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Data Communications
Chapter 16,Exploring the Digital Domain
In this chapter . . .
chief elements of a communications system
how data is encoded and transmitted
classifying the variety of computer networks
getting “connected” at home
You will learn about
Communicating Information
broadcasting one sender, many receivers one-way communication
networking one sender, one receiver pair
(multiples) two-way communication
Shannon’s Data Communication Model
Shannon’s Data Communication Model
an information source generates a message a transmitter encodes the message as a signal the signal is transmitted over a communications
channel—a medium that bridges the distance between
the receiver extracts a signal from the communications channel and converts it back into the form of a message
the destination receives the message a source of noise is usually present in the
communication channel—this is a random element that modifies the signal in unpredictable ways
Encoding Messages
messages are encoded as a stream of binary numbers (0s and 1s)
signals are transmitted as electro-magnetic energy (electrical, optical, or radio waves)
rate of reliable transmissions depend on the properties of the channel and the complexity of the message
Transmitting Digital Data
analog (modulated signals) amplitude frequency phase (calibrated timing)
digital
Two types of signals:
Amplitude Modulation
Frequency Modulation
Phase Modulation
Bandwidth
the bandwidth of a communication channel determines its capacity to transmit data—analogous to the size of plumbing pipes
analog signals may contain independent waveforms of various frequencies
the number of frequencies supported by a channel determines its bandwidth
Limiting Factors in Data Transmissions
bandwidth
signal strength
noise, i.e., signal-to-noise ratio
Characteristics ofComputer Networks
distance or expanse covered by the network
media used for transmission of signals
type of signal used for transmission type of switching performed to route
the signal
Networks Classifiedby Distance
LAN (Local Area Network) WAN (Wide Area Network)
usually a large area often owned by a single
corporation internetwork
Networks Classified by Media bounded media
copper wire optical fiber cable
unbounded media “wireless” radio frequency waves
Networks Classified by Signal
baseband (narrowband) employs entire bandwidth for one signal
broadband multiple signals on the same channel
simultaneously channel is divided into separate
frequency bands, each capable of carrying a signal
Frequency-Division Multiplexing
bandwidth is divided into separate channels each signal occupies a specific portion of
the bandwidth
Time-Division Multiplexing
signal occupies entire bandwidth time divided into frames, slots pieces of the signal are sent in slots
Networks Classified by Switching
devices in a network called “nodes” arrangement of nodes and links
called “topology” point-to-point connectivity vs.
shared connectivity shared connectivity requires
“switching” routing data over common links
Circuit Switching
a continuous connection or circuit is made between transmitter and receiver
ordinary telephone connections are made by circuit switching
Packet Switching the message is made of separate data packets
each addressed to the destination Packets are transmitted over any available
connection to the destination receiving node reassembles the message
Packet vs. Circuit Switching
circuit packet
mode synchronous, continuous
asynchronous, connectionless
messages handles lengthy
suited for short
processing very little required at both ends
robustness connection-sensitive
fault-tolerant
Analog Modems “modem” = modulate-demodulate
Cable Modems employ CATV communications upstream, downstream speeds differ downstream bandwidth shared
Digital Subscriber Line (DSL)
DSL employs telephone communications
DSL uses bandwidth not required for voice
connection is continuous Asymmetric DSL is common today
ADSL
maximum length (5.4 km)
lines must be “DSL-ready”
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