Computer Science Chapter 4.9 - Fundamentals of communication and networking

Serial Data Transmission

Single bits are sent one after another along a single wire.

Parallel Data Transmission

Multiple bits are sent down several wires simultaneously.

What happens when data skew occurs?

bits sent together may not be received together

Bandwidth

the range of signal frequencies that may be transmitted.

Baud rate

The rate at which signals on a wire may change.

Bit rate

The number of bits transmitted per second.

Latency

Time delay between the moment something is initiated and the moment the effect begins

Crosstalk

unwanted transfer of signals between communication channels

Synchronous data transmission

a clock signal is used to time when signals are sent.

Asynchronous data transmission

sender and receiver have separate clocks which are not kept synchronised - a start bit is used to signal the arrival of data and to synchronise the transmitter and receiver temporarily

Advantages of a network (5)

• Sharing resources such as printers makes it cheaper

• site licenses are likely to be cheaper than several standalone

• files can be shared easily

• work can be easily accessed using any computer on the network

• data is easy to backup as all the data is stored on the server

Disadvantages of a network (4)

• Purchasing the network cabling and file servers can be expensive

• Managing a large network is complicated - requires training and usually a network manager

• if the file server breaks down the files on the file server become inaccessible

• viruses can spread to other computers throughout a computer screen

Advantages of wired networks (4)

• reliable

• faster (than wireless)

• more secure

• can extend further easier

Disadvantages of wired networks (3)

• can be expensive

• less flexible (cannot easily move devices)

• trip hazards

Advantages of wireless networks (3)

• cheaper

• very flexible

• no trip hazards

Disadvantages of wireless networks

• not always reliable

• slower

• less secure

• don’t extend very far

Where are Ethernet cables usually used for

You will usually find them in buildings - they connect the school computers together and are widely used in businesses

Where/What are fibre optic and copper cables used (for)

usually used to join together houses, businesses and other locations externally

What do copper cables use to work

electrical signals travelling along the wire (sometimes slow)

What do fibre optic cables use to work

Fibre optic cables use light to transmit data, and light moves at the speed of light! So these cables are a lot faster (than copper cables)

What does PAN, LAN and WAN stand for?

• Personal area network

• Local area network

• Wide area network

LAN

when computers or devices are connected together over a single geographical area, such as within a home, a building or one site. (eg. the school/home)

WAN

when computers or devices are connected together over multiple geographical areas. (eg. the internet)

PAN

network on a very small scale. (eg. Bluetooth)

What are the 4 components that allow you to access the internet

The modem, the router, the switch, the WAP (Wireless access point)

What does the modem do?

Connects the device to the internet through a phone line

What does the router do?

A router is a networking device that forwards data packets between computer networks

What does the switch do?

Allows you to connect multiple devices to the router

What does the WAP do?

Lets your wireless devices connect to the network

Whats the purpose of a stop bit?

ensures that the receiver has time to process each byte before receiving the next one

Whats the purpose of a start bit?

temporarily synchronises the clocks in both the receiver and the transmitter

Advantages of a bus topology

• Cheap as it uses the least amount of cable

• Adding another computer is easily done

Disadvantages of a bus topology

• Slow with many computers (has to check with every computer)

• If the backbone breaks the whole system breaks

Advantages of a ring topology

• Cheap to expand

• Does not rely on a central hub

Disadvantages of a ring topolgy

• slow if a lot of computers are on the system

• If one computer breaks the whole network breaks

Advantages of a star topology

• If a cable fails it only affects one computer

• Computers can be easily added

Disadvantages of a star topology

• more cabling needed

• fail if the centre hub fails 

Topology definition

The structure of the connections that connect devices to the network.

Physical topology definition

The physical layout of the cabling between the devices on the network

Logical topology definition

How the data flows around a network

How does a logical bus topology work

The network will physically be set up in one way however it will logically operate like a bus topology. Using a bus protocol meaning it will check with each computer if the packet is meant for it like a bus topology

Client server definition

A system in which some computers (the clients), request services provided by other computers, the servers

Peer-to-peer network definition

A network that has no dedicated servers. All computers are of equal status and can both share resources themselves and use resources from other computers

Server definition

A computer that provides shared resources to network users

3 examples of servers

• file server

• mail server

• database server

3 examples where peer-to-peer networks are used

• file sharing

• online gaming

• streaming media

3 examples where client servers are used

• the internet

• in businesses (file servers, database etc)

• cloud services

What is Wi-Fi

A family of related protocols that define how devices should communicate over wireless connections (WLAN)

What does Wi-Fi allow you to do

Allows you to create a WLAN thats based on internation standards

What two (extra) components required for wireless networking (compared to wired)

• wireless network adaptor

• wireless access point

Whats a wireless network adaptor

a piece of hardware, such as a device or a card installed inside a computer, that allows a device to connect to a wireless network (like a Wi-Fi network) or the internet without using an Ethernet cable

What are the 3 ways to make wireless networks more secure

• Disabling SSID broadcast - means that in order to connect to the network you must already know the name of the network (SSID)

• MAC address whitelist - You can filter out unwanted devices

• Encryption (WPA,WPA2)

 What is the SSID

The service set identifier - a unique ID that consists of up to 32 characters and is used for naming wireless networks

Whats the MAC address

Media access control - Used to uniquely identify pieces of hardware

Whats the advantages of a client-server network over a peer-to-peer network?

• The server computer can manage security

• Files can be stored on the server - accessed at all times even when computers are off

• processing some tasks can be performed on servers instead of having to purchase more powerful computers

What does CSMA/CA stand for?

Carrier sense multiple access 

How does CSMA/CA work?

It allows a computer to listen out for transmissions - and if it hears one it waits. Then it checks again after some time and if it doesn’t hear anything it’ll wait a random short amount of time before sending the data over

What does RTS stand for?

Request to send

What does CTS stand for?

Clear to send

What does RTS and CTS do?

RTS - when the computer sends an RTS signal it is telling the WAP it is ‘ready to send’ data 

Then the WAP temporarily stops all over transmissions 

CTS - The WAP sends a CTS signal to the computer telling it that the network is clear then the computer sends over the data 

What is the internet?

A network of interconnected computer networks

What do internet service providers (ISPs) do?

Companies that provide their customers with access to the internet. National internet service providers provide internet access to smaller regional and local ISPs from whom homes and businesses can buy from

What happens in packet switching?

Data is split up into packets and transmitted over networks through routers. Routers send to the recipient via the fastest route → routers hold tables with information relating to the fastest routes to certain devices which are frequently updated

What is TTL?

Time to live → each packet can only pass through a finite number of routers/only perform a finite number of hops. When a packets TTL expires the packet is said to be dropped meaning it is deleted. The recipient will notice a missing packet and request that the sender transmits the missing packet again

What are the 5 main parts of a packet?

Senders address, Receivers address, Packet contents, TTL, Sequence number

What do gateways do?

Where two networks use different protocols packets must be modified so as to conform to both protocols. Gateways strip away most of the packet details leaving just the packet contents. The gateway will then give packets new sender and receiver addresses which comply to the new protocol 

What does URL stand for?

Uniform resource locator

Why are domain names used?

As humans cannot understand IP addresses so domain names are used as we can understand them

How does a domain name server system work?

Each domain name has a direct relationship with an IP address. A domain name server stores a table of domain names and their corresponding IP address.

What happens when a domain name isn’t found in a DNS server?

The request will be passed onto another DNS -> some very small and rarely visited websites will require numerous changes of servers before a record can be found. Makes these websites slower to access

What does an internet registry do?

They assign IP addresses to devices. There are only 5 internet registries in operation (each for a different geographical area). They will look for an unused previously allocated IP address which can then be reused instead of creating a new one 

What do firewalls do?

They sit in between a device and the internet and regulates the packets that pass through it. 

Whats packet filtering?

Firewalls use packet filtering to accept and block packets on their source IP address or the protocol that they are using. A networks administrator can specify particular IP addresses or protocols to block or use automatic filtering software that can block suspicious packets 

Whats stateful inspection?

It actually examines the contents of a packet before deciding whether to allow it through the firewall. some firewalls keep a record of current connections in a network allowing them to filter out packets that aren’t related to activity on the network. 

Whats the hierarchy structure in DNS systems?

Each server tells the resolver how to resolve the next level down. The root server tells the resolver where to find the TLD server which then tell the resolver where to find the authoritative name server.   

Whats a proxy server?

A server that sits in between a public network and a private network. When a device in a private network sends a packet through a firewall and into a public network the packets sender address is that of the firewall rather than the devices private IP address. IP addresses on a LAN are generally not route able to the internet (as all IPs on internet must be unique) → also provides anonymity 

Whats symmetric encryption?

Both the sender and the receiver share the same private key. This key is used to both encrypt and de-crypt data. If the key is exchanged over a network it is vulnerable to interception

Whats asymmetric encryption?

When both person A and B have a pair of keys - one public and private. They encrypt messages for each other using the other persons public key and decrypt using their own private key.

How does caeser cipher work?

Things are encrypted by shifting the letters either clockwise or anti-clockwise along the alphabet

Whats a one time pad cipher?

It is a completely random one time use key used to encrypt data

What are the 4 rules needed for a one time pad to work securely?

• It must be the same length or longer than the plaintext message 

• It must be completely random

• It must be destroyed after being used

• it must be kept securely 

How does Vernam cipher work?

A key is created the same length or longer than the plaintext. Each letter in both the key and message are turned into binary (usually using ASCII) and then the XOR is performed to create a new binary number. All the numbers are then turned back into letters to create a new string of encrypted letters 

What are digital signatures?

Can be used to verify that the sender is who they said they are and also that the message hasn’t been tampered with during transmission

How does it work?

• A hash function is performed on the plaintext message to create a message digest

• Person A uses their private key to encrypt this message digest which creates the digital signature

• this digital signature is attached to the end of the plaintext message and it is all encrypted using person B’s public key

• When person B recives the message they use their private key to decrypt the whole message with their private key

• They then extract the digital signature. then they take the plaintext message they recieved and use a hash function on it

• They then use this regenerated message digest to compare it to the one sent in the message 

• If they match that means that the message came from the right sender and that it hasn’t been tampered with 

Whats a digital certificate? 

It verify’s ownership of a key pair (in this circumstance) and can be used to check that a fake key isn’t being used by an imposter. Issued by a certificate authority → they contain → A serial number, the owner name, an expiry date, the owners public key and the certificate authority’s digital signature

What are worms?

They are self-replicating pieces of malware that can move between computers either within a network of by users downloading and running a malicious file 

Whats a trojan?

A type of malware that is disguised as a benign file that users can be tricked into opening. These are often spread as email attachments or downloaded from malicious websites 

Whats a virus?

They require a host file in which to reside. These files are typically executable files meaning they can lie dormant in a computer until their host file is opened or run 

How are some ways to prevent malware?

• malware often exploits bugs in code → good quality code is an important factor in preventing malware

• installing antivirus software → specialist pieces of software that scan the files on a computer and remove suspicious files

• employees can be trained about the risks of opening suspicious email attachments in order to reduce the ris