OSI – Open System Interconnection Networking Model
In a land a long time ago, actually in the 1970′s computer networks and devices had a hard time communicating with each other. Before the OSI model or the Open System Interconnection networking model was ever created there was only proprietary networking architectures such as IBM’s Systems Network Architecture (SNA) networking model. This meant that major computer vendors controlled the network market, since they produced and sold more devices.
In order for a brand new company to allow their devices to be able to talk to each other, they would either have to create their own network model, or lease a proprietary model from a company such as IBM. A network model is nothing more than a set of rules that break up the process of being able to send information from one computer to another computer on a computer network. You can compare a network model to a spoken language.
For instance you would have a pretty hard time trying to communicate if you spoke English and the other person spoke Spanish, so all a network model does allow like and unlike devices to agree upon a language before they ever start talking to each other. So as you can see in the early days of networking, that having only proprietary network models could cause a problem.
Seeing that there was a need for a standardized network model, two different organizations started working on standardizing the network model. The two standardized models that ended up being created were the OSI model and TCP/IP. The OSI model was created by the International Organization for Standardization (ISO) and TCP/IP was created through a Department of Defense contract (DOD).
The Open System Interconnection (OSI) networking model consist of seven layers. The layers of the OSI networking model allow the process of networking to be broke down into smaller sections, allowing developers and vendors to produce devices and software that will be able to have reliable communication on a network. The seven layers of the Open System Interconnection OSI model consists of the Application, Presentation, Session, Transport, Network, Data Link and Physical layer. Understanding the OSI model will later help you understand the TCP/IP model if you are trying to take you Cisco CCNA exam, or any other network exam.
Open System Interconnection (OSI) Network Model
|Application||Layer 7 – L7 PDU||DNS, FTP, HTTP, NFS, SMTP, SNMP, Telnet, SIP, NNTP||Web Browser, FTP Client, Firewall, and Software that connects to the internet or lan.||The actual application that you use to interface with the network. Many applications also control layers 5 and 6.|
|Presentation||Layer 6 – L6 PDU||MIME, TLS, SSL, XDR||File Types||The function of the presentation layer is to figure out how information will be displayed. For instance .txt, .bmp, .png are examples of the presentation layer.|
|Session||Layer 5 – L5 PDU||NetBIOS, SAP||See Application||Controls on how to start, control and end a session between two computers|
|Transport||Layer 4 – L4 PDU||TCP, UDP, SCTP, DCCP||Provides on how to provide reliable or unreliable communication, error recovery and flow control. Information located at the transport layer is known as a segment.|
|Network||Layer 3 – L3 PDU||IP, ICMP, IGMP, IPX, IPsec, AppleTalk||Router||Provides logical/device addressing, routing, and path determination. Transport information when pc are not on the same network. Information located at the network layer is known as a packet.|
|Data Link||Layer 2 – L2 PDU||Ethernet, PPP, Frame Relay, ARP, CSLIP, SLIP, L2TP, PPTP||Switch, WAP, Cable and DSL Modem||Provide physically addressing and error detection, not error recovery. Provide local network topology. Data is this level is known as a frame.|
|Physical||Layer 1 – L1 PDU||RJ-45, T1, E1, USB, DSL, Bluetooth||Hub, Repeater||Standardizes the transmission medium, voltage, light modulation and other physical characteristics. Data is the physical layer is knows as a bit, so when you hear the term binary, you are talking about the physical layer.|