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Introduction to Networks

Ethernet Definition: Ethernet is a physical and data link layer technology for  local area networks(LANs). Ethernet was invented by engineer Robert Metcalfe.

When first widely deployed in the 1980s, Ethernet supported a maximum theoretical data rate

of 10 megabits per second (Mbps). Later, so-called "Fast Ethernet" standards increased this

maximum data rate to 100 Mbps. Today, Gigabit Ethernet technology further extends peak 

 performance up to 1000 Mbps.

Higher level network protocols like Internet Protocol (IP) use Ethernet as their transmission

medium. Data travels over Ethernet inside protocol units called frames.

The run length of individual Ethernet cables is limited to roughly 100 meters, but Ethernet

networks can be easily extended to link entire schools or office buildings using networkbridge devices.

adapter By Bradley Mitchell, About.com Guide

See More About:

  types of network hardware

  wireless network adapters

  vpn

Definition: A network adapter interfaces a computer to a network. The term "adapter" was

 popularized originally by Ethernet add-in cards for PCs.

Modern network adapter hardware exists in several forms. Besides traditional PCI Ethernet

cards, some network adapters are PCMCIA devices (also know as "credit card" or "PC Card"

adapters) or  USB devices. Some wireless network adapter gear for laptop computers are

integrated circuit chips pre-installed inside the computer.

Windows and other operating systems support both wired and wireless network adapters

through a piece of software called a "device driver." Network drivers allow application

software to communicate with the adapter hardware. Network device drivers are ofteninstalled automatically when adapter hardware is first powered on.

A few network adapters are purely software packages that simulate the functions of a network 

card. These so-called virtual adapters are especially common in virtual private networking

(VPN). 

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Defining a Network 

In the simplest terms, a network is a group of two or more computers interconnected by cable

or some other media, enabling them to share information and resources. Networks make it

more efficient to share information and resources than stand-alone computers. As businesses

grow, the need to share information and use common applications increases. With a network,

a large number of people can simultaneously work on different parts of the same database,

share information and resources and communicate across the network. A network serves to

integrate the business operation and improve efficiency and productivity.

Sharing files is the largest shared resource on a network. Everyone in the company can work 

from the same consolidated database application, fully integrating all facets of the business

operation while greatly reducing manual data transfer by each individual department and

reducing the possibilities of human errors and miscommunication.

Sharing common printers is the second largest shared resource. For example, say that you

wanted everyone to print company correspondence on a high quality laser printer. Without a

network, you would have to purchase one of these printers for every computer. With a

network, all computers can share a single printer. For a small office, the cost of networking

the computers together is probably LESS than the cost of one additional printer. Other shared

resources on a network can include scanners, fax machines, modems and copiers.

 Networks enable us to share information throughout the company, between other companies

and all over the world.

Network Types

THERE ARE TWO PRIMARY TYPES OF NETWORKS TODAY-CLIENT SERVER ANDPEER TO PEER.

Peer to Peer Network 

This is a simple and inexpensive form of networking. Each computer (workstation) on the

network acts as both a client (using information) and a server (providing information to

others). Each user can access data and other resources on other work stations and can share

their own resources by setting share rights on their own directories through password

 protection known as share level security. This network is well suited to a smaller 

environment with ten workstations or less and where share level security is sufficient. In

addition, all computers with Windows 95/98 or Windows NT as the operating system already

have peer to peer networking built in. All you need to establish a network are network adapter cards, properly installed network cabling to connect them and, depending on the type

of cabling you choose, a network hub.

Advantages

  Works well for smaller offices of ten or less workstations

  Easy to set up

  Easy to maintain as long as it stays small

   No need for a dedicated network administrator 

   No need for a file server 

  Software already built in

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Disadvantages

  Becomes more difficult to administrate as it gets larger 

  Individual users must do data backups

  Share level security becomes inadequate as network grows

  More dependent on individual user training

   Not well suited for large database applications due to lack of dedicated file server 

Client Server Network 

This is the most popular type of network today. In this type of network, one or more

dedicated file servers will handle file requests from workstations, store and manage files,

databases, printers and other network devices. This type of network is more suited for larger 

environments, where an integrated database is used to run operations and where tighter 

security is required. The file servers are much more efficient at handling large numbers of filerequests. In addition, critical data is stored and backed up at one central location. Finally, this

type of network employs user level security. The password of each individual user defines

which files, applications and other network resources the user is permitted to access

throughout the entire network. To establish this type of network, you will need to purchase a

file server, network adapter cards for all workstation, client server software, a network hub

and have network cabling professionally installed.

Advantages

  Much more efficient at handling large databases and managing files

  User level security makes network easier to use while providing much tighter security

  Critical data is backed up at one central location

Disadvantages

  Dedicated file servers are more expensive than workstations

   Need to have at least a part time administrator to maintain the network 

   Need to purchase client server network software

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Network Topologies

THERE ARE THREE PRIMARY TYPES OF NETWORK TOPOLOGIES, WHICH REFER 

TO THE PHYSICAL AND LOGICAL LAYOUT OF THE NETWORK CABLING. THEY

ARE BUS, STAR AND RING. BUS AND STAR ARE THE MOST WIDELY USED FOR 

ETHERNET NETWORKS AND RING IS USED FOR TOKEN RING NETWORKS.

Low Level Standards

  Prior to discussing network topologies, it is necessary to define low level standards.

These are guidelines that describe how data (frames) are transmitted across the

 physical and data link layers of a network. They are developed by the Institute of 

Electrical and Electronic Engineers. The 802.x standard describes guidelines for 

Ethernet and Token Ring networks. Standards such as 10 BASE T and 10 BASE 2

describe a specific cable type and other limitations for Ethernet, such as Category 5

unshielded twisted pair for 100 BASE T, or Fast Ethernet.

Bus topology

Bus topology refers to a single cable that connects all the workstations, servers, printers and

other devices on the network. The cable runs from device to device by using tee connectors

that plug into the network adapter cards. Each end device has a terminator on one end of the

tee and a cable going out to the next device on the other end, while all devices in the middle

have one cable coming in and one going out. The terminators on each end device simply stop

the network signal from reflecting back into the cable and colliding with other transmissions.

The most common type of network cable used for a bus topology is RG-58 thin net. The

network speed is limited to 10 megabits per second, making it a suitable media for only 10

BASE 2 Ethernet. There are also network size limitations. You may have a maximum of 

twenty network devices on a segment, and the segment cannot exceed 185 meters in total

length. By using a device called a repeater that boosts the signal, you can have up to fivesegments on a network. However, only three of these segments can have devices attached to

them. The other two segments are used to link the three populated segments, giving you a

maximum number of sixty devices with a total network length of 925 meters. This topology

works equally well for either peer to peer or client server.

Advantages

  Less expensive than a star topology due to less footage of cabling and no network 

hubs

  Good for smaller networks not requiring higher speeds

Disadvantages  Limited in size and speed

  One bad connector can take down entire network 

  Difficult to troubleshoot

Star Topology

In a star topology, each network device has a home run of cabling back to a network hub,

giving each device a separate connection to the network. If there is a problem with a cable, it

will generally not affect the rest of the network. The most common cable media in use for star 

topologies is unshielded twisted pair copper cabling. Category 3 is still found frequently in

older installations. It is capable of 10 megabits per second data transfer rate, making itsuitable for only 10 BASE T Ethernet. Most new installations use Category 5 cabling. It is

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capable of data transfer rates of 100 megabits per second, enabling it to employ 100 BASE T

Ethernet, also known as Fast Ethernet. More importantly, the brand new 1000 BASE T

Ethernet standard will be able to run over most existing Category 5. Finally, fiber optic cable

can be used to transmit either 10 BASE T or 100 BASE T Ethernet frames.

Two variations of the star topology used by most larger Ethernet networks today are the star 

 bus and star tree topologies. Essentially, the star bus topology has multiple data closets

interconnected by bus trunk lines of thin net, while the star tree topology links multiple data

closets with twisted pair or fiber optic. These types of network topologies allow a network to

cover a much larger physical area.

There are size limitations to star topologies utilizing Ethernet. The maximum number of 

network devices is 1,024 and the maximum number of data closets is four. When using

Category 3 or 5 twisted pair cabling, individual cables cannot exceed 100 meters. In regard to

total network length, the maximum when linking data closets with twisted pair is 500 meters

 between the furthest two devices. If multi-mode fiber optic is used to link closets, then the

distance between closets can be up to 2,000 meters.

Advantages

  More suited for larger networks

  Easy to expand network 

  Easy to troubleshoot because problem usually isolates itself 

  Cabling types can be mixed

Disadvantages

  Hubs become a single point of network failure, not the cabling

  Cabling more expensive due to home run needed for every deviceRing Topology

Ring topologies are used on token ring networks. Each device processes and retransmits the

signal, so it is capable of supporting many devices in a somewhat slow but very orderly

fashion. A token, or small data packet, is continuously passed around the network. When a

device needs to transmit, it reserves the token for the next trip around, then attaches its data

 packet to it. The receiving device sends back the packet with an acknowledgment of receipt,

then the sending device puts the token back out on the network. Most token ring networks

have the physical cabling of a star topology and the logical function of a ring through use of 

multi access units (MAU). In a ring topology, the network signal is passed through each

network card of each device and passed on to the next device. All devices have a cable homerunned back to the MAU. The MAU makes a logical ring connection between the devices

internally. When each device signs on or off, it sends an electrical signal which trips

mechanical switches inside the MAU to either connect the device to the ring or drop it off the

ring. The most common type of cabling used for token ring networks is twisted pair, although

there are nine different types that can be used. With IBM Type 1 Shielded cable, you can

have up to 33 network segments with 260 devices on each. Transmission rates are at either 4

or 16 megabits per second.

Advantages

  Very orderly network where every device has access to the token and the opportunity

to transmit

  Performs better than a star topology under heavy network load

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  Can create much larger network using Token Ring

Disadvantages

  One malfunctioning workstation or bad port in the MAU can create problems for the

entire network 

  Moves, adds and changes of devices can affect the network 

   Network adapter cards and MAU's are much more expensive than Ethernet cards and

hubs

  Much slower than an Ethernet network under normal load