The OSI Model

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Contents 1 The Open Systems Interconnection (OSI) Model 2 Physical (Layer 1) 3 Data Link (Layer 2) 3.1 Sublayers 3.2 Protocols 4 Network (Layer 3) 5 Transport (Layer 4) 6 Session (Layer 5) 7 Presentation (Layer 6) 8 Application (Layer 7) 9 Conclusion

[edit section] The Open Systems Interconnection (OSI) Model

At the core of the Network+ exam is the OSI "Seven" Model. The model describes the ways and means that networks use to operate and for communication . Though you will seldom (if ever) encounter or use it in practice, and although you will find it to be a mundane, ambiguous, or even arbitrary model, it is heavily tested on the Network+ exam and therefore for our purposes, it is immediately relevant.

The OSI Model is like a seven-layered cake. Just like the cake the OSI Model is a LIE and will never help you in life. But I digress, as the cake starts from the bottom and becomes more ornate as it reaches the top, the OSI Model begins with the most basic layer, the Physical layer, and ends at the layer that we as users encounter, the Application layer. Like a cake, each of the layers depends on the layers below it to operate - for example, Outlook Express cannot receive email when the network cable is disconnected - without the cable, the "cake" crumbles. (OK, this analogy is a bit of a stretch, but just go with it!) The order of the layers, therefore, is quite important and you should have it intimately memorized.

Here are the layers, from bottom to top:

Name	Mnemonic	Layer Number
Application	All	7
Presentation	People	6
Session	Seem	5
Transport	To	4
Network	Need	3
Data Link	Data	2
Physical	Processing	1

Of course, you are free to come up with your own mnemonic device, but we do recommend you find some way of remembering the layers because it will be a subject of the test. Which is not a lie, unfortunately.

More important than the order of the layers, however, is the function of the layers. Most exam questions on the OSI model ask you, "Which layer does so-and-so operate in?" or something to that nature. Another type of question that occurs frequently is, "A problem has occurred (Problem description). Which layer is to blame ?" These questions can feel ambiguous or difficult, but most of the time, there is a single, clear answer that makes itself known provided that you are aware of the model and the place each layer takes in the model. We will now cover each layer in detail.

[edit section] Physical (Layer 1)

At the base of the OSI model is the physical layer. This one is the easiest to understand - it encompasses most of the physical aspects of the network; for example, a repeater (a piece of equipment that amplifies signals) operates at the physical level because it is only concerned with transmitting the electric signal on the wire - it does not try to interfere with, encode/decode, or otherwise logically manipulate the signal. Think of the physical layer as the "electrical" layer of the model - the physical layer is the layer of low-level networking equipment, such as some hubs, cabling, and repeaters . The physical layer is never concerned with protocols or other such higher-layer items.

Examples of hardware in this layer:

• Network adapter
• Repeater
• Network hub
• Modem
• Fiber Media Converter

[edit section] Data Link (Layer 2)

The Data Link Layer transfers data between adjacent nodes in a Wide Area Network, or between nodes on any given Local Area Network. This layer also provides the procedural means to transfer data between network entities and sometimes to detect and correct errors that have occurred within the Physical Layer.

Since the Data Link layer is concerned primarily with local delivery within a LAN, data link frames do not cross the boundaries of a local network segment and instead focus on local delivery, addressing, and media arbitration.

Switches operate in the Data Link layer.

[edit section] Sublayers

The Data Link Layer is often subdivided into two sublayers, the LLC Sublayer and the MAC Sublayer:

• The LLC (or Logical Link Control) Sublayer multiplexes protocols running atop the Data Link Layer, and provides flow control, acknowledgement, and error control. It also specifies the mechanisms to be used for addressing stations and for controlling the data exchanged between machines.
• The MAC (or Media Access Control) Sublayer determines who is allowed to access the media at any one time (as in CSMA/CD) and provides frame synchronization, which determines where one frame ends and the next begins.

[edit section] Protocols

Protocols in the Data Link Layer include:

• Ethernet for LANs
• PPP
• HDLC
• ADCCP for point-to-point connections
• PTPPD Point-to-Point Portal Device

[edit section] Network (Layer 3)

The Network layer is where the frames of the Data Link layer become packets. It can be described as the puberty of the OSI model. It is where the boys of the Data Link layer become men. The best way to think of the Network layer is as the mailroom clerk of the OSI model. The clerk receives mail and directs it to the appropriate couriers. In similar fashion, the Network layer translates the frames it receives from the Data Link layer into more logical packets which can be routed to other networks (like sending it to a courier). At the Network layer, you can begin to actually communicate across a Network, but the service is called "unreliable" because no connection can be established. Communication over the Network layer is something like throwing a message in a bottle into the sea or hollowing out the wall and writing cryptic references to the Companion Cube at the Aperture Science enrichment center- you cannot verify that the other person ever reads the message. The Network Layer is the layer that uses IP addresses. Most of what we call "routing" occurs at the network layer - that is,network traffic is routed from one network to another at this layer, allowing for inter-network (as opposed to intra-network) communication.

[edit section] Transport (Layer 4)

At the Transport layer, the Network layer's packets are sorted and organized into "segments." This is different from the idea of packets in the Network layer in one fundamental way: the segments of data over the Transport layer contain information on the connection and the transmission of data. The Transport layer removes the uncertainty of "throwing the message in the bottle" that we experience over the Network layer by attaching to segments (which are basically continued packets) information about the state of a connection. Thus, the Transport layer uses the LLC sub-layer of the Data Link layer to establish connections between hosts. The protocols that are typically associated with the Transport layer are:

• TCP: Connection-Oriented, reliable - unlike the "message in the bottle," or "hidden Companion Cube reference," it can verify that a segment or packet reaches the location, or note failure
• UDP: Connectionless, unreliable - it is like a "smarter" message in the bottle service; it makes a best-effort delivery but does not establish a connection or verify receipt

[edit section] Session (Layer 5)

The Session layer is the layer that initiates and terminates the transport layer connection-oriented services . While the Session layer is not widely used by protocols, it is important in that it is responsible for managing the connections that we value and that the Transport layer provides. In other words, the Session layer is like the GLaDOS of the transport layer, just like GLaDOS tells the robots what to do and promises them eternal damnation in case of insubordination, the Session layer also bosses the Transport layer around.

[edit section] Presentation (Layer 6)

The Presentation layer translates the segments of information from the Transport layer into data that can be used at the Application layer. It is like the C3-PO of the OSI model. It is something of an intermediary between the network node's processing area and the network node's actual networking area - it can interpret the segments or packets it receives and change them into "data formats" that we all know and that the PC can recognize.

[edit section] Application (Layer 7)

This is the top of our cake, utilizes the layers below it, and includes the functions that we are most familiar with - the end-user application protocols such as FTP and HTTP, the vital services like DHCP and DNS, and several obscure applications. This is where the sockets are defined. The Application layer does indeed cover a wide variety of protocols and services, but don't let this overwhelm you. In general, when trying to decide if a service or protocol is an application level one, ask: "Does this facilitate networking, or does networking facilitate the service?" If the answer is the latter, you know it is an Application layer service.

[edit section] Conclusion

The OSI model permeates the theoretical foundation of all networking hardware, software, and standards. It is the common denominator of networking (at least as far as the Network+ exam is concerned) and will reappear in just about any discussion of any networking topic. How can you expect to survive the daily IT department's water cooler conversations without knowing about the OSI model? So, be sure to keep the OSI model in the back of your head and continue to study it! Your reputation among the noble workers of your company's help desk depends upon it!

Also See: Interactive OSI Model Tutorial