By KailinAchesonIn Part 1 of The Seven Layers of Networking, John started a discussion about the layers of Open Systems Interconnection’s (OSI). You can read about the first four layers in that blog: the Physical layer (Data Link layer), the Network layer (Network layer), and the Transport layer (Transport layer). The first four layers are the most common in network discussions. Although not as often discussed, the Session, Presentation, and Application layers are essential for a network. This blog will discuss these layers and the encapsulation process which occurs when one host communicates to another. Then you’ll have to stay tuned for Part III, which will cover a related-yet-different layered model, the Transmission Control Protocol/Internet Protocol (TCP/IP) model.
Layered models, such as the OSI model, are used to make data networks more efficient. Each layer is dedicated to a specific set of functions. This framework helps us analyze, discuss, and simplify communication. Each layer acquires services from the layers below and exposes a consistent interface to the layer above in order to facilitate vendor interoperability.
Figure 1 shows the seven OSI layers. It represents, at its most basic level, one computer communicating to another.
Figure 1 – The OSI Model
Here’s a summary of the four layers that were discussed in the previous blog.
The Physical layer, also known as Layer 1, is what can be seen and measured in physical world. A Layer 1 problem could be due to cables or connectors linking devices together. The Protocol Data Unit (PDU), which is what each layer refers to, is the binary bit (bit) at Layer 1.
Layer 2, or Data Link, deals with Media Access Control addresses and data movement within the same network. A Layer 2 problem could be due to configuration errors like link speed or duplex setting. The frame is the PDU at Layer 2.
Layer 3, or Network, deals with Internet Protocol addresses and data movement from one network to the next. A Layer 3 problem could be due to router configurations or errors in IP addresses. The Layer 3 PDU is the packet.
The Transport layer (or Layer 4) provides services to applications at higher levels. It can either provide connection-oriented communication, or connectionless communication. Layer 4 problems could be caused by access control lists (ACLs), firewalls blocking or redirecting ports required by a service. PDUs in Layer 4 are the segment (for connections-oriented communications) or the datagram (for connectionsless communications).
Let’s now discuss the last three layers. I won’t discuss each layer in detail as Part I of this series. Instead, I will give a brief overview of each layer so that you can better understand what they mean next time someone says Device X is at Layer Y or that a particular problem is a Layer Z issue.
The Session Layer
The Session layer manages the establishment, maintenance, and termination of communication sessions between applications and devices. These sessions can include service requests and replies, which can be sent back-and-forth between two parties. The Session Layer, also known by Layer 5, is home to several protocols. These protocols include the Session Control Protocol (SCP), Remote Procedure Call(RPC) and Zone Information Protocols (ZIP).
The Presentation Layer
The Presentation layer ensures that data that is transmitted between devices can be readable by both devices.