OSI, DoD & TCP/IP

Some notes on this topic, made during CCNA preparation
OSI model
There are 7 layers:

Application layer
Application layer is acting as an interface between the actual application programs. It chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery. The Application layer is responsible for finding the network resources broadcast from a server and adding flow control and error control

VoIP needs better jitter, loss and delay, then for a example HTTP. VoIP typically requires less bandwidth than data applications.

Presentation layer
Presents data, for example encryption.

Session layer
Keeps different applications data separate.

Transport layer
Provides reliable or unreliable delivery, performs error correction before retransmit.

The Transport layer segments and reassembles data into a data stream.
Flow Control - The purpose of flow control is to provide a means for the receiver to govern the amount of data sent by the sender. Types of flow control:

• Buffering
• Windowing
• Congestion avoidance
• Connection-Oriented Communication
• Multiplexing/Demultiplexing
• Windowing - Windows are used to control the amount of outstanding, unacknowledged data segments.

Acknowledgments
TCP uses a concept called forward acknowledgment, in which the acknowledgment field in the header lists the next-expected byte, not the last-received byte.

P.S UDP provides only multiplexing/demultiplexing and it is very useful, when no error recovery is needed(VoIP). UDP also require less network and CPU resources. UDP does not provide ordered data transfer.

Network layer
Provides logical addressing

Datalink layer

• Combines packets into bytes and bytes into frames
• Provides access to media using MAC(Media Access Control) address
• Performs error detection, BUT not correction
• This layer handles error notification, network topology, and flow control(optional)
• Media Access Control (MAC) 802.3 defines how packets are placed on the media
• Logical Link Control (LLC) 802.2 is responsible for identifying Network layer protocols and then encapsulating them.
• An LLC header tells the Data Link layer what to do with a packet once a frame is received.
• Data Link (LLC sublayer) uses service access points.

Physical layer
Moves bits between devices, specifies voltage, wire speed and pin-out of cables.

DoD model

There are 3 layers:

• Application
• Host to host
• Internet

TCP/IP model

There are 4 layers:

• Application
• Transport
• Internet
• Network access