Revision topic: Communications and Networking
Introduction
I am going to be setting revision guides for topics that I feel will get you the most
marks as well as making sure we cover the other topics we have not looked at yet.
I will go through a list of different activities and I will include hints and clues to help
you answer the questions taking from different exam papers. It is important that you
attempt to try and answer the question first before looking for the answer on here or
online.
What this revision is going to be covering:
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Networks
Topologies
IP address
Router
Wireless: Wi-Fi & Bluetooth
P2P and Server model
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Communications and networking
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June 2013
exam
To be able to tackle this diagram we need to know the following:
Research Task 1
1. Find out what a segment is
2. Find out the difference between physical and logical topology
Things you should already know: (if you don’t, you need to do a quick revision on
them)
LAN / WAN
Topologies
IP addresses
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IP address classes
IP addresses are identified by the classes they use. The IP address is defined by the
subnet mask, although classful addresses are no longer used, an IP address can be
categorised into a class unless the subnet mask tells us otherwise.
Some examples of different classes:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
Class D: 224.0.0.0
Start add: 0.0.0.0
Start: 128.0.0.0
Start: 224.0.0.0
Finish add: 126.255.255.255
Finish: 191.255.255.255
Finish: 223.255.255.255
Finish: 239.255.255.255
This is an example of a class C IP address:
192.168.2.8
I know this IP address is a class C address because it starts with 192, which is
between 192 – 223 in the class list. I can only make this guess because there is no
subnet mask that has been given to us, such as: 255.255.255.0. If there was a subnet
mask given to us, then we cannot guess and we would have to rely on the subnet mask
to tell us other wise. For example, the above address could have the subnet mask
255.255.0.0
The IP address above is broken down into 4 octets. This is 8 bits which means that
each section can have 2^8 address (0 – 255). The address is then splits into two
sections, the Network and the Host portion. The left hand side of the IP address is
always the network address and the right hand side is always going to be the host
portion. The host address can change but the network address will usually stay the
same. The more bits that are given to the network, the more networks there will be but
the fewer hosts within each network.
Questions section 1
Q1. Find out the class that computer D is going to have
Q2. Find out the starting IP address for a class C address
Q3. If you are given a class C address of 192.168.2.1, identify the network portion of
this address and the host portion
Q4. State the number of addresses that can be given to the host in this class (Class C)
Q5. What is so special about an IP address that ends with .255 or .0?
Advice (To help with the research/questions)
A router, which is a device that works in the layer 3 of the OSI model, is responsible
for making sure the packets are routed to the correct destination. It will usually have
several ports. It will have ports for serials, fast ethernet and also gigabit. These ports
are like tiny slots where a cable would be connected to. Each slot or port will have a
different address or segment. Imagine a router having two ports, which means, that
particular router is going to have two different IP addresses and it will be two separate
networks. This is useful to know when asked to label a port on the router.
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For example:
192.168.2.1 & 192.168.2.2 >>>> Are both on the same network. They are class C, so
the network address is going to be 192.168.2.0 and three octets, 192, 168 and 2 is
going to be the same on both of them.
We can also have ports on a router with IP addresses which are on a separate network.
This means that the network address is going to be different for each of the addresses
that is assigned to a device. In order for these devices with different network
addresses to communicate, they will need to have a router as they have two separate
network addresses. The router is the device that allows separate networks to
communicate with each other.
192.168.2.1 & 192.168.0.1
I know they are different networks because the first 3 octets, or the first three numbers
before the dot do no match together.
It seems weird having two separate networks on the same physical device but that is
the beauty of a router as it can be used to stop broadcast domains. This is essentially
when a network has too many devices and you want to break the network down into
smaller chunks so the broadcast is reduced i.e. the network is not slowed down.
The router or computer cannot have an IP address ending with 255, this is because
this is the last IP address or the broadcast address. The broadcast address is a unique
address that cannot be assigned to any devices. You may want to look at why this is
the case. The router or host device cannot also have an address ending with 0 for the
most part as this is another unique address assigned for network address.
Questions section 2
Refer to the diagram above:
Suggest a suitable address:
Q1a) The “router 1” port labelled A
Q1b) Which two addresses can this router not have?
Q2. The router labelled port B
Q3. List 3 addresses that the network adapter on the student’s computer can have and
one IP address that it cannot have
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Revision
LAN
LAN stands for local area network. It is a type of network that uses the standard IEEE
802.3, which is also known as Ethernet. This means that the computer is part of a
network. If the computer is not connected to a network, then we refer to them as
standalone computers. It is a network that is used in homes, offices and small
buildings. The term you need to remember is that it is a network used in close
proximity i.e. computers close to each other. This type of network by in large is
connected by wire and it is a network designed for performance.
WAN
WAN stands for wide area network and it is a type of network that consists of
multiple computers connected together over a large geographical region. LANs are
perfect for sharing resources within one building whereas, WAN, is just a collection
of LANs connected together. These networks would be connected via satellite, radio
waves, fibre-optic cable etc… The Internet is an example of a WAN network. It is the
biggest public network available.
Topology
The topology of a network is how all of the computers are connected together. It is the
physical layout.
Research Task 2
If you are not familiar with topologies then I would look at Star, Ring and Bus as a
minimum and find out how they communicate with each other as well as their
strengths and weaknesses.
Questions section 3
Q1. Explain the difference between LAN and WAN
Q2. Give a brief description of the following topologies and explain one advantage
and one disadvantage
Bus
Ring
Star
Q3. Explain the difference between physical and logical topology
Q4. Explain the role of a network adapter and why it is needed in communication
Q5. Give a definition of the Internet
Q6. Explain the role of a buffer and why it is used
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Revision
Network adapter
A device can only participate in the communicate within the network if it has a
network adapter. The network adapter can be integrated or it can be external adapter
connected to the computer via USB. They can be wired using Ethernet or wireless
using Wi-Fi. Network adapters convert the digital data produce by computers into a
form that can be transmitted or received over the cable. The data from the computer
motherboard is transmitted to a memory area called buffer, from here it then goes
through a checksum which is a procedure for error checking and other bits of data are
added such as source address, destination address etc… Each network adapter is
assigned a unique address which was added during the manufacturing process. This
address is known as the MAC address and it is unique to network adapters.
The data that the network adapter transmits is known as a frame. This operates in the
second layer of the OSI which is the data link layer. This is where MAC addresses
operate as well.
Questions section 4
Q1. Explain the difference between P2P and server-based networks
Q2. When would you use a P2P network?
Q3. When would you use a server-based network?
Q4. List the strengths and weaknesses of the two networks
P2P
A peer to peer network is a type of network where all of the computers have equal
power. This means that the computers are not central managed by anyone so there
isn’t any policies that have been set. It is a type of network that is setup where you
have up to 10 computers. It is a network that is easy to setup, cheap and doesn’t
require maintenance or a lot of specialist equipment i.e. hardware or software. This
type of network does have its drawbacks when it comes to management and
implementation of security policies. As all of the computer have equal privileges it is
difficult to setup a policy and there might be many accounts with different usernames.
There is no central location to manage things so when the computers are sharing
resources, they are acting as client and server at the same time. A machine could be
accessing another computer for data whilst its resources maybe access by another
computer.
The machines within this network can take on different roles. The role of a file server,
application server, print server etc…
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Server-based networks
A server-based network is a type of network that is used where the requirements are
to support 10 or more users within the network. This type of network is more
expensive and it requires management as dedicated servers are needed to provide
different services. If we compare this with P2P, this type of network is more secure,
easier to manage and implement policies as well as make changes.
The server offers the following:
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The ability to login users and authenticate them to the network and give them
access to their user area
The ability to share files and manage quotas and security of files
The ability to share resources such as printers and applications
Questions section 5
Q1. Some other students using laptops are connected to the LAN by Wi-Fi through
the Wireless Access Point that is labelled E on Figure 8. Wireless communication is
less secure than communication using cables.
Describe one measure that could be implemented by the Wireless Access Point to
improve the security of the network.
Q2. Explain why Wi-Fi has been chosen for this connection rather than Bluetooth.
Revision: Wired and wireless communication
Wired
There are two main ways to transfer data in data communication across the network.
It can either be done by wired or wireless. If we are going to be using wired, we are
using a standard called IEEE 802.3 which is also known as Ethernet. Ethernet will be
using a cable called ‘Unshielded Twisted Pair’ and it is a popular cable used in homes
and offices. The cable is broken down into categories which define the speed i.e.
throughput and the distance the data can travel in. For example, you may have heard
of CAT5, which is ‘Fast Ethernet’ i.e. 100 MBPS or Cat 6 which is known as
‘Gigabit’, 125 M/bytes per second.
Wired connections are quite secure because you have to have a device physically
connect to it, in order for communication to take place. If someone wanted to
intercept data, again they will have to be physically connected on the network. Wired
is also good when it comes to performance and is usually prone from electromagnetic
interferences.
Wireless
The other method used for communication is wireless. This uses the IEEE 802.11
standard which defines wireless communication. Wireless uses RF frequency to
transmit data wirelessly. It is popular for businesses or home users that want
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portability so they can access the Internet from different locations within the building
or even outside.
There are many factors that can affect the performance of wireless communication.
For example, the distance from the wireless access point, i.e. the further you are from
it, the slower it becomes. The interferences that can affect the signal and performance
as well as the actual building and its structure. Buildings with thick walls tend to have
issues and repeaters are generally used to rebroadcast the signal i.e. amplify the signal
so it covers the whole building or location.
The performance of wireless is measured by the following standards, 802.11 a /b/g/n.
The higher the letter, the better it will perform, such as the 802.11n which was
recently in draft version supports distances of up to 300 metres and 300+ Mb/ps.
Bluetooth is another example of wireless connection, you have to remember that it is
only used over a short distance to connect to devices to share resources. It is quite
slow compared to wifi so you can’t transfer huge files, the word we are looking for is
low bandwidth. The other problem with Bluetooth is that it only allows two devices to
connect at any one time whereas with Wi-Fi you can have multiple connections.
Security is very important and wireless access points are secured with a password so
no one can access the actual setup but they are also secured by using encryption such
as Wireless Equivalent Protocol (WEP), the use of Wi-Fi protected Access (WPA) or
Advance Encryption System (AES). These are all the security solutions available in
wireless networks where for some a key for passphrase is used and where the
password is used for accessing the network. You may also consider the fact that a
router will also have other security features such as once the key is entered to check if
the mac address is set in the filtered section i.e. whether it should allow the user to the
network or not.
Some people think that not broadcasting the SSID, which is the name that everyone
will see when they are searching for a Wi-Fi will improve security but this is actually
false, neither would reducing the power transmitted by the router or using two way
authentication.
Notes section
If you need to make any notes, do so here. If there is a topic you are still not sure, then
make a note here and remind of it during lesson.
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Communications and networking
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The student now uploads a file from her computer to a web server over the Internet.
Question section 6
Write a detailed description of how one packet of data that the student is uploading to
the web server will be routed from her computer in the United Kingdom to the web
server that is located in Chicago in the United States of America. You may assume
that the web browser software on the student’s computer has already looked up, using
a domain name server, the IP address of the web server.
Your description should cover:
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how the packet will be routed within the LAN from the student’s computer to
the gateway and
how, once it has reached the gateway, the packet will be routed across the
Internet to the web server that the data is being uploaded to.
In your answer you will be assessed on your ability to use good English, and to
organise your answer clearly in complete sentences, using specialist vocabulary where
appropriate.
Note:
Answer the question, and then you can look at the revision tip to see if there is
anything you have missed.
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Explain the difference between routable and non-routable IP address.
Revision tip
Something we should already know:
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the IP address is not a local address
the DNS lookup has already been performed
It important that you understand that a LAN is like a room. If you want to
communicate with people within the room, i.e. local IP addresses, then you do not
need to pass those messages to a router. The student’s computer performs its own
sophisticated checks. It will check the destination address and it will do a Bitwise
AND with its own subnet mask and identify whether the destination IP address is in
the same subnet.
If it was in the same subnet, then it will just pass the packets on but as in this case it is
not, it will send the packets to the router as we do not know how to reach Chicago so
we will send the packet to the gateway (router).
The packet has arrived at router 1, the router will check the packet and strip the
destination address and check to see if is a local address, if is not a local address then
the router will send the packet to the gateway. The reason why it is sending to the
gateway and not router 2 is because router 1 and router 2 are on the same network, i.e.
192.168.0 network.
Once the gateway receives the packet, it will go through a hierarchical organisation of
routers. It will go to the ISP, the ISP will then send it to the national router which will
then pass it on to international routers and that then passes it down based on the
hierarchical structure. The reason why routers are configured like this is to make it
more efficient and it is broken down so one router does not have to deal with national
and international packets. You have to remember routers are beautiful, intelligent and
caring devices and if a packet cannot reach its destination due to congestion or any
technical issues, it will re-route that particular packet.
Other things to consider
As the packet goes through each router, the information is stripped, it has to know
where it has come from and where it is going, this process is known as decapsulation.
The opposite of decapsulation is encapsulation. This is where data is added as it is
coming down each of the OSI layer models. This is done so the router knows where
the packet is going and the router will be using the network address of the destination
address to find out where to send the data. Don’t forget that each time the packet
passes a router, the source and destination change at each router before as it is going
through each router, you would expect this to change.
The router
Each time the packet passes a router it is called a hop. Think of a hop like a jump the
packet will make as it passes each router. A maximum of 15 hops can be made before
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the packet is destroyed. This is because if the packet is not destroyed it can be lost and
it will loop endlessly.
Write about each of the OSI layers and their roles.
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Resources to look at
http://www.firewall.cx/networking-topics/the-osi-model/179-osi-dataencapsulation.html
Questions on communication
Did okay on these ones
Q1. Particular long-distance data transmission system transmits data signals as
electrical voltages using copper wire.
Q2. What is the relationship between the bandwidth of the copper wire and the bit rate
at which the data can be transmitted?
Q3. The system is affected by latency. What is latency in the context of data
communications?
Q4. Explain what Bluetooth is and give an example of a task for which a laptop user
might use Bluetooth.
What Bluetooth is:
Example use:
More questions
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Extra bits
Q1) After considering other alternatives, the analyst finally decides to use a thin-client
network.
Explain how a thin-client network works and how the use of a thin-client network
instead of a traditional rich-client (thick-client) network will affect the selection of the
hardware to be used by the system.
In your answer you will be assessed on your ability to use good English, and to
organise your answer clearly in complete sentences, using specialist vocabulary where
appropriate.
Q2) The system will be networked within the college. This network will then be
connected to the Internet so that staff who are out of the college can log in and access
the system. This connection will use a gateway.
What is the purpose of the gateway?
Q3) A home desktop computer is connected to a number of peripherals including a
printer
and a keyboard. It is also connected to the Internet and to a wired Local Area Network
(LAN). The keyboard is connected to the computer using a serial connection at a
speed of 9,600 bits per second with a baud rate of 9,600 baud.
Explain what is meant by baud rate.
Q4) The computer is connected to a small LAN using a wired baseband connection
and to the Internet using a broadband connection.
Explain the difference between baseband and broadband connections and justify why
the LAN connection is baseband whereas the Internet connection is broadband.
In your answer you will also be assessed on your ability to use good English, and to
organise your answer clearly and coherently in complete sentences, using specialist
vocabulary where appropriate.
Q5. Data can be transmitted using either serial or parallel data transmission.
State one advantage of serial data transmission over parallel data transmission.
Q6. Explain what asynchronous data transmission is and why start and stop bits are
required when it is used.
Q7. The computers in segment 192.168.1 use Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) to determine when to transmit data.
Explain how the CSMA/CD method is used, including what happens in the event of a
collision occurring.
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In this question you will also be assessed on your ability to use good English and to
organise your answer clearly in complete sentences, using specialist vocabulary where
appropriate.
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