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Friday, September 4, 2015

Communications

Communications

Digital communications describes a process in which two or more computers or devices transfer
data, instructions, and information.Today, even the smallest computers and devices can communicate directly with one another, with hundreds of computers on a corporate network, or with
millions of other computers around the globe — often via the Internet.

Communications system. Some communications involve cables and wires; others are sent wirelessly through the air. For successful communications, you need the following:

• A sending device that initiates an instruction to transmit data, instructions, or information.
• A communications device that connects the sending device to a communications channel.
• Transmission media, or a communications channel, on which the data, instructions, or
information travel.
• A communications device that connects the communications channel to a receiving device.
• A receiving device that accepts the transmission of data, instructions, or information

All types of computers and mobile devices serve as sending and receiving devices in a communications system. This includes servers, desktops, laptops, smartphones, portable media players, handheld game devices, and GPS receivers. Communications devices, such as modems, wireless access points, and routers, connect transmission media to a sending or receiving device. Transmission media can be wired or wireless.

Communications System
A simplified example of a communications system. Some devices that serve as sending and receiving devices are
(a) servers, (b) desktops, (c) laptops, (d) tablets, (e) smartphones and headsets, (f) portable media players,
(g) handheld game devices, and (h) GPS receivers in vehicles. Transmission media consist of
phone and power lines, cable television and other underground lines, microwave, and satellites.



Networks


A network is a collection of computers and devices connected together via communications devices and transmission media. A network can be internal to an organization or span the world by connecting to the Internet. Many home and business users create a network to facilitate communications, share hardware, share data and information, share software, and transfer funds.

Facilitate communications. Using a network, people communicate efficiently and easily via
email, instant messaging, chat rooms, blogs, wikis, online social networks, video calls, online
meetings, videoconferencing, VoIP, wireless messaging services, and more. Some of these
communications occur within an internal network. Other times, they occur globally over the
Internet.
Share hardware. Each computer or device on a network can be provided access to hardware
on the network. For example, each computer and mobile device user can access a printer on
the network, as they need it. Thus, home and business users create networks to save money on
hardware expenses.
Share data and information. Any authorized user can access data and information stored on
a network. A large company, for example, might have a database of customer information.



Reasons Businesses Use a Network
Networks facilitate communications; enable sharing of hardware, data and
information, and software; and provide a means for transferring funds.

Any authorized employee can access the database using a computer or mobile device connected to
the network.
Most businesses use a standard, such as EDI (electronic data interchange), that defines how
business documents transmit across transmission media. For example, businesses use EDI to
send bids and proposals, place and track orders, and send invoices.

Share software. Users connected to a network have access to software on the network. To
support multiple users’ software access, vendors often sell versions of their software designed
to run on a network. These network versions usually cost less than buying individual copies
of the software for each computer. Recall from Chapter 5 that a network license is a legal
agreement that allows multiple users to access the software on a server simultaneously. The
network license fee usually is based on the number of users or the number of computers
attached to the network.
Transfer funds. Electronic funds transfer (EFT) allows users connected to a network to
exchange money from one account to another via transmission media. Both businesses
and consumers use EFT. Examples include wire transfers, use of credit cards and
debit cards, direct deposit of funds into bank accounts, online banking, and online bill
payment.

Instead of using the Internet or investing in and administering an internal network, some
companies hire a value-added network provider for network functions. A value-added network
(VAN) provider is a third-party business that provides networking services such as EDI services,
secure data and information transfer, storage, or email. Some VANs charge an annual or
monthly fee; others charge by the service used.

What is an intranet?
Recognizing the efficiency and power of the Internet, many organizations apply Internet and web technologies to their internal networks. An intranet (intra means within) is an internal network that uses Internet technologies.
Intranets generally make company information accessible to employees and facilitate collaboration within an organization.
One or more servers on an intranet host an organization’s internal webpages, applications, email messages, files, and more. Users locate information, access resources, and update content on an intranet using methods similar to those used on the Internet. The difference is that the host name of the intranet server differs from the host name of the company’s public web server (recall that the host name of public web servers often is www). 
Sometimes a company uses an extranet, which allows customers or suppliers to access part of its intranet. Package shipping companies, for example, allow customers to access their intranet via an extranet to print air bills, schedule pickups, and track shipped packages as the packages travel to their destinations.

LANs, MANs, WANs, and PANs
  Networks usually are classified as a local area network, metropolitan area network, wide area
network, or personal area network. The main difference among these classifications is their area of
coverage.

LAN A local area network (LAN) is a network that connects computers and devices in a
limited geographical area such as a home, school, office building or closely positioned group of buildings. Each computer or device on the network, called a node, often shares resources such as printers, large hard drives, and programs. Often, the nodes are connected via cables.

A wireless LAN (WLAN)
Computers and devices on different floors access the same
LAN in an office building.





A wireless LAN (WLAN) is a LAN that uses no physical wires. Computers and devices that
access a wireless LAN must have built-in wireless capability or the appropriate wireless network
card, USB adapter or other wireless device. A WLAN may communicate with a wired LAN for
access to its resources, such as software, hardware, and the Internet.

A wireless LAN (WLAN)
Computers and mobile devices on a WLAN may communicate via
a wireless access point with a wired LAN to access its hardware,
software, internet connection, and other resources.




MAN A metropolitan area network (MAN) is    
MAN A metropolitan area network
A simplified example of a WAN
a high-speed network that connects local area
networks in a metropolitan area, such as a city
or town, and handles the bulk of communications
activity across that region. A MAN typically
includes one or more LANs, but covers a
smaller geographic area than a WAN.
A MAN usually is managed by a consortium
of users or by a single network provider
that sells the service to the users. Local and
state governments, for example, regulate some
MANs. Phone companies, cable television
providers, and other organizations provide
users with connections to the MAN.

WAN A wide area network (WAN) is a 
network that covers a large geographic area
(such as a city, country, or the world) using 
a variety of wired and wireless transmission 
media. A WAN can be one 
large network or can consist of multiple 
LANs connected together. The Internet is 
the world’s largest WAN.

PAN A personal area network (PAN) is a 
network that connects computers and devices 
in an individual’s workspace using wired and 
wireless technology. Devices include smart-
phones, digital cameras, printers, and more. 
A PAN may connect devices through a router 
using network cables or directly using special USB cables. PANs also may use Bluetooth or Wi-Fi technology. 

Client/Server On a client/server network, one or more computers act as a server, and the other computers on the network request services from the server. A server, sometimes called a host computer, controls access to the hardware, software, and other resources on the network and provides
a centralized storage area for programs, data, and information. The clients are other computers and
mobile devices on the network that rely on the server for its resources. For example, a server might store an organization’s email messages. Clients on the network, which include any users’ connected computers or mobile devices, access email messages on the server.

Client/Server On a client/server network
On a client/server network, one or more computers act as a server, and the
client computers and mobile devices access the server(s).

Although it can connect a smaller number of computers, a client/server network typically provides an efficient means to connect 10 or more computers. Most client/server networks require a person to serve as a network administrator because of the large size of the network. some servers are dedicated
servers that perform a specific task. For example, a network server manages network traffic (activity), and a web server delivers requested webpages to computers or mobile devices.

Peer-to-Peer A peer-to-peer network (P2P) is a simple, inexpensive network that typically connects fewer than 10 computers. Each computer, called a peer, has equal responsibilities and capabilities, sharing hardware (such as a printer), data, or information with other computers on the peer-to-peer network. Each computer stores files on its own storage devices. Thus, each computer on the network
contains both the operating system and applications. All computers on the network share any peripheral device(s) attached to any computer. For example, one computer may have a laser printer and a scanner, while another has an ink-jet printer and an external hard drive.

Peer-to-PeerA peer-to-peer network (P2P)
Each computer on a P2P network shares its hardware and software
with other computers on the network.
P2P networks are ideal for very small businesses and home users. Some operating systems include a P2P networking tool that allows users to set up a peer-to-peer network. Many businesses also see an advantage to using P2P. That is, companies and employees can exchange files using P2P, freeing the company from maintaining a network server for this purpose. Business-to-business e-commerce websites find that P2P easily allows buyers  and sellers to share company information such as product databases.

Network Topologies

A network topology refers to the layout of the computers and devices in a communications network.    Three basic network topologies are star, bus, and ring.  Most networks, including the Internet, use combinations of these basic topologies. Thus, you should be familiar with the layout of communications in these topologies.                     

Network Topologies
A star network contains a single, centralized device through which all
computers and devices on the network communicate.
Star Network On a star network, all of the computers and devices (nodes) on the network connect
to a central device, thus forming a star. All data that transfers from one node to another passes through the central device.  
        Star networks are fairly easy to install and maintain. Nodes can be added to and removed from the network with little or no disruption to the network.

Bus Network A bus network consists of a single central cable, to which all computers and other devices connect. The bus is the physical cable that connects the computers and other devices. The bus in a bus network transmits data, instructions, and information in both directions. When a sending device transmits data, the address of the receiving device is included with the transmission so that the
data is routed to the appropriate receiving device.

Bus Network
Devices on a bus network share a single data path.
Bus networks are inexpensive and easy to install. One advantage of the bus network
is that computers and other devices can be attached and detached at any point on the
bus without disturbing the rest of the network.
  Another advantage is that failure of one device usually does not affect the rest of
the bus network. The greatest risk to a bus network is that the bus itself might become
inoperable. If that happens, the network remains inoperative until the bus is back in working order. 

Ring Network On a ring network, a cable forms a closed loop (ring) with all computers and devices arranged along the ring. Data transmitted on a ring network travels from device to device around the
entire ring, in one direction.

Ring Network
On a ring network, all connected devices form a continuous loop.

If a computer or device on a ring network fails, the entire network potentially could stop functioning. A ring network can span a larger distance than a bus network, but it is more difficult to install. The ring topology primarily is used for LANs, but it also is used in WANs.
 
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