Layer 3 of the OSI Model. Network Layer

Layer 3: Network Layer
The network layer provides the functional and procedural means of transferring variable length data sequences (called packets) from one node to another connected in “different networks”. A network is a medium to which many nodes can be connected, on which every node has an address and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver the message to the destination node, possibly routing it through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. It may, but does not need to, report delivery errors.

Message delivery at the network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it need not do so.



Layer 2 of the OSI Model – Data Link Layer

The 2nd layer of the OSI layer is called the Data Link Layer.  This is where the method of networking is determined.  (wired or wireless or token ring etc)
Data Link Layer (Layer 2)

The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer, often abbreviated “DLL” (though that abbreviation has other meanings as well in the computer world). The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The set of devices connected at the data link layer is what is commonly considered a simple “network as opposed to Internetwork

Data Link Layer Sublayers: Logical Link Control (LLC) and Media Access Control (MAC)The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.

Data Link Layer Functions

The following are the key tasks performed at the data link layer:

Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.

Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.

Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.

Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.

Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.

Networking OSI Layers

The part of networking that I always have problems with is the OSI model.  Because of this, I am documenting my study of those layers here.  To help with this, I have copied the Dummies guide description of the explanation.  They always say that typing things out helps in memorization so over the next week or so I am going to translate the “Dummies'” definition to the complete Idiot’s definition that I need to finally understand this stuff.

Wish me luck

The layers of the OSI model

Under its official name, the Open Systems Interconnection Reference Model, or the OSI model, was developed by the International Organization for Standardization, which uses the abbreviation of ISO. And, yes, the full acronym of the OSI is ISO OSI.
The OSI model is a layered model that describes how information moves from an application program running on one networked computer to an application program running on another networked computer. In essence, the OSI model prescribes the steps to be used to transfer data over a transmission medium from one networked device to another. The OSI model is a seven-layer model developed around five specific design principles:
Whenever a discrete level of abstraction is required, a new layer should be created.
Each layer of the model should carry out a well-defined function.
The function of each layer should define internationally standardized protocols.
The boundaries of the layers should be placed to minimize the flow of information across interfaces.
There should be a sufficient number of layers defined to prevent unnecessary grouping of functions and the number of layers should also be small enough so that the model remains manageable.

Moving down through the layers

The OSI model breaks the network communications process into seven separate layers. From the top, or the layer closest to the user, down, these layers are:
Layer 7, Application: The Application layer provides services to the software through which the user requests network services. Your computer application software is not on the Application layer. This layer isn’t about applications and doesn’t contain any applications. In other words, programs such as Microsoft Word or Corel are not at this layer, but browsers, FTP clients, and mail clients are.
Layer 6, Presentation: This layer is concerned with data representation and code formatting.
Layer 5, Session: The Session layer establishes, maintains, and manages the communication session between computers.
Layer 4, Transport: The functions defined in this layer provide for the reliable transmission of data segments, as well as the disassembly and assembly of the data before and after transmission.
Layer 3, Network: This is the layer on which routing takes place, and, as a result, is perhaps the most important OSI layer to study for the CCNA test. The Network layer defines the processes used to route data across the network and the structure and use of logical addressing.
Layer 2, Data Link: As its name suggests, this layer is concerned with the linkages and mechanisms used to move data about the network, including the topology, such as Ethernet or Token Ring, and deals with the ways in which data is reliably transmitted.
Layer 1, Physical: The Physical layer’s name says it all. This layer defines the electrical and physical specifications for the networking media that carry the data bits across a network.

Other interesting OSI layer stuff

Layers 5 through 7 are generally referred to as the upper layers. Conversely, Layers 1 through 4 are collectively called the lower layers. Seems obvious, but you’ll see these references on the test.
You need to know the seven layers in sequence, either top-to-bottom or bottom-to-top. Here are some mnemonic phrases to help you remember the layers of the OSI model:
“Please Do Not Throw Salami Pizza Away” — this works for bottom-to-top. If you don’t like salami pizza, then how about seafood or spinach pizza instead?
“All People Seem To Need Data Processing” — a top-to-bottom reminder.
“APS Transports Network Data Physically” — APS refers to Application, Presentation, and Session. This one separates the upper and lower layer groups.
“Please Do Not Tell Secret Passwords Anytime” — Shh! Another bottom-to-top phrase.

Packaging the data

Each layer of the OSI model formats the data it receives to suit the functions to be performed on that layer. In general, the package of data that moves through the layers is called a Protocol Data Unit (PDU). However, as the data is reformatted and repackaged, it takes on unique names on certain layers. Table 1 lists the name each layer uses to refer to a message.