AQA Computer Science A-Level
4.9.2 Networking
Advanced Notes
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Specification:
4.9.2.1 Network topology:
Understand:
● physical star topology
● logical bus network topology
and:
● differentiate between them
● explain their operation
4.9.2.2 Types of networking between hosts:
Explain the following and describe situations where they might be used:
● peer-to-peer networking
● client-server networking
4.9.2.3 Wireless networking:
Explain the purpose of WiFi
Be familiar with the components required for wireless networking
Be familiar with how wireless networks are secured
Explain the wireless protocol Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA) with and without Request to Send/Clear to
Send (RTS/CTS)
Be familiar with the purpose of Service Set Identifier (SSID)
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Network topology
Topology refers to the structure of a network. There are two types of topology to consider:
physical and logical.
Physical network topology
Physical network topology refers to the actual architecture of a network. Networks using
one physical topology will interconnect components differently to those networks that use
another physical topology.
There are two types of physical network topology to learn: star and bus.
Physical star network topology
In a physical star network, each client (that is,
a device connected to the hub) has its own
direct connection to the central hub. The hub
receives packets for all of the clients
connected to it and is responsible for
delivering them to the correct recipient.
A server can be added to the network in the
same way that clients are connected to the
central hub.
Advantages Disadvantages
Packets are sent directly to their recipient, Should the central hub fail, all
over a cable that is connected only to the communication over the network is
recipient. Other clients on the network stopped.
cannot see packets that aren’t intended for
them.
It is easy to add and remove clients to and Expensive to install thanks to the amount
from the network. of cable required.
Each cable has just one device
communicating over it, eliminating the
possibility of collisions.
The failure of one cable does not affect the
performance of the rest of the network.
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Physical bus topology
A physical bus connects clients to a single cable called a backbone. A device called a
terminator is placed at either end of the backbone.
There is no need for a central hub like in physical star networks and a server can be
connected to the backbone just like a client.
Advantages Disadvantages
There is no central hub, reducing the Packets are sent through the shared
chances of a network failure and backbone, allowing every client on the
decreasing the cost of installation. network to see packets that aren’t intended
for them.
Inexpensive to install as a minimum length The backbone is used for communication
of cable is required. by multiple clients, introducing the risk of
collisions.
Should the backbone fail, the entire
network becomes unusable.
Logical network topology
In contrast to a network’s physical topology, a network’s logical topology refers to the flow
of data packets within a network. A logical bus network delivers packets to all clients on
the network whereas a logical star network delivers packets only to their recipient.
Mixing topologies
If a network is set up as a physical star, it can still behave as a logical bus. Even if the
physical connections between clients and the central hub follow that of the physical star
topology, running a bus protocol on the hub allows it to distribute packets to all of the
connected clients so as to act like a bus network.
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Types of networking between hosts
A host is a device on a network that provides services. This is often a server, which can
provide services such as file storage, printer sharing and internet access but can also be
the clients on a network themselves.
Client-server networking
In a client-server network, one or more central servers provide services to the clients on
the network. Servers are connected to the network in the same way as clients, but are
often more powerful machines than the clients.
The clients on the network request services from the servers, which then respond to the
client with the requested service. Services provided by servers in a client-server network
could include file storage as well as management of emails, user accounts and print
queues.
Most schools, colleges and businesses use client-server networks to allow for central
management of clients on the network. This can improve security but requires a fair
degree of expertise to set up and manage.
Peer-to-peer networking
Peer-to-peer networks do away with a shared server. Instead, services are provided by the
clients themselves and every client has equal status. For example, one computer on the
network might manage print queues, another manage storage and a third manage emails.
The primary disadvantage of peer-to-peer networking is that all of the clients which provide
services must be running in order for the network to be fully operational. If the computer
responsible for managing storage is turned off or faulty, none of the clients on the network
can access their files.
On the other hand, peer-to-peer networking is more cost effective than client-server
networking as there is no need for a powerful server to provide services. Furthermore,
peer-to-peer networks are easier to set up and maintain than their client-server
counterparts
Large file-sharing networks and multimedia providers use peer-to-peer networking to
provide high-performance services without the requirement for a server.
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Wireless networking
Wireless networks allow clients to communicate
within a network without being physically
connected to it.
Wireless networks require a wireless access
point, which connects to a wired network just like
any other client would, and a wireless network
adapter in the device that connects to the wireless
network.
WiFi
WiFi is widely used to provide wireless networks
and refers to a wireless local area network that is
based on international standards. This allows a
device made in one part of the world to connect seamlessly to wireless networks all over
the world.
Wireless networks are secured by encrypting transmitted data using WPA or WPA2. WPA
stands for WiFi protected access and requires that a new wireless client enters a
password in order to connect to the network.
Another method of securing a wireless network is disabling SSID broadcast. SSID stands
for service set identifier and is the name that identifies a wireless network. Disabling SSID
broadcast stops wireless devices within range of the network from displaying that the
network is available, only allowing those who know the SSID to connect.
A third method of securing a wireless network is to set up a MAC address filter. MAC
(which stands for media access control) addresses are assigned to every wireless device
by their manufacturer and are unique to that device. MAC address whitelists can be
created to allow only specific devices to connect to a network. Likewise, MAC address
blacklists can be used to block specific devices from connecting to a network.
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Carrier Sense Multiple Access with Collision Avoidance
Carrier sense multiple access with collision avoidance
(CSMA/CA) is a protocol used in wireless networks to avoid
data collisions caused by multiple devices communicating
simultaneously.
When a device is ready to transmit, it listens to
its communication channel to check whether it
is idle. If so, then the data is transmitted. If the
channel is busy, the device waits for a random
period of time before checking the channel
again. An exponential backoff algorithm can be
used to increase the time period for which the
device waits with each check of the channel.
While CSMA/CA is effective at eliminating
collisions in small networks, it cannot overcome
hidden nodes: a problem that arises when the
device checking for an idle channel cannot
“see” some parts of the network on which
communication may be occuring.
To get around the problem of hidden nodes, a
protocol called request to send/clear to send (or
RTS/CTS) is used. This protocol adds an
additional step into the CSMA process. Once the
transmitting device has checked whether the
channel is idle, it sends a “request to send”
message to the server.
If the server is indeed idle, it will respond with a
“clear to send” message at which point the
transmitting device can begin communication with
the server. If no “clear to send” message is
received, the server is busy communicating with a
hidden node and the transmitting device must wait
before starting the CSMA process again.
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