Sending an email may seem simple, but it involves a complex journey through the OSI (Open Systems Interconnection) model. In this article, I will explain how data travels through each OSI model layer when Alice, a user, sends an email to Bob using an email client.

Visual Overview of OSI model

Scenario: Sending an Email

Alice wants to send an email to Bob using her email client.

The travel of data flow

1. Application Layer (Layer 7)

Role: Interacts with the user’s software.

• Alice types her email message in an email client (e.g., Gmail or Outlook).

• The email client uses an Application Layer protocol, such as SMTP (Simple Mail Transfer Protocol), to format the email.

• Data at this stage: Human-readable email text and metadata, such as the recipient’s email address.

2. Presentation Layer (Layer 6)

Role: Data formatting, encryption, and compression.

• The email client encodes the message in a standard format, such as ASCII or Unicode.

• If encryption is enabled, the email is encrypted using protocols like SSL/TLS to ensure security.

• Data at this stage: Encoded and possibly encrypted email data.

3. Session Layer (Layer 5)

Role: Manages communication sessions.

• A session is established between Alice’s email client and her email server.

• Protocols like SMTP, IMAP, or POP3 ensure reliable communication.

• Data at this stage: Session data and control information.

4. Transport Layer (Layer 4)

Role: Ensures reliable delivery of data.

• The Transport Layer breaks the email message into smaller packets for transmission.

• TCP (Transmission Control Protocol) ensures that all packets are delivered correctly and in the right order.

• Data at this stage: Packets with TCP headers containing sequence numbers and error-checking information.

5. Network Layer (Layer 3)

Role: Determines the best route for data.

• Each packet is assigned a source and destination IP address (e.g., Alice's computer and Bob's email server).

• Routers use these IP addresses to forward packets to the correct destination.

• Protocol used: IP (Internet Protocol).

• Data at this stage: Packets with IP headers containing source and destination addresses.

6. Data Link Layer (Layer 2)

Role: Handles physical addressing and prepares data for the physical network.

• Packets are framed with MAC (Media Access Control) addresses for the local network (e.g., Alice’s Wi-Fi router).

• Error detection mechanisms, such as CRC (Cyclic Redundancy Check), are added.

• Data at this stage: Frames containing MAC addresses and error-checking information.

7. Physical Layer (Layer 1)

Role: Transmits raw bits over the physical medium.

• The email data is converted into electrical signals, light pulses, or radio waves, depending on the transmission medium (e.g., Ethernet cable, fiber optic cable, or Wi-Fi).

• These raw bits travel through the network to Bob’s email server.

• Data at this stage: Raw binary bits (0s and 1s).

Reception at Bob’s End - The Reverse Journey

The process is very simple. It's just the reverse process of Alice. This process is done at Bob’s email server.

Physical Layer: Bits are received and converted back into frames.

Data Link Layer: Frames are checked for errors and passed up.

Network Layer: IP addresses are used to verify the destination.

Transport Layer: Packets are reassembled into the complete email message.

Session Layer: The session is established for Bob’s email client to retrieve the message.

Presentation Layer: Data is decrypted (if encrypted) and formatted for display.

Application Layer: The email client displays the email to Bob.

Conclusion

The OSI model ensures data gets from point A to point B smoothly. It adds and removes layers of information like wrapping and unwrapping a gift, making sure Alice’s email makes it to Bob securely and in one piece.