B Year 12 Communications and Networks

Communications and connectivity

Computer communications - is the process of sharing data, programs, and information between
two
or more computers.
Connectivity - is a concept related to using computer networks to link people and resources.

A communication system is made of four elements:

Sending and receiving devices , Communication channels, Connection Devices and Data
transmission specifications

1.Sending and receiving devices

-are simply computers, mobile devices such as PDAs, iPad, iPhone, tablets, mobile phones that
transmit data to each other.

2. Communication channels

Is the actual medium through which data gets transmitted from one
device to another. This medium can be physical or wireless.

Physical mediums:

Are actually wires/cables used to connect two or more devices such as twisted pair, coaxial and
fiber-optic cables.

i. Twisted pair cables - are usually referred to as telephone lines and at 5/6 cables. It is called
twisted pair because it consists of thin strands of intertwined copper wires.

ii. Coaxial cable -has a solid copper core in the middle which is Twisted Pair insulated thick
rubber coating. Coaxial cables are usually used for transmitting television signals as well as
connect computers. It is approximately 80 times faster in transmitting data than a twisted pair.

iii. Fiber-optic cable is usually used as a backbone cable for extremely high speed data transfer.
It transmits data as pulses of light through tiny Fibre –optic tubes of glass. Since fiber-optic cable
uses pulses of light, it is capable of transmitting data approximately 26,000 times than the
twisted pair.

Wireless mediums:
-are connections that do not use any physical wires/cables, instead it uses radio frequency,
microwave, satellite and infrared to transmit data over the air.
i. Radio frequency (RF)- uses radio signals to communicate between wireless devices. The
radio frequency standards are known as Bluetooth, Wi-Fi and WiMax.

Bluetooth - is a short range RF communication that can transmit data in any direction within ten
metres.

Wi-Fi (Wireless Fidelity)- also uses RF to communicate over short distances. It is based on
certain
standard and each standard has a different transmission speed (11Mbps – 600Mbps). It covers a
range of 32 metres

WiMax (Worldwide Interoperability for Microwave Access) simply extends the distance
offered
by Wi-Fi. It covers a maximum range of 30 miles

ii. Microwave -uses high frequency radio waves and requires line of sight access between the
two
communicating devices similar to infrared.
iii. Microwave dish and satellites act as relay station to transmit microwaves where line of sight
is not possible.
iv. Infrared uses infrared light waves to transmit data over very short distances approximately
thirty centimeters requires the communicating devices to be in line of sight with no obstacles in
between.

3. Connection Devices

-are devices that connect the devices to the communication channels for data to be transmitted.
For example, a modem, network interface card (NIC) switch and routers and routers are devices
that connect several computers and mobile devices together. Since the computer understands
digital language, everything that is sent and received must be in 0‘s and 1‘s.

The NIC, switch and routers send and receive everything in 0‘s and 1ss. However, if you are
connected to the internet from your home/office via a telephone line then a modem is required
for
data transmission to take place.
Since the telephone lines were designed to carry voice data i.e analog signals, everything that
leaves a computer system must be converted into analog form.

Modems - convert the digital signal from the computer into analog signal from the sending
computer and
converts the analog signal back into digital signal on the receiving computer.
This process is known as modulation-demodulation, hence the name modem (modulator-
demodulator).
While modems usually connect a single computer to the internet, a switch and a router can
connect
hundreds to thousands of computers in a network.
Analog – telephone signals; continuous electronic wave.
Digital – computer signals; presence or absence of an electronic pulse; on/off

Types of Modems
i. Telephone- can be either internal or external; connects computer directly to a telephone line.
ii. DSL (Digital Subscriber Line) – uses standard phone lines; external and uses either USB or
Ethernet
ports.
iii. Cable uses coaxial cable – same as your television; uses either USB or Ethernet ports.

iv. Wireless does not use cables and also known as WWAN (wireless wide area network
modem). Signals are sent through the air where transfer speed or transfer rate is measured in bits
per second; the higher the speed the faster the transfer rate.
4. Data transmission specifications
– specify the rules and speeds at which data is transmitted over the network. Protocols and
bandwidth specify the rules and speed respectively.
Protocols-are rules for exchanging data between computers. The standard protocol for the
internet is
TCP/IP(transmission control protocol/Internet protocol).

TCP/IP
The higher layer, Transmission Control Protocol, manages the assembling of a message or file
into smaller packets that are transmitted over the Internet and received by a TCP layer that
reassembles the packets into the original message. This process is known as packetization.

The lower layer, Internet Protocol, handles the address part of each packet so that it gets to the
right destination. Each gateway(router/server) on the network checks this address to see where to
forward the message. Even though some packets from the same message are routed differently
than others, they will be reassembled in the original order at the destination.
This process is known as identification. TCP/IP uses domain name servers (DNS) that converts
the
numeric based IP address into text based address .

Bandwidth
-is the capacity of the communication channel that determines the volume of data that can be
transmitted in a given amount of time.
Four categories of bandwidth:

1. Voice band also known as low bandwidth, is used for standard telephone communication.
It is
effective for transmitting text documents however it is too slow for many types of
transmission, including high-quality audio and video.
 2. Medium band is used in special leased lines to connect servers as well as transmit data
over long distances. This band width is capable of very high-speed data transfer.

3. Broadband is widely used for DSL, cable and satellite connections to the internet.
Several users can simultaneously use a single broadband connection for high-speed data
transfer.

4. Baseband is widely used to connect individual computers that are located close to one
another. Like broadband, it is able to support high-speed transmission. Unlike broadband,
however, baseband can only carry a signal at one time.

Two methods of data transmission

1. Asynchronous transmission
- method frequently used with microcomputers, data is sent and received one byte at a
time.
-often used for terminals with slow speeds.
- Advantage is that the data can be transmitted whenever convenient for the sender.
-Disadvantage is relatively slow rate of data transfer.

2. Synchronous transmission
- is used to transfer great quantities of information by sending several bytes or a block at a
time.
-For the data transmission to occur, the sending and receiving of the blocks of bytes must
occur at carefully timed intervals. Thus, the system requires a synchronized clock.
- Advantage is that data can be sent very quickly.
-Disadvantage is the cost of the required equipment.

Three modes(directions) of data transmission in Communication Channels

The message source is the transmitter, and the destination is the receiver.

1. Simplex channel

- Whose direction of transmission is unchanging.

- For example, a radio station is a simplex channel because it always transmits the signal to
its listeners
and never allows them to transmit back.

2. Half-duplex channel
-is a single physical channel in which the direction may be reversed.
-Messages may flow in two directions, but never at the same time, in a half-duplex system.
-Example: In a radio telephone (RT) call, one party speaks while the other listens. After a
pause, the other party speaks and the first party listens.

3. Full-duplex channel
-allows simultaneous message exchange in both directions.
 -consists of two simplex channels, a forward channel and a reverse channel, linking the
same points.
-The transmission rate of the reverse channel may be slower if it is used only for flow
control of the forward channel. Fiber-optic, twisted pair and all wireless communication
channels are full duplex.

4.3 Computer Networks

A computer network- is formed when two or more computers are connected to each other either
to exchange data, share information and resources.
Purpose of network
 Easily share files and data
 Share resources such as printers and Internet connections
 Communicate with other network users (e-mail, instant messaging, video-conferencing, etc.)
 Store data centrally (using a file server) for ease of access and back-up
Purpose of Internet
 Make use of on-line services such as shopping (e-commerce) or banking
 Get access to a huge range of information for research
 Access different forms of entertainment (games, video, etc.)
 Join on-line communities (e.g. MySpace, Facebook, etc.)
Specialized terms
Network Interface Card (NIC) - these are expansion cards located within the system unit that
connect the computer to a network.
Network Operating System (NOS) - control and coordinate the activities of all computers and
other devices on a network.
Client – a node that requests and uses resources available from other nodes.
Server – a node that shares resources with other nodes. Host – any computer system that can be
accessed over a network.
Switch – a central node that coordinates the flow of data by sending messages directly between
sender and receiver nodes.
Router- is a network device that connects together two or more networks. A common use of a
router is to join a home or business network (LAN) to the Internet (WAN).
The router will typically have the Internet cable plugged into it, as well as a cable, or cables to
computers on the LAN.
Bridge is a network device that typically links together two different (independent) parts of a
LAN.

Firewall - blocks unauthorized connections being made to your computer or LAN. Normal data
is
allowed through the firewall (e.g. e-mails or web pages) but all other data is blocked.

NB
When designing networks, you must first describe the network type to be used, then
the topology and finally the strategy.

4.4 Network Types

There are five types of network. They are LAN, WLAN, MAN, WAN and PAN.
1. Local Area Network (LAN)
– are networks that connect nodes which are usually within the same room, offices,
rooms within the same building and even homes.
-can be spread within a radius of one hundred meter’s from the main building.
-Usually owned or controlled by a single organization.
- commonly used by individuals in their homes and apartments. These LANs, called home
networks, allow different computers to share resources, including a common internet
connection.

2. Wireless LAN (WLAN)

- is a LAN that uses radio signals (WiFi) to connect computers instead of cables.
- At the centre of the WLAN is a wireless access point or base station that interprets
incoming radio frequencies and routes communications to the appropriate devices.

3. Metropolitan Area Network (MAN)

- is a network that extends far beyond the reach of a LAN.
- It usually connects offices that are spread around a city or even across multiple cities
within an approximate distance of one hundred and fifty kilometers.
- typically owned by a group of organizations who set/select the protocols and bandwidth
in place for data transmission to take place.

4. Personal Area Network (PAN)

- is a wireless network formed spontaneously within a very small area.
- It can be formed by devices like our laptop, mobile phone and cordless mouse and
keyboards which can be connected to each other spontaneously.
- This can also be regarded as an ad-hoc network which sets Bluetooth RF for data
transmission.
NB
Bluetooth devices contain small, low-power radio transmitters and receivers. When devices
are within a maximum range of 30 feet of other Bluetooth devices, they detect each other and
can be 'paired' (connected).
 5. Wide Area Network (WAN)
- is a network that spans the entire globe such as the internet.
- are often geographically spread over large areas and links between computers over long
distances, they often use quite exotic connections technologies: optical fiber (glass)
cables, satellite radio links, microwave radio links, etc.

Advantages and disadvantages of types of networks

TYPE Advantage Disadvantage

LAN 1. Resource/internet Sharing 1. High Setup Cost (initial cost)
2. Software Applications Sharing 2. Privacy Violations
3. Easy and Cheap Communication 3. Data Security Threat
4. Centralized Data 4. Covers Limited Area
5. Data Security

WLA 1. It's easier to add or move 1. When the number of computers that use
N workstations. the network increases, the data transfer to
2. easier to provide connectivity the computer each will be reduced.
3. Installation is quick and easy 2. Security is more difficult to guarantee
4. access to the network can be 1. The device operates on a limited
from anywhere within range of the distance from the access point
access point.

MAN 1. high bandwidth (potential speed) of 1. cost of the latest technology employed
the connecting links. 2. high cost involved due to the
2. allow multiple users to share the same geographical range.
high-speed Internet connection
PAN 1. Portable 1. Health issue: Long time working
2. secure file transfer between with this radio signals will leads
devices. some brain related problems,and
heart related problems
2. Expensive: expensive devices like
smartphones,laptops,Digi camera,
PDA... etc.

WAN 1.Centralizes IT infrastructure 1. High setup costs

2.allows you to share sensitive data with 2. Security Concerns
all your clients 3. Maintenance Issues
3. sites without having to send the
information over the Internet.
4.Increases bandwidth
5. allow companies to expand their
networks through plug-in connections
over locations and boost
interconnectivity by using gateways,
 bridges, and routers.

4.5 Network Architecture

Network architecture - describes how a network is arranged and how the resources are
coordinated
and shared. It is divided in two major parts: topology and strategy.

Network topology
- describes how a network is configured and arranged while strategies define how resources are
shared.

Six types of network topologies

1. Bus network is where each device is connected to a common cable called a bus or a
backbone and
all communication travel along this bus.
Bus topology passes the message from device to device till it reaches its correct
destination.
Advantages
-It works well when you have a small network
-Easiest network topology for connecting computers or peripherals in a linear fashion.
-Requires less cable length than a star topology.
Disadvantage
-if the main cable fails or gets damaged the whole network will fail
- as more workstations are connected the performance of the network will become slower
because of data collisions.

2. Ring network is where each device is connected to two other devices, forming a ring.
Ring topology passes the message from device to device till it reaches its correct
destination.
Advantages
- All data flows in one direction, reducing the chance of packet collisions.
- A network server is not needed to control network connectivity between each
workstation.
- Data can transfer between workstations at high speeds.
Disadvantage
-Each packet of data must pass through all the computers between source and destination.
This makes it slower than Star topology.
- If one workstation or port goes down, the entire network gets affected.

3. Star network is where each device is connected to a central device usually a switch. When a
node
 sends a message, it first goes to the switch which then verifies and forwards the message
to the correct destination device. The star network is the most widely used network
topology in the world.

Advantages

- As compared to Bus topology it gives far much better performance, signals don’t
necessarily get transmitted to all the workstations. A sent signal reaches the intended
destination after passing through no more than 3-4 devices and 2-3 links. Performance of
the network is dependent on the capacity of central hub.
- Easy to connect new nodes or devices. In star topology new nodes can be added easily
without affecting rest of the network. Similarly components can also be removed easily.
- Centralized management. It helps in monitoring the network.
- Failure of one node or link doesn’t affect the rest of network. At the same time its easy
to detect the failure and troubleshoot it.

Disadvantage

- Too much dependency on central device has its own drawbacks. If it fails whole
network goes down.
- The use of hub, a router or a switch as central device increases the overall cost of the
network.
- Performance and as well number of nodes which can be added in such topology is
depended on capacity of central device.

4. Tree network

-is also known as a hierarchical network where each device is connecting to

another device which may be connected to other devices. It forms an upside down tree
where the root device is usually the most powerful.

Advantage

 Scalable as leaf nodes can accommodate more nodes in the hierarchical chain.
 A point to point wiring to the central hub as each intermediate node of a tree topology
represents a node in the bus topology
 Other hierarchical networks are not affected if one of them gets damaged
 Easier maintenance and fault finding

Disadvantage

 Huge cabling is needed

 A lot of maintenance is needed
 backbone forms the point of failure.
5. Hybrid network is a mixture of all other topologies. Big organizations have lots of smaller
networks based on differing topologies which are then interconnected. For example, USP has
twelve member countries and in each campus there are several computer labs. Each of these
labs are setup using the star configuration which then is connected via a backbone cable,
microwave dish and satellite forming a tree and mesh network as well.

Advantages

Reliable : fault detection and troubleshooting is easy AND required corrective measures
can be taken, WITHOUT affecting the functioning of rest of the network.
Scalable: Its easy to increase the size of network by adding new components, without
disturbing existing architecture.
Flexible: Hybrid Network can be designed according to the requirements of the
organization and by optimizing the available resources. Special care can be given to
nodes where traffic is high as well as where chances of fault are high.
Effective: Hybrid topology is the combination of two or more topologies, so we can
design it in such a way that strengths of constituent topologies are maximized while there
weaknesses are neutralized. For example we saw Ring Topology has good data reliability
(achieved by use of tokens) and Star topology has high tolerance capability (as each node
is not directly connected to other but through central device), so these two can be used
effectively in hybrid star-ring topology.

Disadvantages

Complexity of Design: One of the biggest drawback of hybrid topology is its design. Its
not easy to design this type of architecture and its a tough job for designers. Configuration
and installation process needs to be very efficient.
Costly Hub: The hubs used to connect two distinct networks, are very expensive. These
hubs are different from usual hubs as they need to be intelligent enough to work with
different architectures and should be function even if a part of network is down.
Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a
lot of cables, cooling systems, sophisticate network devices, etc.

6. In Mesh network there is no particular configuration; Rather the devices are connected to
each other on the fly especially in the wireless domains. It is best suited for WLANs and
PANs.

Advantages
- Data can be transmitted from different devices simultaneously. This topology can withstand
high traffic.
- Even if one of the components fails there is always an alternative present. So data transfer
doesn’t get affected.
- Expansion and modification in topology can be done without disrupting other nodes.
Disadvantages
 - There are high chances of redundancy in many of the network connections.
- Overall cost of this network is way too high as compared to other network topologies.
- Set-up and maintenance of this topology is very difficult. Even administration of the
network is tough.

4.6 Network Strategies

A network strategy - describes how resources and information is coordinated and shared.

Four types of Network Strategies

2. Terminal server network

- processing power is centralized in one large computer (server), which can handle large
number of connections.
- All nodes connected to this server are either terminals with no or little processing power or
microcomputers running special terminal emulation such as Windows Remote Desktop.
- For example, most airline reservation systems are terminal server networks.
The server maintains all airlines schedules including rates, seat availability, bookings and light
arrival/departures. Travel agents use terminals to connect to the server to make reservations.
Although the tickets may be printed along with travel itineraries at the agent‘s desk, nearly all
processing is done by the server.
Advantages
-Access to systems from anywhere and anytime
-Single point of maintenance
-The Installation of applications only happens once and everyone receives the upgrade at the
same time
-Businesses will see a reduction in application costs with concurrent licenses instead of per
device licenses.
The network administrators can lockdown file and system access from a single point for security
purposes.
Disadvantages
-Should the central computer crash, the entire system will "go down"
-central computing relies heavily on the quality of administration and resources provided to its
users

3. Client/server network
- the host computer usually the server coordinates resources to other computers (clients)
connected to it.
-The software running on the client requests for information and services from the server
which then responds to individual clients.
- In a network, the client/server model provides a convenient way to interconnect programs
that are distributed efficiently across different locations.
-For example, to check your bank account from your computer, a client program in your
computer forwards your request to a server program at the bank. That program may in turn
 forward the request to its own client program that sends a request to a database server at
another bank computer and to retrieve your
account balance. The balance is returned to the bank data client, which in turn serves it back
to the client in your personal computer, which displays the information for you.

Advantages

-Central control over resources

-Easier to manage and locate files
- data backup and recovery is easy
- new resources and systems can be added by making necessary changes in server.
- From various platforms in the network, server can be accessed remotely.
- All the changes are made only in central computer on which server database exists.
- security and access rights can be defined at the time of set-up of server.
- Servers can play different roles for different clients

Disadvantages
- Congestion in network.
- if the server fails, the whole network goes down.
- It is very expensive to install and manage this type of computing.
-need professional IT people to maintain the servers and other technical details of network.

3. Peer-to-peer network (P2P)

-nodes have equal processing capabilities and can act as both the client and a server.
- it allows a group of computer users with the same networking program to connect with
each other and directly access files from one another's hard drives.
- Napster and Gnutella are examples of this kind of peer-to-peer software. Major producers
of content, including record companies, have shown their concern about what they consider
illegal sharing of copyrighted content by suing some P2P users.

Advantages - users can share files without the expense involved in maintaining a centralized
server and as a way for businesses to exchange information with each other directly.
-Easy to set up

Disadvantages

-it is difficult to administer. That is one person cannot determine the whole accessibility
setting of whole network.
- malwares can easily transmitted over this P-2-P architecture.
 - Data recovery or backup is very difficult. Each computer should have its own back-up
system
- Lot of movies, music and other copyrighted files are transferred using this type of file
transfer.

4.distributed processing networks

- processing capability is shared across multiple nodes usually servers dispersed at different
locations. -common for very large organizations like banking industries.
For example, ANZ bank has it main server in Melbourne Australia, but for Fiji customers,
ANZ house has its own server that does the processing for its Fiji customers. The account
details are then replicated to the main headquarters servers in Melbourne.

Advantages

- speed, better communications, decreased costs and lower risk of single-point failures.
Disadvantages

-reduced performance
-increased traffic

Three Types of Organizational Networks

-Internet provides the network connections that link computers together.

-Intranet is the name given to a private network that provides similar services to the internet: e-
mail, messaging, web pages, etc. However, these services are only for the users of the intranet –
they are private, not public (unlike Internet services which are generally public). Businesses and
other organizations often have intranets for use by their employees.
-Extranet is a private network that connects more than one organization. It can be viewed as an
extension for an organization‘s intranet that is extended to users outside the organization.

4.8 Network Security

Proxy server
- is a computer setup to share a resource, usually an Internet connection. Other computers can
request a web page via the proxy server. The proxy server will then get the page using its internet
connection, and pass it back to the computer who asked for it.
- are often used instead of router since additional software can be easily installed on the
computer such as anti-virus, web filtering etc.

Firewall
-is a device, or a piece of software that is placed between your computer and the rest of the
network (where the hackers are!!!)
Intrusion detection systems (IDS)
- work with firewalls to inspect all incoming and outgoing network traffic and identifies
suspicious patterns that may indicate a network or system attack from someone attempting to
break into or compromise a system.
Virtual private networks (VPN)
- create a secure private connection between a remote user and an organization‘s internal
network. It enables a computer to send and receive data across shared or public networks as if it
is directly connected to the private network, while benefiting from the functionality, security and
management policies of the private network.