OSI Layer 1 conveys the bit stream - electrical impulses, light, or radio signals - through the network at the electrical and mechanical level. In doing so, it provides the hardware means of sending and recieving data. This includes defining:
- Cables
- Interface cards
- Physical aspects
Ethernet
Ethernet LAN connections can be carried over different cable types coaxial cables, twister copper pair, fiber cable, or wireless. The most common type of cable is Copper Unshielded Twister Pair (Copper UTP). The connector type is RJ45.
UTP Cable Categories
| Category | Max Bandwith |
|---|---|
| Cat 5 | 100 Mbps |
| Cat 5e | 1 Gbps |
| Cat 6 | 10 Gbps |
| Cat 6a | 10 Gbps |
| Cat 7 | 10 Gbps |
| Cat 8 | 40 Gbps |
Straight-Through vs Crossover UTP Cable
The more common type of cable is straight-through cables, which are used to connect end devices to other network elements - such as a PC or router to a switch.
On the other hand, crossover cables are used to connect devices directly, commonly done for devices of the same type. For instance, in a pinch you could connect to separate PCs with a crossover cable to transfer files (through obviously not often done these days). The more realistic scenario is connecting switches to each other via crossover cables.
Note
Modern switches support Auto MDI-X, where the RX and TX signals can be reconfigured automatically; this means we don’t really need to worry about the type of UTP cable we need to use.
Fiber cables
Fiber optic cables can be used for higher length requirements or higher bandwidth requirements; for example, they may be used between separate buildings within a campus, or for switch to switch connections within a building.
Single Mode vs Multi-mode fibre
There are two main types of fibre cable: single mode supports higher bandwidth and distance, but is more expensive. In particular, multi-mode fibre can support several hundred meter cabling, while single-mode can do kilometers.
Note
There are many types of fiber connectors. Often, fibre connectors are connected into a transceiver that itself then uses RJ45 to connect to end devices.