OSI segments networking tasks, processes, and services into different layers.  Each layer has its own responsibilities when it comes to how two computers communicate over a network.  Each layer has certain functionalities and the services and protocols that work within that layer to fulfill them.

Each protocol at a specific OSI layer communicates with a protocol that operates at the same OSI layer on another computer.  This happens through encapsulation.  A message is constructed at the application layer and then passed down through the protocol’s stack.  Each layer adds its own information to the message; thus, the message grows in size as it goes down the protocol stack.  The message is then sent to the destination computer and the encapsulation is reversed by taking the message apart through the same steps as the source computer that encapsulated it.

 

This is how computers communicate logically.  Each layer adds its own information to the packet and then each layer at the destination computer only takes information that pertains to it.

 

Layer 7	Application	Security: Confidentiality, authentication, data integrity, non-repudiation   Technology: gateways   Protocols: FTP, SMB, TELNET, TFTP, SMTP, HTTP, NNTP, CDP, GOPHER, SNMP, NDS, AFP, SAP, NCP, SET	Includes the protocols that support the application.   Responsible for all application-to-application communications. User information maintained at this layer is user data.
Layer 6	Presentation	Security: confidentiality, authentication, encryption   Technology: gateway   Protocols: ASCII, EBCDIC, POSTSCRIPT, JPEG, MPEG, GIF	Receives information from the application layer protocols and puts it into a format that all computers following OSI can understand.   Responsible for the formatting of the data so that it is suitable for presentation. Responsible for character conversion (ASCII/EBCDIC), Encryption/Decryption, Compression, and Virtual Terminal Emulation. User information maintained at this layer is called messages.
Layer 5	Session	Security: None   Technology: gateways   Protocols: Remote Procedure Calls (RPC) and SQL, RADIUS, DNS, ASP	Responsible for the setup of the connection, maintaining of the connection, and the connection tear-down.   Simplex – communication takes place in one direction   Half-duplex – comm. takes place in both directions, only 1system can send info at a time.   Full-duplex – comm. takes place in both directions, both systems send information at the same ime.
Layer 4	Transport	Security: Confidentiality, authentication, integrity   Technology: gateways   Protocols: TCP, UDP, SSL, SSH-2, SPX, NetBios, ATP	Responsible for the guaranteed delivery of information. It is also responsible for error detection, correction, and flow control.  Information at this layer is called datagrams.
Layer 3	Network	Security: confidentiality, authentication, data integrity   Technology: virtual circuits (ATM), routers   Protocols: IP, IPX, ICMP, OSPF, IGRP, EIGRP, RIP, BOOTP, DHCP, ISIS, ZIP, DDP, X.25	Responsible for the routing of data from one node to another through the network including the path selection. Logical addresses are used at this layer. Information maintained at this layer is called packets.
Layer 2	Data Link	Security: confidentiality   Technology: bridges, switch   Protocols: L2F, PPTP, L2TP, PPP, SLIP, ARP, RARP, SLARP, IARP, SNAP, BAP, CHAP, LCP, LZS, MLP, Frame Relay, Annex A, Annex D, HDLC, BPDU, LAPD, ISL, MAC, Ethernet, Token Ring, FDDI	Responsible for translating data into LAN or WAN binary format for proper line transmission.  Formats the data frame…puts the correct info into the header.   Responsible for the physical addressing of the network via MAC addresses. There are two sublevels to the Data-Link layer. MAC and LLC. The Data-Link layer has error detection, frame ordering, and flow control. Information maintained at this layer is called frames.
Layer 1	Physical	Security: confidentiality   Technology: ISDN, Hubs, Repeaters, Cables   Protocols: 10BaseT, 100BaseT, 1000BaseT, 10Base2, 10Base5, OC-3, OC-12,  DS1, DS3, E1, E3, ATM, BRI, PRI, X.23	Responsible for the physical transmission of the binary digits through the physical medium. This layer includes things such as the physical cables, interfaces, and data rate specifications. Information maintained at this layer is called bits (the 1s and 0s).   Bits are converted into voltage for transmission.