Unit 6 Communications Flashcards
What is the difference between serial and parallel transmission?
Serial data cables include the USB (Universal Serial Bus)
Parallel cables are a ribbon of several smaller cables used primarily for connecting internal components
Serial data transmission: bits are sent one after another along the same data line
This means that only one line is needed to transmit data in one direction. (Two lines are required for two-way communication, plus a common signal ground wire)
Parallel data transmission: several bits (e.g. one byte of data) are sent simultaneously along separate lines or channels
This means that an 8-bit block of data will require 8 data channels, plus some extra for control information
How does serial transmission work?
Using serial communication, bits are sent one after the other
For two-way communication, an extra line is required
Serial connections were reliable but slow
How does parallel transmission work?
Multiple bits are sent simultaneously
For two-way communication, twice the number of lines are required
What are the problems with parallel transmission?
Skew and crosstalk
What is skew?
As each individual wire has slightly different properties, bits travel at slightly different speeds along each of the wires. This produces a problem known as skew
What is crosstalk?
This refers to electromagnetic interference between two adjacent channels or parallel wires
It gets more pronounced as the frequency (speed of transmission) increases
If the data is corrupted, it has to be retransmitted
What is more reliable over long distances, parallel or serial?
Parallel transmission can be used only over very short distances of up to about 2m
Serial transmission is reliable over very long distances at very high frequencies
Which has better connections serial or parellel?
Serial connectors such as USB (Universal Serial Bus) connectors are much smaller and cheaper than parallel connectors
A PC has several USB ports for connecting peripherals
What is Synchronous data transmission?
Synchronous data transmission: all data transfers are timed to coincide with an internal clock pulse
The data is sent as one long stream or block of data, with no gaps in the transmission
The receiver counts the bits and reconstructs bytes
What is asynchronous data transmission?
Using asynchronous data transmission, each byte is sent separately the moment it is available instead of waiting for a clock signal
Each byte is preceded by a start bit and ends with a stop bit or stop ‘period’ – a short time gap between each set of bits
What is the parity bit?
The parity bit or check bit is added as the 8th bit as a form of error detection
Odd or even parity may be used
This bit is set to a 1 or 0 to make the total number of 1s or 0s in the byte (including the parity bit) odd or even depending on the machine
What are positives and negatives of asynchronous data?
Asynchronous transmission is relatively slow owing to the increased number of bits being sent
It is a cheap and effective form of serial transmission well suited to low-speed connections such as keyboard and mouse
What is latency?
Latency is the time delay between the moment the first byte or packet of a communication starts and when it is received at its destination
What is a protocol?
The set of rules relating to communication between devices is called a protocol
What does the protocol need to define?
The protocol needs to define, for example:
Standards for physical connections and cabling
The rate of transmission (bit rate or baud rate)
Data format
Whether transmission is synchronous or asynchronous
Error checking procedures, e.g. odd or even parity
Any pieces of equipment which use the same communication protocol can be linked together
What is bit rate?
Bit rate is the number of bits (0’s and 1’s) transmitted in one second over a wired or wireless data link
What does a faster signal change indicate?
The faster the signal change, the greater the number of bits per second (bps)
What is baud rate?
Baud rate is the rate at which the signal in a communications channel changes state
It is usually measured in megabits per second (Mbit/s)
What is the difference between baseband and broadband?
Baseband carries one signal at a time. A bit is represented by a high or low voltage in the cable
In baseband, bit rate is the same as baud rate
At each signal change, one symbol is transmitted
A symbol in this case is 1 bit
Broadband carries multiple signals on a fixed carrier wave. Bits are sent as variations on the wave
What are the advantages of serial transmission over parallel?
No risk of skew (bits travelling at different speeds and confusing the original message)
- No risk of crosstalk (electromagnetic interference between wires)
- Cheaper to implement and occupies less space
What is the relationship between bit rate and baud rate?
Baud rate is the rate at which the signal changes
Bit rate is bits per second
Bit rate = baud rate x number of bits per signal
What is bandwidth?
Bandwidth is a measure of the maximum capacity of a communications channel
In computer networks, the term bandwidth is used as a synonym for data transfer rate
It is usually expressed in bps, Mbps or Gbps
There is a direct relationship between bandwidth and bit rate
What is a local area network (LAN)?
Local Area Network: two or more computers connected together within a small geographical area, for example confined to one building or site
What is a bus topology?
Bus topology: an arrangement where nodes are connected in a daisy chain by a single central communications channel
What is a star topology?
Star topology: an arrangement where a central node or hub provides a common connection point for all other nodes
How does a bus network operate?
All nodes are connected to a single backbone cable
Each end of the backbone is connected to either a terminator or a computer which stops signals ‘bouncing back’
Each node is passive
Data is sent in one direction at a time only
Only one computer can transmit successfully at any one time
Carrier Sense Multiple Access (CSMA/CD) technology is used to enable nodes to transmit on the same cable
What are advantages of a bus topology?
Inexpensive to set up.
Devices can be easily added.
Good for small networks.
What are disadvantages of a bus topology?
Main cable is a port of failure.
Limited cable length.
Performance degrades with heavy use, owing to data “collisions”
Poor security
How does the operation of a star topology work?
Computers are connected to a central node. This is often either a hub or a switch
What is the difference between a hub and a switch?
A switch sends each communication to the specific computer it is intended for.
A hub broadcasts the message to every computer on the LAN
What is MAC addressing?
Every networked device contains a Network Interface Card (NIC)
Each NIC is attributed a unique Media Access Control (MAC) address hardcoded in manufacture
A switch holds all of the MAC addresses for each device connected to it and uses these to direct packets of data to the correct device
What are advantages of a star network?
Easy to isolate problems
Good performance
More secure if a switch is used as data is sent only to the recipient
What are disadvantages of a star network?
Can be expensive to set up because of the length of cable required
Central device is point of failure
What is the physical topology?
The physical topology of a network defines how the devices are physically connected
What is the logical topology?
The logical topology defines how the devices communicate across the physical topologies
How can a network wired in a star topology behave logically as a bus topology?
The logical topology defines how the devices communicate across the physical topologies
What are the two types of network architecture?
Client server
Peer-to-peer
What is a client server network?
The server is a powerful computer which provides services or resources required by any of the clients
A client is a computer which requests the services or resources provided by the server
What is the role of the client in the client server network?
The client sends requests to the server
Waits for a reply
Receives the reply
What is the role of the server in the client server network?
The server waits for requests from the clients
It may need to pass the request (e.g. for a web page) to another server – in which case, it becomes the “client”
What are the features of a client server network?
Features of a client-server network include:
A central server is used to manage security
Some files are held on the central server
Some processing tasks are performed by the server
Clients issue requests to the server for services such as email, file storage, backup and printing
Suitable for many different types of organisation, small and large
Can require specialist IT staff to administer the network
What are some examples of specialised servers in a client server network?
Web server
Print server
Mail server
File server
Database server
What is peer to peer architecture?
A peer-to-peer network has no central server
What are features of a peer to peer network?
Features of a peer-to-peer network include:
Suitable for a small companies with fewer computers
No central server controls files or security
All computers can easily see files on all other computers
All computers can communicate with each other without going through a server
If a computer is switched off, data cannot be retrieved
How does peer to peer work on a WAN?
The P2P configuration can also be used for file-sharing websites
BBC iPlayer, for example, uses P2P networking
This means that with thousands of people downloading, data can be passed between computers rather than just from the server
Peer-to-peer networks are often used for the illegal distribution of copyright material such as music and films, as they are very difficult to close down
How does peer to peer file sharing work?
A file request can return several parts of the same file from multiple copies on multiple computers
What is cloud computing?
“The cloud” is simply an imaginative term for Internet computing, using remote servers run by organisations such as Dropbox, Microsoft, or thousands of other specialist software companies
Instead of maintaining their own networks, many organisations now rely on cloud computing to share both software and files at different sites
What are advantages of cloud computing?
Some software does not have to be downloaded to the user’s computer. All the software and data files are held by the software manufacturer “in the cloud”
Software and data is accessible from any computer anywhere in the world
The data is automatically backed up so there is no danger of losing it
The software and data does not occupy space on the user’s hard drive
What is the purpose of WiFi?
Wi-Fi is a wireless networking technology providing high-speed Internet and network connections
Devices connect to the Internet via a Wireless network Access Point (WAP)
What is needed to connect to a wireless network?
ISP (Internet Service Provider)
Modem and wireless router (Often combined)
Device with a network interface card (NIC), also known as a network interface controller
What is a station?
A station consists of a computer and an NIC
Stations share a radio frequency channel
What is a Service Set IDentifier (SSID)?
Identifies each network by a unique name
Must be used by all devices on that network
Can be set manually or automatically
Can be hidden to make it harder to detect
Can be set to broadcast to wireless devices in range of an access point
What is CSMA/CA protocol?
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a network protocol designed to manage how data packets are transmitted over a shared communication medium. It operates in the Medium Access Control (MAC) layer and is particularly important for wireless networks
What happens if two devices transmit simultaneously?
The CSMA/CA protocol is needed to prevent this from happening
What is the hidden node problem?
The hidden node problem is a problem that occurs in wireless networking when a node can communicate with a wireless access point (AP), but cannot directly communicate with other nodes that are communicating with that AP. This can happen when two nodes in a wireless network can communicate with a third node, but cannot communicate with each other directly due to obstacles or being out of range.
What can the hidden node problem cause?
It can lead to collisions at the third node when both nodes transmit simultaneously.
What is RTS/CTS?
RTS/CTS is an additional method to implement virtual carrier sensing in carrier sense multiple access with collision avoidance (CSMA/CA). By default, 802.11 relies on physical carrier sensing only, which is known to suffer from the hidden node problem. RTS stands for Request To Send, and the sending device uses it to tell the other end to get ready to receive, and to set its CTS line when ready. CTS stands for Clear To Send, and the receiving end uses it to tell the far end to start sending the characters
How are wireless networks secured?
Wireless encryption: Scrambles data into a secret code that only authorized users can read.
Authentication: Verifies the identity of users and devices trying to connect.
Access control: Set rules on who can connect and what they can do.
Device security: Protects devices through updates and restricted access.
Intrusion detection: Watches for suspicious activity and blocks threats.
Describe the components required for wireless networking
Wireless Router
Wireless Access Point (WAP) or Access Point (AP)
Wireless Adaptors or NICs
Wireless Bridge
Wireless Controller
What is a wireless router?
This device allows wireless and even wired devices to connect to the internet.
Most modern wireless routers integrate a wireless access point, router, and ethernet switch into one device. This allows the three functions of these components to be configured and managed in the router. This is done through the router’s management interface and hardware.
A wireless router can thus permit a wireless or wired device to access the internet through a cable, DSL, or ADSL modem.
Some advanced wireless routers have a parallel port to allow them to function as print servers.
What is a wireless access point (WAP)?
An access point is a device that contains a radio card that communicates with individual user devices on a wireless local area network or WLAN. Access points also have a wired NIC that interfaces to a network distribution system such as Ethernet.
System software installed in the access point bridges the WLAN and wired distribution side. Therefore, the access point provides a wireless connection to the remaining wired LAN, receiving and transmitting data to all the wireless devices in a particular area.
Access points also manage multiple connections within the wireless network. However, if one WAP gets overloaded with numerous devices, the network’s overall performance will drop. One or more access points are typically enough for a small office or building.
Most access points look like a small box with multiple aerials, synonymous with the looks of a router. And despite modern routers also integrating a wireless access point, some devices serve this role exclusively.
What are wireless adaptors (or NICS)?
Wireless adaptors, also known as wireless network interface cards (NICs), act as the radio receiver and transmitter for the computer or device connected to a wireless network. This component is coupled with the computer’s operating system using a software driver.
Many modern computers and laptops come with Wi-Fi capability built-in. You may have to install this feature using a network card connected to your computer’s motherboard or externally through a USB interface for older generation computers.
NICs are designed chiefly to connect to an access point. However, some NICs allow a peer-to-peer connection. This type of connection allows computers with compatible network cards within range to communicate.
Peer-to-peer connections are helpful for a small roaming workgroup of devices that don’t need LAN or the internet to operate.
What is a wireless bridge?
A wireless bridge aims to improve your Wi-Fi coverage or extend it. Their primary purpose is to receive an existing Wi-Fi signal, relay requests, and respond back and forth between your device and the main access point or Wi-Fi router.
Wireless bridges are at times referred to as wireless repeaters. They can double the range of the WLAN.
Thus, a wireless bridge provides a high-speed, long-range, outdoor link between buildings. Most bridges have a range of about twenty-five miles. They’re not usually affected by water bodies, railroads, or physical obstacles since they’re based on line-of-sight.
However, the range of most WLANs will typically be affected by the access points in the network. Most WLANs have access points with omnidirectional aerials or antennae which transmit the signal in all directions.
On the other hand, some WLANs use access points with directional antennae that produce a more concentrated signal covering a narrower area. You may thus need to replace the antennae on your wireless access point to get a better range.
What is a wireless controller?
Wireless controllers are necessary when a wireless network needs many wireless access points. The job of a wireless controller is to provide a centralized Wi-Fi management platform that manages all the access points in the network.
With wireless controllers, you get advanced features and management capabilities such as access control, centralized authentication, and the ability to restrict bandwidth on a group or user basis. You can also perform traffic and Quality of Service (QoS) prioritization
What are some privacy issues?
Developments in communications technology mean that organisations can:
Monitor behaviour
Collect and analyse personal information
Distribute, publish, communicate and disseminate personal information
What are some negative aspects of social media sites?
Internet trolling
Cyberbullying
Anonymous blogs
Hate sites
What are social, legal and ethical issues to do with software and algorithms?
Decisions and choices previously made by humans are now often delegated to algorithms
Profiling algorithms determine what information can be accessed
Online advertisements may be delivered according to perceived ethnicity, interests or pages viewed
Algorithms give users directions about how and when to exercise, what to buy, what route to take
An algorithm may be used for bail and sentencing decisions, to identify potential crime hotspots and to predict the likelihood of people reoffending
Do algorithms always make ethical decisions?
Algorithms are based on complex rules that make it difficult to predict or explain the decision-making logic
The ethics of an algorithm may be ethically questionable because of this complexity
Machine-learning algorithms, given new training data, can define or modify their decision-making
The human operator does not need to understand how or why the algorithm makes a particular decision
What are ethical concerns with algorithms?
Algorithms may give rise to ethical concerns because of:
Inconclusive evidence – the algorithm may not be infallible and this limitation has to be recognised
Inscrutable evidence – the human operator may not know what data and what reasoning a decision is based on
Inadequate algorithm or data – the conclusions can only be as reliable as the data they are based on
What are some challenges facing legislation?
Enforcement is difficult and the criminals are often very hard to trace
There may not be suitable laws in place to deal with new types of behaviour on the Internet that could be viewed as criminal
The Internet is international but laws vary in different countries, so what is illegal in one country may be legal in others
New uses of technology arise so fast that the law cannot keep up – e.g. types of wearable technology
