Data Comm Chp 3
phase
refers to the direction in which the wave begins; it's measured in number of degrees
time division multiplexing (TDM)
share a communication circuit among 2 or more computers by having them take turns dividing the circuit vertically; one character is taken from each computer in turn, transmitted down the circuit, and delivered to the appropriate device at the far end; time is allocated even when data isn't being transmitted; it's more efficient than FDM because it doesn't need guardbands, which use "space" on the circuit which could otherwise be used to transmit data
analog transmission
occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern much like the human voice; modems translate the digital binary data produced by computers into the analog signals required by voice transmission circuits.
digital subscriber line (DSL)
one approach to changing the way data are transmitted in the local loop to provide higher speed data transfer; it combines analog transmission and FDM to provide a set of voice and data circuits; with DSL a modem is needed, the modem is first an FDM device that splits the physical circuit into 3 logical circuits: a standard voice circuit used for telephone calls, an upstream data circuit from the customer to the telephone switch, and a downstream data circuit from the switch to the customer
baud rate
a baud is a unit of signaling speed used to indicate the number of times per second the signal on the communication circuit changes; baud rate and symbol rate are the same thing
byte
a group of consecutive bits that's treated as a unit or character; one byte is composed of 8 bits and usually represents one character; 3 predominate coding schemes: ~ ASCII- most popular code for data communications and is the standard code on most microcomputers, there are 2 types- 7bit code that has 128 characters and an 8bit code that has 256 combinations ~ ISO 8859- standardized by the International Standards Org, it's an 8bit code that includes ASCII codes plus non-English letters (European) ~ Unicode- many different versions of Unicode, UTF-8 is an 8bit version which is very similar to ASCII, UTF-16 uses 16bits per character (i.e. 2 bytes, called a "word")
frequency modulation (FM)
a modulation technique whereby each 0 or 1 is represented by a number of waves per second; amplitude doesn't vary; one frequency (i.e., a certain number of waves per second) is the symbol defined to be a 1, and a different frequency (a different number of waves per second) is the symbol defined to be a 0
quadrature amplitude modulation (QAM)
a popular technique; involved splitting the symbol into 8 different phases (3 bits) and 2 different amplitudes (1 bit), for a total of 16 different possible values; one symbol in QAM can represent 4 bits, while 256-QAM send 8 bits per symbol.
serial transmission
a stream of data is sent over a communication circuit sequentially in a bit-by-bit fashion; the transmitting device sends one bit, then a second bit, and so on, until all the bits are transmitted; it takes 'n' iterations or cycles to transmit 'n' bits; serial transmission is considerably slower than parallel transmission- 8 times slower in the case of 8bit ASCII
half-duplex transmission
a two-way transmission, but can transmit in only one direction at a time; similar to a walkie-talkie link; computers use control signals to negotiate which will send and which will receive; the time it takes to switch between sending and receiving is called turnaround time.
coaxial cable
a type of guided media that is quickly disappearing; it has a copper core (the inner conductor) with an outer cylindrical shell for insulation; because they have additional shielding provided by their multiple layers of material, coaxial cables are less prone to interference and errors than basic low cost twisted pair wires
wavelength division multiplexing (WDM)
a version of FDM used in fiber-optic cables; it works by using lasers to transmit different frequencies of light through the same fiber-optic cable; as FDM, each logical circuit is assigned a different frequency, and the devices attached to the circuit don't "know" they are multiplexed over the same physical circuit; dense WDM (DWDM) is a variant of WDM that further increases the capacity of WDM by adding TDM to WDM; DWDM permits up to 40 simultaneous circuits, each transmitting up to 10 Gbps, giving a total network capacity in one fiber-optic cable of 400 Gbps
data rate
calculated by multiplying the number of bits sent on each symbol by the maximum symbol rate
data compression
can increase throughput of data over a communication link by literally compressing the data; V.44 is the ISO standard for data compression; the reduction by V.44 compression depends on the actual data sent but usually averages about 6:1
full-duplex transmission
can transmit in both directions simultaneously with no turnaround time; this means that the available capacity in the circuit is divided- half in one direction and half in the other
voice over internet protocol (VoIP)
commonly used to transmit phone conversations over digital networks; it's a relatively new standard that uses digital telephones with built-in codecs to convert analog voice data into digital data.
frequency division multiplexing (FDM)
described as dividing the circuit "horizontally" so that many signals can travel a single communication circuit simultaneously; it's divided into a series of separate channels, each transmitting on a different frequency- like TV and radio; guard-bands are the unused portions of the circuit that separate these frequencies from each other
phase modulation (PM)
most difficult to understand; phase refers to the direction in which the wave begins
microwave transmission
extremely high-frequency radio communication beam that is transmitted over a direct line-of-sight path between any 2 points; a microwave signal is an extremely short wavelength; microwave radio transmissions perform the same functions as cables; because they approach frequency of visible light waves, they exhibit many of the same characteristics as light waves, such as reflection, focusing, or refraction; microwave signals can be focused into narrow, powerful beams that can be projected over long distances; this transmission medium is typically used for long-distance data or voice transmission
fiber-optic cable
instead of carrying telecommunication signals in the traditional electrical form, this technology uses high-speed streams of light pulses from lasers or LEDs that carry info inside hair-thin strands of glass called optical fibers; one of the main advantages of fiber optics is that it can carry huge amounts of info at extremely fast data rates; the earliest fiber-optic systems were multimode, meaning that the light could reflect inside the cable at many different angles- which were plagued by signal weakening and dispersion.
twisted pair cable
insulated pairs of wires that can be packed quite close together; the wires are usually twisted to minimize the electromagnetic interference between one pair and any other pair in the bundle
channel
it's a separate logical circuit, and the devices connected to them are unaware that their circuit is multiplexed; FDM is used to divide one circuit into 4 channels
point-to-point circuit
it's named so because it goes from one point to another; this type of configuration is used when the computers generate enough data to fill the capacity of the communication circuit.
codec
it's possible to translate analog voice data into digital form for transmission over digital computer circuits using a device called codec; a device or program that compresses data to enable faster transmission and decompresses received data.
multi-point circuit
many computers are connected on the same circuit- which means that each must share the circuit with the others; the disadvantage is that only one computer can use the circuit at a time- when one computer sends/receives data, all others must wait; the advantage is that they reduce the amount of cable required and typically use the available communication circuit more efficiently.
radio transmission
one of the most commonly used forms of wireless media is "radio"; when people used the term "wireless" they usually mean radio transmission; each device or computer on the network has a radio receiver/transmitter that uses a specific frequency range that doesn't interfere with commercial radio stations.
simplex transmission
one-way transmission, such as that with radios and TVs
hertz
same as cycles per second; for example, 3,000 Hz is 3,000 cycles per second
satellite transmission
similar to microwave transmission except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles up in space; one disadvantage is the "propagation delay" that occurs because the signal has to travel out into space and back to earth, a distance of many miles that even at the speed of light can be noticeable; low earth orbit (LEO) satellites are placed in lower orbits to minimize propagation delay
physical circuit
the "actual" wire used to connect two devices; we are referring to the physical media that carry the message we transmit.
pulse code modulation (PCM)
the North American telephone network uses PCM; the input voice signal is sampled 8,000 times per second; each time the input voice signal is sampled, 8 bits are generated; therefore transmission speed on the digital circuit must be 64,000 bps (8 bits per sample x 8,000 samples per second) to transmit a voice signal when it's in digital form
amplitude modulation (AM)
the amplitude or height of the wave is changed; in the AM the highest amplitude symbol (tallest wave) represents a binary 1 and the lowest amplitude symbol represents a binary 0; AM is more susceptible to noise (more errors) during transmission than is frequency modulation or phase modulation
bandwidth
the difference between the highest and the lowest frequencies in a band of set of frequencies
modulation
when we transmit data through the telephone lines, we use the shape of the sound waves we transmit to represent different data values; we do this by transmitting a simple sound, wave through the circuit and then changing its shape in different ways to represent a 1 or 0; the technical term used to refer to these "shape changes"
statistical time division multiplexing (STDM)
the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines; it allows more terminals or computers to be connected to a circuit than does FDM or TDM; it's called statistical because selection of transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed; provides more efficient use of the circuit and saves money; there's 2 problems: can cause time delays- the multiplexed circuit cannot transmit all the data it received because of lack of sufficient capacity (therefore needs internal memory), second is that because logical circuits aren't permanently assigned to specific devices as they are in FDM and TDM the data from one device are interspersed with data from another.
coding
the groups of bits representing the set of characters that are the "alphabet" of any given system
amplitude
the height of the wave; measured in decibels
frequency
the length of the wave, usually expressed as the number of waves per second; expressed in hertz
bit rate
the number of bits transmitted; a bit is a unit of info; bps = bits per second
quantizing error
the original signal had a smooth flow, but the digitized signal has jagged "steps"; the difference between the 2 signals is called quantizing error; there are 2 ways to reduce quantizing error and improve the quality of the digitized signal: first is to increase the number of amplitude levels, second is to sample more frequently (it'll improve signal quality)
digital transmission
the transmission of binary electrical or light pulses in that it only has 2 possible states, a 1 or 0; digital signals are usually sent over wire of no more than a few thousand feet in length; ~ unipolar signaling- voltage is always positive or negative ~ bipolar signaling- the 1s and 0s vary from a +voltage to a -voltage; there's non-return zero (NRZ), return to zero (RZ), and alternate mark inversion (AMI); bipolar signaling experiences fewer errors than unipolar signaling because the symbols are more distinct
guardbands
the unused portion of the circuit that separates these frequencies from each other
parallel transmission
the way the internal transfer of binary data takes place inside a computer
multiplexing
to break one high-speed physical communication circuit into several lower-speed logical circuits so that many different devices can simultaneously use it but still "think" that they have their own separate circuits; two multiplexers are needed for each circuit: one to combine the 4 original circuits into the one multiplexed circuit and one to separate them back into 4 separate circuits; primary benefit is to save money by reducing the mount of cable or the number of network circuits that must be installed
symbol rate
to successfully send and receive a message, both sender and receiver have to agree on how often the sender can transmit data
modem
translates the computer's digital data into analog data that can be transmitted through the voice communication circuits, and a second modem at the receiver's end translates the analog transmission back into digital data for use by the receiver's computer.
Manchester encoding
used by Ethernet; a special type of bipolar signaling in which the signal is changed from high to low or from low to high in the middle of the signal; a change from high to low is used to represent a 0, whereas the opposite (a change from low to high) is used to represent a 1.
logical circuit
used to connect two devices, which refers to the transmission characteristics of the connect, such as when we say a company has a T1 connection into the internet.
