Packet Switching

This lesson covers how data is transmitted across networks using packet switching, including packet structure, routing, reassembly, and a comparison with circuit switching. These are key concepts for the OCR H446 specification.

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What is Packet Switching?

Packet switching is a method of data transmission where data is broken into small units called packets, which are independently routed across the network and reassembled at the destination.

This is the method used by the Internet and most modern networks.

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How Packet Switching Works

1. The sending device divides the data into packets of a fixed maximum size.
2. Each packet is given a header containing routing information (source and destination IP, sequence number, etc.).
3. Packets are sent independently across the network — they may take different routes depending on network conditions.
4. Routers at each hop examine the destination IP address and forward the packet toward the destination using their routing tables.
5. Packets may arrive at the destination out of order.
6. The receiving device uses the sequence numbers in the headers to reassemble the packets in the correct order.
7. If any packets are missing or corrupted, the receiver requests retransmission (using TCP).

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Packet Structure

A packet consists of three main parts:

Header

Field	Purpose
Source IP address	Identifies the sender
Destination IP address	Identifies the intended recipient
Sequence number	Indicates the packet's position in the original data (for reassembly)
Time to Live (TTL)	Maximum number of hops before the packet is discarded (prevents infinite looping)
Protocol	Identifies the transport protocol (TCP or UDP)
Packet length	Total size of the packet
Checksum	Error-detection value for the header

Payload (Data)

The payload is the actual data being transmitted — a portion of the original message. The maximum payload size is determined by the Maximum Transmission Unit (MTU), which is typically 1500 bytes for Ethernet.

Trailer

The trailer (or footer) contains:

• Error-checking data (e.g., CRC — Cyclic Redundancy Check) to verify that the packet was not corrupted during transmission.
• End-of-packet marker.

[HEADER | PAYLOAD (DATA) | TRAILER]

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Routing

Routing is the process of determining the path a packet takes from source to destination.

How Routers Route Packets

1. A packet arrives at a router.
2. The router reads the destination IP address from the header.
3. The router consults its routing table to determine the best next hop.
4. The router forwards the packet to the next router (or the destination if directly connected).
5. This process repeats at each router until the packet reaches its destination.

Routing Tables

A routing table contains:

Field	Purpose
Destination network	The network address the route leads to
Next hop	The IP address of the next router on the path
Metric/cost	A measure of the "distance" or cost of the route (e.g., hop count, latency)
Interface	Which physical port to send the packet out of

Routers use routing protocols (e.g., OSPF, BGP, RIP) to share information about network topology and build optimal routing tables.

Why Packets May Take Different Routes

• Network congestion: A router may choose a less congested path.
• Link failure: If a link goes down, packets are rerouted around it.
• Load balancing: Traffic is distributed across multiple paths.
• Dynamic routing: Routing tables are updated in real time as network conditions change.

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Reassembly at the Destination

When packets arrive at the destination: