Docstoc

Telecommunications - DOC

Document Sample
Telecommunications - DOC Powered By Docstoc
					                Telecommunications and
                       Networks
COMMUNICATIONS CONCEPTS
       Communications : the transmission of a signal by way of a medium from a
sender to receiver




                SENDER         SIGNAL          RECEIVER




                              Transmission medium
                         Figure 1 Elements of communications




    Computer                                                                 Computer
   system and                                                               system and
   equipment     sig                                                        equipment
                 n
                              Telecommunications devices

   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
                     value
2. Digital signal : the signal is always high or low value (discrete signal).

Transmission capacities
       - 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.
Transmission Modes
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
     ่
   ทีจะใช้ฟังหรือพูดในเวลาเดียวกันไม่ได้
3. Full-duplex transmission : the data can flow in both directions at the same time. เช่น
   การพูดคุยทางโทรศัพท์

TPYE OF COMMUNICATION MEDIA
Physical Cables
       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
        beams.


Microwave and Satellite Transmission


    Microwave                             Microwave                             Microwave
                        microwave                           microwave
   relay station                         relay station                         relay station
                   line-of –sight path
                    (about 30 miles)
                                Figure 3 Microwave transmission




       Microwave                                                                 Microwave
         station            signals         Communication        signals           station
                                               satellite



                            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
TELECOMMUNICATION DEVICES
Modems
            ่                                                  ่    ่ ้
         เนืองจากการส่งสัญญาณผ่านทางสายโทรศัพท์ สัญญาณทีจะใช้สงนันต้องเป็น analog
              ั        ่
signal แต่สญญาณทีออกมาจากเครื่องคอมพิวเตอร์เป็น digital signal
     ้
ดังนันจึงต้องมีการแปลงสัญญาณดิจิตอลจากเครื่องคอมพิวเตอร์เป็นสัญญาณอนาล็อกก่อน
แล้วจึงส่งออกไปตามสายโทรศัพท์ได้ ซึ่งกระบวนการนีเ้ รียกว่า การ modulation
และในส่วนของภาครับ
   ้                                                                      ่
ก็ตองทาการแปลงสัญญาณอนาล็อกจากสายโทรศัพท์ให้เป็นสัญญาณดิจิตอลทีคอมพิวเตอร์เข้าใ
                                                      ี่
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.

                                                                       Job 1


            Main                  Front-end
           system                 processor                            Job 2



                                                                       Job 3


       - 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
                                              quality.                  everywhere.
                                              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
                                                                     modem.
T1                    1.544 Mbps             Broad bandwidth for     High cost; high
                                             digital,data ,and image installation fee, and
                                             transmissions.          subscribers pay
                                                                     monthly fee based on
                                                                     distance.
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
configured.
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
another.
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
Network Types
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”
terminal.
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
program execution.


                  “Dump”                                Host
                  terminal                            computer



                         Figure 6 Terminal-to-Host connection.


                                File downloaded to user
              User’s                                                  Host
             Personal                                               computer
             computer


                             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
networks.
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
difference manufacturers.
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
protocol.
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
destination.
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.

TELECOMMUNICATIONS APPLICATIONS
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
network.
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
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
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.



                Vendor                                                  Customer
                                    EDI link

                                        (a)

                                                 Third-party
                                               clearinghouse


                         EDI link                                           EDI link



                     Vendor                                           Customer


                                        (b)
                   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.