Communication and Networks Flashcards
What is data communication?
Data communication involves sending and receiving data from one device or computer to another
What are the two ways in which data can be sent?
- Serial Data transmission
- Parallel data transmission
What is serial transmission?
Data can be sent via an interface, one bit at a time over a single wire
What is Parallel transmission?
- Data is transmitted several bits at a time (simultaneously) using multiple wires
Parallel Transmission is only reliable over short distances. Explain why
- When parallel transmission is used, each wire has slightly different properties.
- This means bits could travel at slightly different speeds over each of the wires, leading to a problem known as skew
- Skew can get worse over longer distances, which is why parallel transmission is only suitable for short distances
What are the advantages of serial transmission over parallel transmission?
- Low Cost - Reduced size and complexity of connectors used in serial transmission results in lower cost
- No interference - Cross talk causes interference between parallel wires, leading to data corruption. This worsens with increasing signal frequency and communication link length
- Reliability - Serial transmission is reliable over long distances as there is no skew of data that can occur in parallel transmission
- High net data transfer - Serial transmission can transmit data at high frequencies without interference. This means there is a higher rate of data transfer, even though less data is transmitted per cycle
Serial transmissions can allow data to be transmitted at a higher data transfer rate at higher frequencies. Explain why
When transmitting data at high frequencies via parallel transmission, crosstalk occurs, which causes interference in the data transmitted by each wire. This causes data to be corrupted.
However, this problem does not occur in serial transmission and so it is possible to transfer data efficiently at higher frequencies, resulting in more bits transmitted per second without interference and therefore a higher data transfer rate.
What is bit rate?
Bit rate is the number of bits transmitted serially per second
What is baud rate?
Baud rate is the rate at which the signal changes
How is bit rate calculated?
Bit rate = Baud rate x number of bits per signal
What is baseband mode?
Two voltage levels are used, one to represent ‘0’, and the other to represent ‘1’.
In baseband mode, bit rate and baud rate can be the same. Explain why
bit rate = baud rate x number of bits per signal. Since 1 bit is used to store each signal, the bit rate would be baud rate x 1 which is equal to the baud rate.
Explain how it is possible for the bit rate to be higher than the baud rate
The bit rate can be higher than the baud rate if the number of bits used to store each signal is greater than 1.
In a particular communications system, 32 voltage levels are used. How many bits can be in a group that is encoded by a voltage level?
5 bits, as 2^53 = 2
What is bandwidth?
Bandwidth is the range of frequencies that a transmission medium can carry, measured in bits per second or Hz. It is the rate at which a transmission medium can send or receive bits per second.
Explain why a higher bandwidth results in a higher data transfer rate
A higher bandwidth supports higher frequencies, which means more bits can be transmitted per second. This leads to a higher data transfer rate.
Why is internet upload speed different from download speed?
Most users spend more time downloading than uploading so internet providers design their systems to give priority to downloading.
What is latency?
Latency is the time delay between the moment that an action starts and the moment where its effect is observed.
In the context of data transmission it is the time delay between the moment that transmission of the first packet of data of a communication starts and when it is received at its destination.
What is parity? Explain the parity system and what a parity bit is?
The parity system is a system to verify the correctness of data that could be erroneously changed during transmission.
A parity bit is an additional bit used to verify the correctness of other bits during data transmission. The parity bit is set at the transmitting end and checked at the receiving end.
What is odd parity?
The total number of 1s in every byte, including the parity bit, is an odd number
What is even parity?
The total number of 1s in every byte, including the parity bit, is an even number
What is synchronous data transmission?
Using synchronous transmission, data is transmitted at regular intervals timed by a shared clocking signal, allowing for a constant and reliable transmission for time sensitive data.
What are some examples where synchronous transmission is used?
- Time sensitive data
- In the CPU, the clock emits a signal at regular intervals and transmissions along the data bus, address bus and control bus start on a clock signal
What is asynchronous transmission? Briefly explain the principles of asynchronous transmission
- In asynchronous transmission, one byte at a time is sent, preceded by a start bit and followed by a stop bit.
- The start bit and stop bit must always be different
- The baud rates of the sender and receiver must be set up to be the same or the signal won’t be received correctly.
The stop bit in an asynchronous transmission can be arbitrarily long. Explain why?
The stop bit is a “stop period”, which can be arbitrarily long. This allows the receiver time to identify the next start bit and gives the receiver time to process the data before the next value is transmitted.
What are the differences between Synchronous and Asynchronous transmission?
- Synchronous transmission needs a common clock signal, whereas in asynchronous transmission there is no clock sharing.
- Synchronous transmission has a more complex interface because of the additional clock signal, whereas asynchronous transmission is cheap and easy to implement.
- Synchronous transmission allows for faster transmission, whereas asynchronous transmission is slower as extra bits need to be transmitted
- All parallel transmission is synchronous e.g. CPU, video/audio streaming, whereas asynchronous transmission can be used when data is sent sporadically e.g. via a mouse or keyboard.
What is a protocol?
A protocol is a set of rules relating to communication between devices.
Why is a communications protocol necessary when communicating over a network?
A protocol allows equipment from different supplies to be networked
What standards can protocols cover?
- Physical connection
- Cabling
- Mode of transmission
- Data format
- Error detection and correction
What is a LAN?
A Local Area Network consistes of a number of computing devices such as PCs, printers, scanners and a central server, on a single site or a single building, connected together by cables.
What is a LAN used for?
A LAN allows user on the network to:
- Communicate with each other
- Share hardware devices such as printers and scanners
- Share data
What is a topology?
A topology is the layout of a network
What are the characteristics of a Physical Bus Topology
In a physical bus topology:
- All network devices are connected to a single backbone cable
- The ends of the cable are plugged into a terminator
What are the characteristics of a Physical Star Topology?
In a physical star topology:
- Each node is connected to a central node
- The central node may be a switch or a computer that acts as a router
- The switch keeps a record of the unique MAC address of each network device, which helps it to identify where to route messages
What is the difference between a Physical and Logical topology?
Physical topology refer to the actual architecture of a network, whereas a logical topology refers to flow of data packets on the network
What are some advantages and disadvantages of Bus Topology?
- Cost: Bus Topology is inexpensive to install as it requires less cabling and no additional hardware (Advantage)
- Fault isolation: If the main cable fails, network data can no longer be transferred to any of the nodes. This makes it harder to identify any faults in the network (Disadvantage)
- Performance: Performance degrades with heavy traffic and high transmission speeds (Disadvantage)
- Low security: All computers on the network can see all data transmissions (Disadvantage)
What are some advantages and disadvantages of Star Topology?
- Cost: Star topology is more expensive to install due to increase cable length and additional hardware i.e. central node (Disadvantage)
- Fault isolation: If one cable fails, only one node is affected so easy to isolate faults (Advantage)
- If the central node goes down, the whole network goes down (Disadvantage)
- Performance: Consistent performance, even when network is being heavily used and at high transmission speeds (Advantage)
- Security: Highly secure as messages are sent directly to the central node and cannot be intercepted by other nodes (Advantage)
- Easy to add devices to the network without disruption (Advantage)
- No problem with data collisions as each node has its own cable to the central node (Advantage)
What is a NIC?
A Network Interface Card provides a computer with a full time, dedicated connection to a network. Each card represents a device and can prepare, transmit and control the flow of data on the network. A computing device must have an NIC in order to connect to a network.
What is a MAC address?
A MAC address is a unique, 48 bit (12 hex digits) long address that is hardcoded into the NIC of a device during manufacture. It uniquely identifies a particular device so data packets can be routed to and from them.
How can a MAC address be used to track a device?
When you walk around with a device containing an NIC, the device will scan for nearby Wi-Fi networks and broadcasts its MAC address
The MAC address is then logged by the network.
What are implications of MAC addresses on personal privacy?
Devices scan for nearby Wi-Fi networks and broadcast their MAC address.
As the devices pass by enough networks, broadcasting their address each time, the information could be used to track your complete movements throughout the day.
This is an infringement on personal privacy.
How could advertisers use tracking information?
Advertisers could know the areas you visited and could try to target ads specifically towards you
Who else might use this information?
Security services and police can use this for:
- Investigation of criminal cases
- Keeping track of the movements of individuals they are interested in
What is MAC address filtering?
Networks use MAC address filtering to allow only devices with specific MAC addresses to connect to a network.
What is a Client- server network?
In a client- server network, one or more computers known as clients are connected to a powerful, central computer known as a server.
Each client may hold some of its own files and resources such as software and can also access resources by the server.
What are some examples of client-server networks?
File server: Holds and manages data for all the clients
Web server: Manages requests for clients to access the web
Print server: Manages print requests sent by clients
Database server: One or more database clients accessing a database server and running queries against them
What are some advantages of client-server networks?
Sharing: Data and expensive/powerful resources can be shared between a large number of clients
Security: Security is better as files/data are stored in a central location and access rights are managed centrally
Backup: Backups are done centrally, recovery procedures recover lost data
Data consistency: Data consistency is maintained as only one copy of the data is held. Data consistency means each client sees a consistent view of the data, including visible changes made by the clients own transactions and transactions of other clients/
What are some disadvantages of client-server networks?
Expensive: Client server networks are expensive to install and manage as the hardware involved is expensive (expensive server-grade hardware and software), and professional IT staff are needed to maintain the servers and run the network.
What is a peer-to-peer network?
In a peer to peer network there is no central server i.e. decentralised. Individual computers and connected together either in a LAN or WAN so they can share files
In a small LAN, such as a home or small office, a peer to peer netweork is a good choice because:
It is cheap to set up
Not difficult to maintain
Enables users to share resources such as a printer or router
Scalable
How do P2P networks offer scalability?
Each peer potentially acts as a server.
This avoids the bottleneck of most distributed systems by causing the number of servers to increase linearly with the number of clients
What are some downsides of peer-to-peer networks?
- Widely used for online piracy as it’s impossible to track the files which are being illegally downloaded
- Security Risks
- Lack of centralised control
Where are client-server networks used?
- Schools
- Hospitals
- Small/medium large businesses
Where are peer to peer networks used?
- Home
- Small office
- Video on demand
Why is it illegal to download music/movies from some internet sites?
- Most songs and movies that appear on file-sharing sites are protected by copyrights e.g. “Copyrights, Designs and Patents Act 1988”
- It is illegal to download any movies or music that is copyright
What are the consequences of online piracy?
- Financial loss to songwriters, filmmakers
- The artists do not get royalties for their work
- This is more significant in the case of budding artists
-Filmmakers and music companies will have less funding for new works
- This has a wider impact on the industries as a whole and everyone who works in them
What is the internet?
The Internet is defined as:
○ a network of interconnected computer networks
○ which uses an end-to-end communication protocol
Explain the physical structure of the internet
Each continent uses backbone cables connected by trans-continental leased lines fed across the sea beds.
ISPs connect directly to this backbone and distribute the internet connection to small providers who in turn provide access to individual homes and businesses
What is a URL?
A Uniform Resource Locator is the full address for an internet resource. It specifies a resource on the internet, including its name and a file type, so that a browser can go and request it from a web server
Explain why internet registrars are needed
Internet registrars are needed to ensure that a particular domain name is only used by one organisation, and they hold records of all existing website names and the details of those domains that are currently available to purchase. All registrars must be accredited by their governing registry
What are internet registries?
Internet registries are five global organisations governed by ICANN, with worldwide databases that hold records of all the domain names currently issued to individuals and companies, and their details. They also allocate IP addresses and keep track of which addresses a domain name is associated with as part of DNS
What details are stored by internet registries?
- Registrant’s name
- Type (company or individual)
- Registered mailing address
- The registrar that sold the domain name and date of registry
Explain the purpose of a domain name
A domain name identifies the area or domain that an Internet resource resides in. These are structured into a hierarchy of smaller domains and written as a string separated by full stops.
Each domain name corresponds to one or more IP addresses
Explain how DNS is used when a user requests a web page from its URL
The DNS catalogues all domain names and IP addresses in a series of global directories that domain name servers can access in order to find the correct IP address location for a resource.
When a webpage is requested using the URL a user enters, the browser requests the corresponding IP address from a local DNS. If that DNS does not have the correct IP address, the search is extended up the hierarchy to another larger DNS database. The IP address is located and a data request is sent by the user’s computer to that location to find the web page data.
Why are IP addresses not used to access websites instead of alphanumeric addresses?
Domain names are much easier for humans to remember and type in than a string of numbers in an IP address.
What is a FQDN?
A Fully Qualified Domain name is one that includes the host server name, for example www. or ftp. or mail.
What is an IP address?
An Internet Protocol address is a unique address that is assigned to a network device. It indicates where a packet is to be sent to or has been sent from
What is Wi-Fi?
Wi-Fi is a local area wireless technology
It enables you to connect a device to a network resource or the internet via a wireless access point (WAP)
