NETWORK BASICS - OSI LAYERS

OSI Layer:  Important Functions

 Application

¨        Provides an interface between a host's communication software and any necessary external applications.

¨        Evaluates what resources are necessary and the available resources for communication between two devices.

¨        Synchronizes client/server applications.

¨        Provides error control and data integrity between applications.

¨       Provides system-independent processes to a host.

Presentation

¨        Presents data to the Application layer.

¨        Acts as a data format translator.

¨        Handles the structuring of data and negotiating data transfer syntax to Layer 7.

¨       Processes involved include data encryption, decryption, compression, and decompression.

Session

¨        Handles dialog control among devices.

¨        Determines the beginning, middle, and end of a session or conversation that occurs between applications (intermediary).

Transport

¨        Manages end-to-end connections and data delivery between two hosts.

¨        Segments and reassembles data.

¨       Provides transparent data transfer by hiding details of the transmission from the upper layers.

Network

¨        Determines best path for packet delivery across the network.

¨        Determines logical addressing, which can identify the destination of a packet or datagram.

¨        Uses data packets (IP, IPX) and route update packets (RIP, EIGRP, and so on).

¨        Uses routed protocols IP, IPX, and AppleTalk DDP.

¨       Devices include routers and Layer 3 switches.

Data Link

¨        Ensures reliable data transfer from the Network layer to the Physical layer.

¨        Oversees physical or hardware addressing.

¨        Formats packets into a frame.

¨        Provides error notification.

¨       Devices include bridges and Layer 2 switches.

Physical

¨        Moves bits between nodes.

¨        Assists with the activation, maintenance, and deactivation of physical connectivity between devices.

¨       Devices include hubs and repeaters.

Table 1.9. Protocols for Each Layer of the TCP/IP Model
TCP/IP Layer	Protocols
Application	Telnet	HTTP/HTTPS	FTP	TFTP
	DNS	SMTP	POP3	NFS
	NNTP	SNMP	NTP	DHCP
Transport	TCP		UDP
Internet	ICMP	ARP	RARP	IP
Network Interface	Ethernet	Fast Ethernet	Token Ring	FDDI