Communication and Networking Flashcards
What is a symbol?
A symbol is a particular pattern of bits that are represented by a signal.
What is communication?
The transfer or exchange of information between two parties via a medium.
What is meant by the bit rate?
The number of bits that are transmitted over a medium per second.
What is the baud rate?
The number of signal changes that take place across a medium per second.
What is the bandwidth of a communication medium?
The frequency range available within a communication medium. The difference between the highest and lowest frequency. Measured in Hertz (Hz).
What is the bit rate dependent on?
1) Bandwidth (and therefore baud rate)
2) Bits per symbol
Bit rate = Baud rate x Bits per symbol
What is the relationship between bit rate and bandwidth? What is the relationship between bit rate and bits per symbol?
1) Directly proportional
2) Directly proportional
What is latency?
The time delay between a message being transmitted and received.
What is a protocol?
A protocol is a set of rules that falicitate interaction or exchange of information between communicating parties or devices. This occurs regardless of there being different manufacturers or different countries.
What is serial data transmission?
Data is sent over a single wire, one bit at a time.
What is parallel data transmission?
Data is sent across multiple wires, sending multiple bits simultaneously.
In what conditions should parallel data transmissions be used?
- Over very short distances
- e.g. buses within computers (data, address, control bus)
What are the main problems with parallel data transmission?
1) Data Skew
2) Crosstalk
3) Expensive price of cabling
What is data skew?
In parallel communication, each communication medium will have slightly different electrical properties. This causes slightly different times taken to transfer data. This causes a problem called skew which is worse over long distances. It leads to pulses overlapping and causes corruption.
What is crosstalk?
Parallel communication lines cause interference with each other when tightly packed. This causes data corruption.
How does synchronous data transmission work?
The sender and receiver use a shared clock signal to keep continually synchronised.
Signals are sent at regular intervals will be received in the same order in which they are sent.
Where is synchronous data transmission used?
Within the buses of a computer’s processor in the fetch-execute cycle.
Suitable for real-time systems.
How does asynchronous transmission differ from synchronous communication?
Sender and receiver are synchronised only for the duration of the transmission.
How does asynchronous transmission occur?
1) The channel is idle. The sender sends the start bit, which is the opposite of the idle bit, to alert the receiver that they are about to transmit data and so they must synchronise their clock.
2) They will then serially transmit data and possibly include a parity bit.
3) They finally transmit the stop bit which is the opposite of the start and idle bits to indicate that they no longer have to synchronise clocks.
What is required to be agreed before transmission for asynchronous?
Baud rate (frequency at which to read data from the channel)
What is overhead?
The number of additional bits transmitted beyond the data itself.
What is a computer network?
Computer networks are two or more computing devices connected together in order to exchange information and share resources.
What is a local area network?
Computer networks where multiple computers are connected together within a small geographic area.
What is a wide area network?
A network where multiple computers are connected together over a large geographic area.
What are the differences between LAN and WAN?
1) LAN is baseband and WAN is broadband - meaning multiple communications take place on the channel.
2) WAN is slower and has higher latency due to less capacity.
3) Microwave and satellite used for WAN and cabled connection for LAN.
What is the physical topology of a network?
The physical connections between devices on a network.
What are the key features of a physical bus topology?
1) The nodes all connect to a single cable called a central backbone.
2) There is no central hub to manage the flow of traffic.
3) Data is broadcast to all connected devices in all broadcasts.
What are the advantages of bus topologies?
- Relatively inexpensive to implement with only one backbone with short connections to nodes
- Easy to add and remove nodes
What are the disadvantages of bus topologies?
- There is multiple points of failure as the whole network fails if the backbone fails
- Performance degrades as traffic increases due to increasing number of collisions
- Less secure as all nodes receive transmissions
What are the main features of the star topology?
A network that adopts a star topology has devices that are individually connected to a central hub or switch.
What is the role of the switch in a star topology?
- Stores the MAC addresses of all connected devices
- Creates direct, temporary connections between communicating to devices which allows simultaneous data transmission without collisions
- Makes a smaller collision domain
What are the advantages of star topologies?
- Network unaffected if connection to one device fails
- Easy to isolate faults
- Switch allows packets to be broadcast without collisions
- Switch can screen and remove corrupted packets
- Switch (not hub) allows data sent only to intended devices
What are the disadvantages of star topologies?
- Single point of failure - whole network fails if central switch fails
- Expensive to lay cable from switch to each node
What occurs in a collision?
- Two or more devices transmit data simultaneously which occupy the network bandwidth
- The electrical charges of the transmissions interfere which corrupts their signals
What are logical topologies?
Logical topologies describe the flow of data packets across a network as controlled by protocols and network hardware.
What is the key features of logical bus topology?
- Data behaves as it were being transmitted on a physical bus network
- Only one device can communicate at a time with data being broadcast to all nodes / hosts
- Only intended recipient accepts and processes data
How can a physical star topology still be a logical bus topology?
A bus protocol allows broadcasting to all hosts from hub. This makes it a logical bus topology.
What does the host do in a network?
A host is a device on a network that provides services.
Give an example of a LAN.
Network at home
Give an example of a WAN.
The Internet
How does Ethernet work? (not specifically on spec)
1) Data sent over Ethernet in Ethernet frame which contains MAC address of destination and data itself
2) Broadcast to network after checking carrier is free using CSMA/CD
3) If in physical star topology, switch or hub forwards data to appropriate host
4) Ethernet still acts as a logical bus topology.
What is baseband?
The entire network bandwidth is used in data transmission.
What is broadband?
Only a part of the entire network bandwidth is used for data transmission.
What are the different types of networking between hosts?
1) Client-server
2) Peer-to-peer
What is client-server networking?
Computers are connected together with a single central server and many clients.
What is the role of the central server in client-server networking?
It provides resources and services to clients.
If a client requests resources, the server processes the request and then returns a response to the client.
What are some example of servers that are used in a network?
- Mail server
- File server
- Wireless access point
- Router
- Printer server
- Scanner server
What type of network cable is used in a bus network?
Serial
How are client-server networks drawn?
Similar to star topology, but centralised server instead of hub and clients direct connection.
Remember to use context for networking questions and add all hosts as required
What are the advantages of a client-server network?
- Files are stored centrally and can be accessed by any connected computer
- Backups made centrally
- Centralised security
- Multiple clients can work on the same version of a file (e.g. concurrent access)
- Updates can be managed centrally (e.g. timestamp ordering)
What are the disadvantages of client-server networks?
- If server unavailable, users cannot access their files
- Server hardware is more expensive
- Cyberattacks can focus on the server
What are peer-to-peer networks?
Each computer is a peer and has equal status on the network.
How are resources shared on peer-to-peer networks?
Each peer has resources stored. To share these resources, peers can communicate directly. This means that files are distributed across the network.
How are peer-to-peer networks managed?
Each peer is responsible for its own maintenances.
What are the advantages of a peer-to-peer network?
- Decentralised with no dependence on a central server
- Easy to set up
- Inexpensive to set up
- No single point of failure
What are the disadvantages of a peer-to-peer network?
- Without central server, no centralised management
- No centralised back up
- Files are duplicated in transfer
- Files cannot be accessed if some peers are offline
What is the purpose of WiFi?
- Wireless LAN
- based on international standards
- enables any device in any part of the world to connect to a network wirelessly
What two components are required to set up a wireless network?
1) Wireless access point
2) Wireless network adapter
What is the role of the wireless access point?
- it connects to a wired LAN just like any other client
- broadcasts an SSID
- encrypts communication with WPA2
What is the role of the wireless network adapter?
- provides the interface between a computer and wireless network
What sort of frequency communications does WiFi use?
Broadband as capacity shared among devices
What are the ways of securing wireless network?
1) WPA - WiFi Protected Access or WPA2
2) Disabling SSID broadcast
3) MAC address filter
How does WPA work?
WiFi Protected Access (WPA) requires new wireless clients to enter a pre-shared key (password) to connect to the network.
Also, it encrypts communications between hosts using AES encryption.
What does WPA protect against?
Stops packets from being decrypted but not stopping them from being intercepted.
Why does WiFi need securing?
- broadcast using radio waves
- any device in range can eavesdrop on communication
How does the MAC address filter secure wireless networks?
MAC (media access control) addresses are unique to each device.
The whitelist only permits known addresses to enter the network.
How does disabling SSID broadcasting secure a wireless network?
SSID (Service Set ID) is the public identifier of the network. Disabling it means that only those who know the SSID can connect, instead of anyone in range.
What is CSMA/CA?
Protocol for avoiding collisions on wireless networks.
What is the CSMA/CA process without RTS/CTS?
1) Transmitting device senses the local network to check if another device is currently transmitting.
2) If the channel is idle, it will transmit the data. Else, it will use an exponential backoff to wait a random period of time before sensing again.
3) After transmitting, the transmitting device will wait to receive an acknowledgement. If no acknowledgement is received, it is assumed that a collision has occurred and the packet is sent again after sensing after an exponential backoff.
4) Once the acknowledgement is received, the device goes idle.
What is the CSMA/CA process with RTS/CTS?
1) Transmitting device senses the local network to check if another device is currently transmitting.
2) If the channel is idle, it will send a Request to Send (RTS) signal, which causes all devices in range to go silent.
3) Receiving device gets RTS and transmits Clear to Send (CTS) signal, causing all devices on the network to go silent.
4) Upon receipt of the CTS, the sending device will transmit the data.
5) After transmitting, the transmitting device will wait to receive an acknowledgement. If no acknowledgement is received, it is assumed that a collision has occurred and the packet is sent again after an exponential backoff.
4) Once the acknowledgement is received, the sending device goes idle.
What is the use of RTS/CTS?
Solves the problem of hidden nodes as not all nodes may be in range of sending device but still in range of the receiver.
What is most usually the receiving device in CSMA/CA?
WAP / Wireless Access Point
Flowchart for CSMA/CA without RTS/CTS
Flowchart for CSMA/CA with RTS/CTS
