Communications : the transmission of a signal by way of a medium from a
sender to receiver
SENDER SIGNAL RECEIVER
Figure 1 Elements of communications
system and system and
equipment sig equipment
Figure 2 Telecommunications device act as relay points for data-bearing signals
between computer systems and equipment
CHARACTERISTICS OF TELECOMMUNICATIONS
Type of Signals
1. Analog signal : the signal is continuous fluctuation over time between high and low
2. Digital signal : the signal is always high or low value (discrete signal).
- The speed of a transmission medium is measure in bit per second (bps)
- Signal frequency is the number of times the signal cycles per second
มีหน่วยเป็น Hertz (Hz)
- Bandwidth : the range of signal frequencies that can be sent over a given
medium at the same time.
: the measure of transmission capacity.
1. Simplex transmission : data can only flow in one direction เช่น การฟังวิทยุ
2. Half-duplex transmission : the data can flow in both directions, but it can only flow
in one direction at any point in time. เช่น CB radio
3. Full-duplex transmission : the data can flow in both directions at the same time. เช่น
TPYE OF COMMUNICATION MEDIA
1. Twisted-pair wire cable is a cable consisting of pairs of twisted wires.
แบ่งออกเป็น 2 ชนิดคือ
- Shielded twisted-pair wire cable มีชั้นของสารตัวนาพิเศษภายในฉนวนหุม ้
ซึ่งสามารถลดการรบกวนทางไฟฟ้า และลดการเกิด noise
- Unshielded twisted-pair wire cable (UTP)
2. Coaxial cable : consist of an inner conductor wire surrounded by insulation
(dielectric). The dielectric is surrounded by a conductive shield, which is turn
covered by a layer of nonconductive insulation, called the jacket.
3. Fiber-optic cable : consisting of many extremely thin stands of glass or
plastic bound together in a sheathing (a jacket) transmits signals with light
Microwave and Satellite Transmission
Microwave Microwave Microwave
relay station relay station relay station
line-of –sight path
(about 30 miles)
Figure 3 Microwave transmission
station signals Communication signals station
Figure 4 Satellite transmission
Communications satellites are relay stations that receive signals from one earth station
and rebroadcast them to another
Cellular transmission : a local area , such as a city, is divide into cells. As a car or
vehicle with a cellular device, such as a mobile phone, move from one cell to another,
the cellular system passes the phone connection from one cell to another. ดูรูปในหนังสือ หน้า
248 (figure 6.9)
Infrared transmission : requires line-of-sight transmission and short distances under a
few hundred yards
่ ่ ่ ้
เนืองจากการส่งสัญญาณผ่านทางสายโทรศัพท์ สัญญาณทีจะใช้สงนันต้องเป็น analog
signal แต่สญญาณทีออกมาจากเครื่องคอมพิวเตอร์เป็น digital signal
แล้วจึงส่งออกไปตามสายโทรศัพท์ได้ ซึ่งกระบวนการนีเ้ รียกว่า การ modulation
จ หรือเรียกอีกอย่างว่า การ demodulation และอุปกรณ์ทใช้ในการทา
modulation/demodulation นี้ ก็คอ modems
Facsimile devices (Fax) : allow businesses to transmit text , graphs, photographs, and
other digital files via standard telephone lines.
Special purpose modems : เช่น Cellular modems, Expansion slots, PC modems.
Multiplexers : allow several telecommunications signals to be transmitted over a single
communications medium at the same time.
Front-end processors : are special-purpose computers that manage communications to
and from a computer system.
system processor Job 2
- Figure 5 A front-end processor takes the burden of communications
management away from the main system processor.
Carriers and Services
Common carriers : long distance telephone companies.
Value-added carriers : companies that have developed private telecommunications
systems and offer their services for a fee
Switched lines : lines that use switching equipment to allow one transmission device
(e.g. your telephone) to be connected to other transmission device (e.g. the telephones
of your friends and relatives).
Dedicated line (leased line) : a line that provides a constant connection between two
points. No switching or dialing is needed; two device are always connected.
Private branch exchange (PBX) : a communications system that can manage both voice
and data transfer within a building and to outside lines.
Integrated Services Digital Network (ISDN) : a technology that uses existing common
carrier lines to simultaneously transmit voice , video, and image data in digital form.
Digital Subscriber line (DSL) : a line that uses existing phone wires going into today’s
homes and businesses to provide transmission speeds exceeding 500 Kbps
ตารางแสดงการเปรียบเทียบข้อดีข้อเสียของ Several Line and Service types
Line / service Speed Advantages Disadvantages
Plain old telephone 56 Kbps Low cost. Too slow for video.
service (POTS) Broadly available.
ISDN 67-128 Kbps Broad bandwidth for High cost to install and
video, voice, and data. connect costs.
Good to very good Not available
Wide product support.
DSL 500 Kbps Broad bandwidth for a Requires expensive
1.544 Mbps variety of applicationsmodem.
Not broadly available.
Cable modem Receive at up to 500 Broad bandwidth for a Just becoming
Kbps; send at up to 64 variety of applications available.
Kbps. Requires special
T1 1.544 Mbps Broad bandwidth for High cost; high
digital,data ,and image installation fee, and
transmissions. subscribers pay
monthly fee based on
NETWORKS AND DISTRIBUTED PROCESSING
Basic Data Processing Strategies
Centralized processing : data processing that occurs in a single location or facility.
Decentralized processing : data processing that occurs when devices are placed at
various remote locations
Distributed processing : data processing that occurs when computers are placed at
remote locations but are connected to each other via telecommunication deviced.
Network Concepts and Considerations
Network topology : a logical model that describes how networks are structured or
ลักษณะโครงสร้างของ network สามารถแบ่งได้ดงนี้ ั
1. Ring network : a type of topology that contains computers and computer devices
placed in a ring, or circle. With a ring network , there is no central coordinating
computer. Messages are routed around the ring from one device or computer to
2. Bus network : a type of topology that consists of computers and computer devices
on a single line. Each device is connected directly to the bus and can communicate
directly with all other devices on the network. The bus network is one of the most
popular types of personal computer networks.
3. Hierarchical network : a type of topology that use a treelike structure with messages
passed along the branches of the hierarchy until they reach their destination.
4. Star network : a type of topology that has a central hub or computer system, and
other computers or computer devices are located at the end of communications
lines that originate from the central hub or computer.
5. Hybrid network : คือ network แบบผสม ของ network 4 แบบข้างต้น
Local Area Network (LAN) : a network that connects computer systems and devices
within the same geographic area.
Wide Area Network (WAN) : a network that ties together large geographic regions using
microwave and satellite transmission or telephone lines
International Network : a network that links systems between countries.
Terminal-to-Host, File Server, and Client/Server Systems
Terminal-to-Host : an architecture in which the application and database reside on one
host computer, and the user interacts with the application and data using a “dumb”
File Server : an architecture in which the application and database reside on one host
computer, called the file server.
Client/Server : an architecture in which multiple computer platforms are dedicated to
special functions such as database management, printing , communications, and
Figure 6 Terminal-to-Host connection.
File downloaded to user
Figure 6 File server connection
Communication software and Protocols
Communication software : software that provides error checking, message formatting,
communications logs, data security and privacy, and translation capabilities for
Network Operating System (NOS) : system software that controls the computer
systems and devices on a network and allow them to communicate with each other.
Network Management Software : software that enables a manager on a networked
desktop to monitor the use of individual computers and shared hardware (like printer) ,
scan for viruses, and ensure compliance with software licenses.
Protocol : rule that ensure communications among computers of difference types from
Open Systems Interconnection model (OSI) : a standard model for network
architectures that divides data communications functions into seven distinct layer to
promote the development of modular networks that simplify the development, operation
and maintenance of complex telecommunications networks.
The Seven Layers of OSI model
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
Transmission Control Protocol / Internet Protocol (TCP/IP) : a standard originally
developed by the U.S. government to link defense research agencies; it is the primary
communications protocol of the Internet
Bridge, Routers, Gateway and Switches
Bridge : a device that connect two or more networks at the media access control
portion of the data link layer; the two networks must use the same communication
Router : a device that operates at the network level of the OSI model and features
more sophisiticated addressing software than bridges. Whereas bridges simply pass
along everything that comes to them , routers can determine preferred paths to a final
Gateway : a device that operates at or above the OSI transport layer and link s LANs
or networks that employ different , higher-level protocols, thus allowing networks with
very different architectures and using dissimilar protocols to communicate.
Switch : a device that routes or switches data to its destination.
Voice and Electronic mail
Voice mail : technology that enables users to leave ,receive and store verbal messages
for end from other people around the world.
Electronic mail (e-mail) : technology that enables a sender to connect his or her
computer to a network , type in a message,and send it to another person on the
Electronic software and Document distribution
Electronic software distribution: a process that involves installing software on a file
server for users to share by signing onto the network and requesting that the software
be downloaded onto their computers over a network.
Electronic document distribution : a process that involves transporting documents.
Telecommuting : enables employees to work away from the office using personal
computers and networks to communicate via electronic mail with other workers and pick
up and deliver results
Videoconferencing : systems that combine video and phone call capabilities with data
or document conferencing
Electronic Data Interchange
Electronic Data Interchange (EDI) : an intercompany , application-to-application
communication of data in standard format , permitting the recipient to perform the
functions of a standard business transaction.
EDI link EDI link
Figure 7 Two approaches to Electronic data Interchange
Public Network Services
Public Network Services : services that give personal computer users access to vase
databases and other services, usually for an initial fee plus usage fees.