X) Networks and Communication Flashcards
Serial Data Transmission
Serial data transmission is when data is transmitted one bit at a time, down one data line.
For two way communication, an extra data line is required.
They can transmit at a rate of 1Gps ( 1000 million bits per second).
Parallel Data Transmission
Parallel data transmission is where data is transmitted several bits simultaneously along separate data lines.
The more wires there are the more data that can be sent simultaneously, this creates large network channels, (processor and memory).
Serial Vs Parallel Data Transmission
Serial:
- Only one-way data transmission
- Slower than parallel
- Less number of cables required
- Data can be transmitted over longer distances
Parallel:
- Two-way data transmission
- Faster than serial
- More prone to developing connection problems (Skew).
- Can only transmit data over short distances
Connection problems
Skew and Crosstalk
Skew is a problem caused when each wire has slightly different properties, leading to different data speeds along each wire.
Crosstalk is the electromagnetic interference between two adjacent wires. This issue can become pronounced as the frequency of transmission increases.
Synchronous Transmission
Synchronous data transmission is when all data transfers are timed to coincide with an internal clock.
The data is sent as one long stream of data with no gaps in the transmission. The receiver counts the bits and reconstructs bytes.
Asynchronous Transmission
Asynchronous data transmission is when each byte is sent separately instead of waiting for a clock signal.
Each byte is preceded by a start bit and a stop bit, with a short gap between each byte.
This transmission is relatively slow as to the short gap between each byte. But is a cheap, serial transmission suited to low transmission connections.
The parity bit
The parity bit is added as the 8th bit, to form an error detection. Can either be a 0 or 1 and is placed at the end of the byte, before the stop bit.
Latency
Latency is the time delay between the moment the first byte is sent to when it is received at its destination.
Causes for latency:
- the time it takes data to pass through a communication medium.
- the time it takes for data to pass through a network depending on how many devices it has to pass through.
Protocols
Protocols are a set of rules for governing data transmission across devices and networks.
The protocol defines:
- Standards for physical connections and cables
- The rate of data transmission
- How data is formatted
- Whether the transmission is synchronous or asynchronous
Bit Rate
Bit rate is the rate at which data is transmitted, measured in bits per second (bps). The bit rate is directly proportional to bandwidth.
Using baseband, at each signal change one bit is transmitted, the faster the signal change, the greater the bit rate.
Baud Rate and Broadband
Baud rate is the rate at which the signal in a communications channel changes state. Measured in megabits per second (Mbs).
Broadband carries multiple signals on a fixed carrier wave, where bits are sent as variations on the wave.
How to calculate bit rate?
Bit rate = baud rate x number of bits per signal
Bandwidth
Bandwidth is a measure of the capacity of the channel down which the data is being sent, measured in hertz (Hz).
Local Area Networks (LANs)
A LAN is two or more computers connected together within a small geographical area.
- Devices can be sent data over to each other faster over a LAN.
- Computers safer from threats as data is not sent directly from a client, sent through a server.
Bus Topology
A Bus network topology is where all nodes are connected to a single backbone cable.
Each node is passive and data is sent in one direction at a time only.
Only one computer can transmit successfully at any one time.
+ives:
Cheaper as only one cable is needed.
Easier to add new clients.
-ives:
Less secure as all data is transmitted down one cable.
If the main cable fails then all clients are affected.
Star Topology
A Star network topology is where computers are connected to a central hub or 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.
+ives:
Fast connection speed as each client has a dedicated cable.
If one cable fails then only that client affected.
-ives:
Expensive to set up and increase capacity.
The central server can get congested as all communications go past it.
Carrier Sense Multiple Access (CSMA)
CSMA with Collision Detection (CSMA/CD) detects two nodes attempting to transmit simultaneously.
Both nodes cease transmission and then wait a random amount of time before reattempting.
Media Access Control Addressing (MAC addressing)
Each networked device has a Network Interface Card (NIC), a NIC enables a device to connect to a network.
The type of NIC determines the speed at which data can be transmitted between the device and the network.
Physical and Logical Topology
The physical topology of a network is how the devices are physically connected.
The logical topology of a network is how the devices communicate across the physical topology.
Client-server network
A client is a computer that requests the services or resources provided by a server.
The client sends requests to the server and waits for a reply.
A server is a powerful computer that provides services or resources required by any of the clients.
The server waits for requests from clients, then it may pass the request to another server, which in effect becomes a client.
Features of a client-server network
- A central server is used to manage security.
- Some processing tasks are performed by the server.
-Suitable for both small and large types of organisations. - Can require specialist IT staff to administer the network.
-Many specialised servers, each with a dedicated task:
Web Server
File Server
Database Server
Print Server
Peer-to-peer network
A peer-to-peer network is a network where all devices share resources between them rather than having a central server.
Features of a peer-to-peer network
- Suitable for small organisations with few computers.
- All computers share the same files.
- No central server to control files and security.
- All computers can communicate with each other without going through a central server.
Peer-to-peer on a WAN
Peer-to-peer (P2P) can be used for file-sharing websites.
This means with people downloading, data can be passed between computers rather than through a central server.
P2P is used for the illegal distribution of copyright material such as films and music.
Cloud Computing
Cloud computing is the storing and accessing of data through the internet.
Whereby using remote servers run by organisations, data can be held or accessed on different networks.
Advantages of Cloud Computing
- Software and data is accessible from any computer around the world.
- Data is automatically backed up, so no danger of losing data.
- Software and data doesn’t occupy space on the users hard drive.
Wi-Fi
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).
Connecting to a wireless network
Each device that wants to connect to the internet must require a NIC (Network Interface Card) which then enables the device to communicate to a WAP.
WAP requires a connection to a router, the router requires a connection to a modem, the modern converts digital signals from a computer to analogue signals which are then sent down a network line.
Service Set Identifier (SSID)
SSIDs identify each network by a unique name. SSIDs are used by all devices on that network.
SSIDs can be set to broadcast to wireless devices in range of an access point.
Securing a wireless network
Wireless networks can be less secure than wired networks. Unauthorised users can be hard to spot and transmitted data can easily be intercepted.
Security measures used are WPA and WPA2
Collisions and avoidance
When several devices are transmitting via a single WAP, a protocol is needed to prevent two devices from transmitting simultaneously.
CSMA/CA
CSMA/CA using RTS/CTS
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol.
Collisions are avoided by each station transmitting only when the channel is idle.
CSMA/CA
- Assemble a frame
- Is channel Idle?
- No- wait a random time until data can be transmitted.
- Yes- transmit data
Using RTS/CTS
- Assemble a frame
- Is channel Idle?
- No- wait a random time until data can be transmitted.
- Transmit RTS (Request To Send)
- CTS (Clear To Send) received?
- No- wait a random time
- Yes- transmit data
