Overview of Computer Science
CSC 101 — Summer 2011
Data Communications
LAN and
LANs
d WANs
WAN
Lecture 19 — August 1, 2011
Announcements
• Final Exam next Tuesday (8/9/11).
– This Room – 2pm – 5pm
• Final Review on Mondayy – 8/8/11
• Quiz #3 (?) on Friday – Lectures 19 – 22
• Lab #7 Tomorrow
– Prelab and Lab-Report is online
• WA#7 due Tomorrow @ 5pm
• WA#8 due Thursday 8/4/11 @ 5pm
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Objectives
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Data communications systems
Data encoding, transmission and error checking
Interconnection of computers: data networks
Classification of data networks
LAN Topologies
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1
Information Exchange
• Two ways to think about exchanging information
– Broadcasting – one-to-many, one-way communication (asymmetric)
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Radio and television
Print media – magazines, newspapers, etc.
Bulk messages (e.g. spam email)
The Web (published Web pages)
– Networking – one-to-one, two-way communication (symmetric)
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Telephone
Postal mail
Email, IM, texting
Computer resource sharing (disks, printers, P2P file sharing, etc.)
The Web (interactive or dynamic Web pages), Gchat, Facebook Chat
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Information Exchange
• Data communication systems exist to exchange
information between two agents
• Claude Shannon developed
p a
general model for information
exchange (1948)
– A simple yet fundamental way of
looking at data communication
– Six basic components of all communications systems
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Shannon’s Data Communication Model
• The six components of a data communications
system:
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Shannon’s Data Communication Model
1. An information source generates
a message
2. A transmitter encodes the message
as some kind of signal
g
3. The signal is transmitted over a
communications channel
– Some medium that bridges the distance between transmitter and receiver
4. A receiver extracts the signal from the communications channel
and decodes it back into a message
5. The destination receives the message
6. Some noise source is usually present in the channel
– A random element that affects the signal in unpredictable ways
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Shannon’s Data Communication Model
• The six components of information transfer
apply, even to very low-tech messages…
• …as well as messages that use advanced,
digital technologies
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Data Encoding
• Messages in digital communications systems are encoded as a
stream of bits (0s and 1s)
– Computer files already exist as binary data
– Other information can be digitized as it is being sent
• These bits can be transmitted as some type of energy
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Sound (modem)
Electricity (Ethernet)
Light (fiber-optic cables)
Radio waves (wireless)
Etc.
• All of these forms of energy are analog phenomena
– Yet they can carry digital information
• The physical properties of the communications channel determine
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the maximum data transfer rate (maximum bandwidth)
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Transmitting the Signal
• Digital data may be transmitted as analog signals
• Analog signals are composed of waves
– Repeating waveforms are modulated (altered) to encode information
– Amplitude modulation (AM) and frequency modulation (FM) are common
• Digital data must be translated into modulated analog signals for
transmission over many types of media
– Sound, electrical signals, radio waves are analog phenomena
– Even so, they can carry digital data as an analog signal
• The analog signal must be translated back into digital data at the
receiver
• A modem (modulator/demodulator) is a device that performs
these translations
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Modulating Digital Data
• With amplitude modulation (AM), the amplitude
(“loudness”) of the signal is increased to
represent a binary “1”
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Modulating Digital Data
• With frequency modulation (FM), the frequency
(“pitch”) of the signal is increased to represent a
binary “1”
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Data Encoding and Error Checking
• ASCII (American Standard Code for Information Interchange) was
designed for text communications
– A way to digitize text data using a binary code for each letter or character
– Need to ensure that the correct character is received
• Error checking can accommodate communication noise
– Minor errors in transmission can change the message
• To be able to identify errors in ASCII data:
– 8 binary bits used to represent each text character
– An extra parity bit is often used to detect transmission errors
– 8 data bits + 1 parity bit = 9 bits (1 byte + 1 extra bit) per character
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Parity Bit Error Detection
• Sending system sets the parity bit
– Two types of parity error detection
– Odd parity error detection
• Transmitter sets the parity bit so that the total number
of “1” bits in the bbyte
te is an odd number
n mber
– Even parity error detection
• The parity bit is set to make the total number
of “1” bits an even number
• Receiver checks the parity
– If the total number of “1” bits in the byte is not correct
(odd or even), this indicates an error, and the byte is discarded
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Parity Bit Error Detection
• Data in an 8-bit byte with parity
– Eight bits of encoded data
– Ninth bit (the parity bit) provides error checking
Parity Bit
8-bit ASCII code
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Parity Bit Error Detection
• Example:
• The ASCII code for “N” is “01001110
01001110”
• In odd parity, what should the parity bit be?
– It should be set to “1
1” to make an odd number of 1s
1 0 1 0 0 1 1 1 0
Parity Bit
8-bit ASCII code
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Parity Bit Error Detection
• If one bit is in error, the parity bit disagrees and
the error is detected
– Odd parity, but an even number of “1” bits
• This byte must be in error!
• (But, this does not provide a way to correct the error)
1 0 1 0 0 1 0
1 1 0
Discard
this byte!
Parity Bit
8-bit ASCII code
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Parity Bit Error Detection
• If there are two single-bit errors, the parity bit
cannot detect the error
– Odd parity, but still an odd number of “1” bits Æ
data appears to be valid, even though it is not
1 0 1 1
0 0 1 0
1 1 0
Parity Bit
8-bit ASCII code
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Checksum
• Parity works well to find single-bit errors
– Parity can’t detect two-bit errors, but these are very uncommon
– Almost all errors are single-bit errors, so parity works well as a
‘first-line’ error detection method
• Higher-level error detection methods include calculating the
checksum of a data packet
– Sender: Add together the numerical values of all the bytes in the
packet and include that checksum with the packet when it’s sent
– Receiver: Add together the numerical values of all the bytes in the
received packet and compare it to the checksum sent by the sender
– If they are different, there was an error; receiver requests a resend
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Bandwidth
• The bandwidth of a communication channel is its
capacity to transmit data
– Analogous to pipe sizes in plumbing
• A
An analog
l signal
i l may simultaneously
i lt
l contain
t i different
diff
t
waveforms of various frequencies
– Each waveform carries some information
• The number of frequencies simultaneously supported by
a channel determines its overall bandwidth
– Similar to the radio dial – many stations simultaneously
broadcasting, each at a different frequency
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Types of Data Networks
• There are many ways to connect devices
– The word network implies many different connections and
types of connections
• Data communication networks can be classified by their
various properties
– Scope or expanse covered by the network
– Type of medium used for transmission of signals
– Type of switching performed to route the signal
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Networks Classified by Scope
• LAN (Local-Area Network)
– Small geographical area – from a room up to a few buildings
• WAN (Wide-Area Network)
– May span across states, nationwide, even worldwide
– Often serves a single organization,
organization acting like a single network
• MAN (Metropolitan-Area Network)
– Medium-sized, such as a city, campus, or multiple sites of a single institution
• PAN (Personal-Area Network)
– Up to a few meters in size, usually one person’s devices
• An internet
– An interconnection of autonomous networks
– The largest of all internets is the Internet
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Basic Characteristics of LANs
• A local area network (LAN) provides:
– Computer connectivity with 2-way communication
• Between servers and nodes or between individual nodes
– Resource sharing
h i
• Servers provide resources to other nodes
– Files, backups, software licenses, security validation, printing, etc.
– A server might be a computer or just a stand-alone peripheral
– Transparency of use
• LANs are designed to be almost plug-and-play
• Remotely shared resources often appear as virtual local
resources
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Networks Classified by Medium
• Wired media
– Specific, physical connections
• Copper wire
• Coaxial cable
• Optical fiber cable
• Wireless networking
– Connections can come and go
• Infrared waves (IR)
• Radio frequency waves (RF)
– Sometimes called “Wi-Fi”
– Includes wireless now deployed on campus
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Transmission Media
• Wired media
– “Twisted-pair” cable (copper wires)
• Limited to about 100 meters max
– Optical fiber cable
• Pulses of laser light over very thin glass strands
• One cable can connect up to 25 miles or more
• Low error rates and very high data bandwidth (up to Tbps)
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Transmission Media
• Wireless networking – Wi-Fi
– Radio frequency (RF) connection
to an access point
• Speeds of up to 54 Mbps are common (“802.11g”)
(“802 11g”)
– 100 Mbps is becoming widely available (“802.11n”)
• Limited range
– Tens to hundreds of feet
– Reduced by walls or other obstructions
– Possible interference from portable phones, microwave ovens,
etc.
Transmission Media
• Wireless networking – Wi-Fi
– Public “hotspots” are common
• All WFU buildings and some outdoor areas (‘WFUvisitor’)
St in downtown Winston-Salem
Winston Salem
• Public areas such as 4th St.
(free)
• Other public or commercial locations (free or pay)
– Private access points
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All buildings and some outdoor campus areas (‘WFUstudent’)
Businesses, etc., that use wireless networking
Personal (in-home) wireless
Security issues must be considered
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Transmission Media
• Wireless networking – cellular
– Blackberry; cellular modem; “3G” and “4G” phones
• Long range Wireless connectivity
– Good coverage in most populated areas
areas, depending on provider
• Currently ~1–10 Mbps
Transmission Media
• Wireless networking
– Bluetooth
• Short range “personal area network”
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Computer peripherals
C ll phone
Cell
h
andd music
i player
l
headsets
h d t
Automobile dashboards
Home networking (gaming and entertainment)
Etc.
• 1-2 Mbps
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Transmission Media
• Wireless networking
– IR (infrared)
• Like television remote controls
• Requires
q
line-of-sight
g
– Works well only within enclosed spaces
• Sometimes used for simple
networking tasks (“beaming”)
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Laptop-to-PDA (e.g. Palm Pilot)
PDA-to-printer
Digital camera uploads
Etc.
• 115 Kbps (slow!)
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