Data Packets
What are packets?
- Information on the internet is broken down into packets and are created by TCP and transmitted over the internet
- Packets are small chunks of information/data
- TCP stands for Transmission Control Protocol and is used for organising data transmission over networks
- Small chunks of data are easier and quicker to route over the internet than big chunks of data
- Routing involves finding the most optimal path over a network
- Data can include anything from text, images, audio, video, animations, etc, or any combination of these
What do packets contain?
- Packets are “chunks” of information. This information is called the “payload”
- Packets act like postage letters, each one has:
- a delivery address (destination IP address)
- a return address (source IP address),
- and a message (data payload)
- Packets are split into three parts:
- Packet header
- Payload (the actual data)
- Trailer
- The header contains:
- Source IP
- Destination IP
- Packet number
- Error checker e.g. a checksum or parity bit
- The trailer contains:
- Additional error checks
- End of packet notification
Figure 1: To transmit the message “This is a message :)”over the internet, the TCP might break the message down into 4 packets
- Each packet in figure 1 contains a source IP address, destination IP address, payload (the data) and a packet number. Error checking and end of packet notifications have not been included in this example
- Individual packet structure depends on the protocol used to create them; variations exist
- Error checks make sure that when a packet is received there is minimal or no corruption of the data
- Corruption is where packet data is changed or lost in some way, or data is gained that originally was not in the packet
- A parity bit checks that no bits have been flipped from 0 to 1 or vice versa
- A checksum performs a calculation and compares the result to the checksum value. If the values are different then the data has been corrupted
How are packets sent across the internet?
- Sending packets over the internet is called packet switching and is more efficient than circuit switching
- Packet switching involves:
- Routers know which nearby router is closer to the destination device
- Like normal car traffic, data traffic builds up on the internet. Routers can see this and decide to send a packet down a different route that avoids traffic.
- Packets from the same message can take different routes from the sender to the receiver, and may arrive in different orders. The receiver's computer reassembles the message by reordering the packets using the packet numbers
- Breaking down a file into packets and sending these packets down different routes over the internet (via routers) from a source to a destination and reassembling them at the end
- Packet numbers allow for the original message, which has been broken down into smaller parts, to be re-assembled in the correct order, or assembled together like a jigsaw, once all of the packets have been received
- Routers contain routing tables which keep track of nearby routers like a map or contacts list
- If a packet does not reach its destination the receiver can send a resend request to the sender to resend the packet
Figure 2: Packets take different routes across a network from the source address to reach the destination address as shown by the green, blue and red routes taken
- The advantages of packet switching are:
- Interference and corruption are minimal as individual packets can be resent if they are lost or damaged
- The whole file doesn’t need to be resent if a corruption occurs, only the individual packets that were corrupted need to be resent. This saves time and internet bandwidth
- Packet switching is quicker than sending a large packet as each packet finds the quickest way around the network
- It's harder to hack an individual's data as each packet contains minimal data, and travels through the network separately
Worked example
A local market shop wishes to arrange a delivery of goods from a supplier. Anna, the shop owner, decides to send an email to request the delivery of the goods at a certain date and time.
Describe how packet switching is used to send this email and how it can be protected from corruption.
[8]
- The business email is first broken down into packets which are given a source address (where its come from) and a destination address (where its going to) [1]
- Each packet receives a packet number so that the email can be reassembled when it reaches its destination [1]
- Each packet also receives an error check such as a parity bit. A parity bit checks whether any bits have been flipped due to corruption [1]
- Each packet is sent over the internet via routers. Routers contain routing tables that determine the next closest router to the destination [1]
- Packets may take different routes depending on internet traffic and arrive at their destination in any order [1]
- Packets are checked for errors using the error checks and missing packets can be requested to be resent [1]
- Once all packets have been received then they can be put together in order using the packet numbers [1]
- Once assembled the original email can be read by the other business [1]
Exam Tip
- For high marks make sure your answer is coherent, that is it follows logically from one point to the next. Some marks depend on previous points you have made. Explaining parity bits without mentioning error checking first may not gain you additional marks
