CCNA 1.3 The OSI Reference Model

• OSI has seven layers, divide into two groups: Application, Presentation and Session layer; Transport, Network, Data Link and Physical Layer
• The top three layers define applications communication between end stations
• The bottom three layers define data transmission between end points
• Application Layer provides interface between actual application programs
• Presentation Layer presents data to Application layer and responsible for data encryption, compression and translation services
• Session Layer separates different applications' data from other applications' data, provides dialog control between nodes and organize systems communication through simplex, half duplex and full duplex mode.
• Transport Layer provides reliable (TCP) or unreliable (UDP) end-to-end delivery by reassembling data from upper layers into a data stream and performs error correction before retransmit
• Reliable networking (connection-oriented) at the Transport Layer can be achieved through flow control, sequencing and acknowledgment
• Flow control allows receiver to govern the amount of data sent by sender through buffering, windowing and congestion avoidance.
• Buffer acts a memory section of receiver to store data from sender
• Window is the quantity of data (bytes) that the sender is allowed to sent without acknowledgment from receiver
• Acknowledgment requires receiver to send ack to sender when it receives data
• Network Layer provides routing service within an internetwork with two types of packets: data (IP, IPv6) and route update (RIP, RIPv2) packet
• Data Link Layer provides access to device in a LAN through hardware address, format packets into frames and perform error detection
• Data Link Layer has two sublayers: Media Access Control (802.3) and Logical Link Control (802.2)
• MAC defines how packets are placed on the media
• LLC identifies network layer protocol and then encapsulates them
• Physical Layer activates, maintains and deactivates physical link between actual communication media through sending and receiving bits