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”