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OSI Seven Layer Model

The Open Systems Interconnect (OSI) model was developed circa 1981 by the International Standards Organization (ISO). The OSI model comprises seven functional layers, which provide the basis for communication among computers over networks.

The seven layers of the OSI model, from highest to lowest, are Application, Presentation, Session, Transport, Network, Data Link, and Physical (you can easily remember them, using the mnemonic phrase All People Seem To Need Data Processing). Table 11-1 lists these layers, their general functions, and corresponding protocols, services, or standards.

The seven layers of the OSI Basic Reference Model are (from bottom to top):

1. The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. E.g. this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.
   
   The NIC cards in your PC and the interfaces on your routers all run at this level since, eventually, they have to pass strings of ones and zeros down the wire.
    
2. The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. E.g. providing a means for communicating open systems to establish, maintain and terminate network connections. The IP protocol lives at this layer, and so do some routing protocols. All the routers in your network are operating at this layer.
   
   On an Ethernet, of course, access is governed by a device's MAC address, the six byte number that is unique to each NIC. Devices which depend on this level include bridges and switches, which learn which segment's devices are on by learning the MAC addresses of devices attached to various ports.
   
   This is how bridges are eventually able to segment off a large network, only forwarding packets between ports if two devices on separate segments need to communicate. Switches quickly learn a topology map of the network, and can thus switch packets between communicating devices very quickly. It is for this reason that migrating a device between different switch ports can cause the device to lose network connectivity for a while, until the switch, or bridge, re-ARPs.  
    
3. The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. E.g. this layer defines the addressing and routing structure of the Internet.
    
4. The Transport Layer describes the quality and nature of the data delivery and is where TCP lives. The standard says that "The Transport Layer relieves the Session Layer [see Layer 5] of the burden of ensuring data reliability and integrity". It is for this reason that people are becoming very excited about the new Layer 4 switching technology.
   
   Before these devices became available, only software operated at this layer. Hopefully, you will now also understand why TCP/IP is uttered in one breath. TCP over IP, since Layer 4 is above (over) Layer 3. It is at this layer that, should a packet fail to arrive (perhaps due to misrouting, or because it was dropped by a busy router), it will be re-transmitted, when the sending party fails to receive an acknowledgement from the device with which it is communicating.
   
   The more powerful routing protocols also operate here. OSPF and BGP, for example, are implemented as protocols directly over IP.
    
5. The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. E.g. this layer describes how request and reply packets are paired in a remote procedure call. It provides for two communicating presentation entities to exchange data with each other.  
   
   The Session Layer is very important in the E-commerce field since, once a user starts buying items and filling their "shopping basket" on a Web server, it is very important that they are not load-balanced across different servers in a server pool. This is why, clever as Layer 4 switching is, these devices still operate software to look further up the layer model. They are required to understand when a session is taking place, and not to interfere with it.
    
6. The Presentation Layer describes the syntax of data being transferred. E.g. this is where application data is either packed or unpacked, ready for use by the running application. Protocol conversions, encryption/ decryption and graphics expansion all takes place here.  
    
7. The Application Layer describes how real work actually gets done. E.g. this is where you find your end-user and end-application protocols, such as telnet, ftp, and mail (pop3 and smtp).