1.3 Computer Networks Flashcards
OCR GCSE Computer Science J277
1
Q
Network
A
a group of devices connected together, either using cables or wirelessly, so that they can communicate with each other and share resources.
2
Q
Topology
A
the physical or logical arrangement of connections and devices in a network.
Networks can be structured in different ways called topologies.
3
Q
Network standards and protocols
A
allow a wide range of devices from different manufacturers to communicate with each other.
4
Q
Network performance
A
The speed at which a network operates at it is most important to users.
Network performance is affected by:
Bit rate
Bandwidth
Number of devices
Latency
5
Q
Benefits of networks
A
Shared files
Multi-user access
share devices
central backup
central deployment
6
Q
NIC
A
Network interface card. Hardware built into the device
A network interface controller (NIC) is a component that allows a device to connect to a network.
Wired and wireless versions are available.
Nearly all new computers have an NIC built directly into the motherboard.
7
Q
Network manager
A
in larger organisations, makes decisions about the network and run its day to day in the IT team
8
Q
LAN
A
Local Area Network is a single network that is located in a small geographical area such as someone’s home, an office, or to cover a school site. Uses its own infrastructure, cabling and equipment.
9
Q
WAN
A
A Wide Area Network (WAN) is a network that connects two or more networks over a wider geographical area. WANs connect LANs together to form larger networks. Uses third party infrastructure such as telephone lines, satellites.
10
Q
Public telecoms companies
A
Many Wide Area Networks (WANs) will make use of public telecommunications infrastructure. These are links managed by other companies.
Organisations that run their own WANs will often lease (loan) bandwidth from these companies, who in turn maintain telephone cables, ducts, cabinets and exchanges on a national network.
Examples of these companies:
Openreach (owned by BT)
Virgin Media
11
Q
Data collisions
A
In a LAN, if two or more devices are attempting to transmit at the same time along the same channel, data collisions occur, and the data has to be retransmitted.
This is like two people talking on the telephone at the same time.
12
Q
Bit rate
A
the measure of the amount of data (in bits) that is being transferred per second. The abbreviation bps is commonly used in place of bits/s, which is the standard unit of measurement.
Because communication channels carry millions or billions of bits per second, the unit is usually prefixed.
13
Q
Bandwidth
A
is the theoretical maximum amount of data that a communications channel can carry at one time.
14
Q
Latency
A
the delay between the time a signal is transmitted and the time a signal is received.
15
Q
Client-server
A
allow you to access your files and emails from any authorised device.
- A client sends a request to a server
- The server processes the request
- The server sends a response back to the client
a networking architecture where devices are designated as clients or servers. Clients make requests for services from servers that respond to these requests.
These classifications explain the role that devices have within a network, and are not related to whether the network is a LAN or a WAN.
16
Q
Peer-to-peer
A
(P2P) is a networking architecture where all devices have equal status and can share resources directly with each other
In a peer-to-peer network, there are no central servers. Every computer can be configured to shares its resources.
Peers communicate directly with each other. Computers must be switched on to share resources.
The owner of each device gives permission to access some or all of the device’s resources.
These classifications explain the role that devices have within a network, and are not related to whether the network is a LAN or a WAN.
17
Q
server
A
a powerful computer which provides services or resources required by its clients
A server is a program on a computer that shares resources with or provides services to any authorised client.
Most servers will run 24 hours a day, 7 days a week, so that the client can send a request for resources at any time.
In large organisations servers will usually run on high-powered, dedicated, computers that are able to handle a lot of requests at any one time.
In smaller organisations, multiple servers may run on a single computer system.
18
Q
client
A
a computer that requests services or resources provided by the server (such as your desktop computer, phone, laptop or tablet)
A client is a program that typically runs on a device used by an end-user, such as a laptop or a mobile phone.
19
Q
Email server
A
Store and forward emails. Some services filter out spam
20
Q
Print server
A
Provide printing services across the network and manage print queues
21
Q
Database server
A
Store databases, run queries and allow data to be manipulated
22
Q
File server
A
Store and provide files
23
Q
Web server
A
Store and provide web pages
24
Q
WAP
A
Wireless Access Point or Wireless AP enables wireless devices to connect to a LAN.
It creates a wireless network using radio waves. A Wi-Fi enabled device can connect to it to communicate with the network or Internet.
25
Router
A router is a networking device with two or more interfaces, each in a different network.
You will find routers where communication happens between two separate networks, that are required to connect.
A router examines the destination IP address of incoming packets (data) to decide which interface to use to forward the packet.
26
Network Access Device
In your home network, the network access device (sometimes called the home hub) that is typically provided by your Internet Service Provider (ISP) will incorporate a router to connect your home network to that of the ISP (WAN).
Network access devices typically contain:
A wireless access point (WAP)
A switch to allow all the wired devices to be networked
A router to act as the network gateway and direct external requests out of the LAN
Media to transmit data
such as an ADSL (telephone) cable or even fibre-optic. If there is no cable, the external connection may be 3G/4G/5G or even satellite.
27
Core Routers
On the wider internet, core routers are used to forward packets to their destination along the backbone of the internet.
These routers examine the destination of each packet and consult a routing table of known networks.
The table indicates which connection is best to use to send the packet towards it intended destination.
28
Hub
A hub will simply repeat messages to all connected devices.
This is inefficient because it sends data to places other than its intended destination.
a hub is a network device that is used to connect devices on a LAN. It is used to simplify cabling systems and acts as a repeater.
When data arrives at a port, it is broadcast to the devices connected to every other port.
29
Switch
a network component that is used to connect devices on a LAN. A switch learns which device is connected to each of its ports to that it can switch traffic directly to the correct recipient.
A switch only sends messages to its intended destination device.
The switch builds up a table with the MAC address of every device that its connected to each of its ports.
When a signal is received, the data is analysed to determine the destination MAC address. The data is then sent to the port connected to the device with the MAC address.
30
MAC Address
Each NIC has a unique identifier called a Media Access Control (MAC) address.
is a unique hexadecimal identification number assigned to every network interface card used in networked devices.
It is 48 bits long, and it usually referred to in hexadecimal notation, for example:
cd:f1:24:e4:89:a1
The MAC address is allocated by the manufacturer and cannot be changed.
31
UTP
Unshielded Twisted Pair cable. A type of cable where two conductors are twisted together to reduce interference. Also known as Ethernet cable.
32
Ethernet Cable
An unshielded twisted pair (UTP) cable is a common type of cable for connecting computers. It is often referred to as an ‘Ethernet cable’ thought it is not technically an Ethernet cable, it can support Ethernet.
33
Fibre-Optic Cable
a cable that can be used to transport electromagnetic waves in the infrared region. These waves are modulated to carry digital data at high speeds over long distances.
Fibre-optic cables transmit data as pulses of light through a single strand of glass fibre which can transmit large amount of data over long distances and without interference.
Fibre-optic cables support data transfer speeds of up to 10 gigabits per second and can transmit data for up to 2km.
34
Coaxial Cable
a cable made up of an inner conductor and an outer conducting shield, separated by an insulator.
Coaxial cables have numerous issues with poor connections, and these issues, combined with the development of cheaper alternatives and improved electronics, has led to this type of cable falling out of use.
35
The internet
The internet is a global system of interconnected computer networks
The internet facilitates a wide range of services, including:
The World Wide Web
Email
Internet telephony (VOIP)
File sharing (FTP)
36
Communication backbone
At the heart of the internet is the communication backbone - fibre-optic cables and industrial routers that move data at superfast speeds.
Most of the backbone is provided by giant telecommunication companies.
37
ISP
Internet Service Provider. You can usually pay monthly to use their connection to the internet
38
Packet Switching
When you use the internet to view a webpage, a request to retrieve the data that makes up the page is first sent to the web server where the page is stored.
The fields (information) that is sent in return are broken down into smaller data packets, called IP packets.
The packets are forwarded around the internet using a method called packet switching.
Different packets from the same file may be sent over different routes.
Routers examine the destination IP address of each incoming packet and decide on which route the packet should be sent until the destination is reached.
At the destination, the packets are reassembled into the correct order.
39
Packet Header
The data stored in the header of each packet is required for the packets to reach their destination and then be reassembled back into their source files.
Each header contains:
The internet protocol (IP) address of the packet’s destination
The IP address of the packet’s origin (source)
The sequence number of the individual packet
The number of packets in the whole file
Data that is used to check the packet for errors (checksum)
40
Control information
information, typically contained in a packet header, that provides data for packet routing, sequencing and delivery.
41
The world wide web
The World Wide Web (WWW) and the Internet have become synonymous in modern times.
It is important to know that the web is not the only service that runs over the internet.
Many different services use the internet, for example, email and video calling.
42
Hypertext
Hypertext allows users to display text and embed resource locations with that text as hyperlinks (often just called ‘links’ or ‘clickable links’.
43
HTML
HTML or HyperText Markup Language allows users to dictate the layout and presentation of hypertext documents (web pages).
Using simple tags (coding) a user can format text to look however they want. There are tags for almost anything, like bold, italics, headings, images etc.
44
Domain name
are usually chosen to represent the name of the company or organisation that is providing a service.
Wherever you want to browse a website, play an online game, collect your email, you need to access the relevant server via its specific IP address.
Most servers have a user-friendly name that saves us from remembering its IP address.
These are called domain names and each domain name maps to a specific IP address.
Google.co.uk -> 172.217.14.195
45
Domain Name System
a hierarchy of servers that work together to resolve domain names to corresponding IP addresses
The Domain Name System converts domain names to the IP addresses needed to locate resource on the internet. This saves you from having to remember the IP addresses.
The domain name system comprises multiple domain name servers.
There is a hierarchy of DNS servers based on the hierarchical structure of the domain names.
If the URL is not in the domain name server lookup table, then it passes it up the hierarchy.
46
Domain Name Server
a server that works to resolve domain names to corresponding IP addresses
The role of a Domain Name Server is to convert a URL (website address) like google.co.uk to an IP address (e.g. 172.217.14.195) so that a browser (like Google Chrome) on a client machine can make a request to the correct server hosting that web page.
If the DNS server does not have an entry for the domain name, it passes the request to another more authoritative DNS server.
An error is sent back if no match is found at all.
47
URL
URL - uniform resource locator. The address of a resource on the internet (the web page’s address). It indicates the location of a resource as well as the protocol (http / https) used to access it.
48
TLD
Top-level Domain
There are separate servers for most of the top-level domains such as .com, and each are administered by a different authority.
The top-level domain server will probably know the IP address of the full URL and will reply with the IP address.
49
The Cloud
The term ‘the cloud’ refers to a set of computing services that are accessed over the internet.
Cloud computing services are run out of server farms (or data centres) which are huge warehouses full of servers, attached to extremely large storage devices.
The Cloud is a term used for a file server accessible via the internet. It is a remote server often owned by a third-party company which offers storage space for your files as well as backup and security services.
50
SaaS
Software as a Service
SaaS is a subscription based service that will give you access to application software, such as a word processor, spreadsheet and presentation software.
Example: Google Apps - Google Docs, Sheets, Slides.
The service provider you subscribe to will host the application online so you do not have to install the application software on your local computer.
51
IaaS
Infrastructure as a Service
IaaS provides web hosting, data centres, servers and storage networks that are all hosted by the service provider.
Similar to SaaS, if you use these services you do not have to manage the services locally.
This is often referred to as hosting services in the cloud.
52
Star
Each device on the network is connected to a central switch which directs transmissions to the correct device using its unique MAC address.
In this topology, each device on the network is connected to a central communication device such as a:
Hub
Switch
This is the most commonly used topology in businesses and organisations where performance and security is essential. They are also found in smaller offices and home networks owing to their simplicity.
In a star network, the switch or hub is a central point of failure.
53
Mesh
In a mesh network, there is no central communications point. Instead, there are multiple paths between the communication devices (e.g. switches and routers) that form the network.
Each communications device connects directly, and non-hierarchically, to as many other communication devices as possible, to provide many communication pathways.
This means there is no critical point of failure as every communication path has several alternative pathways.
Mesh networks can be used to connect small offices or entire cities.
Wireless examples are most common, providing very large networks supporting traffic management and home automation systems.
54
Partial Mesh
More common is the partial mesh topology where some the devices may be connected to only one or two others. This is less costly and reduces redundancy.
55
Wired Transmission Media
Wired networks can use:
Coaxial cable (unlikely)
Unshielded Twisted Pair (Ethernet) cable
Fibre-Optic cable
Most wired networks are Ethernet networks.
56
Trunking
Cables are often run inside plastic trunking or are inside wall and ceiling cavities to keep them tidy and prevent them becoming a safety hazard.
57
Frames
Devices communicating over Ethernet divide streams of data into shorter pieces called frames. (Comparable to ‘data packets’)
Each frame contains:
Source address (MAC address)
Destination address (MAC address)
Remember the MAC address is encoded onto each network interface card.
58
Wireless Technologies
Wireless networks use radio waves to connect devices.
There are a range of wireless technologies but the most common are:
Wi-Fi
Bluetooth
59
WLAN
A WLAN is a Wireless Local Area Network. The network is based around a central communications device, a wireless access point (WAP)
In your home the Wireless AP is probably part of the network access device provided by your ISP.
In larger buildings there will probably be several discrete wireless APs that allow a wider area to be covered.
60
Bluetooth
Bluetooth is a radio-wave technology commonly used to connect devices within 10 metres of each other. Common uses include hands free phone kits and smart speakers.
The central device in a Bluetooth network can communicate with up to seven other devices over a typical distance of 10m.
Bluetooth is designed to use very little power, allowing some devices to use standard batteries and most use batteries that need only occasional charging.
61
Hybrid
Most local area networks have a mix of wired and wireless connections. Wired connections will generally be used for larger desktop computers and devices that are not often moved, such as printers.
At home, you will probably have a network access device (commonly referred to as ‘the router’) that is provided by your internet service provider (ISP). This device includes a wireless access point with a few Ethernet ports for wired connections.
62
Encryption
the process of applying an encryption algorithm to plaintext to produce ciphertext that cannot be understood (without decryption)
Encryption is the process of encoding data so that it cannot be easily understood if it is discovered, stolen or intercepted.
63
Encode
To encrypt data
64
Decode
To decrypt data
65
Encryption Algorithm
An encryption algorithm is a set of instructions you follow to encrypt a messages (change plain text into the cipher text). This is often referred to as a cipher.
66
Key
As well as the plain text, the encryption algorithm takes another input - a key. The key determines the output of the algorithm, only those who know the key can decrypt the message.
In modern-day encryption, keys are most often very long strings of numbers and letters, generated by an algorithm.
As long as the key used to encrypt a message is kept secret, only the people who know the key should be able to decrypt the message correctly.
67
Symmetric Encryption
With a symmetric cipher technique, you encode (encrypt) the message with the same key (technique) as you use to decrypt it.
68
Asymmetric Encryption
With an asymmetric cipher technique, the sender and receiver use different keys (techniques) to encrypt and decrypt the message, which makes them far harder to break.
Asymmetric encryption means using an algorithm with two keys: a public key and a private key.
Anyone with the public key can encrypt a message, however the public key cannot decrypt that message, only the second private key can do that.
69
Polyalphabetic Encryption
Polyalphabetic encryption is more challenging for a human to decrypt as the shift changes with each character of the message.
70
Public Key
Used in asymmetric encryption.
Anyone with the public key can encrypt a message, however the public key cannot decrypt that message
71
Private Key
Used in asymmetric encryption.
Only the private key can be used to decrypt a message that has been encoded with the public key.
72
IP Address
IP Address - Internet Protocol address. A binary number assigned to a node in an internet network that is made of a network ID and a host ID.
73
Internet Protocol
IP - Internet Protocol. An internet (network) layer protocol that ensures that packets can be transferred between the source and the destination using the IP address in in the packet header.
74
IP Standards
IPV4
IPV6
75
IPV4
Addresses are 32 bits long
Displayed as four sets of denary numbers
Separated by dots
E.g. 192.168.1.105
76
IPV6
Addresses are 128 bits long
Displayed as eight sets of hexadecimal numbers
Separated by colons
E.g. 2a00:23c1:f002:ec01:690e:4024:ed81:3fee
77
Standards
technical specifications which specify how software and hardware components must work.
Data transmission and communication standards have been developed to ensure that all connected devices can communicate seamlessly with each other using the same set of rules.
78
Protocols
a set of rules that enable two processes to communicate.
Each standard will include one or more sets of protocols (rules).
A protocol is a set of rules that determines how two devices will communicate.
Most network communications rely on sets of protocols that are organised into layers.
79
Layers
an abstraction of a part of the communication system used by computers to exchange data with each other. The OSI model has seven layers. The TCP/IP model has four layers.
To allow communication systems to be as open as possible, the different processes are divided up into layers.
Each layer has a defined responsibility.
The layers are ordered to that the data moves from one layer to the next in its journey from the application (that the end-user interacts with like Google Chrome) to the point at which it can be transmitted across a network.
80
TCP/IP Model
The TCP/IP model has four layers.
Application
Transport
Internet
Link
81
Specification
The specification for the interface between each layer is well documented.
This means that software developers just need to make sure that the data is in the correct format (so that it can move from one layer to the next).
The advantage of this is that each layer is independent of the others. For example, you can use the same web browser regardless of whether you connect to the internet using a local area network or through a mobile phone mast.
82
TCP/IP Stack
a conceptual four-layer model and set of protocols for internet communications.
83
TCP/IP
Transmission Control Protocol / Internet Protocol - a set of protocols (rules) operating on different layers.
84
TCP
Transmission Control Protocol
When data is to be sent (whether from client or server) the data is split into packets and each packet is given a sequence number.
This is a reliable transmission protocol. At the receiving end the packets are checked, and if any packets have gone missing, they will be re-sent.
Part of the application layer.
85
IP - Internet Protocol
IP - Internet Protocol
Creates a new packet (imagine putting the packet from the transport layer into a new envelope).
Adds the source and destination addresses to allow the packet to be delivered. These are the packets that are routed across the internet.
Part of the network (internet) layer.
86
HTTP
HTTP - HyperText Transfer Protocol
Used to make a request for a webpage.
The server returns the page or an error code if there was a problem with the request.
Part of the application layer.
87
HTTPS
HTTPS - HyperText Transfer Protocol Secure
Sends an encrypted request for a webpage. The server returns the encrypted page or an error code if there was a problem with the request.
Part of the application layer.
88
FTP
FTP - File Transfer Protocol
Used to upload or download a file.
The server opens a data connection (over which the file will be transferred) or sends an error code if there was a problem with the request.
Part of the application layer.
89
POP
POP - Post Office Protocol
Used to retrieve an email from an email server to your device.
It deletes the message on the email server once it has been downloaded to single device.
Part of the application layer.
90
IMAP
IMAP - Internet Message Access Protocol
Used to synchronise a client email account with an account on a mail server.
The server returns new emails (if there are any) and deletes any emails that were deleted locally on the client application. This allows a user to use multiple devices to access their email account. This is like your school email.
Part of the application layer.
91
SMTP
SMTP - Simple Mail Transfer Protocol
Used to send an email to an email server.
The server returns a code indicating whether or not the email could be delivered.
Part of the application layer.
