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