communication by yaofenji


									Topic 7 – Connectivity

Upon completion of this topic, the students should be able to:

   Define what a network means.
   List the basic components of a data communication system.
   Explain the different types of data transmission methods, including
    types of signals, modulation and choices among transmission
   Differentiate the various kinds of communication links, and
    appreciate the need for protocols.
   Explain the different types of network configurations or topologies.
   Know the components, types and protocols of a local area network.

What is Internet?

The Internet is a gigantic collection of millions of computers, all linked
together on a computer network. The network allows all of the
computers to communicate with one another. A home computer is
usually linked to the Internet using a normal phone line and a modem
that talks to an Internet Service Provider (ISP). A computer in a
business or in a university (that's us in MMU) has a Network Interface
Card (NIC) that directly connects it to a Local Area Network (LAN).
LAN is usually connected to an ISP using a high-speed phone line like
a T1 line. A T1 line can handle approximately 1.5 million bits per
second, while a normal phone line using a modem can usually handle
30,000 to 50,000 bits per second.

     Network is collection of computers and devices connected by
      communication channels
     Standalone PC is PC that is not connected to the network
     Types of Networks:
         o Wide Area Network: network that covers a wide
            geographical area such as a country. WAN consist of 2 or
            more LANs connected by router.
         o Metropolitan Area Network: network covering a
            geographic area the size of a city or suburb
         o Local Area Network: privately owned communications
            network that serves users within a confined geographical
     Most LANs are confined to a single building or group of buildings.
      However, one LAN can be connected to other LANs over any
      distance via telephone lines and radio waves. A system of LANs
      connected in this way is called a wide-area network (WAN).
     The basic components of a communications via internet are:
         o A sending/receiving device e.g. PC, laptops
         o A communication device that converts the data into signal
            that can be carried by communication channel e.g. modem
         o A communications channel/ link e.g. cables, radio waves
     Node - a device that is attached to a network, it can be a
      computer, terminal or some peripheral device.
     Download - you retrieve files from another computer and store
      them in your computer
     Upload - you send files from your computer to another

Protocol of the Internet

Every machine on the Internet has a unique identifying number, called
an IP Address. The IP stands for Internet Protocol, which is the
language, that computers use to communicate over the Internet. A
protocol is the pre-defined way that someone who wants to use a
service talks with that service. The "someone" could be a person, but
more precise it is a computer program like a Web browse, MIRC, ICQ
used by the person. A typical IP address looks like this:
(PC from MMU)

In 1983, the University of Wisconsin created the Domain Name System
(DNS), which maps text names to IP addresses automatically. This

way you only need to remember the address instead
of IP address which is

Internet servers make the Internet possible. All of the machines on the
Internet are either servers or clients. The machines that provide
services to other machines are servers. And the machines that are
used to connect to those services are clients. There is Web servers, e-
mail servers, FTP servers and so on serving the needs of Internet
users all over the world. When you connect to the address above to
see the content, you are a user sitting at a client's machine. You are
accessing the MMU web server. The server machine finds the page you
requested and sends it to you. Clients that come to a server machine
do so with a specific intent, so clients direct their requests to a specific
software server running on the server machine. For example, if you
are running a Web browser on your machine, it will want to talk to the
Web server on the server machine, not the e-mail server.

A server has a static IP address that does not change very often. A
home machine that is dialing up through a modem, on the other hand,
typically has an IP address assigned by the ISP every time you dial in.
That IP address is unique for your session -- it may be different the
next time you dial in. This way, an ISP only needs one IP address for
each modem it supports, rather than one for each customer. The
simplest thing to do to check the server is running or down is by using

MS-Dos is useful after all :)

PING is the abbreviation for Packet Internet Groper, a utility to
determine whether a specific IP address is accessible. It works by
sending a packet to the specified address and waiting for a reply. PING
is used primarily to troubleshoot Internet connections. By pinging a
URL (uniform Resource Locator) like (don't include
the http and path), you can know the IP address and the time taken
for the data to make a round trip from your PC to the server and back
again to your PC. If the server is down or if u are not connected to
internet, u'll get the msg Request Timed Out. If you are chatting using
MIRC, you can run the PING command from MIRC to check whether
you are lagging or not.

How data travels the Internet?

When you, the user, look at a web page through the Internet, many
things happen along the way. There are various ways to get from your
house to your destination: modem dialup, ISDN, DSL, cable modem,
wireless, leased line, etc. These various technical methods may
provide speeds anywhere from very slow (a few hundred bits per
second) to very fast (billions of bits per second).

In order to upload (send) text or pictures, or request to download
data, your data is chopped up into small packets, which are routed
through the Internet. But first they have to go from you to your local
ISP. This local ISP is a possible point of failure. If something goes
wrong at your local ISP, it may look to you like the Internet is broken.
It's not. Only one small piece of it is broken. The rest of the Internet
will not break because one ISP does.

To reach a web server, your local ISP sends your packets of data to
another ISP, which may send them to another ISP, or through an
Exchange Point (IX) or a National Access Point (NAP) or Local Access
Point (LAP) to get to another ISP. Thus your packets pass through a
chain of ISPs. Your packets may pass through fiber optic cables in
the ground, satellites in the sky, undersea cables, or radio links. They
may travel at speeds including T-1 (1.544 Mbps), T-3 (45Mbps), or
faster (or slower). The Internet Protocol (IP) ties all of those links
together, enabling your packets travel through the Internet.

Eventually your packets arrive at the web server, and the web server
sends responses back along a similar path (almost definitely not the
same one). It’s like driving a car from KL to Shah Alam, u have a lot of
options either using Federal Highway, KESAS Highway , PLUS Highway

, Lebuhraya Elite , Jln Kelang Lama etc. If one of it is congested... u
can take the other route :)

Analog and Digital Transmission

     An analog signal is a continuous electrical signal in the form of
      a wave and it is called as a carrier wave.
     Telephones, radios, TV and older form of communication
      technology were designed to work with analog signal. Two
      characteristics of analog carrier waves that can be altered are:
     Frequency - the number of times a wave repeats during a
      specific time interval--that is, how many times it completes a
      cycle in a second
     Amplitude - the height of a wave within a given period of time
     Digital signal use on-off or present absent electrical pulses in
      discontinuous, or discrete, bursts, rather than a continuous
      wave. The two-state kind signal (the presence of an
      electrical/light pulse) works perfectly in representing the two-
      state binary language of 0s and 1s that computers use


     Computers uses digital signal, but most communication channels
      are still analog. Modem (short for modulate/demodulate) is used
      to converts a digital signal to an analog signal and vice versa.
     Conversion from digital to signals is called modulation, and the
      reverse process - reconstructing the original digital message at
      the other end of the transmission-is called demodulation.

      PC Card modem - used for portable computers like laptops. A
       size of a credit card
      The speed at which modem transmit data measured in kilobits
       per second (Kbps). The faster the better. Now the standard is
       56Kbps. Older modems are 14.4Kbps, 28.8Kbps and 33.6Kbps

Types of Communications Links

      Twisted-Pair Cable is the most common communication link
       especially in telephony.
      The wire pairs are twisted together to reduce noise (electrical
       disturbance), which is then insulated.
      Coaxial cable is a single conductor wire wrapped in a solid or
       braided metal shield, then in an external cover. An example is
       the cable that connects your vcr to the TV.
      Fiber optics cable consist of hundreds or thousands of thin
       strands of glass, each thinner than a human hair, that can send
       data via light beams.
      Carry more signal than wire cables and less susceptible to noise.
       Though the size is smaller but it is faster. The speed is 100Mbps
       to 2Gbps compared to twisted pair 1-128Mbps and coaxial
      Disadvantage - cost more than twisted pair and coaxial, difficult
       to install and modify
      A range of frequencies a device can handle is called its
       bandwidth. The bandwidth is the difference between the lowest
       and highest frequencies transmitted


      Acronym for Integrated Services Digital Network
      Consists of hardware and software that allow voice, video and
       data to be communicated as digital signals over traditional
       copper-wire telephone. With ISDN, the same twisted-pair
       telephone lines can carry 3 or more signals at once.

T-Carrier Lines

Most of us are familiar with a normal business or residential line from
the phone company. A normal phone line like this is delivered on a pair
of copper wires that transmit your voice as an analog signal. When you
use a normal modem on a line like this, it can transmit data at perhaps
30,000 bits per second. The phone company moves nearly all voice
traffic as digital rather than analog signals. Your analog line gets
converted to a digital signal so that it can be transmitted using fiber
optic lines like T1 lines from one ISP to another.

If an organization has a T1 line, it means that the phone company has
brought a fiber optic line into the organization office. A T1 line can
carry 24 digitized voice channels, or it can carry data at a rate of
1.544 megabits per second. If the T1 line is being used for telephone
conversations, it plugs into the office's phone system. If it is carrying
data it plugs into the network's router.

A T1 line is roughly 60 times faster data than a normal residential
modem. It is also extremely reliable .For general browsing; hundreds
of users are easily able to share a T1 line comfortably. T1 line might
cost between around RM 100,000 per annum.

Microwave transmission

      Microwave is radio waves. Uses what is called line-of-sight
       transmission of data signals through the atmosphere.
      Line-of-sight means that there must be an unobstructed view
       between transmitter and receiver.

Satellite transmission

      Uses earth stations to send and receive signals from satellites in
      A satellites are usually placed 22,300 miles above earth, where it
       reaches geosynchronous orbit- the place that allows it to

      remain positioned over the same spot on earth (because the
      speed of the satellite is the same as earth)
     Satellites are used in TV/radio broadcast, video conferencing
      and Global Positioning System (GPS).

Local Area Network

     A collection of computers, usually personal computers, that
      shares hardware, software, and data. Each computer can share
      the resources of the others
     Usually cover short distances, such as one office or building, or
      group of building
     Each individual computer in a LAN has its own CPU with which it
      executes programs, but it also is able to access data and devices
      anywhere on the LAN. This means that many users can share
      expensive devices, such as laser printers, as well as data. Users
      can also use the LAN to communicate with each other, by
      sending e-mail or engaging in chat sessions.
     LAN have several topologies (topology = physical layout)

Three basic topologies for LAN

1. Star network

     All devices in the network are connected to a central computer
      via a hub. All data transferred, must pass this hub.
     If one device fails, the other devices still operate normally, but if
      the hub fails, the entire network is inoperable.

2. Ring network

     Links all nodes together in a circular manner and data messages
      travel in only one direction around the ring
     There is no central server and any data that passes by is
      examined by the node to see if it is the addressee; if not, the
      data is passed on to the next node in the ring
     if one node fails then the entire network fails

3. Bus network

     Has a single line, to which all the network nodes are attached.
     Nodes can be attached to or detached from the network without
      affecting the network
     If one node fails, it does not affect the rest of the network

Local Area Network Components

     Connection and cabling system. LANs do not use the
      telephone network; instead, they use some other cabling or
      connection system. They may use wire (twisted-pair wiring,
      coaxial cable or fiber-optic) or wireless (infrared or radio-wave
     Microcomputers with interface cards. A network interface
      card, which is inserted into an expansion slot in a
      microcomputer, enables the computer to send and receive
      messages on the LAN
     Networking operating system - manages the activity of the
      network. Depending on the type of network, the operating
      system software may be stored on the file server or on each
      microcomputer on the network
     Similar networks can be connected by a bridge, a
      hardware/software combination that recognizes the messages on
      a network and passes on those addressed to nodes in other
     Route is the path between a source and destination station on a
      network. Router is device that can connect LANs that have
      different network topologies, such as Ethernet and Token Ring.
      Routers choose the best path for a packet, optimizing the
      network performance.

Routers are often described as being traffic cops for the data
traveling across network connections and Internet highways. Router
guides a packet of data from one network to another. Packets are
small blocks of data that contain three separate parts: header with
the address of the source and destination computers, the data and
code used to correct any errors. Packets are received as a whole
by a router or switch, and then forwarded on to a destination. For
example, when you send an e-mail message to a friend across the
country, the message is actually broken into several smaller
packets that travel on any number of different routes to the
destination computer. The role of TCP/IP: TCP breaks the message
into data packets and reassembles them in the proper order once
they reach their destination. IP is used to assign each packet a
unique address. When a packet reaches a router, the router reads
the address and uses information it has gathered in a database to
intelligently decide which path is the best one for the packet to take
to its destination.

     Gateway is a collection of hardware and software resources that
      lets a node communicate with a computer on another dissimilar
      network (e.g. star to an Ethernet)
     When message reaches its destination network, router passes it
      to the network gateway. Gateway can be a computer or even a
      router itself that is assigned to receive all incoming messages.
      The gateway transfer the message to the network server to the
      proper recipient

  Types of LANs

  Peer-to-Peer Network

                                         Network Neighborhood is a
tool to browse the peer-to-peer network for Windows 95/98/2000
users. Each computer with a shared resource manages its own
security. To share an item simply uses the windows explorer. Right
click on the item and choose Sharing. To keep network management
simple, Microsoft implemented share-level security in all of its

Windows 9x OSes where user can specify whether the access to the
shared item is read/full/depend on password. Many LANs mix element
from both client-server and peer-to-peer models. Napster is another
example of P-to-P network that works in WAN. It allows the users to
share their MP3s

      All microcomputers on the network communicate directly with
       one another without relying on a server
      All computers have equal status, and users share each other's
       data and devices as needed.
      Simple, inexpensive, suitable to connect less than 10 computers
      The main disadvantage is lack of speed - most peer-to-peer
       networks slow down under heavy use.
      Popular NOS for peer-to-peer: Windows95, Windows98

Client / Server Network
If you're working with small files and only require networking for some
file sharing, peer-to-peer is a workable option that's easy to set up but
when the number of users grows, you probably will want to migrate to
a client/server network. A client/server network is a more flexible and
secured network option, but it's also more expensive and more difficult
to set up.

      A client/server LAN consist a server, which is a computer that
       controls the network and other computers called clients that
       can request service from the server such as accessing database
       and printing.
      The server usually is the fastest and has more disk storage than
       the clients. Therefore most processing task are done by the
       server for the clients. The results are then send back to the
      You can have more than 1 server in a network. Some server are
       dedicated to a specific task like print server
      Popular NOS for client/server: Unix, Novell Netware, Windows
       NT, Banyan, OS/2

Popular Local Area Network Protocols

   The most important Internet protocol is Transmission Control
    Protocol/Internet Protocol (TCP/IP). It defines how data travel
    through the Internet. But if the data travel within a LAN only,
    TCP/IP is not used.
   LAN has its own protocols like Ethernet and Token Ring.
    Ethernet (a.k.a 10BaseT) is the most popular LAN protocol,
    uses a high-speed network cable. It uses bus topology but most
    Ethernet networks employ a star topology where single Ethernet
    cable runs from each PC to the network hub. It is inexpensive
    and relatively simple. The maximum transfer rate is 10Mbps.
    There're other versions of Ethernet like Fast Ethernet a.k.a
    100BaseT (100Mbps) and Gigabit Ethernet / gigaPoP

   Ethernet uses carrier sense multiple access with collision
    detection or CSMA/CD, in which a node, before transmitting
    data a node "listen" to find out if the cable is in use
   A Token Ring network - based on ring topology. There's only 1
    token moving around the ring. Only the device with the token
    can transmit data at a time. If the device has nothing to
    transmit, the token is passed to the next device in the ring.
   Most Apple Macintosh networks are based on Apple's AppleTalk
    network system.

Popular Network Applications

Telnet - is terminal emulation software. It allows a terminal
(refer chapter Input & Output) to connect to a telnet server. If the
terminal that are connected to the server is not a dumb terminal
(may be it's a PC or laptop), then terminal emulation software

must be executed to make it appear as dumb. Telnet are no longer
popular among windows user but it is still widely used by Unix
FTP - file transfer protocol allows clients to transfer and retrieve
files from server
HTTP - hypertext transfer protocol, combines telnet, ftp, email,
gopher etc together.
Instant Messaging (IM) - Most student in MMU know how to use
this software :) The most popular are Yahoo, MSN and AOL
messengers. The good thing about it is u can also send SMS to any
user connected to Yahoo Messenger.


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