MIS 140 Chapter 3
One commonly used VoIP standard
G.722 wideband audio a version of ADPCM that operates at 64 Kbps - samples at 8,000 per second and produces 8 bits per second
Time division multiplexing (TDM)
a communication circuit is shared among two or more computers by having them take turns, dividing the circuit "vertically" A character is taken from each computer in turn doesn't require guardbands
Coaxial cable
a type of guided media that is quickly disappearing copper core as an inner conductor with an outer cylindrical shell for insulation - outer shield, just under the shell, is the second conductor way more costly than twisted pair cable with no extra benefit
Circuit configuration
the basic physical layout of the circuit
Symbol
the pattern of electricity, light, or radio wave that is used to represent a 0 and a 1
In clean, error free, and interference free analog transmission, what three basic factors could be changed to obtain a higher data rate?
--Boost frequency range on the transmission line --Go from two waves per symbol to one wave per symbol --Increase our modulation to send more bits per symbol
Gigabit Ethernet
--SymRate = 125 MHz --Bits/Symbol = 2 (x 4 lines parallel) ---Detection = Voltage levels ---Data Rate = "1Gbps" (Actually 800 Mbps because every 5th bit is a control bit with no actual data)
When sending four bits at a time using frequency modulation, the number of different frequency levels that would be needed would be _______.
16?
4B5B encoding
4 bits are mapped into 5 bits no more than 4 1s or 0s in a row - 5th bit will be the opposite in order to delineate each 4 bit section - helps detect errors
Bandwidth of a standard telephone line
4000 Hz
How many bits are required to sample an incoming signal 4000 times per second using 64 different amplitude levels?
6 bits
Multipoint circuit
AKA *shared circuit* Many computers on the same circuit - only one can use circuit at a time
Customer premises equipment (CPE)
A DSL modem It is first an FDM device that splits the physical circuit into three logical circuits: a standard voice circuit for phone calls, an upstream data circuit, and a downstream circuit
Point to point circuit
AKA *dedicated circuit* Goes from one computer to another used when computers generate enough data to fill the capacity of the circuit
Baud rate
A unit of signaling speed used to indicate the number of times per second the signal on the communication circuit changes AKA symbol rate
Fast Ethernet
AKA -* 100Base-T* --SymRate = 125 MHz --Bits/Symbol = 0.8 (4B5B encoding) ---Detection = Change in voltage level ---Data Rate = 100Mbps --Binary 0 = no voltage change --Binary 1 = step voltage
Manchester
AKA -* 10Base-T* --SymRate = 10 MHz --Bits/Symbol = 1 ---Detection = Direction of voltage change ---Data Rate = 10Mbps
Bandwidth on Demand Interoperability Networking Group
AKA BONDING an IMUX standard that allows clients and hosts to communicate using IMUX most common use of BONDING is room-to-room video conferencing
Amplitude modulation (AM)
AKA amplitude shift keying (ASK) the amplitude is changed - one wave height representing 1 and another representing 0 most suseptible to noise (errors)
Data rate
AKA bit rate calculated by multiplying the number of bits sent on each symbol by the maximum symbol rate
Frequency modulation (FM)
AKA frequency shift keying (FSK) A different wave length is used for a 1 and a 0
Phase modulation (PM)
AKA phase shift keying (PSK)
Local loop
AKA the last mile The wires that run from your home or business to the telephone switch that connects your local loop into the telephone network
Statistical time division multiplexing (STDM)
Allows more computers on a circuit than TDM or FDM selection of transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be mutliplexed
Three important characteristics of sound waves that are transmitted through the voice circuit
Amplitude - the height(and depth) of the wave measured in decibles Frequency - length of the wave, measured in waves per second (Hertz or Hz) Phase - refers to the direction in which the wave begins, measured in the number of degrees
What are the two types of data that can flow through the circuit
Analog and digital
Sending multiple bits simultaneously in analog form
Can assign different amplitudes, frequencies, or phase degrees to different bits Can only go so far before differences become too small to distinguish from noise Can also combine modulation techniques in same transmission
Codec
Code/Decode used to send analog voice data over a digital circuit in the same way that a pair of modems is used to translate data to send across the circuit
Data compression and the ISO standard for data compression
Data compression can be used to compress data in order to increase throughput rates in modems V.44 - uses Lempel-Ziv encoding ---Lempel-Ziv encoding produces a dictionary of 2, 3, and 4 character combinations ---every time a repeated character combination occurs, the encoding sends an index to the dictionary, rather than the actual characters ---averages a compression of around 6:1
Take-aways for management
Digital is better (see reasons at beginning of chapter) Because analog and digital data is able to converge in physical networks, voice telecommunications is increasingly under the purview of IS organizations Consolidation of manufacturers of networking equipment for organizations and for telephone companies
5 advantages to digital transmission over analog
Fewer errors Higher maximum transmission rates More efficient More secure Simpler to integrate voice, video, and data on same circuit
Two ways to reduce Quantizing error
Increase the number of amplitude levels, which minimizes the differnce between the levels and results in a smoother signal Sample more frequently, which reduces the "length" of each "step", resulting in a smoother signal
3 levels of Ethernet transmission schemes
Manchester Fast Ethernet Gigabit Ethernet *see lecture notes for more detail*
Guided media
Media in which the message flows through a physical media such as a twisted pair wire, coaxial cable, or fiber-optic cable The media "guides" the signal
Wireless media
Media in which the message is broadcast through the air, such as microwave or satellite
Modem
Modulator/Demodulator converts a digital signal into an analog signal that can be transmitted over phone lines
Symbol rate
Number of symbols sent over the circuit per second
Analog transmission
Occurs when the signal sent over the transmision media continuously varies from one state to another in a wave-like pattern modems translate digital data from a computer into analog signals to be transmitted over telephone circuits
Digital Subscriber Line (DSL)
One approach to changing the way data are transmitted in the local loop to provide higher-speed data transfer combines analog transmission and FDM to provide a set of voice and data circuits
Simplex transmission
One-way transmission, such as that with radios and TVs
Modulation
To represent digital information, a *carrier wave* is sent down the circuit and then *modulated*, or changed in shape to represent a 1 or a 0
POTS
Plain Old Telephone Service Refers to the telephone system
Logical circuit
Refers to the transmission characteristics of the connection
Quadrature amplitude modulation (QAM)
Splitting the symbol into eight different phases (3 bits) and two different amplitudes (1 bit) for a total of 16 possible values This 64-QAM can represent 4 bits in one symbol - 256 QAM can represent 8 bits commonly used in digital TV services and cable modem Internet services
Serial transmission
Stream of data is sent over a circuit sequentially in a bit-by-bit fashion - only one physical wire slower than parallel transmission
Physical circuit
The actual wire used to connect devices
Adaptive differential pulse code modulation (ADPCM)
The alternative used by Instant Messaging and many other applications that provide voice services over low-speed digital circuits Samples voice data at 8,000 times per second and uses same 8 bits per sample that PCM does Difference is that it transmits the *difference* between the current 8 bit value and the previous 8 bit value The slowness of change of voice signals means that they can be represented using only 4 bits
Turnaround time
The amount of time half-duplex communication takes to switch between sending and receiving AKA *retrain time*
Bandwidth
The difference between the highest and the lowest frequencies in a band or set of frequencies
Quantizing error
The difference between the smooth flow of a sound wave and the approximated digital representation - produces a metallic sound
Symbol rate
The frequency at which symbols, representing bits, are sent through a circuit - 64 Hz sends 64,000 symbols per second
Coding Scheme
The language that computers use to represent data
Pulse code modulation (PCM)
The standard used by the North American telephone network input voice signal is sampled 8,000 times per second (2X the 4000 Hz bandwidth) each time the input voice is sampled, 8 bits are generated, making transmission speed 64 Kbps (8 bits per sample x 8,000 samples per second)
Switch
The switch that connects your local loop the the telephone network it contains a codec that converts the analog signal to a digital signal
Digital transmission
The transmission of binary electrical or light pulses in that it only has two possible states, a 1 or 0
Parallel transmission
This is the way the internal transfer of binary data takes place inside a computer Each bit is transferred on its own circuit
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
Codec
Translates analog voice data into digital form for transmission over digital computer circuits
Modem
Translates the computer's digital data into analog data that can be transmitted through the voice communication circuits, then a modem on the other end translates the analog back to digital
Full-duplex transimission
Transmit in both directions simultaneously, with no turnaround time
Half-duplex transmission
Two-way transmission, but can transmit in only one direction at a time similar to a walkie-talkie computers use *control signals* to negotiate which will send and which will receive data
6 Factors in deciding which type of media to use
Type of network Cost Transmission distance Security Error Rates Transmission speeds
5 types of digital transmission
Unipolar - always positive/0 or negative/0 volts Bipolar (NRZ) - positive voltage = 1, negative voltage = 0 - never returns to zero Bipolar (RZ) - voltage returns to zero in between each symbol, which provides an error checking opportunity Bipolar: Alternate Mark Inversion (AMI) - Manchester encoding - used by ethernet - signal is changed from high to low or low to high in the middle of a time splice - less susceptible to errors going unnoticed
3 predominant coding schemes in use today
United States of America Standard Code for Information Interchange (USASCII, or more commonly, ASCII) - most popular for data communications ISO 8859 - includes ASCII codes plus non-English letters - HTML often uses this scheme Unicode - UTF-8 and UTF-16 are two different versions
Guardbands
Unused portions of a FDM circuit that separate the channels from each other to avoid interference
PAM-5 encoding
Used by 1000Base-T 5 diff voltage levels - 4 to send data and 1 as a control bit --00 = -1.0 V --01 = -0.5 V --10 = +0.5 V --11 = +1.0 V More susceptible to noise (finer differences) so some opt for 100Base-T instead
Wavelength division multiplexing (WDM)
a version of FDM used in fiber-optic cables transmit multiple channels using different wavelengths of light Dense WDM (DWDM) is a variant that further increases capacity by adding TDM to WDM
Inverse multiplexing (IMUX)
combines several low-speed circuits to make them appear as one high-speed circuit to the user a common usage is to provide T1 circuits for WANs (T1 combines 24 64 Kbps circuits to transmit at 1.544 Mbps)
Radio transmission
common form of wireless communication a radio receiver/transmitter uses a specific frequency range that does not interfere with commercial radio stations
Voice over Internet Protocol (VoIP)
commonly used to transmit phone conversations over digital networks uses digital telephones w/ built-in codecs - so can be directly connected into a LAN Must be able to operate in emergencies to call 911 ---must have uninterrupted power supplies (UPS)
Frequency division multiplexing (FDM)
dividing the circuit "horizontally" so that many signals can travel a single communication circuit simultaneously divided into channels transmitting at different frequencies - each channel is separate logical circuit
Microwave transmission
extremely high-frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points stations can be place approx. 25 - 50 miles apart
Fiber optic cable
high-speed streams of light pulse from lasers or LEDs and carry information inside hair-thin strands of glass
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
Bit rate
number of bits transmitted per second
Satellite transmission
similar to microwave, except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles in space (low-earth orbit [LEO]) *raindrop attenuation* is an issue in heavy rains