12. Multilevel model of network reference model osi. Data encapsulation.

Moving data across a network is best visualized using the seven layers of the Open Systems Interconnection model, commonly referred to as the OSI model. The OSI model breaks network communications down into multiple processes. Each process is a small part of the larger task.

OSI Model
	Data unit	Layer	Function
Host layers	Data	7. Application	Network process to application
		6. Presentation	Data representation, encryption and decryption, convert machine dependent data to machine independent data
		5. Session	Interhost communication, managing sessions between applications
	Segments	4. Transport	End-to-end connections, reliability and flow control
Media layers	Packet/Datagram	3. Network	Path determination and logical addressing
	Frame	2. Data link	Physical addressing
	Bit	1. Physical	Media, signal and binary transmission

Data Flow and Encapsulation

Data flow occurs when two devices are connected in a network with some kind of shared transmission medium. An application running on the source device creates some kind of data. This happens at the Application layer.

In our case the application needs to add encryption to that data. This will be done at thePresentation layer.

At the Session layer it appends the Session ID. At this point the information is still one block of data.

Next, data goes down to the Transport layer. The Transport layer breaks the data into blocks of data which we call Segments. Every Segment also gets the Port number to identify which upper layer application needs to receive the data on the destination device.

The Segment is then passed to the Network layer. The Network layer takes the Segment, which includes the Port number, and appends the source and destination IP address. At that point the Segment becomes a Packet.

The Packet is then passed to the Data Link layer where the source and destination MAC addressand the CRC is added. At this point we have a Frame.

The Frame then is sent to the physical device where it is translated into some kind of a signal, whether it's electrical, radio wave, light or other signal. This Frame then becomes some kind of a signal that represents a series of zeros and ones. This is why at the Physical layer we often call itBits. The Network Interface Card (NIC) prepares those signals and sends it out on the transmission medium.

13 The physical connection of networks

Networking: The Physical Connection

A network is a way to connect two or more devices together so that they can exchange information. In order to make this work, each device must know when another is trying to connect to it and it must know how to reply. The network consists of a whole suite of protocols to make this work. A protocol is just a set of rules that each device uses to communicate with other devices.

At the most basic level is the wire used to make the physical connection. In most cases this will designed for an "Ethernet" type physical network. Fiber optic cable is just a single wire so it's kind of boring to talk about but it offers the highest speeds.

The most common is a cable containing 8 wires arranged as 4 pairs. This kind of cable (Unshielded Twisted Pair, UTP) is rated according to its reliability at various transmission speeds. Typically the minimum is rated Category 5 with most new installation rated as Category 5e or Category 6.

Once the medium (cable, wireless, fiber, etc.) is installed, there are other rules that determine how it will connect to the devices on the network. Fiber will use connectors called either ST or SC type, copper cable (say Cat 5e) will use RJ45 type connectors and wireless will use no connectors at all. When the transmission medium is in place and the connectors properly installed, the network is almost ready.