Physical
Layer 1 (Physical) specifications define the transmission and reception of RAW BIT STREAMS between a deice and a SHARED physical medium
- simply transmits any data it receives onto the physical medium
- defines things like voltage lvls, timing, rates, distances, modulation, and connectors
- both devices use this physical medium to send and receive raw data
- The physics of the network
- signaling, cabling, connectors
- physical medium can be copper (electrical), fibre (light) or WIFI (RF)
- No concept of protocols, addresses, or error handling.
- Operates purely on physics
- 1 broadcast and 1 collision domain
- not scaled very well, the more devices, the higher chance of collisions and data corruption
- for this to be useful, we need to add layer 2 which works on top of a working layer 1 connection
Responsibilities
- Signaling: Voltage levels, light pulses, RF signals.
- Cabling: Copper, fiber, wireless.
- Connectors: RJ-45, LC, etc.
- Modulation: How bits are represented physically.
Common Issues (“Physical Layer Problems”)
- Bad cabling/punch-downs, faulty connectors/adapters
- Fix: Loopback tests, cable replacement, NIC swaps
Examples
Direct Connection (Point-to-Point)
- diagram
- connecting 2 laptops together in a LAN with a copper network cable, carried unstructured information
- a point-to-point electrical shared medium between these 2 devices (between the 2 network interface cards)
Hub (Multi-Device Shared Medium)
- diagram
- HUB with 4 ports.
- No addresses (Layer 2 needed for that).
- Collisions are a problem
- One device transmits at a time; otherwise, collisions.
- Everything received on one port is retransmitted to all others (including errors/collisions).
- collisions are guaranteed, and it cannot even detect when collisions occur, as this layer just transmits voltage via shared medium