Data Transmission
This is the process of sending data over a network from one peripheral device to another and there are many ways to do this. Data is sent in packets, which is split into a header, a footer and the message. Each packet has a maximum size, defined by the protocol in use and therefore, files are split up into packets, each containing a part of the file. The header contains the source and destination IP address.
This topic covers:
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Error checking techniques
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Types of transmission
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Multiplexing
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Network routing
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Data transfer rates
Types of transmission
Serial
Each bit in a byte is sent separately, one at a time and in succession. This means that only one wire is required, helping to reduce both costs and space. They are actually not slower, due to the fact that the clock speed can be increased to send more data per second.
Parallel
All bits in a byte are sent at the same time. This type of transmission used to be faster, but due to increased serial performance, these are now slower. This is mainly due to interference caused by an induced current between the multiple wires which reduces the quality.
Simplex & Duplex
When a network is established, each party decided on the protocol to use, as well as which duplex mode to use.
Simplex- communication is only one direction, meaning there are no return packets. an example would be satellite connections for a TV, as messages are only sent one way.
Full Duplex – communication in both directions at once, like a telephone, by having a pair of twisted cables, one for sending and another for receiving. This is so that packets to not interfere with each other.
Half Duplex – Same as full with communication both ways, but only one way at a time, like a printer.
Multiplexing & Switching
The bandwidth represents the total theoretical data carrying capacity of a medium. However, the actual data input is always less, due to some reasons.
Multiplexing – Combines data from a number of sources into one single message that can then be transferred over the network.
Packet Switching – Messages are directed over a network, ensuring the whole message reaches its desired destination. Each message is divided up into multiple packets, each with a header, footer and body. Packets are sent separately via many routes, so must be reorganised at the destination. This means the message cannot be intercepted & also contain error checking to ensure the correct and full data has been sent
Circuit Switching – Separate packets are transmitted one after another, each following the same route to the destination.
Communication Networks
When data is transferred, it is sent over as many units of data called packets, allowing the bandwidth to be better shared among users in a network.
Packets – A packet consists of actual data and control data. The control data in a TCP/IP packet (The most common in networks) consists of:
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Source and destination IP addresses
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Control signals
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Error control
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Order number
Collisions– Data collisions will always occur when transferring data over a network, regardless of the topology. It happens when 2 sets of data are detected simultaneously, causing them both to be discarded. To recover the data, the computer waits a random amount of time before attempting to resend the discarded packet once again. This is repeated until the packet is successfully sent and received.
A protocol called carrier sense multiple access/collision detection (CSMA/CD) determines how collisions are detected and dealt with.
Traffic routing & costs – Whenever data is sent over a network, a process must take place in order to ensure that the packets are delivered to their target destination in the shortest amount of time possible. This process is called routing and it determines the path each packet takes, by using a routing table that stores each route the packet can take.
These tables are stored in the memory of network hardware, like a router or a switch, hence why your hose probably has one. The best path is determined by calculating the cost of each route and then choosing the lowest. It can sometimes be cheaper to travel from destination A, to C and then B, rather than going straight from A to B.
Data Transfer Rates
There are a fair few factors that affect transfer rates when sending data over a network and in this section we explore the biggest contributors.
Bandwidth
The measure of how much data can be transmitted over a cable at any one point in time. The higher the bandwidth, the higher the amount of data that can be fit onto the cable. It is important to know that the bandwidth does not affect the speed at which data is sent across, only the capacity. This means that if one connection transmits data at 5 Mbits/s and another at 20 Mbits/s, but the bandwidth of the cables is 5 Kb, then it would take exactly the same amount of time for a packet to be sent. Many broadband companies will advertise the transmission speed and charge more, without the customer knowing that there will often be no difference.
Bit rate
Different software and applications require different bit rates. The bit rate is a measure of how much data will be sent over a given amount of time. When browsing web pages, a low bit rate is required as only a small amount of data needs to be sent. However, when watching a video a higher bit rate is needed due to the need for a stutter-free stream. Bit rates can change in any scenario and used as higher bit rates are more expensive, so maintaining a high bit rate is not suitable.