Technical definition
The OSI Physical layer is layer 1 of the OSI reference model. It defines the mechanical, electrical, functional and procedural characteristics used to establish physical connections and transmit bits through a communication medium.
In plain English: Turns bits into signals and back: voltages on copper, light in fibre, radio waves in the air. Defines cables, connectors and speeds.
What this layer does
A bit is a binary value, 0 or 1. Physical signalling rules describe how those values travel and how a receiver recognises them. Both ends need compatible media and signalling. Full duplex means both ends can send and receive at the same time; half duplex means they take turns.
Legacy shared, half-duplex Ethernet used CSMA/CD to handle collisions. Modern full-duplex Ethernet does not use that collision-detection method. A hub repeats signals rather than choosing a destination from a MAC address table.
A working physical link is necessary, but its link light does not prove that IP settings or the application are correct.
Follow one request from PC-1
PC-1 is a user's computer opening a page on the lab web server. Its IPv4 address is 192.168.1.20/24 and its gateway is R1 at 192.168.1.1. The server is on another subnet, at 203.0.113.20/24, with gateway 203.0.113.1. The /24 means that the first 24 bits identify each subnet. R1 joins the two subnets.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
In this request: PC-1 sends Ethernet signals along its cable to R1 Gi0/0. R1's other Ethernet link carries signals to the server. Both physical links are needed for this request.
The interface labels identify the ends of the cables: Eth0 is the example Ethernet interface on each host, while Gi0/0 and Gi0/1 are R1's two Gigabit Ethernet interfaces. Actual device names vary. An IP address identifies an interface; TCP port 443 identifies the HTTPS service endpoint. It is not a physical socket on the router.
Where it fits in the other model
The four-layer TCP/IP model groups Physical and Data Link functions into its Link or Network Access layer. A common five-layer teaching view separates them again.
The trade-off is between a more detailed vocabulary and a simpler description of deployed protocols. OSI names seven responsibilities; the four-layer TCP/IP model groups some of them together. Neither view requires a separate header for every named responsibility.
When it does not work
If a wired link is down, check the cable, the connected switch port and whether the adapter is enabled. Fibre links also need compatible optics and correct transmit/receive connections. A high negotiated speed does not guarantee that applications get that throughput. Physical impairments can cause errors even while a link remains up, so read interface error counters when symptoms are intermittent.
Try a practical check
Use PowerShell on Windows for this observation exercise. These commands inspect your own connection or request a public example service; they do not configure R1 or contact the documentation server in the figure.
Get-NetAdapter | Format-Table Name, Status, LinkSpeedOn Windows PowerShell, identify the adapter you are actually using. Status and LinkSpeed describe its reported connection. Virtual adapters can also appear. An Up status is useful starting evidence, but it does not verify the IP address, gateway, DNS or web application. Do not unplug a shared connection as part of this observation exercise.
Your task: identify one useful field in the result, state what it shows, then name something it cannot prove. If the command is unavailable or a request is blocked, record that limitation rather than treating it as proof of a failed layer.
Check your understanding
The link light is on and the adapter reports Up. Is a website guaranteed to open?