highest possible data rates, synchronous hardware must contain sufficient data buffers to store transmitted data (for resending if necessary) until a successful transfer is confirmed.

iii) Parallel Communication

The original 8-bit parallel port was developed by IBM in 1981 as a faster interface to dot matrix printers than the then standard one-bit serial port. The parallel port greatly increases transfer speeds by using an eight wire connector which transmits the eight bits in a byte of data simultaneously, thus sending an entire byte of data in the time it takes to send a single bit in a serial system. This byte of data is supplemented by several other handshaking signals, each sent on its own wire, which ensures that data transfer takes place smoothly.

Serial vs. Parallel Communication

The major drawback to the original parallel port or Standard Parallel Port (SPP) was that it allowed for communication in only one direction--computer to printer. While there were wires which the printer could use to indicate its status to the computer, it could do no more than put a positive or negative charge on these wires. 2.6Data Transmission

i) Parallel transmission Parallel transmission is where n wires are used to send n bits at a time. It can therefore increase the transfer speed by a factor of n over serial transmission. Because this is expensive, parallel transmission is usually limited to short distances. Parallel port sends and receives data eight bits at a time over 8 separate wires. This allows data to be transferred very quickly; however, the cable required is more bulky because of the number of individual wires it must contain. Parallel ports are typically used to connect a PC to a printer and are rarely used for much else.

ii) Serial transmission In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices. The concept of serial communication is simple. The serial port sends and receives bytes of information one bit at a time. Although this is slower than parallel communication, which allows the transmission of an entire byte at once, it is simpler and can be used over longer distances. For example, the IEEE 488 specifications for parallel communication state that the cabling between equipment can be no more than 20 meters total, with no more than 2 meters between any two devices; serial, however, can extend as much as 1200 meters.

Typically, serial is used to transmit ASCII data. Communication is completed using 3 transmission lines: (1) Ground, (2) Transmit, and (3) Receive. Since serial is asynchronous, the port is able to transmit data on one line while receiving data on another. Other lines are available for handshaking, but are not required. The important serial characteristics are baud rate, data bits, stop bits, and parity. For two ports to communicate, these parameters must match:

1. Baud rate: a speed measurement for communication. It indicates the number of bit transfers per second. For example, 300 baud is 300 bits per second. When we refer to a clock cycle we mean the baud rate. For example, if the protocol calls for a 4800 baud rate, then the clock is running at 4800Hz. This means that the serial port is sampling the data line at 4800Hz. Common baud rates for telephone lines are 14400, 28800, and 33600. Baud rates greater than these are possible, but these rates reduce the distance by which devices can be separated. These high baud rates are used for device communication where the devices are located together, as is typically the case with GPIB devices.2. Data bits: a measurement of the actual data bits in a transmission. When the computer sends a packet of information, the amount of actual data may not be a full 8 bits. Standard values for the data packets are 5, 7, and 8 bits. Which setting you choose depends on what information you are transferring. For example, standard ASCII has values from 0 to 127 (7 bits). Extended ASCII uses 0 to 255 (8 bits). If the data being transferred is simple text (standard ASCII), then sending 7 bits of data per packet is sufficient for communication. A packet refers to a single byte transfer, including start/stop bits, data bits, and parity. Since the number of actual bits depends on the protocol selected, the term packet is used to cover all instances. 3. Stop bits: used to signal the end of communication for a single packet. Typical values are 1, 1.5, and 2 bits. Since the data is clocked across the lines and each device has its own clock, it is possible for the two devices to become slightly out of sync. Therefore, the stop bits not only indicate the end of transmission but also give the computers some room for error in the clock speeds. The more bits that are used for stop bits, the greater the lenience in synchronizing the different clocks, but the slower the data transmission rate.4. Parity: a simple form of error checking that is used in serial communication. There are four types of parity: even, odd, marked, and spaced. The option of using no parity is also available. For even and odd parity, the serial port sets the parity bit (the last bit after the data bits) to a value to ensure that the transmission has an even or odd number of logic high bits. For example, if the data is 011, then for even parity, the parity bit is 0 to keep the number of logic-high bits even. If the parity is odd, then the parity bit is 1, resulting in 3 logic-high bits. Marked and spaced parity does not actually check the data bits, but simply sets the parity bit high for marked parity or low for spaced parity. This allows the receiving device to know the state of a bit to enable the device to determine if noise is corrupting the data or if the transmitting and receiving device clocks are out of sync

a) Asynchronous transmissionIt is so named because the timing of a signal is unimportant. Information is received and transmitted by agreed-upon patterns, which are based on grouping the bit stream into bytes. One start bit (0) is sent at the beginning and one or more stop bits (1) at the end of each byte. There may be a gap between each byte.

The strategy with this scheme is to avoid the timing problem by not sending long,

uninterrupted streams of bits. Instead, data are transmitted one character al a time,

where each character is five to eight bits in length. Timing or synchronization must only be maintained within each character: the receiver has the opportunity to resynchronize at the beginning of each new character.

Figure 6.1 illustrates this technique. When no character is being transmitted, the line between transmitter and receiver is in an idle state. The definition of idle is equivalent to the signaling element for binary 1. Thus, for NRZ.-L signaling which is common for asynchronous transmission, idle would be the presence of a negative voltage on the line. The beginning of a character is signaled by a start bit with a value of binary 0. This is followed by the five to eight bits that actually make up the character. The bits of the character arc transmitted beginning with the least significant bit. For example, for IRA characters, the first bit transmitted is the bit labeled b. Usually, the data bits are followed by a parity bit, which therefore is in the most significant bit position. The parity bit is set by the transmitter such that the total number of ones in the character, including the parity bit is even (even parity) or odd (odd parity), depending on the convention being used. This bit is used by the receiver for error detection.The final element is a stop element which is a binary 1, A minimum length for the stop element is specified, and this is usually 1, 1.5, or 2 times the duration of an ordinary bit. No maximum value is specified. Because the stop element is the same as the idle state, the transmitter will continue to transmit the stop element until it is ready to send the next character.If a steady stream of characters is sent, the interval between two characters is uniform and equal to the slop element. For example, if the slop element is one bit time and the IRA characters ABC are sent (with even parity bit), the pattern is 0100000101011111000101011000011 111 ... 111. The start bit (0) starts the timing sequence for the next nine elements, which are the 7-bit IRA code, the parity bit, and the stop clement. In the idle state, the receiver looks for a transition from 1 to 0 to signal the beginning of the next character and then samples the input signal at one-bit intervals for seven intervals. It then looks for the next 1-to-0 transition, which will occur no sooner than one more bit time.

The timing requirements for this scheme are modest. For example, IRA characters are typically sent as 8-bit units, including the parity hit. If the receiver is 5 percent slower or faster than the transmitter, the sampling of the eighth character bit will be displaced by 45 percent and still be correctly sampled. Figure 6-1 c shows the effects of a liming error of sufficient magnitude to cause an error in reception. In this example we assume a data rate of' 10,000 bits per second (10 kbps); therefore, each bit is of 0.1I millisecond (ms) duration. Assume that the receiver is fast by 6 percent, or 6 s per bit time. Thus, the receiver samples the incoming character every 94s (based on the transmitter's clock). As can be seen, the last sample is erroneous.

An error such as this actually results in two errors. First, the last sampled bit is incorrectly received. Second, the bit count may now he out of alignment. If bit 7 is a 1 and bit 8 is a 0, bit 8 could he mistaken for a start bit. This condition is termed a framing error, as the character plus start bit and stop clement are sometimes referred to as a frame. A framing error can also occur if some noise condition causes the false appearance of a start bit during the idle state.Asynchronous transmission is simple and cheap but requires an overhead of two to three bits per character. For example, for an 8-hit character with no priority bit, using a 1-bit-long stop element, two out of every ten bit convey no information but are there merely for synchronization; thus the overhead is 20 percent. Of course, the percentage overhead could he reduced by sending larger blocks of hits between the start bit and slop element. However, as Figure 6.1c indicates, the larger the block of bits, the greater the cumulative timing error. To achieve greater efficiency, a different form of synchronization, known as synchronous transmission, is used.

b) Synchronous transmissionIn synchronous transmission the bit stream is combined into longer "frames" which may contain multiple bytes. It is the responsibility of the receiver to group the bits. Byte synchronization is accomplished in the data link layer.

With synchronous transmission, a block of bits is transmitted in a steady stream

without start and stop codes. The block may be many bits in length. To prevent timing drift between transmitter and receiver, their clocks must somehow be synchronized. One possibility is to provide a separate clock line between transmitter and receiver. One side (transmitter or receiver) pulses the line regularly with one short pulse per bit time. The other side uses these regular pulses as a clock. This technique works well over short distances, but over longer distances the clock pulses are subject to the same impairments as the data signal, and timing errors can occur. The other alternative is to embed the clocking information in the data signal. For digital signals, this can be accomplished with Manchester or Differential Manchester encoding. For analog signals, a number of techniques can be used; for example, the carrier frequency itself can be used to synchronize the receiver based on the phase of the carrier.With synchronous transmission, there is another level of synchronization required. to allow the receiver lo determine the beginning and end of a block of data. To achieve this, each block begins with a preamble bit pattern and generally ends with a postamble bit pattern. In addition, other bits are added to the blocks that convey control information used in the data link control procedures. The data plus preamble, postamble, and control information are called a frame. The exact format of the frame depends on which data link control procedure is being used.

Figure 6-2 shows, in general terms, a typical frame formal for synchronous

transmission. Typically, the frame starts with a preamble called a flag which is eight

bits long. The same flag is used as a postamble. The receiver looks for the occurrence of the flag pattern to signal the start of a frame. This is followed by some number of control fields, then a data field (variable length for most protocols), more control fields, and finally the flag is repeated.

For sizable blocks of data, synchronous transmission is far more efficient than

asynchronous. Asynchronous transmission requires 20 percent or more overhead.

The control information, preamble, and postamble in synchronous transmission are

typically less than 100 bits.

2.7Transmission Modes

i)Half Duplex Half duplex means that signals can be passed in either direction, but not in both simultaneously. A telephone channel often includes an echo-suppressor, allowing transmission in only one direction, this renders the channel half-duplex. Echo suppressors are slowly being replaced by echo cancellers, which are theoretically full-duplex devices. With half-duplex transmission, only one of two stations on a point-to-point link may transmit at a time. This mode is also referred lo as two-way alternate, suggestive of the fact that two stations must alternate in transmitting. This can be compared to a one-lane, two-way bridge. This form of transmission is often used for terminal-to-computer interaction. While a user is entering and transmitting data, the computer is prevented from sending data to the terminal, which would appear on the terminal screen and cause confusion.

ii)Full Duplex Full duplex means that signals can be passed in either direction, simultaneously. Full duplex operation on a two-wire line requires the ability to separate a receive signal from the reflection of the transmitted signal. This is accomplished by either FDM (frequency division multiplexing) in which the signals in the two directions occupy different frequency bands and are separated by filtering, or by Echo Canceling (EC). For full-duplex transmission, two stations can simultaneously send and receive data

from each other. Thus, this mode is known as two-way simultaneous, and may be compared to a two-lane, two-way bridge. For computer to computer data exchange,

this form of transmission is more efficient than half-duplex transmission.With digital signaling, which requires guided transmission, full-duplex operation usually requires two separate transmission paths (e.g.. two twisted pairs), while hall duplex requires only one. For analog signaling, it depends on frequency: If a station transmits and receives on the same frequency, it must operate in half-duplex mode for wireless transmission- although it may operate in full-duplex mode for guided transmission using

two separate transmission lines. If a station transmits on one frequency and receives on another, it may operate in full-duplex mode for wireless transmission and in full- duplex mode with a single line for guided transmission.

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