Computer Networks What you’ll learn Types of networks: Ring, bus, star and tree

Local area networks(LANs), wide area networks(WANs), wireless LANs(WLANs)

Network devices- modems, hubs and switches, routers and bridges The internet- web browsers and internet services and providers (ISPs) Intranets Network security- user IDs, passwords, encryption and authentication techniques Communications-fax, email, conferencing and voice over internet protocol(VOIP).

• Computer systems are connected together to form a network.

• Most networks are controlled by the use of servers.

Types of servers:

File Servers This allows the users to save and load data/files.

Application Servers This deals with the distribution of applications software to each computer.

Printer Servers This type of server ensure printing from devices on the network is done in a queue.

Proxy Servers This server is used as a buffer between WANs and LANs.

LAN consists of multiple computers and devices that are connected to hubs or switches. One of the hubs or switches will be connected to a router or modem to allow the LAN to connect the internet. It then becomes WAN(Wide Area Network)

Advantages Disadvantages

• Communications between users• A network administration to

control and monitor all aspects of the network.

• Virus spreading is easier to get through network.

• Slower access to external networks.

• If main servers breaks down, networks will no longer function.

There are four types of LAN network topologies:• Ring• Bus• Star• Tree networks

Ring Network• A ring network is a network topology in

which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring.

Advantages Disadvantages• They work well under heavy loading.• Possible to create a very large network.

• If there’s a problem with the wiring between two computers, it will also affect the

Any system where you can connect to a network or to a single computer through a wireless communication. They rely on some form that uses radio frequency technology to receive and send signal

Examples:

• To WLANs

• PDAs and other handheld devices

• Laptop computers

• Printers, keyboard and mouse

• Bluetooth is an example of wireless personal area working (WPAN)technology. Spread spectrum transmission is used to provide wireless links between mobile phones, computers and other handheld devices and allow connection to the internet.• With this system, it is possible to create a small home network.

WIDE ARE NETWORKS

• A wide area network (WAN) is basically formed by a number of LANs being connected together through either a router or a modem. Some companies will set up private WANs. This is expansive but comes with the advantage of much enhanced security.

NETWORK DEVICES MODEMS

• Modem means modulator demodulator and is a device which converts a computer’s digital signal into an analogue signal for transmission over an existing telephone line.• Modems are used to allow computers to connect to networks over a

long distances using existing telephone networks.• Dial-up modems operate at transmission speeds of about 60 kilobits

per second, which is quite slow by today’s standards.• Although the ADSL modems still use the existing telephone network,

unlike dial-up modems they do not tie up the line while accessing the internet, so the land-line telephone can still be used at the same time.

NETWORK HUBS

Network hubs are hardware devices that can have a number of devices/computers connected to them. Its main task to take any data received via one of the pots and then send out this data from all of the pots. Each computer/device will receive the data, whether it is relevant or not.

SWITCHES

• Switches are similar to hubs but are more efficient in the way they distribute data. A hub learns which devices are connected to which ports. Each device has a media access control (MAC) address which identifies it uniquely. Data packets sent to the switch will have a MAC address giving the source and receiving device.• If a device X is always sending the switch data via port 4 then it learns

that X must be connected to that port. This means that the network traffic only goes to where it is needed and so a switch is more efficient than a hub, especially when the network is very busy.

BRIDGES

• Bridges are devices that connect one LAN to another LAN that uses the same protocol. They decide whether a message from a user is going to another user on the same LAN or to a user on a different LAN.• In network that use bridges, workstation addresses are not specific to

their location and therefore messages are actually sent out to every workstation on the network. However, only the target workstation accept this message. Networks using bridges are interconnected LANs since sending out every message to every workstation would flood a large network with unnecessary traffic.

ROUTERS

• Since large companies often have more than one network there are occasions when the computers in one network want to communicate with the computers in one of the other networks. Routers are often used to connect LANs together and also connect them to the internet.• Routers inspect the data packages sent to it from any computer on any of• the networks connected to it. Since every computer on the same network

has the same first part of an internet protocol (IP) address, the router is able to send the data package to the appropriate switch and it will then be delivered using the mac destination address in the data packet. If this mac address doesn’t match any device on the network it passes on to another switch on the same network until the device is found.

HTTP PROXY SERVERS

• This is a special type of server that acts as a buffer between WAN and a LAN. The server passes on the service requests to the internet and then passes back the requested pages. It therefore retrieves web pages and passes them on to the computer that made the request. Any page retrieved from the internet is stored on the server, which means that when a different computer requests the same page it is available immediately thus considerably speeding up the browsing process.

THE INTERNETWEBSITES

• The web is made up of millions of those websites and millions of web pages . Web pages are documents on a computer screen which may consist of text, pictures, sounds, animation or video. A website consists of many of these pages linked together. The website shows these hyperlinks to allow users to navigate between web pages. These hyperlinks are often shown as blue underlined text. An alphanumeric form is usually used which has the format: protocol://site address/path/filename.

• Where protocol is usually http• Site address consists of: host computer name, domain name, domain type and the country code• Computer name is usually www• Domain name is the name of the website• Domain type is commonly one of the following: .com, .org, .co, .net, .gov• Examples of country code include: .uk, .us, .de, .cy, .vn• Path is the web page• File name is the item of the webpage

ACCESSING THE INTERNET

• An ISP is a company that provides users with access to the internet, usually for a fee. When a user registers with an ISP, an account is set up and they are given login details, which include a username and a password. Most IPs also provide an email account.• Emails allow attachments. An email address contain 2 parts:

example1@yahoo.co.uk OR example2@yahoo.com

INTRANETS

• Many companies use an intranet as well as internet. Intranets is a computer network based on internet technology that is designed to meet the internal needs for sharing information within a single organization/company. Reasons for doing this:• It is safer since there is less chance of external hacking or viruses• it is possible to prevent employees from accessing unwanted websites• It is now worth comparing the internet with intranets:• The term ‘internet’ come from the phrase international network• The term ‘intranet’ comes from the phrase internal restricted access

network

NETWORK SECURITY

• The security problems when using networks such as the internet are well documented. There are various security threats to network and there are many equally varied ways of combating the threat.

USER IDS

• When logging on to any network system, a user will be asked to type in a User ID. The assigns the user privilege once the logon procedure is successful

PASSWORDS

• After keying in the user ID, the user will then be requested to type in their password. This should be a combination of letters and numbers which would be difficult for somebody else to guess. When the password is types in it often shows on the screen as ****** so nobody overlooking can see what the user has typed in. if the password doesn’t match up with the user ID then access will be denied. Many systems ask for the password to be typed in twice as a verification check. To help protect the system, users are only allowed to type in their password a finite number of times – three times is usually maximum number of tries allowed before the system locks the user out.

ENCRYPTION

• Encryption is the converting of data into a code by scrambling it orencoding it. This is done by employing encryption software. Since the data isall jumbled up it appears meaningless to a hacker or anyone who illegally accesses the data. It should be stressed that this technique doesn’t prevent illegal access.

The system works like this:• A user writes a message and the computer sending this message uses an

encryption key to encode the data• At the other end, the receiving computer has a decryption key which its

uses to decode the message. Note that the data when the message was sent is important since this formed part of encryption algorithm.

AUTHENTICATION TECHNIQUES

• There are many ways in which a computer user can prove who they are. This is called authentication, and a type of authentication is used in the banking. Most systems adopt the following authentication logic: something that you know - PIN/password Something belonging to you – your bank card Something unique to you – your fingerprints. At least two of these are needed at the moment when a user has to prove who they are. Something you know – surname, reference number, PIN, date last logged on. Something belonging to you – card put into card reader to produce the 8-digit code

COMMUNICATION METHODSFAX

• With this system, documents are scanned electronically and converted into a bit map image. The receiving fax machine converts this electronic image and prints it out on paper.• It is also possible to generate fax signals from a computer to allow

files and documents to be sent to a fax machine. Fax/modem software in the computer converts the image into a form recognized by a fax machine.

EMAIL• This is an electronic method for sending text and attachments from

one computer to another over a networkThe speed of sending and receiving replies using the email system

Advantages Disadvantages

• The low cost, since stamps, paper and envelopes are not needed.

• Not needing to leave home to send the email.

• The possibility of virus threats and hacking.

• The need for the email address to be completely correct.

• The inability to send bulky objects via emails.

VIDEO CONFERENCING

• This is a method o communication between people at 2 locations. The system works in real time and uses additional hardware. There are potential problems with those systems such as time lag, sound and picture quality can be poor.• However, these systems are becoming more and more popular at the

cost of travelling increases and the risk of terrorist attacks becomes higher.

VOIP• A method that is used to talk to people using the internet. It converts the

sounds into discrete digital packets which can be sent to their destination by the internet.

Prominent Security issues:• Identity and service theft

• Viruses and Malware

• Spamming

• Phishing attacks

Advantages Disadvantages• Free• Local rate around the world

• The sound quality• Security