In data networking, you might have come across this term pretty often, the 7 layers of OSI model (Open Systems Interconnection Model). Although there’s tons of articles out there describing these layers in detail, I’m still struggling to understand (and even memorize) what these 7 layers does exactly.
Let’s see how we can view these 7 layers with 7 pictures to help us understand.
Overview of the OSI Model
Before diving into each layer, let’s first try to understand what the OSI model is about.
The OSI model was created by the ISO (International Standards Organization), composed of members from national standards organizations of many countries. It was patterned after the IBM layered network scheme SNA (Systems Network Architecture) introduced in 1974.
Why OSI Model?
The OSI model is particularly popular because it is simpler and more elegant than SNA.
For comparison, it looks something like this:
Because this standard is open, everyone from network card manufacturers to application developers just need to adhere to this model to allow interconnection between networks. That’s the power of open.
Why Layers?
Layers serve as a convenient and powerful way to define and model communication networks. The layered approach provides a clear definition of the software requirements for each layer.
The software at each level can be developed and implemented independently. Changes of functions or protocols on one layer do not require changes on the other layers, which makes upgrading of services to meet future requirements possible.
The downside of this approach however, is performance penalty.
How It Works
In general, the higher the level, the less components need to care how its data is being transmitted over the network.
Layer 7, the applications layer, such as web browsers and email clients, would only be concerned with presentation of data to end users, for example look-and-feel, data presentation and interaction. Think UI/UX.
While Layer 1, the physical layer, defines the electrical and physical specifications for transmitting signals over a physical medium.
Layer 1: The Physcial Layer
Layer 1 describes the transmission protocols that bits, 1s and 0s, are represented and flows across physical media. It defines electrical interface specifications in attaching devices to networks.
Existing standards are: EIA-232-C, V.35 and X.21.
Layer 2: Data Link
As bits passes across cables, known as a link, the Data Link Layer encapsulates this information by placing a header and trailer around the data, also known as a frame.
Frame headers container information, such as the source and destination address, so that it can be used by the next layer up to route to the next stop in the network.
Common data link protocols are:
- HDLC — High-Level Data Link Country
- SDLC — Synchronous Data Link Country
- ISDN — Link Access Procedure on the D Channel
- LAN — Local area network
- WAN — wide area network
Layer 3: The Network Layer
The network layer handles operations associated with routing a packet of information from one node to another in a network. It might also perform fragmentation, reassembly, and reporting of delivery errors.
Common network protocols are: X.25, IP, and IPX (Internetwork Package Exchange).
Layer 4: The Transport Layer
The main job of the transport layer is to manage connections for specific processes witin a node, including establising and releasing connections, flow control and buffering. It includes negotiation of quality of service (QOS).
Common transaport layer protocols are: TCP, UDP, DCCP, and SCTP.
Layer 5: The Session Layer
The session layer controls the flow of conversation between applications talking to each other over a network.
Serveral sessions and take place during a connection. A session can require several connections to take place.
Common session layer protocols are: NetBIOS, PPTP, RPC
Layer 6: The Presentation Layer
The Presentation Layer provides services that related to the way data is represented, including data repsentation, data security such as encryption and decryption, and data compression.
Some protocols used here can be: MIME, SSL, TLS, XDR.
Standard data encoding techniques can be: ASCII or EBCDIC.
Layer 7: The Application Layer
You will definitely know this — all applications must have their own protocols to communicate across a network.
Web browsers use HTTP and HTTPS protocols, email clients use SMTP and POP protocols, and file transfer applications uses FTP protocol. A HTTP server is also called a web server.
In Summary
Layers 5 through 7 (the Session, Presentation, and Application layers) provide capabilities to allow applications to share data and communicate with over a network.
Each layer encapsulates their own headers as the data is passed down from the layer above, and then transmitted over a network.
On the receiving end, the data are decapulated, passing the data up to the next layer.
Each layer utilizes one of the many protocols as a standardization scheme for treating data presented to it.
