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Computer Science IGCSEPDF 10
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Unit 2: Data Transmission
1. Data Packets
● When data is sent across networks (especially over the internet), it is split
into packets/datagrams instead of being sent as one continuous stream.
● Reason for packetization:
○ Easier to control/manage.
○ Allows more efficient use of network resources.
○ Prevents a single transmission from monopolizing the channel.
Packet Structure
1. Header
○ Contains essential control information.
○ Includes:
■ Sender IP address (origin).
■ Receiver IP address (destination).
■ Packet/sequence number (used to reassemble in correct
order).
■ Size of packet (helps allocate buffer space).
○ Sometimes includes protocol type (e.g., TCP/UDP).
2. Payload
○ The actual data to be delivered (part of the overall message).
○ Could be text, image data, file segments, etc.
3. Trailer
○ Marks the end of the packet.
○ Contains error-checking codes (e.g., CRC – Cyclic Redundancy
Check) to detect corruption.
○ CRC works by counting 1-bits, appending result, and checking at
receiver’s end.
Key Features
● Packets may arrive out of order (different routes).
● Receiver reorders packets using sequence numbers.