The Physical Layer : How Machines Are Connected

Now that we have learned about the layers that make up the OSI model, let’s go ahead and discuss each of them separately. In this chapter, we’ll learn about the first layer: The physical layer.

Physical Layer Components

As we saw in the last chapter, the physical layer is the one responsible for physically transporting data from the source to the destination machine. When the physical layer receives data from the layer above it (Data link layer), it makes sure to convert the bits into the proper form so that they can be sent through the physical transmission medium. The component responsible for this is called Network Interface Card, or simply NIC.

Network Interface Card (NIC)

The NIC is a hardware component that is present in all computers. It is the interface that connects a computer to a network. It looks something like this:

Network Interface Card (NIC)

There are generally two types of NICs that are present in almost all modern computers:

– Ethernet NIC: It connects the computer to an Ethernet cable (We’ll talk about Ethernet in the next chapter. Don’t worry about this for the moment). This type of NICs generates electrical signals that represent data which are then sent through the cable. It converts the binary 1s and 0s into voltage.

– Wireless NIC: This type of card connects the computer to a wireless network (e.g. Wi-fi). It converts the data to a radio signal using an antenna.

Transmission medium

The transmission medium is the support that carries data when it leaves the sending machine. There are many types of transmission mediums, we will only address here the most common ones.

Twisted Pair Cables

The twisted pair cable contains eight wires twisted in pairs.

Twisted Pair Cable

These are the most common cables. You will find them mainly in Local Area Networks. They have the advantage of being cheaper than other types of cables and flexible to use and maintain.

On the other hand, they have a high signal attenuation which impacts the maximum distance that they can cover between two connected points. That’s why, for long distances, we use fiber optic cables.

Fiber Optic Cables

A fiber optic cable comprises a thin strand of glass protected inside a jacket.

Fiber Optic Cable

Unlike the twisted pair cable, fiber optics transmit bits in the form of light. 0 corresponds to no light and 1 corresponds to the presence of light. The fact that it uses light makes fiber optics the fastest medium for data transmission.

Fiber optics also have their own limitations. They are more expensive, less flexible and not so easy to maintain.

One thing to remember. If you have to design a network, make sure that you use twisted pair cables for short distances, and fiber optics for longer distances. Generally, the accepted threshold for using twisted pair cables is 100 meters.

Unguided transmission

Unguided transmission is just a fancy way to say wireless transmission. Data is transmitted in the form of radio waves, and does not require any physical medium. The most common wireless transmission standards are the Wi-Fi family of protocols.


While cables are a good way to connect devices, they are still limited in that they only provide point-to-point connections. A single cable can only link two machines. Designing a large network using only cables would be an impossible task. Just imagine how that would be for a network as large as the Internet.

One way to solve this problem is using a device that provide point-to-multipoint connectivity, allowing multiple machines to be connected at once. One such device is the Hub.

Hub - Physical layer device
Network Hub

A Hub (As shown in the picture above) is a simple device that has multiple ports. Each port can connect to a single machine. This way, all connected machines act as if they share the same cable.

Although a hub provides the advantage of connecting many machines on the same network segment, it still has some drawbacks.

Hubs are “dumb”. This is because whenever a hub receives a message from a port, it sends it back to all other connected machines. This is called a broadcast. With the high number of broadcasts, the quality of network diminishes and no machine is able to talk to an other machine without having to go through a collision.

A collision is when two machines try to talk at the same time, and none is able to do so. All machines that are connected to the hub are in the same collision domain. This is not good, especially if the number of connected machines is high.

This is why you won’t find many hubs in today’s networks. We now use the switch instead. But we don’t need to get ahead of ourselves now, we’ll talk about the switch in the next chapter.

Network Topologies

A Network Topology is the way machines are connected within a single network. There are four main network topologies:

  • Bus
  • Ring
  • Star
  • Mesh

Bus Topology

In the bus topology, all devices connect to a single cable

Bus Topology

Ring Topology

In the ring topology, all devices connect to a single cable, but this cable is connected to itself to form a circle.

Ring Topology

Star Topology

All devices are connected to a single device placed in the center.

Star Topology

Mesh Topology

There are two types of mesh:

  • Full Mesh : Every device is connected to all other devices.
  • Partial Mesh: Every device is connected to some of the other devices.
Full Mesh Topology

Now that we know how to physically connect machines, let’s go ahead and start sending some data between them. In the next chapter, we will discuss the data-link layer.


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