Title: 2.1 OSI Model
Course: Information and Communication Technology: Module B > Section: Section 2 - Network Addressing Basics

2.1.1 OSI Model

The Open System Interconnect (OSI) breaks the network process into seven manageable and easy-to-reference layers. It is used universally as a way to understand network functionality. The functions of each layer are defined by the International Standardization Organization (ISO).

Following the OSI model when creating or troubleshooting a network is always advised.  The OSI model allows greater compatibility and interoperability between various network technologies. It breaks down network operations into less complex elements. This helps engineers focus their design and development efforts on specific functions. Each layer prevents changes in one area from affecting other areas. As a result, testing or troubleshooting a network are easier processes.

The seven layers are:

- Layer 7 – Application layer
- Layer 6 – Presentation layer
- Layer 5 – Session layer
- Layer 4 – Transport layer
- Layer 3 – Network layer
- Layer 2 – Data Link layer
- Layer 1 – Physical layer

Layer 1 is always on the bottom and Layer 7 is always on top. Each layer is only responsible for its own functions and only communicates with the layer directly above or below it. Everything entering or exiting the computer MUST go through the Physical layer.

To help you remember the order of the layers, a mnemonic device or acronym can be useful—something like “Please Do Not Throw Sausage Pizza Away” or “Please Do Not Teach Students Pointless Acronyms.” Can you think of a different one?

2.1.2 Layer 1 – Physical Layer

The Physical layer provides connectivity between hosts. It is responsible for converting information into bits (0’s and 1’s).  The correct term for this is "encoding the signal."

The Physical layer is what connects the medium (twisted pair copper wire, fiber-optic, coaxial, or wireless) to the network. All cabling is done at layer 1.

When a device is not connecting properly to a network, troubleshooting should always start at the Physical layer. Most network problems are layer 1 problems and can be avoided when installing cable.

One of the most common wiring errors is laying cables near other wires, particularly sources of power such as power cables. Power cables emit background noise, which can interfere with the signals on network cables. Fluorescent lights and machines are sources of electromagnetic noise that can also cause problems with signals on wires. This interference can cause signals to weaken or attenuate.

The key words to remember when dealing with the Physical layer are bits and media.

2.1.3 Layer 2 – Data Link Layer

The Data Link layer is responsible for the speed of transmission, flow control (how much data is sent at one time), and error identification (was the complete signal received). The physical topology works at layer 2.

There are two sub-layers found in the Data Link layer:

1. The Media Access Control (MAC)—This sub-layer recognizes special identifiers that are unique to for each network connected device, called media access control addresses (MAC addresses).  It is also known as the physical address or hardware address, which is written in hexadecimal (03-55-1A-F5-3C-4B). A MAC address is 48 bits long. Each character represents 4 bits and are formed in groups of two, followed by a hyphen or colon.

The first 24 bits (03-55-1A-F5-3C-4B) represents the manufacturer, make, and model number of the network card. The last 24 bits (03-55-1A-F5-3C-4B) represents the unique serial number of the card. This number is a “burned-in address”—hard coded and unique to every device on the network.

2. The Logic Link Control (LLC)—This sub-layer acts as an interface between the media access control (MAC) sub-layer and the network layer. The LLC works as a multiplexing feature. LLC header information directs the data link layer on how to handle a packet once a frame is received. The LLC works by telling the received frame to read the LLC header and find out to what protocol the packet is destined (i.e. the IP protocol) and the type of frame it is (i.e. 802.3 or Ethernet protocol).

The key words to remember when dealing with the Data Link Layer are MAC and frames.

2.1.4 Layer 3 – Network Layer

The Network layer is responsible for higher-level addressing schemes (IP addressing) and path determination. This layer deals with the logical topology.

The Network layer examines the IP address of the computer and indicates to which network or subnetwork the computer belongs. Another name for an IP address is a logical address. An additional function of this layer is to determine the best path a data packet will take through the LAN or WAN.

The key words to remember when dealing with the Network layer are IP and path determination.

2.1.5 Layer 4 - Transport Layer

The Transport layer is responsible for segmenting data files into manageable chunks and regulating the flow of information.

There are two important protocols that work in this layer:

Transmission Control Protocol (TCP)—Responsible for delivery of data between two hosts. Provides connection-oriented, reliable delivery of data from a source device to a destination device.  It provided end-to-end control or quality of service (QoS) by the following techniques:

- Sequence numbers—Numbering the segment to properly reassemble it at the destination
- Acknowledgements—Tell the source the information was received
- Windowing—How many segments should be received before sending the acknowledgement

User Datagram Protocol (UDP)—Offers an unreliable and connectionless delivery of data from source to destination. It does not guarantee delivery or acknowledge receipt of data. It is used when speed is more important than accuracy.

The key word to remember when dealing with the Transport layer is reliability.

2.1.6 Layer 5 – Session Layer; Layer 6 – Presentation Layer; Layer 7 – Application Layer

The Session layer is responsible for the establishment, maintenance, and termination of conversations (called sessions) between two or more applications on different computers. This includes starting, stopping, and re-synchronizing two computers as they communicate. This process is called dialogue control. The key word to remember when dealing with the Session layer is communication.

The Presentation layer is responsible for facilitating transparent communications between applications on connected devices. This layer deals with data formatting:

- Syntax—The common language each host must speak (html, .avi, .jpg, .docx)
- Encryption / De-encryption—The process of encoding information in such a way that eavesdroppers or hackers cannot read it
- Compression / De-compression—The process of encoding digital information using fewer bits

The key words to remember when dealing with the Presentation Layer are syntax and data representation.

The Application layer is responsible for providing services to the end user. It is the only layer the end user deals with. It provides services to applications used by the end user:

- telnet
- FTP
- HTTP
- word processing programs
- spreadsheet programs
- E-mail

The key word to remember when dealing with the Application Layer is an application like WWW.

2.1.7 Encapsulation

All network communications start from a source device and end up at a destination device. Any information sent on a network is typically referred to as data in packet format. If one computer wants to send data to another computer, the data must first be packaged through a process called encapsulation. Encapsulation literally means "to package."

Networks must perform the following five conversion steps in order to encapsulate data:

1. Build the data—The end user inputs information in an application. This raw data is used by layer 7, 6, and 5.
2. Segment the data for transport—Divide the raw information into manageable chunks at layer 4.
3. Package the segment—Add the network IP addresses (source and destination) to the header.
4. Frame the Packet—Add the data link layer header that contains the source and destination MAC address and frame check trailer.
5. Convert to bits for transmission—Encode all the information for transport out of the medium.

In order for data to travel from a source device to a destination device, each layer of the OSI model at the source must communicate with its peer layer at the destination. This form of communication is referred to as peer-to-peer. During this process, the protocols of each layer exchange information, called protocol data units (PDUs). Each layer of communication on the source computer communicates with the corresponding layer on the destination computer. The flow of information must travel down through the layers. Information is only sent out from the Physical layer (layer 1).

At the Application, Presentation and Session layers, the PDU is called data.

At the Transport layer, the PDU is called a segment.

At the Network layer, the PDU is called a packet.

At the Data Link layer, the PDU is called a frame.

At the Physical layer, the PDU is called a bit.

To remember the encapsulation process, it may help to memorize the acronym, “Do Sergeants Pay For Bagels?"

Warriors of the Net

Please complete assignment 2.1.1 in the lab manual.