The first four layers of the OSI model (bottom-up) handle everything from raw bit transmission up to end-to-end delivery:

1. Physical Layer

• Transmits raw bits over a physical medium (copper, fiber, wireless).
• Defines mechanical/electrical specs: voltage levels, cable/connector types, pin layouts.
• Handles bit synchronization, line coding, and data rate (bits/sec).
• Defines transmission mode: simplex, half-duplex, full-duplex.

2. Data Link Layer

• Organizes raw bits into frames for transmission.
• Physical (MAC) addressing — attaches sender/receiver hardware addresses to frames.
• Flow control — prevents a fast sender from overwhelming a slow receiver.
• Error control — detects (and sometimes corrects) bit errors using checksums/CRC.
• Access control — determines which device gets to use the shared medium at a given time (via MAC sub-layer protocols like CSMA/CD).

3. Network Layer

• Logical addressing — assigns and uses IP addresses, independent of physical hardware.
• Routing — determines the best path for packets across multiple interconnected networks.
• Packet forwarding — moves packets from source to destination, hop by hop, through routers.
• Fragmentation — splits packets to fit within a network segment's Maximum Transfer Unit (MTU), reassembling at the destination.

4. Transport Layer

• Process-to-process delivery — uses port numbers to deliver data to the correct application/process on the destination host, not just the correct host.
• Segmentation and reassembly — splits data into segments at the sender, reassembles in order at the receiver.
• Connection control — connection-oriented (TCP) or connectionless (UDP) service.
• Flow and congestion control — manages the rate of data transfer to avoid overwhelming the receiver or the network.