Introduction to Computing
Session 4: Data Communications and Networks
Objectives
Understanding Data Communication
Concepts of Data Communication
Data Transmission & Signaling
Networks and Physical Media
Communication
■ Communication
“ transfer of information from a ‘source’ to a ‘destination’
“
– implies ‘two’ parties and ‘transfer’ of information
– there must be a ‘means’ by which information is
‘transferred’
– does not specify what ‘type’ of ‘information’
Telecommunication
Any process that permits the passage from a sender to one
or more receivers of information of any nature, delivered in
any easy to use form (printed copy, fixed or moving
pictures, visible or audible signals) by any electromagnetic
system (electrical transmission by wire, radio optical
transmission, guided waves) which includes telegraphy,
telephony, video-telephony, data transmission and others.
Data Communication
Part of telecommunication that relates to the use
of computers for the transmission and receiving of
computer readable information.
Elements of Data Communication
■ Senders
– any equipment that compose and transmit message
■ microcomputers
■ video terminal
■ bar code scanner
■ Receivers
– any equipment that receive and interpret message
■ computer, terminal, printer
■ most can send response back to sender indicating
message receipt
Elements of Data Communication
■ Media
■ messages can be transmitted using a variety of media
■ Guided media telephone wires, network cables (twisted pair)
– TV cable (coaxial cable)
– fiber optic cables
■ Unguided media
– radio waves, microwave signal, cellular communication
signals
– Method used to ‘transmit’ messages is known as signaling
– Data can be analog or digital and transmitted using analog or digital
signaling
Elements of Data Communication
■ Messages
– text or numeric data
– document images, photographs, video
■ Message encoding
– data codes are the “languages” used in data communication
– Common data codes : ASCII, UNICODE, EBCDIC
– translation required if sender/receiver understands different
codes
■ Feedback
– whether receiver receives message
– error detection mechanisms can be implemented
Devices in Communication
■ Computers– e.g. PC’s …
■ Peripherals – e.g. Printers, Scanners …
■ Physical Interfaces – e.g. Serial, Parallel
■ Communication Devices – e.g. Modems,
■ Network Interfaces - NIC, PCMCIA Card
■ Cable connectors : RJ45, BNC, Transceiver, ...
■ Cable Media : UTP, STP, Coaxial, Fiber, ....
■ Cabling Equipment : Wiring Closet, Hubs, Concentrators, ...
■ Connectivity Devices : Repeaters, Bridges, Routers, .....
Concepts in Data Communication
■
Data (information) is ‘encoded’ using some ‘coding’ method to enable it to be
transmitted over some medium
–
■
e.g. Jack wants to ask Mary where she stays
■
Jack’s brain formulates this into a question (message)
■
Jack asks - encodes into speech with his vocal chords (transmitter)
– ‘Where do you stay, Mary ?’ (message)
■
Jack ‘transmits’ this as sound vibrations (signals) across the air
(medium)
■
Mary hears – receives – these sound vibrations with her ear (receiver)
■
Mary ‘decodes’ (understands) the question in her brain
Data Communication at its most basic level consists of transferring bits of
information one at a time
Concepts in Data Communication
■ Transmission Modes
– Simplex
■ Transmission can only travel in one direction
– Half-Duplex
■ Transmission can travel in both directions but only
one way at a time – e.g. walkie-talkie
– Full Duplex
■ Transmission can occur in both directions at the
same time
– e.g. telephone conversation
Half-Duplex Transmission
■ Half-duplex : only one can transmit at a time.
Must take turns.
– Walkie-talkie
A
Time 1
Only one side
May send
A does
B
A
Time 2
Only one side
May send
Neither does
B
Duplex Transmission
■ Full-duplex transmission: both can transmit simultaneously
– Even if only one sends, still full-duplex line
– Even if neither is sending, still full-duplex line
A
B
Time 1
Both may send.
Both do
A
B
Time 1
Both may send.
Only A does
A
B
Time 1
Both may send.
Neither does
Concepts in Data Communication
Serial & Parallel Transmission
– Serial Transmission
■ Bits are transmitted over a single wire one bit at a
time
– Parallel Transmission
■ Bits are Transmitted in sets over a number of wires
(typically eight at a time) e.g. parallel cable to printer
Serial and Parallel Transmission
■ Serial: one wire, one bit per clock period*
1
0
One
Bit in
Clock
Cycle
One
One
Bit in
Clock
Cycle
Two
*For simplicity, we assume
binary transmission (2 states)
Serial and Parallel Transmission
■ Parallel
– N bits per second on N wires
– Parallel is faster than serial
1
1
0
1
1
0
0
0
Eight Bits
In Clock
Cycle One
1
1
0
1
1
0
0
0
Eight Bits
In Clock
Cycle Two
Serial and Parallel Transmission
■ Parallel Transmission
–
N bits per second on N wires
–
N=8 in this example
–
N can also be 2, 4, 16, 32, etc.
1
1
0
1
1
0
0
0
1
1
0
1
1
0
0
0
Synchronous versus Asynchronous Transmission
–
Synchronous Transmission
■
A long stream of bits is transmitted (typically a frame) separated by a
start sequence of several bits (known as preamble bits) and possibly
followed by an end sequence of several bits (known as postamble bits)
■
This causes less ‘overhead’ in terms of extra framing bits and is thus
more efficient
■
However, this requires more reliable lower level transmission since
errors detected may need re-transmission of the whole sequence.
10 Data
Bytes
10 Preamble
Bytes
10 Postamble Bytes
In the example above, only 10 bytes ‘overhead’ for transmitting 500 bytes data i.e. 2%
Synchronous versus Asynchronous Transmission
–
Asynchronous Transmission
■
Every sequence of 8 bits is preceded by one start bit and followed by
one stop bit
■
This causes high ‘overhead’ in terms of extra framing bits and is thus
more inefficient
■
However, this could be better in the case of less reliable lower level
transmission since errors require only retransmission of one set of
eight bits
1 Data bit
1 Start bit
1 Stop bit
In the example above, 10 start and stop bits ‘overhead’ for transmitting 40 bits data i.e. 25%
Channel Bandwidth
■ Channel bandwidth: rate at which data is exchanged over a
communications channel
– Usually measured in bits per second (bps)
■ Broadband communications: telecommunications system in which a
very high rate of data exchange is possible
Guided Transmission Media Types
Table 6.1: Guided Transmission Media Types
Unguided Transmission Media Types (continued)
Table 6.2: Wireless Technologies
Telecommunications Hardware (continued)
Figure 6.6: How a Modem Works
Telecommunications Hardware (cont.)
■ Multiplexer: device that encodes data from two or more data sources
onto a single communications channel
– Reduces number of communications channels needed
– Lowers telecommunications costs
Modem: a device that translates digital signals to analog and vice-versa
■ Front-end processors: special-purpose computers that manage
communications serving hundreds or even thousands of users
Distributed Processing
■ Centralized processing: all processing occurs at a single
location or facility
■ Decentralized processing: processing devices are placed at
various remote locations
■ Distributed processing: processing of a single task is
performed on all computers connected over a network
Client/Server Systems
■ Client/server: multiple computer platforms are dedicated to special
functions such as database management, printing, communications,
and program execution
– Each server is accessible by all computers on the network
– A client is any computer that sends messages requesting services
from servers on the network
Communications Protocols/Hardware
Table 6.14: Wi-Fi Network
Communications Hardware
■ Switch: uses physical device address in each incoming
message to forward message to another device on the
same network
■ Bridge: connects one LAN to another LAN that uses the
same telecommunications protocol
■ Router: forwards data packets across two or more distinct
networks toward their destinations through routing
■ Gateway: serves as an entrance to another network
Communications Software
■ Network operating system (NOS): systems software that controls
computer systems and devices on a network and allows them to
communicate with each other
■ Network-management software
– Monitors the use of individual computers and shared hardware
– Scans for viruses
– Ensures compliance with software licenses