| The Application Layer (Layer 7) refers to | | | | included. Examples include: TCP, UDP, and SPX. |
| communication services to applications and is the | | | | Network Layer (Layer 3) defines end-to-end |
| interface between the network and the | | | | delivery of data packets and defines logical |
| application. Most application layer protocols offer | | | | addressing to accomplish this. It also defines how |
| services that programs use to access the | | | | routing works and how routes are learned. The |
| network, such as the Simple Mail Transfer | | | | OSI Reference Model fragments a packet into |
| Protocol (SMTP). | | | | smaller packets to accommodate media with |
| Application layer protocols often include the | | | | smaller maximum transmission unit sizes. The |
| session and presentation layer functions. A typical | | | | Internet Protocol (IP) is the cornerstone of the |
| protocol stack consists of four separate | | | | Transmission Control Protocol/Internet Protocol |
| protocols, which run at the application, transport, | | | | (TCP/IP) suite and the most frequently used |
| network, and data-link layers.Examples include: | | | | network layer protocol. Other examples include: |
| Telnet, HTTP, FTP, Internet browsers, NFS, SMTP | | | | IPX, AppleTalk DDP, NetBIOS, NetBEUI and ICMP. |
| gateways, SNMP, X.400 mail, and FTAM. | | | | Data Link Layer (Layer 2) is the channel between |
| The Presentation Layer (Layer 6) defining data | | | | the computer's networking hardware and |
| formats, such as ASCII text, EBCDIC text, | | | | networking software. This layer is concerned with |
| binary, BCD, and JPEG and is responsible for | | | | getting data across one particular link or medium. |
| formatting data exchange.. | | | | The data link protocols define delivery across an |
| Encryption also is defined as a presentation layer | | | | individual link. These protocols are necessarily |
| service. Examples include: JPEG, ASCII, EBCDIC, | | | | concerned with the type of media in use. To |
| TIFF, GIF, PICT, encryption, MPEG, and MIDI. | | | | implement a data-link layer protocol, the following |
| The Session Layer (Layer 5) defines how two | | | | hardware and software is needed: Network |
| computers establish, synchronize, maintain, and | | | | interface cards (NICs), which are also called |
| end communication sessions. This includes the | | | | Network adapters; Network adapter drivers; |
| control and management of multiple bidirectional | | | | Network cables and ancillary connecting hardware; |
| messages so that the application can be notified if | | | | and Network hubs. NICs and hubs are designed |
| only some of a series of messages are | | | | for specific data-link layer protocols, and it cannot |
| completed. This allows the presentation layer to | | | | mix with other products or different kinds of |
| have a seamless view of an incoming stream of | | | | protocols. Some network cables are |
| data. | | | | protocol-specific, while others can be used with |
| The presentation layer can be presented with | | | | different protocols. The most popular data-link |
| data if all flows occur in some cases. There are | | | | layer protocol is Ethernet while Token Ring is also |
| no separate session layer protocols as there are | | | | common. Data-link layer protocol specifications |
| at the lower layers. Session layer functions are | | | | include the three basic elements: A format for the |
| integrated into other protocols that include | | | | frame, A mechanism for controlling access to the |
| presentation and application layer functions. The | | | | network medium, One or more physical layer |
| transport, network, data-link, and physical layers | | | | specifications for use with the protocol Examples |
| are concerned with the proper transmission of | | | | include: IEEE 802.3/802.2, HDLC, Frame Relay, PPP, |
| data across the network. The session layer and | | | | FDDI, ATM, and IEEE 802.5/802.2. |
| above are not involved in that part of the | | | | Physical Layer (Layer 1) deals with the physical |
| communications process. The session layer | | | | characteristics of the transmission medium. |
| provides 22 services. The most important of | | | | Connectors, pins, use of pins, electrical currents, |
| these services are called dialog control and dialog | | | | encoding, and light modulation are all part of |
| separation. Examples include: RPC, SQL, NFS, | | | | different physical layer specifications. It defines |
| NetBIOS names, AppleTalk ASP, and DECnet SCP. | | | | the nature of the network's hardware elements. |
| Transport Layer (Layer 4) defines several | | | | It also defines what kind of network interface |
| functions, including the choice of protocols. The | | | | adapter should be used and the hub that should |
| most important Layer 4 functions are error | | | | be used. Different kinds of copper or fiber optic |
| recovery and flow control. The transport layer | | | | cable can be used, and a wide variety of wireless |
| may provide for retransmission, i.e., error | | | | solutions. Usually when a LAN is set up, the |
| recovery, and may use flow control to prevent | | | | physical layer specifications are related to the |
| unnecessary congestion by attempting to send | | | | data-link layer. The data-link layer protocol, |
| data at a rate that the network can | | | | Ethernet, helps with the different kinds of options |
| accommodate, or it might not, depending on the | | | | in the physical layer. Examples includes: |
| choice of protocols. Multiplexing of incoming data | | | | EIA/TIA- 232, V.35, EIA/TIA-449, V.24, RJ-45, |
| for different flows to applications on the same | | | | Ethernet, 802.3, 802.5, FDDI, NRZI, NRZ, and |
| host is also performed. Reordering of the incoming | | | | B8ZS. |
| data stream when packets arrive out of order is | | | | |