The Lower OSI Layers
- Whereas network engineers are not particularly interested in the upper OSI layers, we are very concerns with the lower 4 layers of the OSI model.
- Each of these layers have their own dedicated section later and you will learn much more detailed information about them throughout this tutorial.
Layer 4 - The Transport Layer
We will begin our discussion with Layer 4 (L4), known as the Transport Layer. This layer is primarily defined by the choice between TCP and UDP for data transport, as well as the associated port number. In scenarios where reliable communications between two hosts is essential, TCP will be the preferred protocol of choice.
Conversely, if the priority is on speed rather than reliability, such as in the case of voice or video traffic, UDP will be utilized. Another significant aspect of this layer is the port number; for instance, port 80 is designated for HTTP web traffic, while port 25 is used for SMTP email communications.
- The main characteristics of the Transport Layer are whether TCP or UDP transport is used, and the port number.
- Definition:
- The transport layer defines services to segment, transfer, and reassemble the data for individual communications between the end devices.
- It breaks down large files into smaller segments that are less likely to incur transmission problems.
Layer 4, known as the Transport Layer, is responsible for providing services that segment, transfer, and reassemble data for distinct communications between end devices. This layer divides large files into smaller segments, which reduces the likelihood of transmission issues.
Layer 3 - The Network Layer
Following Layer 4, is Layer 3 (L3), referred to as the Network Layer. The primary focus at this layer is the source and destination IP addresses. Additionally, the Layer 3 header contains a significant amount of other information.
- The most important information at the Network Layer is the source and destination IP address.
- Routers operate at Layer 3.
- Definition:
- The Network Layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.
- The Network Layer is the layer that manages the connectivity of hosts by providing logical addressing.
- The Network Layer is the layer that manages the connectivity of hosts by providing logical addressing.
Routers function as devices operating at Layer 3: the Network Layer (which can also be shortened simply tp: 'L3') of the OSI model. This layer facilitates connectivity and determines the most optimal path for communications between two host system endpoints that may reside on distinct geographical networks. The Network Layer is responsible for managing host connectivity through the provision of logical addressing, with IP addressing serving as the primary form of this logical addressing.
Layer 2, referred to as the Data Link Layer, follows as the next level in the OSI model. The critical information at this layer pertains to the source and destination Layer 2 addresses. Similar to Layers 3 and 4, the Layer 2 header also encompasses additional information.
- The most important information at the Data Link Layer is the source and destination Layer 2 address.
- For example the source and destination MAC address if Ethernet is the Layer 2 technology.
- Switches operate at Layer 2.
- Definition:
- The Data Link Layer defines how data is formatted for transmission and how access to physical media is controlled.
- It also typically includes error detection and correction to ensure a reliable delivery of the data.
1. For instance, in the context of Ethernet as the Layer 2 technology, the source and destination MAC addresses are utilized. Various Layer 2 technologies employ distinct formats for their addressing schemes. An example of this is the older Frame Relay technology, which utilizes DLCI or DLCI numbers for addressing purposes.
In Ethernet, which is the predominant technology in our Local Area Networks, the MAC address serves as the identifier. Switches function at Layer 2, and our switches are devices that are aware of this layer.
The Data-Link Layer is defined by its role in specifying how data is formatted for transmission and how access to the physical medium is managed. It generally encompasses mechanisms for error detection and correction to guarantee the reliable delivery of data.
Layer 1, known as the Physical Layer, pertains to the tangible components of the network. This includes the actual physical cables employed. The Physical Layer facilitates the transmission of bits, represented as 1s and 0s, between end devices.
It outlines the specifications necessary for the activation, maintenance, and deactivation of the physical link between end devices, including aspects such as voltage levels, physical data rates, maximum transmission distances, and the types of physical connectors used.
- The Physical Layer concerns physical components of the network, for example the cables being used.
- definition: The physical link enables bit transmission between end-devices.
- It defines specifications needed for activating, maintaining, and deactivating the physical link between end-devices.
- For example, voltage levels, physical data rates, maximum transmission distances, physical connectors, and more.
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