Advantages of Using Networks
Network Classifications
Ethernet & Network Layers
Networks are very complicated in the way they work. There are physical parts to them, such as the cables and wires, and there is also software. To make the process simpler a standard was created which splits up the different functions of a network into layers.
To transport data across a large network, such as a WAN, the data may travel across lots of different connections. These connections are called links because they ‘link’ the devices together. Each type of link has a different frame structure, and uses different addresses and protocols.
The process of encapsulating data, transporting it across a link, and decapsulating at the end of the link, is repeated time and time again as the data makes its journey.
Imagine a world in which we send letters by placing them in different kinds of envelopes for different transportation systems (i.e. types of links); the addresses we write on the envelopes only get the envelope across a particular section of the transportation system.
The transport system between houses and post offices uses white envelopes. The only things that can be written on white envelopes is a house number, a street, and the name of a local post office.
The transport system between post offices use red envelopes. The only things that can be written on red envelopes are the names of post offices.
At each stage, the envelope is in the correct wrapper for that section of the journey and just has the local addresses to get it across that section or link.
Note that unlike what it looks like here, the post offices don’t get to read the contents of our letter - more on this later in the course.
How does this connect to real world networks? How do we transport data across one link?
We refer to the set of processes above as part of the data link layer. This is a combination of software and hardware built into the network interface card (NIC) or network adapter. A modern computer typically has three separate data link layer for Ethernet, wireless, and Bluetooth, and switches in the appropriate layer as needed.
In the part of a device’s operating system that deals with networking, the data link layer sits between the network layer and the physical connectors that connect the device to the transmission media.
You may be imagining that the data link layer on one device communicates directly with the data link layer on another device. However, the connection is via the physical media.
The final part of the data link layer converts the bits of the frames into the electrical, wireless, or optical signals that are sent along the link medium.
When the frame is received, the receiving device’s data link layer decapsulates the data from the frame and passes it up to the receiver’s network layer.
We say that the data link layer is providing a service to the network layer by transporting its data in frames across the link.
We can now start to build our layered model from the bottom up.
You can see that the data link layer is just the first, or bottom, layer of a layered model called the TCP/IP model. TCP stands for Transmission Control Protocol, and IP stands for Internet Protocol. They both are foundational telecommunications protocols
The TCP/IP model allows new link technologies to be developed and integrated into our networks without having to scrap everything and start again. All we’d need to do is use updated NICs in our devices. (As NICs are often embedded, we end up changing the devices anyway.)
Thanks to this model, we didn’t have to shut down the internet and build a new one when new wireless standards or 3G/4G came out.
In this step, you’ll learn more about IP packets and how they are transported in frames. This is visualised as the next layer in the TCP/IP model.
Last week, we looked at how frames operate at the data link layer, particularly at how Ethernet frames transport data.
To transport IP packets across all the various links, there are two theoretical options:
Option 1 would require a vast amount of redesign that would make much of the existing network infrastructure redundant — option 2 is the logical choice.
So the data transmitted in data link frames (such as Ethernet frames) is IP packets. These IP packets contain (encapsulate) the actual data that one device is sending to another.
It’s like an envelope inside an envelope: double encapsulation! (The attached activity can help you and your students visualise this)
So in the same way that the data link layer encapsulates data from the network layer, the network layer encapsulates data from another Layer. This ‘higher’ layer is called the transport layer, which you’ll learn about later in the course.
NOTE: the network layer is sometimes referred to as the internet layer. We will use the term network layer here.
In the same way the data link layer provides a service to the network layer by transporting its data in frames, the network layer also provides a service to the transport layer by transporting its data.
One device’s transport layer wants to send some data to the equivalent transport layer on a distant device.
The transport layer uses the services of the network layer to transport the data inside IP packets.
The network layer uses the services of the data link layer to transport the IP packets as frames across the local links. In practice this involves intermediary devices (e.g. routers) that check each IP packet’s destination address and choose the next link.
The data link layers connect to the media and transmit the frames as bits encoded into electrical, optical, or electromagnetic (wireless) signals. If the media is Ethernet, a switch will be involved.
The transport layer identifies the type of data being transported. It offers two protocols, depending on the need for reliable or unreliable delivery. It is the connection between the network and application layers.
Reliable delivery uses the transmission control protocol (TCP) and creates TCP segments. It’s typically used when re-transmission delays are acceptable. TCP is a connection-orientated protocol, meaning the two ends require a connection to keep track of the numbers of each sent IP packet.
Unreliable delivery uses the User Datagram Protocol (UDP) and creates UDP datagrams. It’s typically used for real-time audio and video. UDP is a connectionless protocol, meaning data is sent to the destination with no additional dialogue.
The top layer of the TCP/IP stack is not the transport layer, where TCP resides, it’s the application layer.
Protocols such as HTTP, FTP, SMTP, POP, and IMAP are all application layer protocols.
The application uses the services of the TCP/IP model to encapsulate its message in various layers of protocols. Ultimately, the data is transferred over the media to the internet.