IS 3413 M03
With which type of digital signaling does the signal always return to zero volts after each bit?
return-to-zero signaling
How Modems Transmit Data:
takes digital data (electrical pulses) from computer and convert to analog signal Several types and standards exist; for data to be transmitted, sender and receiver MUST USE THE SAME TYPE OF MODEM.
Symbols:
> The modulated carrier wave has encoded SYMBOLS >Send 1 bit/interval by defining two different symbols > Send more bits/interval by defining additional symbols
Voice over Internet Protocol (VoIP)
A technology that allows you to make voice calls using a broadband Internet connection instead of a regular (or analog) phone line. Uses the same wiring and network as computers -Must operate even when Power falls for emergency calls (with uninterruptable power supplies (UPS) on all circuits - Common standard: G.722 Wideband audio
Implications for Cyber Security
Unauthorized access to physical hardware cannot be protected by firewalls, encryption, or other software measures. Portable equipment, particularly USB drives, are particularity vulnerable. -Hackers often place malware on USB drives Router, servers, and mobile devices must also be protected
A(n) ___________ divides one high-speed communication circuit into several lower speed circuits for the primary reason of saving communication line costs.
multiplexer
Symbol rate:
number of symbols transmitted per second (baud)
Telephone:
digital transmission except for in the last loop
Full Duplex
data flows simultaneously in both directions
Three ways to modulate a carrier wave:
-Amplitude Modulation (AM) ///Amplitude Shift Keying (ASK)// -Frequency Modulation (FM) ////Frequency Shift Keying (FSK) -Phase Modulation (PM)// Phase Shift Keying (PSK)
Throughput data Depends on:
-Modems' data transmission rate -use of data compression (e.g. Lempel-ziv encoding)
Benefits of Digital Transmission:
-Produces fewer errors than analog transmission -Easier to detect and correct with binary values -Permits higher maximum transmission rates _ e.g. fiber optic cable. -More secure - easier to encrypt -Far simpler to integrate voice, video, and data on the same circuit
Reducing quantizing error can be done by:
-Sampling more frequently -Using more levels of amplitude in encoding
Analog Transmission of Digital Data
-Telephone system built for analog data >Electrical signals mimic sound waves (i.e., voice) >Analog transmissions take on range of values (vs. discrete values of digital transmissions) >NEED A MODEM (modulator/demodulator) to convert from analog to digital and vice versa
Wireless Media - Radio
-Wireless transmission of electrical waves through the air -Includes each device on network with a radio transceiver operating at a specific frequency range -Enables mobile network communication Characteristics: Distance: depends on frequency and power Use: wireless LANs, Cellular and cordless phones, baby monitors
Signal Transmission (basics)
-sender conveys a message to receiver by varying the voltage on the circuit over time. Digital (binary) data might be conveyed by changing the voltage from +15V (representing one) TO -15V (representing zero) Analog data might be convey by changing the voltage to any value between +15V and -15V
More complex data coding schemes:
1 bit of info >>> 2 symbols 2 bits of info>>>4 symbols 3 bits of info >>>8 Symbols n bits of info>> 2^n symbols
A coding scheme that uses 10 bits to represent each character is capable of representing _________ different characters.
1024
When sending four bits at a time using frequency modulation, the number of different frequency levels that would be needed would be _______.
16
Multiplexing
A form of transmission that allows multiple signals to travel simultaneously over one medium. Divides high-speed circuit into several slower (LOGICAL) circuits ADV: cost efficiency by reducing the amount of equipment
Statistical Time Division Multiplexing (STDM)
A variation of TDM Designed to reduce idle time slots by allocating slots based on statistical network usage Disadvantages: -Potential time delays when actual usage does not match statistically allocated time slots -Additional logical addressing requirements
Instant Messaging
Adaptive differential pulse code modulation (ADPCM) -Like PCM, but transmits only how the amplitude (8-bit value) has changed since the last transmission.
Binary coding scheme needed for text messages
American Standard Code for Information InterChange (ASC II) Unicode: UTF-16 used by Windows)
Two types of data that flow through circuits
Analog (wave-based) Digital( binary: 0 or 1)
Circuit Data types:
Analog- Continuous, can be any value within a range -used to carry voice data over POTS Digital -Discrete, can only have specific values -binary computer data has 2 lvls (1 or 0)
Multiplexing-Inverse
Combines many low speed circuits into one high-speed circuit Example: -two T-1 lines multiplexed -creates capacity of 2 x 1.544 MBPS = 3.088 MBPS
Guided Media: Coax cable
Consists OF: 1)Outer plastic Sheath 2)Woven Copper Shield 3) inner dielectric insulator 4) Copper Wire Less prone to interference Characteristics: Price: inexpensive (more than TP) Distance: up to 2 km (1.2 miles) Use: cable TV/Internet
Circuits: Data Translation and Transmission
Data can be translated from analog to digital or vice versa for part or ALL of transmission: Digital computer data can be transmitted over an analog telephone circuit by using a modem Sender and receiver must agree on the symbols used for transmission and on the symbol rate
Half-Duplex
Data flows both directions, but only one at a time
Simplex
Data flows in one direction
Fomula for Data Rate
Data rate = symbol rate * (#bits/symbol) Ex: symbol rate = 16000 symbols/sec #bits/symbols = 4bits/symbol Data Rate= 16000 symbols/sec * 4 bits/symbol = 64000 bits/sec =64Kbps
Quadrature Amplitude Modulation (QAM)
Digital modulation scheme that changes the phase and the amplitude. -More sophisticated modulation method -used in TV and cable modem internet Services QAM- represents symbols using 8 different phases (3 bits) and two different amplitudes (1 bit) for a total of 16 (2^4) different possible values Standard: 802.ac
Which Media Are Best?
Factors to consider in media selection: -Type of Network -Cost -Transmission Distance -Security -Error Rates -Transmission speeds
Categories of Multiplexing
Frequency/Wavelength Time
Guided Media: Twisted-pair (TP) cable
Insulated pairs of wires bundled together Wires twisted to reduce electromagnetic interference Some use additional Shielding (STP) Commonly used for telephones, LANs Characteristics: -Price: Inexpensive -Distance : typically up to 100m -Use: telephones and LANs
Wireless Media - Microwave, Satellite
Microwave- -High frequency radio Communication -Requires line of sight (large antennas and towers) -Affected by weather Characteristics -Distance: ~60 km ( due to curvature of the Earth) -Use: Trunk Line/ Back bone, long distance Satellite transmission- -Special means of microwave communication -Long distance leads to propagation delays
Guided Media, Fiber optic cable
Optical core made of glass or plastic Data Transmitted using light from lasers or LEDs Resistant to interference and corrosion Extremely fast data rates Characteristics Price: expensive Distance: 500m -100 km Use: trunk line/ Backbone, long distance circuits (e.g. undersea cables) Types: Multimode ( about 50 micron core) Graded index multimode Single mode (about 5 micron core)
Digital Transmission of Digital Data - Transmission Modes
Parallel: Multiple bits transmitted simultaneously Serial : bits are transferred sequentially, one at a time
Circuit
Physical: connects devices & includes wires Logical: describes transmission characteristics of the connection
Character
Represented by a group of bits -7 bit ASCII uses groups of 7 bits / can represent 2^7 or 128 Characters 8-bit ASCII uses groups of 8 bits (AKA 8 bits = 1 byte)/can represent 2^8 or 128 characters UTF-16 uses 2 byte WORDS can represent 2^16 = 65536 characters Extensive character set includes Cryllic, chinese, etc
Digital Transmission of Digital Data - Symbols
Sender and receiver must agree upon 1)Set of symbols: how bits are encoded as voltages or light pulses/ +5V might represent "1" Symbol Rate: (Baud): How often symbols are sent -Ex: symbol rate of 64 Kilohertz (khz) mean one symbol is sent every 1/64,00 0 of a second A symbol may contain MULTIPLE BITS
Data Flow Examples:
Simplex -TV/Radio Half Duplex - Walkie-Talkie Full Duplex -Conversation
Manchester
Special type of Bipolar signaling voltage is positive or negative and bits are indicated by a MID-BIT transition - This technique is most resistant to undetected errors because *** if there is no transition in midsignal, the receiver knows than an error must have occured
Radio data transmission requires that each device has a radio receiver/transmitter and uses a specific frequency.
TRUE
Physical Layer:
The lowest, or first, layer of the OSI model. Protocols in this layer generate and detect signals so as to transmit and receive data over a network medium. These protocols also set the data transmission rate and monitor data error rates, but do not provide error correction. -network hardware with a focus on circuits
Data rate (bit rate)
The number of bits transmitted per second.
Media
The physical material (whether it be paper, hard disk, CD-ROM, DVD, or anything similar) used for storing computer-based information. Physical matter used to carry voice or data transmissions
Pulse Amplitude Modulation (PAM)
The process of sampling an analog waveform many times to determine numeric electric amplitude values for digital conversion; PAM is typically combined with pulse-code modulation (PCM). 1) sampling the analog at signal at regular intervals 2) Measuring the amplitude of each sample
Baud
Unit of signaling speed. Now obsolete and replaced by the term symbol rate. The speed in baud = the number of signal elements per second. If each signal represents only 1 bit, baud = the same as bits per second (bps) . When each signal contains more than 1 bit, baud does not equal bps. Baud rate- the number of times per second the signal on the communication circuit changes
Bipolor NRZ:
Voltage is positive or Negative , but NOT ZERO. -Fewer errors than unipolar because signals are more distinct -has synchronization errors
Bipolor RZ:
Voltage is positive or negative, returning to zero between EACH BIT -Fewer synchronization errors than Bipolor NRZ
Two-bit Amplitude Modulation
With 4 levels of amplitude defined as symbols, 2 bits can be transmitted per symbol
Bit
a unit of information expressed as either a 0 or 1 in binary notation. Bit rate - the number of bits transmitter per second
3 Characteristics of waves:
amplitude,- height of waves (decibels) frequency- waves per second (HERTZ) Phase: wave direction (degrees) or the point at which the wave begins
In general, older networks and local loops designed to transmit voice data are likely to be _
analog
Full-Duplex and Half-Duplex
are both "bidirectional"
Time Division Multiplexing
assigns different frequencies to different channels on the same network medium Circuit is divided "VERTICALLY" by taking turns Traditional TDM, all have equal turns -more efficient than FDM but may have idle time slots
Frequency Division Multiplexing
assigns different frequencies to different channels on the same network medium. Divides the circuit bandwidth "HORIZONTALLY" Creates separate channels within frequency band Guardbands separate channels to prevent interference
Circuit Configuration
basic physical layout of the circuit: Point-to-Point: include most wired connections today; Multi-point Circuits: are most commonly used in wireless today. -shared circuits (multipoint) are less expensive
Carrier Wave
basic wave transmitted through a circuit
Pulse Code Modulation (PCM)
converts analog to digital by: 1. Sampling the analog signal at regular intervals 2. Measuring the amplitude of each sample 3. Encoding (quantizing) the amplitude as 8-bit binary data Used in North America telephone networks
Analog Transmission
involves continuous waves with continuous range of values
Digital Transmission
involves discrete binary value i.e., 0 or 1
Codecs (COde, DECode)
is a device or software that converts an analog signal (e.g., voice) into digital form and the reverse.
Wavelength Division Multiplexing (WDM)
is a version of FDM used in fiber-optic cables. Divides Circuit capacity into channels by wavelengths Wavelength is inversely proportional to frequency (W = 1/f *c) Associated with colors in visible spectrum Dense WDM can divide circuit into more than 100 channels per fiber each transmitting at 10 Gdps
The acronym, Modem, stands for _________________
modulator-demodulator
In which type of modulation is a 1 distinguished from a 0 by shifting the direction in which the wave begins?
phase modulation
Which of the following is not an important factor to consider when selecting media to be used in a network?
prestige value of the type of media
Capacity of a Circuit:
the data rate send on a circuit is calculated in bits/sec - number of bits * max symbol rate The number of bits depends on the modulation technique Max symbol rate is typically the same as the bandwidth in Htz - the difference b/w the highest and lowest frequencies in a band or set of frequencies
quantizing error
the difference between the original analog signal and the approximated, digital signal
Modulation:
the modification of a carrier wave's fundamental characteristics in order to encode information
data flow
the movement of data among processes, stores, sources, and destinations.
Modem:
translates digital signals into analog data that can be transmitted through voice communication circuits, and translates it back at the receivers end Translates analog voice data into digital form and transmission using a CODEC
Guided Media
transmission flows along physical medium
Wireless (radiated) media
transmission flows through the air
Unipolor
voltage is 0 or a positive number representing binary bits( in some circuits, 0 and negative voltage could be used)
Bipolar AMI
voltage is 0, positive, or negative, returns to zero between each bit, and ALTERNATES between positive and negative voltage
