detailed discussion of network technology would be incomplete without a shared understanding of the Open Systems Interconnection (OSI) model [1]. This is an often used conceptual model which allows us to create highly interoperable, open, and scalable communication systems by categorising the technological functions required by the network into a stack of layers that are numbered from 1 to 7, each of which has a set of interfaces to communicate with its directly adjacent layers (see Table 3.1).
This model is very powerful, as by isolating a set of technological functions into a specific layer that has interfaces to talk to the layers it is directly adjacent to, changes to any one specific layer do not need to impact the operation of any other layer in the system, allowing asynchronous evolution of the entire stack of network technologies where one or more layers experience more rapid change than their neighbouring layers. An example of this can be seen with each new generation of Wi‐Fi; significant advances in speed can be achieved by changing only layers 1 and 2, without any of the upper layers being aware of the change. Consider what the situation would be like if the entire network technology stack had to be remade to accommodate a change at any layer. The stack that would result would be highly inflexible as even an isolated change would require significant work. Over time, this would become a key barrier to keeping up with the edge of technological progress and prevent open contribution to the stack by other companies or individuals, limiting innovation.
Now that the reasoning behind the OSI model has been established, we will briefly describe the functionality of each layer as it is relevant to infrastructure edge computing, and the number of each layer will be used throughout this book to quickly refer to the concepts that they represent. In this example, we will be taking the perspective of a network endpoint receiving data that has been transmitted across the network, so our progression will be from layer 1 through layer 7. When considering the process of sending data, this progress through the layers is reversed in order as data flows from layer 7 down to layer 1 to be transmitted across the physical network.
Table 3.1 OSI model layer numbers, names, and examples.
| OSI layer number | OSI layer name | Example entity or protocol |
|---|---|---|
| 1 | Physical | Copper cabling |
| 2 | Data link | Ethernet |
| 3 | Network | IPv4 |
| 4 | Transport | TCP |
| 5 | Session | NFS |
| 6 | Presentation | PNG |
| 7 | Application | HTTP |
3.3.1 Layer 1
From a physical perspective, the first layer of the OSI model is at the bottom, layer 1. This layer is known as the physical layer and is responsible for encapsulating the functions of the network which are concerned with the physical transmission medium that is being used to send and receive data, whether that be radio frequency (RF), in the case of many wireless networks, copper cabling, fibre optic cabling, or another medium of choice. Although these three options are the most commonly used today, other options such as infrared (IR) light exist and are used in some specific use cases.
As well as the physical transmission medium itself, layer 1 includes the functions that are needed to encode and decode all data transmissions using that medium. For example, to send and receive data across copper cabling, an encoding scheme must be used to convert some digital application data into a sequence of physical electrical energy, which is then applied to the copper transmission medium, which carries those electrical signals to the remote end of the transmission medium where a corresponding decoding scheme reverses the process, turning these electrical signals back into the digital application data which they represented and passing this data up to layer 2 of the stack.
3.3.2 Layer 2
Layer 2 is known as the data link layer and is responsible for a few key functions of the network stack. One of these is medium access control (MAC), which is the process of determining the means by which a network endpoint may access the transmission medium to send data. In many networks, this is not as simple as merely transmitting immediately whenever layer 3 communicates to layer 2 that it has data to send. Consider a wireless network as an example: The radio spectrum used by these networks is a shared medium, where transmissions from one network endpoint are able to be received by many other network endpoints. If one endpoint were to transmit without checking to see if the “coast is clear” and transmitted at the same time as another endpoint, it is likely that both transmissions would be garbled, resulting in data being lost or being retransmitted. This is to be avoided wherever possible as it reduces the efficiency of the network considerably.
Protocols operating at layer 2 are also responsible for providing link local addressing, a form of network endpoint identification where endpoint interfaces are given locally unique identifiers that enable intranetwork communication but which are not intended to be used as globally unique; this means that they are not suitable for use between networks but can uniquely identify endpoints on the same network. Through various means such as broadcast and multicast data transmission, these link local addresses are often used to allow an endpoint to discover the globally unique addresses of the endpoints it needs to communicate with, or at least that of the next hop in the path to get there.
3.3.3 Layer 3
Referred to as the network layer, layer 3 is the only layer of the OSI model which is responsible for internetwork addressing, where network endpoints can be assigned globally unique addresses. This is crucial as without this function, endpoints that are located in other networks would not be reachable and the internet (as a network of networks) would not be able to exist as it does today.
The globally unique addresses which layer 3 provides are mapped on to the link local addresses that are used by protocols operating at layer 2 by a process that is specific to each of those lower layer protocols. This ensures that when data sent from an endpoint on an external network targeted for an endpoint on the local network enters the local network, it can be correctly forwarded using its link local address (such as a MAC address in the case of Ethernet) as if that traffic had originated on the local network to begin with. The external network endpoint is unaware of this process entirely.
3.3.4 Layer 4
Layer 4 is named the transport layer. This label may seem odd, but it is for good reason: Above the means to transcode digital data into physical signals at layer 1, the MAC functions and link local addressing provided by layer 2, and the globally unique endpoint addressing capabilities enabled by layer 3, there is a need for a group of functions which determine how to send and receive data on the network at a layer of abstraction, which enables them to perform tasks such as flow control and integrity checking of any data received without reliance upon any specific set of layer 1, 2, or 3 technologies. This transport layer is the bridge between the application at higher layers of the stack and the underlying network technologies below, whatever they may be.
Two
