The Seven OSI's Layers ---------------------- REM An understanding of the TCP & IP protocols would help you understand the 7 OSI's layers easier. In 1984, the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) reference model as a better way to describe the sofisticated / complex network layering. This model has been widely used to describe the flow of data between various network layers and the user applications. There are 7 OSI layers: 1. Physical (basis) 2. Data Link 3. Network 4. Transport 5. Session 6. Presentation 7. Application The Physical Layer ------------------ The Physical Layer is the Network Interface Card [NIC] + the network cable. The NIC transfers / receives data frames to / from other computers on the network. Determining which NIC to use is based on the physical network media wich may be infrared, copper, fiber optics or an antenna transmitting and receiving radio frequency (RF) data. 2 major design criteria for the media choice are how far and how fast you need to send data. Masking the media, the physical layer connects the Data Link Layer to the network. Data Link Layer --------------- The data link layer is the one that sends data frames that are received from the network layer to the physical layer and is responsible for the error free transfer of data from one puter to another through the network. In the receiving box, the data link layer organizes the received data bits from the physical layer into network layer compatible data frames. After the data link layer passes the data frames to the network layer, it waits for acknoledgment that the data was transferedcorrectly. If the data link layer doesn't receive an acknoledgment then it re-transmits the data frame. (the data link layer is similar to tcp) ... The timing and retries are governed by the protocol and communications settings. To make this even more complicated to understand, the Institute of Electrical and Electronics Engineers (IEEE) divided the data link layer into 2 sub-layers: the Media Access Control (MAC) and the Logical Link Control (LLC). The LLC manages communications services through the use of SAP (Service Access Points). Using SAP, the LLC can determine/decide where to send the data in the upper OSI model layer. The LLC is also responsible for error handling. If MAC notifies LLC about a network error, LLC retransmits the lost data frame(s). MAC is responsible for the error-free transmission and reception of data frames to and from the physical layer. The MAC is a lower level sub-layer that contains the NIC software driver. Network Layer ------------- The Network Layer is like a network cop that manages traffic problems on the network. It can switch, rout and controll data congestion as well as translate node names into network addresses. The Network Layer also assembles (packs) small data frames together for transmission across the network and restructures larger frames into smaller packets. When the packets are received by the remote computer, the network layer reassembles them into the original frame structure. Transport Layer --------------- As the names says, this layer is responsible for the error-free transfer of data between higher levels of the OSI model. This layer works almost as the network layer ... it helps to the transmission of packets across the network, but when the packets are received, the transport layer unpacks the packets, removes the messages, checkpoints, reassembles with the network layer's help the original msg and sends a receipt acknoledgment. Session Layer ------------- The session layer establishes a connection link between 2 or more computers. This connection link is usually called a session. In order to establish a link, the session layer executes some functions, like: perform name and user rights recognition, creates data checkpoing (purpose: increasing data security) and controls which computer has clear access to send data. Presentation Layer ------------------ This layer translates the obtained data from the application layer into its own intermidiate format. When it receives data from the lower layers, the presentation layer performs reverse translation and passes the result to the waiting applications. The presentation layer's intermediate format is designed to reduce / compress the overhead by packing data. In addition to compression, the presentation layer is equipped to convert protocols, switch the character set of ASCII data, and expand the graphics data. This layer also performs a security management, including data encryption. Application Layer ----------------- The application layer contains the application processes that use network services. It also contains the application programming interface (API). The API is a library of standard functional utilities that can be used by some applications, like : notepad, wordpad, word, visual basi, c++ etc. Well, this is the OSI 7 layer model ... have fun using ur knowledge. This is more for general information ... 2:30PM 1/19/99 Greets to pROcon {ur da man}, LadyinBlk, phooey, Biosone, koala7,syn|ack,Ledge and all my friends in the lair ;) Written by HB3^ aka PhantomDialer