Vocab Ch. 5-9 Flashcards
multiplexing
The technique or transmitting multiple signals over a single medium. Makes a connection more efficient by combining the signals from multiple sources.
medium division methods
1) division of frequencies
2) division of time
3) division of transmission codes
Frequency Division Multiplexing
(FDM) - The assignment of non-overlapping frequency ranges to each “user” of a medium. Signal is often analog.
Pros -
1) Simple
2) Popular with radio, TV, cable TV
3) All the receivers, such as cellular phones, do not need to be at the same location.
Cons -
1) High overhead because each slot must be allocated to one signal.
2) Uses analog signals so noise is a problem.
channel
An assigned set of frequencies that is used to transmit the user’s signal.
dynamic assigned channels
When the users establish a connect, the channels they use as uplink and downlink are assigned based on current network availability.
multiplexor
The general term used for a device that accepts input from one or more users and transmits it over one medium.
demultiplexor
Also called the second multiplexor - The general term used for a device that receives the signal and then splits off each signal to deliver it to the appropriate receiver.
guard band
A set of unused frequencies between two signals that keeps one signal from interfering with another signal.
time division multiplexing
(TDM) - Allows only one user to transmit at a time, and the sharing of the medium is accomplished by dividing available transmission time among users. The user used the entire bandwidth of the channel, but only for a brief moment.
Synchronous time devision multiplexing
(Sync TDM) - Gives each incoming source signal a turn to be transmitted, proceeding through the sources in a round-robin fashion.
Pros -
1) digital signals so noise is less of a problem
2) relatively simple
3) commonly used with T-1, SONET
Con -
1) High overhead because each slot must be allocated to one signal.
T-1 multiplexing
The output stream is divided into 24 separate digitized voice/data channels of 64kbps each. The T-1 can be allocated in full or in part. The T-1 multiplexed stream is a continuous repetition of frames.
DS-1 signaling
The multiplexing technique used by T-1 telecommunications to provide multiplexing of up to 24 separate channels at a total speed of 1.544 Mbps.
256=2^8
The telephone system uses 256 quantization levels to digitize the human voice into 8 bits which equals one byte.
Synchronous Optical Network & Synchronous Digital Hierarchy
(SONET) & (SDH) - Powerful standards for multiplexing data streams over a single medium. They are almost identical standards and for the high-bandwidth transmission of a wide range of data types over fiber-optic cable.
synchronous transport signals
(STS) - A hierarchy of signaling levels, or data transmission rates. Each STS level supports a particular data rate and is supported by a physical specification called an optical carrier (OC). Note: The data rate of an OC-3 is exactly three times that of an OC-1. Often used by Internet Backbone providers.
statistical time division multiplexing
(Stat TDM) - Transmits data only for active users and does not transmit empty time slots. The system used a complex frame that contains data only from those input sources that have something to send.
Pros -
1) More efficient use of bandwidth.
2) Frame can contain control and error information
3) packets can be of varying size
Cons -
1) Increase complexity over TDM, so increased initial cost that is often made up due to decrease operational costs.
fiber exhaust
The inability of a single fiber-optic line to meet a user’s needs.
wavelength division multiplexing
(WDM) - multiplexes multiple data streams onto a single fiber-optic line. It uses different wavelength (frequency) lasers to transmit multiple signals at the same time over a single medium. Pros - 1) good for transmitting multiple concurrent signals over a fiber-optic line. 2) Signals can have varying speeds 3) Scalable Cons - 1) Equipment is costly. 2) Complexity
lambda
The wavelengths of each differently colored laser in a WDM multiplexor.
dense wavelength division multiplexing
(DWDM) - When a WDM multiplexor can support large numbers of lambdas.
coarse wavelength division multiplexing
(CWDM) - a less expensive variation of WDM that is designed for short-distance connections and has only a few lambdas.
discrete multitone
(DMT) - a multiplexing techinique commonly found in DSL systems. It combines hundreds different signals, or sub channels, into one stream; unlike the other multiplexing streams, however, DMT is designed such that all these sub channels are destined for a single user. Each sub channel can perform its own quadrature amplitude modulation (QAM). Pros 1) High speeds Cons - 1) Costly 2) Complex 3) Suffers greatly from too much noise.
code division multiplexing
(CDM) - A relatively new technology that has been used extensively by both the military and cellular telephone companies. Allows multiple users to share a common set of frequencies by assigning a unique digital code to each user. Based on a class of modulation techniques known as spread spectrum technology. Pros - 1) Large capacities 2) Scalable Cons - 1) Complexity
spread spectrum technology
A technique used in the communications industry for modulating a signal into a new signal that is more secure and thus more resistant to wiretapping. Falls into two categories:
1) frequency hoping
2) direct sequence
chip spreading codes
The binary code, 64 bits in length, assigned to each cell phone on the cellular network so that the network can determine what data is sent by which device.
compression
The process of taking data and somehow packing more of it into the same space. whether this is in the form of a storage device such as a hard drive, or a medium such as a fiber-optic line.
compression categories
1) lossless compression
2) lossy compression
lossless compression
No data is lost due to compression. Usually relies on repetitive strings of 1s and 0s.
lossy compression
Some data is lost during the process of compression and decompression. Usually, the lost data is data that is designated as less important. Example, frequencies of sound in a song that most humans can not hear.
run-lenght encoding
Replaces any repetitions of the same bit or byte that occur in a sequence of data with a single occurrence of the bit/byte and a run count, or simply with a run count.
perceptual encoding
Also know as perceptual noise shaping. The process where audio engineers remove the sounds that are at the edge of the normal hearing range.
differential encoding
Used in video encoding (MPEG) where only the differences between frames is encoded.
error-contol options
1) Ignore the error
2) Return an error message to the transmitter (request the packet again)
3) Correct the error without help from the transmitter.
white noise
Also called thermal noise or Gaussian noise, is a relatively continuous type of noise and is much like the static you hear when a radio is being tuned between two stations. It is always present to some degree and is dependent on the temperature of the medium. Because it is relatively continuous, it can be reduced significantly but never completely eliminated. Easier to remove from digital signals.
impulse noise
Noise spike, is a noncontinuous noise and one of the most difficult errors to detect because it can occur randomly and it is difficult to separate the noise from the signal. Easier to remove from digital signals but at times the impulse can overwhelm the digital signal.
error effect of transmission speed
The higher the transmission speed, the more difficult it is to detect and correct for noise.
crosstalk
An unwanted coupling between two different signal paths.Caused when the signal from one wire leaks into another. Can be reduced by twisted pair and shielding.
echo
The reflective feedback of a transmitted signal as the signal moves through the medium. The signal will hit the end of a cable, bounce back, and interfere with the original signal. Often found in coaxial systems without endpoint filters.
jitter
The result of small timing irregularities that become magnified during the transmission of digital signals as the signals pass from one device to another. Often seen as flickering video in video systems.
attenuation
The continuous loss of a signal’s strength as it travels through a medium.
white noise prevention
Install special filters for analog signals; implement digital signal regeneration for digital signals.
impulse noise prevention
Install special filters for analog signals; implement digital signal processing for digital signals.
crosstalk prevention
Shielded and twisted pair in addition to fiber.
echo prevention
Install proper termination of cables
jitter
Use better quality circuitry, use fewer repeaters, slow the transmission speed.
attenuation prevention
Install amplifiers on analog signals and repeaters on digital signals.
simple parity
(Occasionally known as vertical redundancy check) is the easiest error-detection method to incorporate into a transmission system. It comes in two basic forms:
1) even parity
2) odd parity
A check sum is added to a series of bits to create either odd or even parity.
Does not detect swapped bits.
even parity
The 0 or 1 added to the string produces an even number of binary 1s.
odd parity
The 0 or 1 added to the string produces an odd number of binary 1s.
longitudinal parity
Also called longitudinal redundancy checker horizontal parity, tries to solve the main weakness of simple parity - that all an even number of error can not be detected. Uses additional parity check bits. Better than simple but still a high number to check bits.
arithmetic checksum
A sum is added to the data before transmitting and then the receiver adds that same number to the data to check to see if the two sums match.
cyclic redundancy checksum
(CRS) or cyclic checksum, method typically adds 8 to 32 check bits to potentially large data packets and yields an error-detection capability approaching 100 percent.
generating polynomial
An industry-approved bit string used to create the cyclic checksum remainder.
error control
The action the receiver takes once an error is detected.
stop-and-wait error control
A workstation (Station A) transmits one packet of data to another workstation (Station B), and then stops and waits for a reply from Station B. Four possible outcomes:
1) Station B gets a good packet and send back a positive acknowledgement (ACK), Station A transmits the next packet.
2) If the data has an error, Station B responds with a negative acknowledgment (NAK) or (REJ). If Station A receives a (NAK) or (REJ), Station A retransmits the packet.
3) The packet arrives at Station B, B sends an ACK but Station A does not get it. After a timeout, Station A resends the last packet.
4) Station A sends a packet but the packet is lost. Station A waits for the response, times out, and resends.
sliding window protocol
A flow control scheme that allows a station to transmit a number of data packets before receiving some form of acknowledgement.
piggybacking
When a receiver includes an ACK with other data that it needs to transmit to the other station.
forward error correction
Redundant data is sent so that the receiver can correct the errors.
Hamming code
A specially designed code in which special check bits have been added to data bits such that, if an error occurs during transmission, the receiver might be able to correct the error using the included check and data bits.
Hamming distance
The characteristic of a code. The smallest number of bits by which character codes differ. If the character set has a Hamming distance of 1, it is not possible to detect errors, nor correct them. Hamming distance of 2, you can detect errors but not correct them. Hamming distance of 3 allows for single-bit error correction and double-bit error detection.
local area network
(LAN) - A communications network that interconnects a variety of data communications devices within a small geographic area and transmits data at high data transfer rates.
primary function of a LAN
1) file serving
2) database and application serving
3) print serving
4) Internet accessing
5) e-mailing
6) video & music transfers
7) process control and monitoring
8) distributed processing
file server
A workstation with a large storage disk drive that acts as a central storage repository.
print server
Software that provides workstations with the authorization to access a particular printer, accepts and queues print jobs, prints cover sheets, and allows users access to the job queue for routine administrative functions.
common applications for a LAN
1) office environment
2) academic environment
3) manufacturing
LAN advantages/disadvantages
Pros:
- The ability to share resources in an economical and efficient manner.
- component evolution can be independent of system evolution
Cons:
- expensive to implement
- expensive to maintain
- all devices are not always interoperable
- user licensing issues
- maintenance and control requires many hours of dedication and service
bus/tree area network
(bus LAN) - The first physical designs, it is simply a linear coaxial cable into which multiple devices or workstations tap.
Difficult to add a workstation unless a tap already exists.
passive device
A tap in the network that does not alter the signal and does not require electricity to operate.
network interface card
(NIC) - An electronic device, sometimes in the form of a computer circuit board or part of a larger circuit board, that performs the necessary signal conversions and protocol operations that allow the workstation to send and receive data on the network.
types of signaling technologies
1) baseband - typically uses a single digital signal to transmit over the bus, which uses the entire spectrum of the cable so only one signal can be transmitted at a time. It is also bidirectional, so when a signal is introduced in the line, it propagates away from the source in both directions on the cable.
2) broadband - Uses analog signaling in the form of frequency division multiplexing to divide the available medium into multiple channels. It is also possible to split and join broadband cables and signals to create configurations more complex than a single linear bus.
star-wired bus LAN
Looks like a star. Today’s modern version looks like a star but acts logically like a bus.
logical design
The design of the network, it determines how the data moves around the network from workstation to workstation.
physical design
The design of the network that refers to the pattern formed by the locations of the elements of the network, as it would appear if drawn on a sheet of paper.
hub
A relatively non-intelegent device that simply and immediately retransmits the data it receives from any workstation out to all other workstations (or devices) connected to the hub.
medium access control protocol
Software that allows a device to place data onto a hub-based local area network (as well as other networks that require their workstations to compete for access to the network). Because a LAN is essentially a single bus using baseband technology, it is imperative that only one workstation at a time be allowed to transmit its data onto the network.
medium access control protocol categories
1) Contention-based protocols, such as carrier sense multiple access with collision detection.
2) Round-robin protocols, such as token passing.
contention-based protocol
Basically, a first-come, first-served protocol.
carrier sense multiple access with collision detection
(CSMA/CD) - Was found almost exclusively on star-wired bus and bus LANs, and was the most widely used medium access control protocol.
Basically, a half-duplex protocol, the workstations listen to the network waiting for a clear network, and then transmits. While it is transmitting, it continues to listen, normally only hearing its own transmission.
collision
When two or more workstations listen to the medium at the same moment, hear nothing, and then transmit data at the same moment.
collision window
The interval of time that it takes for the signal from the work station to travel, on average, halfway down the bus, collide with the other workstation’s signal, and then propagate back down the bus to the first station.
nondeterministic protocol
A protocol in which you cannot calculate the time at which a workstation will transmit.
switch
A device that uses addresses and processing power to direct a frame out of a particular port, thus reducing the amount of traffic on the network. The switch acts as a filter, instead of sending it out to all locations on the bus, it sends it only to the device it is addressed to.
