Notes

The operating system (OS) controls almost all functions on a computer. In this chapter, you learn about the components, functions, and terminology related to the Windows 7, Windows Vista, and Windows XP operating systems.

Multi-user - Two or more users have individual accounts that allow them to work with programs and peripheral devices at the same time.
Multitasking - The computer is capable of operating multiple applications at the same time.
Multiprocessing - The operating system can support two or more CPUs.
Multithreading - A program can be broken into smaller parts that are loaded as needed by the operating system. Multithreading allows different parts of a program to be run at the same time.


Control hardware access
Manage files and folders
Provide a user interface
Manage applications

An architectural model is a common frame of reference for explaining Internet communications and developing communication protocols. It separates the functions of protocols into manageable layers. Each layer performs a specific function in the process of communicating over a network.

A NOS contains additional features to increase functionality and manageability in a networked environment. A NOS has the following characteristics:

Supports multiple users
Runs multi-user applications
Provides increased security compared to desktop operating systems
A NOS provides network resources to computers, including:

Server applications, such as shared databases
Centralized data storage
Centralized repository of user accounts and resources on the network
Network print queue
Redundant storage systems, such as RAID and backups
The following are examples of network operating systems:

Windows Server
Red Hat Linux
Mac OS X Server

When a computer is passed from one employee to another
When the OS is corrupt
When the primary hard drive is replaced in a computer

To support the immediate delivery of the millions of messages being exchanged between people all over the world, we rely on a web of interconnected networks. The standardization of the various elements of the network enables equipment and devices created by different companies to work together. It is important that IT technicians understand the purpose and function of different network equipment used to support personal and business operations.

A modem is an electronic device that connects to the Internet via an ISP.

The modem converts digital data to analog signals for transmission over a phone line. Because the analog signals change gradually and continuously, they can be drawn as waves. In this system, the digital signals are represented as binary bits.

The digital signals must be converted to a waveform to travel across telephone lines. They are converted back to bits by the receiving modem so that the receiving computer can process the data.

The modem at the receiving end reconverts the analog signals back to digital data to be interpreted by the computer.

The process of converting analog signals to digital and back again is called modulation/demodulation.

The accuracy of modem-based transmission has increased with the development of error detection and correction protocols, which has reduced or eliminated the effects of noise and interference on telephone lines.

An internal modem plugs into an expansion slot on the motherboard. External modems connect to a computer through the serial and USB ports. Software drivers must be installed and connection ports configured for the modem to work properly.

When computers use the public telephone system to communicate, it is called Dialup Networking (DUN). Modems communicate with each other using audio tone signals. This means that modems are able to duplicate the dialing characteristics of a telephone. DUN creates a Point-to-Point Protocol (PPP). A PPP is simply a connection between two computers over a phone line.

To make data transmission more extensible and efficient than a simple peer-to-peer network, network designers use specialized network devices, such as hubs, bridges and switches, routers, and wireless access points, to send data between devices.

Hubs

Hubs, shown in Figure 1, extend the range of a network by receiving data on one port and then regenerating the data and sending it out to all other ports.

A hub can also function as a repeater.

A repeater extends the reach of a network because it rebuilds the signal, which overcomes the effects of data degradation over distance. The hub can also connect to another networking device, like a switch or router that connects to other sections of the network.

Hubs are used less often today because of the effectiveness and low cost of switches. Hubs do not segment network traffic, so they decrease the amount of available bandwidth for all devices connected to them.

In addition, because hubs cannot filter data, a lot of unnecessary network traffic constantly moves between all the devices connected to it.

Bridges and Switches

Files are broken up into small pieces of data, called packets, before they are transmitted over a network. This process allows for error checking and easier retransmission if the packet is lost or corrupted.

Address information is added to the beginning and end of packets before they are transmitted. The packet, along with the address information, is called a frame.

LANs are often divided into sections called segments, similar to the way a company is divided into departments, or a school is divided into classes. The boundaries of segments can be defined using a bridge. A bridge filters network traffic between LAN segments. Bridges keep a record of all the devices on each segment to which the bridge is connected. When the bridge receives a frame, the bridge examines the destination address to determine if the frame is to be sent to a different segment or dropped. The bridge also helps to improve the flow of data by keeping frames confined to only the segment to which the frame belongs.

Switches, shown in Figure 2, are sometimes called multiport bridges. A typical bridge has two ports, linking two segments of the same network. A switch has several ports, depending on how many network segments are to be linked. A switch is a more sophisticated device than a bridge.

In modern networks, switches have replaced hubs as the central point of connectivity. Like a hub, the speed of the switch determines the maximum speed of the network. However, switches filter and segment network traffic by sending data only to the device to which it is sent. This provides higher dedicated bandwidth to each device on the network.

Switches maintain a switching table. The switching table contains a list of all MAC addresses on the network, and a list of which switch port can be used to reach a device with a given MAC address. The switching table records MAC addresses by inspecting the source MAC address of every incoming frame, as well as the port on which the frame arrives. The switch then creates a switching table that maps MAC addresses to outgoing ports. When a frame arrives that is destined for a particular MAC address, the switch uses the switching table to determine which port to use to reach the MAC address. The frame is forwarded from the port to the destination. By sending frames out of only one port to the destination, other ports are not affected.

Power over Ethernet (PoE)

A PoE switch transfers small amounts of DC current over Ethernet cable, along with data, to power PoE devices. Low voltage devices that support PoE, such as Wi-Fi access points, surveillance video devices, and NICs, can be powered from remote locations. Devices that support PoE can receive power over an Ethernet connection at distances up to 330 ft (100 m) away.