3.9 Fundamentals of communication Flashcards
What is serial data transmission
Bits are sent one after another along the same data line
What is parallel data transmission
Several bits are sent simultaneously along separate channels or lines
What are the disadvantages with parallel data transmission (2)
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 meant by the problem of crosstalk (2)
This refers to electromagnetic interference (crosstalk) between two adjacent channels or parallel wires
It gets more pronounced as the frequency (speed of transmission) increases
When can parallel transmission be used
Parallel transmission can be used only over very short distances of up to about 2m
When are serial transmissions used
Serial transmission is reliable over very long distances at very high frequencies
What is an example of a serial connector
Serial connectors such as USB (Universal Serial Bus) connectors are much smaller and cheaper than parallel connectors
What are the two types of serial transmission
Synchronous transmission
Asynchronous transmission
What is synchronous transmission (3)
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
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 bit
How does Asynchronous transmission operate (4)
The start bit starts the receiver clock ticking
The start bit causes the receiver clock to synchronise its clock to the same rate as the sender.
This means that timing of the transmission and receipt are the same on both devices.
The stop bit (or bits) indicate that the data has arrived so the processor on the receiver’s device can now handle those bits, for example, by copying them into memory.
When should we use asynchronous transmission
It is a cheap and effective form of serial transmission well suited to low-speed connections such as keyboard and mouse
What is latency
It 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 are things that need to be defined within a protocol (4)
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
What is meant by bit rate
is the number of bits (0’s and 1’s) transmitted in one second over a wired or wireless data link
What is baseband
at each signal change one bit is transmitted
What is baud rate
is the rate at which the signal in a communications channel changes state
How is a bit represented within a cable
A bit is represented by a high or low voltage in the cable
What is broadband
Broadband carries multiple signals on a fixed carrier wave. Bits are sent as variations on the wave
How do we calculate bit rate using baud rate and bits per signal
bit rate = baud rate * bit per signal
What is bandwidth
A measure of the maximum capacity of a communications channel
What is the purpose of the parity bit?
Check on the accuracy of the transmission.
Explain why the baud rate is always less than or equal to the bit rate but never greater.
What is a 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 meant by network topology
A network topology is the arrangement of the various computing devices which make up a computer network
Describe a bus topology
an arrangement where nodes are connected in a daisy chain by a single central communications channel
Describe a star topology
an arrangement where a central node or hub provides a common connection point for all other nodes
Describe the operation of a bus network
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
What are the advantages of a bus network
Inexpensive to set up
Less cable needed
No reliance on central node (like a star network)
What are the disadvantages of a bus network
Main cable is a point of failure
Limited cable length
Performance degrades with heavy use, owing to data “collisions”
Poor security
Describe the operation of a star network (3)
Computers are connected to a central node. This is often either a hub or 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 meant by MAC addressing (4)
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
Advantages of a star network
Easy to isolate problems
If one cable fails it affects only one computer
More secure if a switch is used as data is sent only to the recipient
Good performance - different computers can transmit at different speeds
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 physical topology
The physical topology of a network defines how the devices are physically connected
what is logical topology
The logical topology defines how the devices communicate across the physical topologies
what is a WAN
requires third party carries to conenct together network over a large geographical area.
What are the three hardware features bus network
t connectors, terminators, main cable
what are the two types of network architecture
client server
peer to peer
Describe a client server network. (2)
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 are terminals of the central server known as
Terminals are known ‘clients’ of the central server
What is the role of the client (3)
The client sends requests to the server
Waits for a reply
Receives the reply
What are the benefits and drawbacks of client-server networking
Centralised management of security
must login to access
Some processing tasks are performed by the server
Resources cannot be accessed if servers turned off
Hardware and software on servers can be optimised for providing services
What is the role of the server (2)
The server waits for requests from the clients
It may need to pass the request
why use client server in an office with a database
System will be storing confidential/personal data
Centralised security management
Centralised backup
Harder for users to change security/sharing settings
Running database from a server will avoid concurrency issues
Running database from server will ensure that it is always available
What are the features of a peer to peer network
Each computer has equal status
No centralised management of security
The same resource can be made available or shared from multiple computers
Suitable for a small companies with fewer computers
All computers can easily access / share files on all other computers
Hardware and software on computers is general purpose
What are examples of specialised servers
Web server
Print server
Mail server
File server
Database server
Why use P2P in a house
Small number of users / devices
Likely that the users will trust each other
Individual users will have ability to choose which files/resources they share and with who
Will avoid additional cost of buying server
No additional expertise required to set up / manage server(s)
How does a wireless network differ from a wired network.
A wireless network varies from a wired network in that it does not use cables to make physical connections between devices. Instead, data is sent using radio waves.
Whats needed to connect to a wireless network
ISP (Internet Service Provider) and a modem and router (Often combined)
A wireless access point.
Device with a wireless network adapter, also known as a network interface card (NIC)
What is a WAP
wireless access point:
This is a device that is either built into a router and provides the necessary hardware to allow devices to connect to the network wirelessly, or it can be a separate, standalone device that allows Wi-Fi devices to connect to the network and can be connected to the router via cable / wireless.
what is a wireless network adapter
allow the device to connect to a wireless access point via radio waves.
What is a SSID (4)
Service Set Identifier, used to identify network by unique name.
Must be used by all devices on 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 are some security measures for wireless networks:
Hide the SSID
Using WPA or WPA2
Using a MAC address list
what is wpa and wpa2
Wi-Fi Protected Access, are protocols for encrypting data so that if any data that is transmitted is intercepted it cannot be understood without the encryption key.
WPA2 is just a more advanced and complex encryption protocol.
what is a wireless nic
a wireless nic is a hardware component, a computer circuit board that is installed in a computer so that it can be connected to a radio based network.
how do mac addresses secure a network
MAC address lists ensure only approved devices can join the network
Describe the operation of the CSMA/CA(5)
Carrier Sense Multiple Access with Collision Avoidance
In networks, data is sent in frames.
When a device attempts to send a frame, it will check to see if the transmission medium is idle, or if another device is using it.
If it is busy it will wait a random amount of time and try again (and repeat until sent)
On receipt of the data, an acknowledgement is sent back to the sending device to confirm the data is received.
If this is not received, it will wait a random amount of time before resending.
Describe the hidden node problem (4)
The hidden node problem is when two nodes are out of range of each other and are unaware of each others presence, and both are trying to send data to the same device that is in range of both nodes. Therefore, they could both be sending data to the WAP at the same time, but would be unaware of the presence of the other device and, as a result, the WAP would not be able to receive any data due to the data collisions.
What can we add to the CSMA/CA to fix the hidden node problem. (4)
RTS/CTS
Once the channel is determined to be idle, a signal is sent called Request to Send (RTS).
The answering device then sends a Clear to Send (CTS) reply to the request.
This way it knows which device asked to send first and can ensure that only that device will send its message, not responding to further RTS signals until the original sender has finished.
What does CSMA/CA stand for
Carrier Sense Multiple Access with Collision Avoidance
Explain the role of a service identifier (SSID) in wireless networking and why some network administrators turn of SSID broadcasting. (3 marks)
1st mark: SSID is a unique identifier
2nd mark: Wireless client needs same ssid as the one access point.
3rd mark: SSID makes public your wireless network, could be a security weakness.
What is the purpose of the start bit in asynchronous serial transmission
