Layers and Numbers

Why do techies talk about layer numbers like, “That’s a layer2 problem or a layer 3 problem”?

The short answer is maybe they expect everyone to know what they know. So let me share what they know about the layers, the numbers, and what happens at each layer.

In the last entry, I gave you some generic layers and the standards that apply to them. Here we’ll talk about the most accepted model for explaining layers: ISO’s Open Systems Interconnect (OSI) model. Let’s start at the bottom and work our way up the layers.

Layer 1 is the Physical layer. It includes the cabling, connectors, wall jacks, and interfaces in the equipment from computers to switches to routers in wired networks as well as the radios, lasers, satellites, dishes, and microwaves used in wireless networks. It supports physical connections without addresses.

Layer 2 is called the Data Link Layer. It links the Physical layer to the Network layer for sending and receiving frames of data. To do that, it uses a Logical Link Control (LLC) functions and Media Access Control (MAC) addresses to identify the source and target devices on the LAN.

The Network Interface Card (NIC) in your computer has a unique burned in MAC or physical address that the switch uses to connect it to the rest of the local network devices. Layer 2 has error detection to keep from wasting the rest of the stack’s time with bad frames. MAC addresses only work on a local network.

The Network Layer is the busiest layer in the model. Layer 3 includes logical addresses to identify the network, subnet, and interface of the source and target of the datagrams being sent and received beyond the LAN into the rest of the Internet. It also uses those logical addresses to route packets through all networks.

There are other protocols at the Network layer to do network diagnostics and identify logical errors. There is also a protocol to match the destination logical address with the target system’s MAC or physical address. Basic security also happens here including packet filtering and access control.

Layer 3’s main device is a router, though, except for routing, the things the network layer does also happen in computers, servers, smart phones, tablets, and other “end devices” which are the senders and receivers of network communications.

Layer 4 or the Transport layer’s task is to get the data from one end device or host to another. It identifies the application and a return socket, detects errors, separates the message into pieces small enough for the application and the network to handle, and makes it possible to have more than one session over one physical link.

The Session layer picks up where the Transport Layer left off by offering logging on and logging out of a network application. Layer 5 works with an application to setup, manage, and shut down a virtual connection. It acknowledges data received and retransmits data as needed. Some protocols combine the Transport and Session layer functions.

Layer 6 is officially known as the Presentation Layer. It works with the application to format the data before sending it down the stack to go out onto the network. This includes enciphering and deciphering for security, compression and decompression for efficiency, graphics formatting, and any format translation to make it possible for different systems to understand the data.

The Application Layer provides a way for the end user to work with the network application. Layer 7 is what the user sees and how users enter data or destinations for applications like file transfers, network printing, messaging, Web browsing, and email.

Some people go beyond the official OSI model. They add some layers of their own that, they believe, have an effect on network applications.

  • Layer 8 — Office Politics
  • Layer 9 — IT Department
  • Layer 10 — Users

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