Chapter 6: Errors, Error Detection, and Error Control

There are three types of error-control options: toss the frame packet, return an error message to transmitter, and correct the error without the transmitter's help. This chapter is a discussion about the many ways to detect errors and what to do to remove or reduce the amount of damage these errors cause.

Noise and Errors:

White noise is a continuous type of noise. An example of this is the sound a radio transmitter makes when changing stations. We hear the static noise as we switch from one channel to another. This kind of noise is dependent on the degree of temperature. The more heat means more white noise. This is due to the increase in activity of the electrons. This type of noise is also called "thermal noise" or "Gaussian noise."

To reduce white noise from a digital signal, we need to pass the signal through a signal re-generator. Reducing white noise from an analog signal, we need to pass the signal through a set of filters.

Impulse noise is the opposite of white noise. The type of noise is a noncontinuous noise and is also considered to be one of the most difficult errors to detect because they occur randomly.

Crosstalk is caused by the coupling between two different signals. An example is when telephone users had that ability to hear other phone users conversations. It is said that high humidity and wet weather causes the increase in electrical crosstalk. This kind of interference can be reduced.

Echo is the reflective feedback of a signal as the signal moves through the specified medium. This is due to open ended coaxial cables or from junctions of connected wires. The signal will move the end of the cable and bounce back from the end of the cable. To reduce the amount of echo, many companies will provide an echo suppressor that restricts the signal to only pass in one direction.

Jitter is when the rises and falls of a digital signal begin to shift. This shifting will cause the signal to become blurry which will cause videos to flicker, audio transmissions to click and break-up, and transmitted computer data to contain various errors. Causes of this phenomena can be due to electromagnetic interference, crosstalk, passing the signal through too many repeaters, and the use of lower-quality equipment. To reduce this kind of interference, proper shielding can be installed and limiting the number of a signal being repeated can help.

Attenuation is the continuous loss of a signal's strength as it travels through a medium. Amplifiers can help reduce the amount of attenuation that occurs.

Error Prevention:

The side effect of noise during transmission is that the transmitting station will have to slowdown its transmission rate. This is because a modem will connect to another modem and conduct a "fallback negotiation." This means that if one of the modems is sending "garbled" data will be instructed to "fall back" to a slower rate to help improve the ability to distinguish one value from the next. However, there are many ways to prevent the occurrence of transmission errors. Many can install wiring with proper shielding, understand that wireless applications will share wireless frequencies, replace older equipment with newer up-to-date equipment, using proper numbering of digital repeaters and analog amplifiers, and to avoid pushing mediums beyond their recommended limits.

Error Detection:

Parity checks are the most basic error-detection techniques that are used with asynchronous connections. However, because these are the most basic types of error-detection these kinds of error checks are rarely used. Most parity checks let too many errors slip by undetected. There are two forms of parity checks: the first is a simple parity and the second is a longitudinal parity.

Simple parity, or also known as a vertical redundancy check, comes in two forms: even parity and odd parity. Parity checking is when a bit is added to a string of bits to create either even parities or odd parities. Even parity is caused when the binary 0's or 1's are added to the string will produce an even number. The odd parity will produce an odd number when the binary numbers are added to teh string.

Longitudinal Parity, or known as longitudinal redundancy check, will use additional parity bits to help solve the issues of the simple parity technique.

Arithmetic checksum is a technique where the characters that are to be transmitted will be "summed" together. These summed numbers are then added to the end of the message that is being transmitted. The receiving end will accept the message and perform the same summing operations to compare the receiving ends value with senders value. If both values match, there were no errors in the message. However, if the values do not match, this means that an error has occurred.

Cyclic redundancy checksum will add 8 to 32 check bits to a large packet of data. This kind of technique will treat the message as a polynomial. Operations to solve the polynomial will be performed on the receiving end of the message. If the division portion of solving the polynomial results in the remainder of 0, this means that there were no errors within the message. But, if the receiving end is unable to produce a remainder of 0, then there was an error within the message.

Error Control:

Error control will either use one of the following techniques to fix an error: toss the frame/packet, return a message to the transmitter to resend the data packet, or correct the error without re-transmission.

Toss the frame/packet is used to detect an error, but the error will be discarded without any thought to the where the error may have been cause. This kind of technique is normally used with mediums such as fiber optic cables. Because the fiber optic cable is used to transmit data from one end to the other, many will use the toss the frame/packet method because the error ratio is so low for this kind of technology.

Return a message is sending a message back to the transmitter to notify the error and to resend the same message without the error. There are two basic versions of returning a message: stop-and-wait error control and the sliding window. The stop-and-wait error control occurs when station A will send station B a message and then will "stop-and-wait" for a response from station B. The response will either confirm to station A that there was no error or it will notify station A of an error. Sliding window error control is produced as a flow control scheme that will allow a station to transmit a number of data packets at one time before receiving any kind of acknowledgement from the receiver.

Correcting the error is when the error is corrected. But, for this technique to occur there must be redundant data present within the message, which is called the forward error correction process.  

Chapter 5: Making Connections: Multiplexing and Compression

For multiple signals to be transmitted through one medium or multiplexing, that medium must be "divided" to give each signal a portion of the total bandwidth. Bandwidth is a range of frequencies within a given band. This medium can be divided into three ways: division of frequencies, division of time, and division of transmission codes.

Frequency Division Multiplexing:

Frequency division multiplexing (FDM) is the assignment of non-overlapping frequency ranges to each "user" of a medium.  The technique used by FDM will assign each user a different channel when sharing the medium for multiple signals. A channel is an assigned set of frequencies that is used to transmit the user's signal. When assigning these channels, the signals are either discrete analog (digital) or analog. Radio stations, cable stations, and various telecommunications companies are all examples of businesses that use the frequency division multiplexing technique for transmitting multiple signals through one medium.

When assigning these channels dynamically to various users, it can be less wasteful to do so. However, when channels are static, much like radio stations, wastefulness can occur. Radio stations will assign guard bands which are frequencies that are inserted between two different signals to a form of insulation. This will prevent the interruption of other signals that are close in range.

Devices that accept incoming transmissions from one or more users, this device is normally called the multiplexor. The device that is attached to the receiving end of the transmission that splits the signal into different channels is called the demultiplexor.

Time Division Multiplexing:

Time division multiplexing allows only one user at a time to transmit signals. To share the medium for transmission, the medium will be divided by available transmission time. The users will have the entire spectrum of the bandwidth, but it will only be for a short amount of time. Time division multiplexing has been divided into two separate technologies: synchronous time division multiplexing and statistical time division multiplexing.

Synchronous time division multiplexing (sync TDM) gives each incoming source signal a turn to be transmitted. The sync TDM accepts one piece of data from device one and transmits it and then accepts another piece of data from a second device and transmits that data. The technique will then start over by accepting another piece of data from the first device and transmitting it and then moving to the second device.

If a device has nothing to transmit, then the sync TDM must still allocate a time slot for the device. The time slot will only be empty when the data its transmitted. If only one device is transmitting data out of five other devices, the sync TDM must still allocate time slots for each device.

To maintain synchronization between the multiplexor and demultiplexor, the data that is being transmitted will be packaged into simple frames that will include synchronization bits. There are two types of synchronous time division techniques which are T-1 multiplexing and SONET/SDH. The T-1 multiplexing technique provides 24 separate digitized voice/data channels of 64 kbps each. Users can use all 24 channels or some can request a fraction of the 24 separated channels. This technique works exactly the same way as we had explained in the beginning of this section. The only difference is that there are 24 digitized voice/data channels.

Synchronous optical network (SONET) and synchronous digital hierarchy (SDH) are two standards for multiplexing data over single mediums. Both techniques are synchronous multiplexing techniques, but they both have a single clock that controls the timing of all transmissions. Both are also capable of transmitting varying speed streams of data onto on fiber optic cable. The SONET technique uses data transmission rates called synchronous transport signals (STS). The STS are supported by physical specifications called optical carriers (OC). The reason for the data rate relationship is to keep SONET simple. Each SONET frame contains 810 bytes which is equivalent to 6480 bits. The frames will also consist of a section overhead, line overhead, path overhead, data, and the synchronous bit.

Statistical time division multiplexing (Stat TDM) transmits data that is from active devices only. This technology does not transmit empty time slots. If there are two devices that are active out of a total of four devices, the multiplexor will only send data from the two active devices. If that were to change, however, the multiplexor will have to provide an address within each byte of data to identify which device had sent the data. To transmit various forms of data from each data, the multiplexor will add a length field that will describe the length of data being transmitted. The frame layout produced by Stat TDM includes a flag bit, control bit, packets that include the length and address, the frame check sequence (FCS), and then another flag bit.

Wavelength Division Multiplexing:

Wavelength division multiplexing(WDM) multiplexes multiple data streams onto a single fiber-optic line. This technique assigns input sources to separate sets of frequencies. Wave division multiplexing uses different wavelengths (frequencies) lasers to transmit various signals at the same time over a medium. Lambda are wavelengths or differently colored frequencies that are transmitted.

The division multiplexing technique is also considered to be scalable.

• Coarse wavelength division multiplexing (CWDM) – a less expensive form of wavelength division multiplexing that involves the transfer of a small number of streams of data over a single optical fiber using multiple lasers of differing wavelengths
• Code division multiplexing (CDM) – a multiplexing technique used extensively by the military and cellular phone companies in which binary 1’s and 0’s are replaced with larger, unique, binary sequences to allow multiple users to share a common set of frequency.
• Compression – the process of manipulating data such that it fits into a more compact space
• Demultiplexor – a multiplexer that un multiplexes that data stream and delivers the individuals streams to the appropriate devices
• Dense wavelength division multiplexing (DWDM) – an expensive form of wavelength division multiplexing that involves the transfer of a large number of data stream over a single optical fiber using multiple lasers of differing wavelengths
• Discrete multitone (DMT) – a modulation technique used in digital subscriber line
• DS-1 signaling – the signaling technique used to transfer 1.544 MBPS over a T-1 system
• Fiber exhaust – what happens when a fiber optic cable is transmitting at its maximum capacity
• Frequency division multiplexing (FDM) – the oldest and one of the simplest multiplexing techniques, FDM involves a signing non-overlapping frequency ranges to different signals or to each user of a medium
• Guard band – a set of unused frequencies between two channels on a frequency division multiplex system
• JPEG – a technique commonly used to compress video images
• Lambda – in wavelength division multiplexing the wavelength of each differently colored laser
• Lossless compression – a compression technique in which data is compressed and then decompressed such that the original data is returned and no data is lost
• Lossy compression – a compression technique in which data is compressed and then decompressed but this process does not return the original data, some data is lost due to compression
• MP3 – a compression technique that allows a high-quality audio sample to be reduced to a much smaller size file
• MPEG – a technique used to compress motion picture images or moving videos
• Multiplexing – transmitting multiple signals on one medium at the same time
• Multiplexor – the device that combines multiple input signals for transmission over a single medium and then demultiplexes the signal back into multiple signals
• Perception encoding – a compression technique applied to audio and video files in which aspects of the data with characteristic that are usually not noticed by the average person are removed from the data or compressed
• Run-length encoding – a compression technique in which a commonly occurring symbol in a data set is replaced with a simpler character and a count of how many times that symbol occurs
• Statistical time division multiplexing (Stat TDM) – a form of time division multiplexing in which the multiplexor creates a data packet of only those devices that have something to transmits
• Synchronous Digital Hierarchy (SDH) – a high-speed synchronous time division multiplexing technology developed in Europe by ITU-T that uses fiber-optic cables for a high-bandwidth transmission in the megabit to gigabit range for a wide variety of data types. Almost identical to SONET
• Synchronous Optical Network (SONET) – A high-speed synchronous time division multiplexing technology developed in the United States by ANSI that uses fiber optic cables for high bandwidth transmission in the megabit to gigabit range for a wide variety of data types. Two common users of SONET are the telephone company and companies that provide an internet backbone service Almost identical to SDH
• Synchronous time division multiplexing (Sync TDM) – a multiplexing technique that gives each incoming source a turn to transmit, proceeding through the sources in round-robin fashion
• Synchronous transport signals (STS) – the signaling techniques that used to support SONET transmissions when the data is transmitted in electrical form and not in optic form
• T-1 Multiplexing – A type of synchronous time division multiplexing (involving T-1 multiplexors) where the data stream is divided into 24 separate digitized voice/data channels of 64 kbps each. Together, T-1 multiplexing and T-1 multiplexers provide a T-1 service.
• Time division multiplexing (TDM) – a multiplexing technique in which the sharing of a single is accomplished by dividing the available transmission time on a medium among the medium users
• Wavelength division multiplexing (WDm) – the multiplexing of multiple data streams onto a single fiber-optic cable through the use of lasers of varying wavelengths