Basics: The OSI Model and Protocol Stacks.
What is the OSI Model:
The OSI Model is a description for layered communications and computer network protocol design. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.
The layers of the OSI Model:
Think of the 7 layers as stages. Each stage prepares data for the next stage.
Application set:
- Layer 7: Application – This layer communicates with the operating system orapplication when the user interacts with the network.
- Layer 6: Presentation – Takes the data from Layer 7 and converts it into a format the other layers can understand.
- Layer 5: Session – Starts, Controls and ends communication with the receiving device.
Transport set:
- Layer 4: Transport – Maintains flow control of data. Flow control is the transport layer looking to see if data is coming from than one application and runs each applications data into a single stream of the physical network. This layer also provides recovery and error checking of data between the devices.
- Layer 3: Network – This layer determines which way the data will be sent to the receiving device. Logical protocols, routing and addressing are handled here.
- Layer 2: Data – In this layer the type of network and packet sequencing is defined. Also, the appropriate physical protocol is assigned to the data.
- Layer 1: Physical – This layer is physical, meaning it’s the connections, v0ltages, timing and actual hardware.
NOTE: ** The OSI Model is more of a reference model and is sometimes referred to as the OSI Reference Model. Actual protocol stacks will often combine one or more of the OSI layers into a single layer.
Protocol Stacks:
A protocol stack is a group of protocols that all work together to allow software or hardware to perform a function. The TCP/IP protocol stack is a good example. It uses four layers that map to the OSI model as follows:
- Layer 1: Network Interface – This layer combines the Physical and Data layers and routes the data between devices on the same network. It also manages the exchange of data between the network and other devices.
- Layer 2: Internet – This layer corresponds to the Network layer. The Internet Protocol (IP) uses the IP address, consisting of a Network Identifier and a Host Identifier, to determine the address of the device it is communicating with.
- Layer 3: Transport – Corresponding to the OSI Transport layer, this is the part of the protocol stack where the Transport Control Protocol (TCP) can be found. TCP works by asking another device on the network if it is willing to accept information from the local device.
- Layer 4: Application – Layer 4 combines the Session, Presentation and Application layers of the OSI model. Protocols for specific functions such as e-mail (Simple Mail Transfer Protocol, SMTP) and file transfer (File Transfer Protocol, FTP) reside at this level.
As you can see, it is not necessary to develop a separate layer for each and every function outlined in the OSI Reference Model. But developers are able to ensure that a certain level of compatibility is maintained by following the general guidelines provided by the model.
ASSEMBLY LANGUAGE:
Assembly language is a low level language for computers, micro processor and other integrated circuits. In Assembly language symbolic names are used to represent operations, registers and memory locations.
A CPU can only execute machine language instructions. So, there is a need to convert assembly language into machine language before CPU can execute it.
Assembler:A program that translates assembly language statement into a machine language instruction.
Compiler: A program needed to translate a high level language program into machine code.
Address: A number that identifies a memory location
Bus: A set of wires or connections connecting the CPU, memory, and I/O ports.
Number System:
1. Binary number system (0,1)
2. Decimal number system (0,1,2,3,4,5,6,7,8,9)
3. Octal number system (0,1,2,3,4,5,6,7)
4. Hexadecimal number system (0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F)
Assembly language is a low level language for computers, micro processor and other integrated circuits. In Assembly language symbolic names are used to represent operations, registers and memory locations.
A CPU can only execute machine language instructions. So, there is a need to convert assembly language into machine language before CPU can execute it.
Assembler:A program that translates assembly language statement into a machine language instruction.
Compiler: A program needed to translate a high level language program into machine code.
Address: A number that identifies a memory location
Bus: A set of wires or connections connecting the CPU, memory, and I/O ports.
Number System:
1. Binary number system (0,1)
2. Decimal number system (0,1,2,3,4,5,6,7,8,9)
3. Octal number system (0,1,2,3,4,5,6,7)
4. Hexadecimal number system (0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F)
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