CCNAX 200-120

CCNAX 200-120

The OSI Reference Model Explained

In the late 1970s, the Open Systems Interconnection (OSI) reference model was created by the International Organization for Standardization (ISO) to break this barrier. The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other.

A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers. When a communication system is designed in this manner, it's known as layered architecture.

The OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. The primary purpose of all such models, especially the OSI model, is to allow different vendors' networks to interoperate. Advantages of using the OSI layered model include, but are not limited to, the following:

  • It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
  • It allows multiple-vendor development through standardization of network components.
  • It encourages industry standardization by defining what functions occur at each layer of the model.
  • It allows various types of network hardware and software to communicate.
  • It prevents changes in one layer from affecting other layers, so it does not hamper development.

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end-to-end. Here are all of the layers and their functions.

OSI Model

The OSI reference model - layers:

  • Application layer (layer 7)
    • The Application layer of the OSI model marks the spot where users actually communicate to the computer. This layer only comes into play when it's apparent that access to the network is going to be needed soon. Take the case of Firefox (FF). You could uninstall every trace of networking components from a system, such as TCP/IP, NIC card, etc., and you could still use FF to view a local HTML document. Application layer is acting as an interface between the actual application program - which isn't at all a part of the layered structure-and the next layer down, by providing ways for the application to send information down through the protocol stack.
  • Presentation layer (layer 6)
    • The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. This layer is essentially a translator and provides coding and conversion functions. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading.
  • Session layer (layer 5)
    • The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialogue control between devices, or nodes. It coordinates communication between systems, and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex.
  • Transport layer (layer 4)
    • The Transport layer segments and reassembles data into a data stream. Services located in the Transport layer both segment and reassemble data from upper-layer applications and unite it onto the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.
  • Network layer (layer 3)
    • The Network layer (also called layer 3) manages device addressing, tracks the location of devices on the network, and determines the best way to move data, which means that the Network layer must transport traffic between devices that aren't locally attached. Routers (layer 3 devices) are specified at the Network layer and provide the routing services within an internetwork.
  • Data Link layer (layer 2)
    • The Data Link layer provides the physical transmission of the data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses, and translates messages from the Network layer into bits for the Physical layer to transmit. The Data Link layer formats the message into pieces, each called a data frame, and adds a customized header containing the hardware destination and source address.
  • Physical layer (layer 1)
    • Finally arriving at the bottom, we find that the Physical layer does two things: It sends bits and receives bits. Bits come only in values of 1 or 0. The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Some use audio tones, while others employ state transitions - changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the physical media's attachment interface. The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). Some old-phone-company employees still call DCE data circuit-terminating equipment. The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU).

The IEEE Ethernet Data Link layer has two sublayers:

  • Media Access Control (MAC) 802.3 - Defines how packets are placed on the media. Contention media access is "first come/first served" access where everyone shares the same bandwidth-hence the name. Physical addressing is defined here, as well as logical topologies. What's a logical topology? It's the signal path through a physical topology. Line discipline, error notification (not correction), ordered delivery of frames, and optional flow control can also be used at this sublayer.
  • Logical Link Control (LLC) 802.2 - Responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the Data Link layer what to do with a packet once a frame is received. It works like this: A host will receive a frame and look in the LLC header to find out where the packet is destined for-say, the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits.