Learning Objectives
- Explain why a large message is divided into packets before transmission.
- Identify the packet header, payload and trailer.
- State the destination address, originator address and packet number stored in the header.
- Explain how packet fields allow a receiving device to reconstruct a message.
- Distinguish the transmitted data from the control information added to it.
Key Terms
- Packet
- A small unit into which a larger message is divided for transmission.
- Packet header
- Control information placed at the start of a packet.
- Payload
- The part of a packet containing a section of the original data.
- Trailer
- Information placed at the end of a packet, often used to mark the end and support checking.
- Destination address
- The address of the intended receiving device.
- Originator address
- The address of the device that sent the packet.
- Packet number
- A sequence value used to identify the packet position in the original message.
Why Data Is Divided Into Packets
When a file, message or stream of data is sent across a network, it is not normally transmitted as one uninterrupted block. The data is divided into smaller units called packets. Each packet can be handled separately by networking devices and then combined with the other packets at the destination.
Breaking data into packets allows communication links to be shared. A very large transmission does not need to occupy an entire route until the whole message has finished. Packets from different users can be interleaved and forwarded as network capacity becomes available.
Packets also make recovery from errors more manageable. If one packet is lost or damaged, the system can arrange for that packet to be sent again rather than retransmitting the whole original message. The syllabus focuses on the packet structure and packet-switching process, so answers should connect the idea of packets to routing, ordering and successful reconstruction.
- A large message is divided into smaller pieces.
- Control information is added to every piece.
- The packets are transmitted through the network.
- The receiving system uses the control information to rebuild the message.
The Three Main Parts Of A Packet
A packet contains a header, a payload and a trailer. These parts have different purposes. The payload is the actual section of the original message. The header and trailer are additional data required so that the network and receiving device can handle the packet correctly.
The packet header is placed before the payload. It contains addressing and sequencing information. The syllabus specifically requires the destination address, the packet number and the originator address. These values do not form part of the user’s original message, but without them the network would not know where the packet should go, where it came from or where it belongs in the reconstructed message.
The trailer is placed after the payload. It indicates the end of the packet and can carry information used when the packet is checked. A candidate does not need to name a particular trailer algorithm unless a question supplies one; the essential point is that the trailer is separate from the payload and supports correct reception.
How Header Fields Are Used
The destination address tells routers and other network devices which receiving system should ultimately receive the packet. The originator address identifies the sender. It can be used when a response or retransmission request must be returned.
The packet number indicates the packet’s position in the complete message. If packets arrive in a different order from the order in which they were sent, the receiving device can place them back into the correct sequence. Packet numbers can also reveal that a packet is missing because a gap appears in the expected sequence.
The same original message therefore produces several packets with related destination and originator addresses but different packet numbers and payload sections. The receiving device removes the additional control information after it has checked and reordered the packets, leaving the original data.
Overhead And Payload
The header and trailer are sometimes described as overhead because they increase the total amount of data transmitted but are not part of the original message. This extra information is necessary for reliable organisation and delivery.
A packet with a large payload carries a greater proportion of useful data, but a damaged large packet may require more data to be resent. A packet with a small payload has proportionally more overhead. The Cambridge syllabus does not require a calculation of ideal packet size, but students should understand that packet structure involves both useful data and control data.
Packet Structure
| Part | Contents Or Purpose |
|---|---|
| Header | Destination address, packet number and originator address. |
| Payload | A section of the original file or message. |
| Trailer | Marks the end of the packet and may support checking for errors. |
How Required Header Fields Help
| Field | How It Is Used |
|---|---|
| Destination address | Directs the packet towards the correct receiving device. |
| Originator address | Identifies the sending device. |
| Packet number | Allows packets to be reordered and missing positions to be noticed. |
Worked Examples
Identifying Packet Parts
Question: A packet contains a source address, sequence number, destination address, part of an image and an error-checking value. Classify the information.
- Place the source, sequence and destination values in the header.
- Identify the part of the image as the payload.
- Identify the checking value as trailer information.
Answer: Header: originator address, packet number and destination address; payload: image data; trailer: checking information.
Explaining Packet Numbers
Question: Packets numbered 1, 2, 4 and 5 arrive. What can the receiving device infer?
- Read the sequence numbers in order.
- Notice the gap between 2 and 4.
- Use the expected sequence to identify the missing packet.
Answer: Packet 3 has not arrived, so it may need to be sent again.
Separating Data From Overhead
Question: A packet is 1024 bytes and its header and trailer total 64 bytes. How many bytes are payload?
- The complete packet contains payload plus overhead.
- Subtract the 64 bytes of overhead from 1024 bytes.
Answer: 960 bytes are payload. This calculation illustrates the distinction, although packet-size arithmetic is not a named syllabus requirement.
Examination Guidance
- Name all three packet parts when asked to describe structure.
- Place destination address, packet number and originator address in the header, not in the payload.
- Explain the use of packet numbers rather than merely listing the field.
- Use the term payload for the section of the original data.
Common Mistakes
- Calling the whole packet the payload.
- Saying the packet number identifies the destination.
- Putting the destination address in the trailer.
- Claiming that every packet contains the complete original file.
Knowledge Check
1. Why is data broken into packets?
2. What is stored in the payload?
3. Name the three required header items.
4. Why is a packet number needed?
5. What is overhead?
