The OSI and TCP/IP Networking Models

The term networking model, or networking architecture, refers to an organized description of all the functions needed for useful communications to occur. Individual protocols and hardware specifications then are used to implement the functions described in the networking model. When multiple computers and other networking devices implement these protocols, which, in turn, implement the functions described by the networking model, the computers can successfully communicate.

OSI Reference Model

OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network. It is the Open System Interconnection reference model for communications. Its purpose is to guide product implementors so that their products will consistently work with other products.

OSI Layers

The OSI reference model consists of seven layers. Each layer defines a set of typical networking functions.the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined. For instance, the IEEE defines Ethernet standards, so the OSI committees did not waste time specifying a new type of Ethernet; it simply referred to the IEEE Ethernet standards.Today the OSI model can be used as a standard of comparison to other networking models.The Seven OSI Layers are :

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

OSI does have a very well-defined set of functions associated with each of its seven layers, you can examine any networking protocol or specification and make some determination of whether it most closely matches OSI Layer 1, 2, or 3, and so on.The upper layers of the OSI reference model (application, presentation, and session—Layers 7, 6, and 5) define functions focused on the application. The lower four layers (transport, network, data link, and physical—Layers 4, 3, 2, and 1) define functions focused on end-to-end delivery of the data.

Functional Description of Layers :-

• Layer 7 : Layer 7 defines the interface between the communications software and any applications that need to communicate outside the computer on which the application resides. For example, a web browser is an application on a computer. The browser needs to get the contents of a web page; OSI Layer 7 defines the protocols used on behalf of the application to get the web page.


• Layer 6 : This layer’s main purpose is to define data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption also is defined by OSI as a presentation layer service. For example, FTP enables you to choose binary or ASCII transfer. If binary is selected, the sender and receiver do not modify the contents of the file. If ASCII is chosen, the sender translates the text from the sender’s character set to a standard ASCII and sends the data. The receiver translates back from the standard ASCII to the character set used on the receiving computer.


• Layer 5 : The session layer defines how to start, control, and end conversations (called sessions). This includes the control and management of multiple bidirectional messages so that the application can be notified if only some of a series of messages are completed. This allows the presentation layer to have a seamless view of an incoming stream of data. The presentation layer can be presented with data if all flows occur in some cases. For example, an automated teller machine transaction in which you withdraw cash from your checking account should not debit your account and then fail before handing you the cash, recording the transaction even though you did not receive money. The session layer creates ways to imply which flows are part of the same session and which flows must complete before any are considered complete.


• Layer 4 : Layer 4 protocols provide a large number of services, as seen in Chapter 5 of this book. Although Layers 5 through 7 focus on issues related to the application, Layer 4 focuses on issues related to data delivery to the other computer—for instance, error recovery, segmentation of large application data blocks into smaller ones for transmission, and reassembly of those blocks of data on the receiving computer.


• Layer 3 : This layer defines end-to-end delivery of packets. To accomplish this, the network layer defines logical addressing so that any endpoint can be identified. It also defines how routing works and how routes are learned so that the packets can be delivered. Chapter 4 of this book examines Layer 3 concepts in detail. The network layer of OSI defines most of the details that a Cisco router considers when routing. For example, IP running in a Cisco router is responsible for examining the destination IP address of a packet, comparing that address to the IP routing table, fragmenting the packet if the outgoing interface requires smaller packets, and queuing the packet to be sent out to the interface.


• Layer 2 : The data link layer (Layer 2) specifications deliver data across one particular link or medium. These protocols are necessarily concerned with the type of media in question; for example, 802.3 and 802.2 define Ethernet for the IEEE, which are referenced by OSI as valid data link layer (Layer 2) protocols. Other protocols, such as High-Level Data Link Control (HDLC) for a point-to-point WAN link, deal with the different details of a WAN link.


• Layer 1 : These physical layer (Layer 1) specifications, which are also typically standards from other organizations that are referred to by OSI, deal with the physical characteristics of the transmission medium. Connectors, pins, use of pins, electrical currents, encoding, and light modulation are all part of different physical layer specifications. Multiple specifications sometimes are used to complete all details of the physical layer. For example, RJ-45 defines the shape of the connector and the number of wires or pins in the cable. Ethernet and 802.3 define the use of wires or pins 1, 2, 3, and 6. So, to use a Category 5 cable with an RJ-45 connector for an Ethernet connection, Ethernet and RJ-45 physical layer specifications are used.

OSI Reference Model—Example Protocols :-

• Application (Layer 7) → Telnet, HTTP, FTP, WWW browsers, NFS, SMTP gateways (Eudora, CC:mail), SNMP


• Presentation (Layer 6) → JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, encryption, MPEG, MIDI


• Session (Layer 5) → RPC, SQL, NFS, NetBIOS names, AppleTalk ASP, DECnet SCP


• Transport (Layer 4) → TCP, UDP, SPX


• Network (Layer 3) → IP, IPX, AppleTalk DDP


• Data link (Layer 2) → IEEE 802.3/802.2, HDLC, Frame Relay, PPP, FDDI, ATM, IEEE 802.5/802.2


• Physical (Layer 1) → EIA/TIA-232, V.35, EIA/TIA-449, RJ-45, Ethernet, 802.3, 802.5, B8ZS

OSI Layering Concepts and Benefits

Many benefits can be gained from the process of breaking up the functions or tasks of networking into smaller chunks, called layers, and defining standard interfaces between these layers. The layers break a large, complex set of concepts and protocols into smaller pieces, making it easier to talk about, easier to implement with hardware and software, and easier to troubleshoot.The following list summarizes the benefits of layered protocol specifications:

• Easier to learn → Humans can more easily discuss and learn about the many details of a protocol specification.


• Easier to develop → Reduced complexity allows easier program changes and faster product evolution.


• Multivendor interoperability → Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network.


• Modular engineering → One vendor can write software that implements higher layers (for example : a web browser) and another can write software that implements the lower layers (for example : Microsoft’s built-in TCP/IP software in its operating systems).
  

OSI Terminology

First, remembering the names of the OSI layers is just an exercise in memorization. You might benefit from the following list of mnemonic phrases, with the first letters in each word being the same as the first letters of the OSI layer names, in order:

• All People Seem To Need Data Processing (Layers 7 to 1)


• Please Do Not Take Sausage Pizzas Away (Layers 1 to 7)


• Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7)

You also should know how to use the names of the layers when discussing other networking models. For example : The OSI model, the TCP/IP model, and a third figure with some sample TCP/IP protocols shown at their respective layers.