I figured this would help a lot of people who are getting ready to go back to school to learn system administrating, networking, or attempting to obtain your MCSEs, this information should be very useful. This is taken from the good book Networking with Microsoft TCP/IP third edition.

The OSI Reference Model

The International Organization for Standardization (ISO) developed the ISO reference model as a guide for defining a set of open protocols. Although interest in the OSI protocols has waned, the OSI reference model remains the most common standard for describing and comparing protocol suites.

There are seven (7) layers to the (OSI) Reference Model. Each layer provides a specific type of network device. The layers are numbered from the bottom of the protocol stack to the top.

The Physical Layer

The Physical layer communicates directly with the communication medium and has two responsibilities: sending bits and receving bits. A Binary digit, or bit, is the basic unit of information in data communication. A bit can have only two values, 1, or 0, which are represented by different states on the communciation medium. Other communication layers are responsible for collecting these bits into groups that represent message data.

Bits are represented by changes in signals on the network medium. Some wire media represent 1s and 0s with different voltages, some use distinct audio tones, and yet others use more sophisticated methods, such as state transitions (changes from high-to-low- or low-to-high voltages).

A wide variety of media are used for data communication, including electric cables, fiber optics, light waves, radio waves, and microwaves. The medium used can vary; a different medium simply necessitates substituting a different set of Physical layer protocols. The upper layers are completely independent from the particular process used to deliver bits through the network medium.

An important distinction is that the OSI Physical layer does not, strictly speaking, describe the media themselves. Physical layer specifications describe how data can are encoded into media signals and the characteristics of the media attachmentinterface, but the specifications do not describe the medium itself. In actual practice, however, many Physical layer standards cover characteristics of the OSI Physical layer as well as characteristics of the medium.

The Data Link Layer

Devices that can communicate on a network are frequently called nodes.
(Other names include station and device.) The. Data Link layer is responsible for providing node-to-node communciation on a single, local network. To provide this service, the Data Link layer must perform two functions. It must translate messages from the upper layers into bits that the Physical layer can transmit.

When the Data link layer receives a message to transmit, it formats the messages into a data frame. (You also hear data frames referred to as packets.) The sections of a frame are called fields.

Start Indicator; A specific bit pattern indicates the start of a data frame.

Source Address; The address of the sending node is also includeed so that replies to messages can be addresses properly.

Destination Address; Each node is identified by an address. The Data Link Layer of the sender adds the destination address to the frame. The Data Link layer
of the receiver looks at the destination address to identify messages it should receive.

Control In many cases, additional control information must be included. The specific information is determined by each protocol.

Data; This field contains all data that were forwarded to the Data link layer from upper protocol layers.

Error Control; This field contains information that enables the receving node to determine whether an error occured during transmission. A common approach is cyclic redundancy checksum (CRC), which is a calculated value that summarizes all the data in the frame.
The sending node calculates a checksum and stores it in the frame. The receiver recalculates the checksum. If the receiver's calculated CRC matches the CRC value in the frame, it can safely be assumed that the frame was transmitted without error.

Frame delivery on a local network is extremely simple. A sending node simply transmit the frame. Each node on the network see every frame and examines the destination address. When the destination address of a frame matches the node's address, the Data Link layer at the node receieves the frame and sends it up the protocol stack.

The Network Layer

Only the smallest networks consist of a single, local network. Most networks must be subdivided. A network that consists of several network segments is frequently called an internetwork.

These subdivisions can be planned to reduce traffic on network segments or to isolate remote networks connected by slower communication media. When networks are subdivided, it can no longer be assumed that messages will be delivered on the local network. A mechanism must be put in place to route messages from one network to another.

For messages to be delivered on an internetwork, each network must be uniquely identifies by a network address. When it recieves a message from upper layers, the Network layer adds a header to the message that includes the source and destination network address. This combination of data plus the Network Layer is called a packet. The network address information is used to deliver a message to the correct network. After the message arrives on the correct network, the receving Data link Layer can use the node address to deliver the message to a specific node.

Forwarding packets to the correct network is called routing, and the devices that route packets are called routers.

End nodes Provide users services. End nodes do use the Network layer to add network address information to packets, but they do not perfom routing. End nodes are sometimes called end systems (the OSI term) or hosts (the TCP/IP term).

Routers Incorporate special mechanisms that perform routing. Because routing is a complex task, routers are usually dedicated devices that do not provide services to end users. Routers sometimes are called intermediate systems (the OSI term) or gateways ( the historic TCP/IP term).

The Network Layer operates independently of the physical medium, which is a concern of the physical layer. Because routers are Network layer devices, they can be used to forward packets between physically different networks. A router can join an Ethernet to a token ring network, for example. Routers also are often used to connect a local area network (LAN) such as ethernet to a wide area network (wan), auch as ATM.

The Transport Layer

All network technologies set a maximum size for frames that can be senton the network. Ethernet, for example, limits the size of the data field to 1,500 bytes. This limit is necessary for the following two reasons:

1.) Small frames improve network efficiency when many devices must share the network. If a device could transmit frames of unlimited size, it might monopolize the network for an excessive period of time. With small frames, devices take turns at shroter intervals, and devices are more likely to have ready access to the network.

2.) With small frames, less data must be retransmitted to correct an error. If a 100KB message encouters an error of a single byte, the entire 100KB message must be retransmitted. However, if the message is divided into one hundred frames, each limited in size to 1KB, a one-byte error requires the retransmission of merely a single 1KB frame.

One responsibility of the Transport Layer is to divide messages into fragments that fit within the size limitations established by the network. At the receiving end, the Transport layer reassembles the fragments to recover the orginal message.

When messages are divided into multiple fragments, the possibility that segments might not be received in the order they are sent increases.

Under the OSI model, the Transport layer assigns a service access poinjt (SAP) IS to each packet. The TCP/IP term for a service access point is port. The SAP ID is an address that identifies the process that orginated the message. The SAD ID enables the Transport layer of the receiving node to route the message to the appropiate process.

Identifying messages from several processes so that the messages can be transmitted through the same network medium is called multiplexing. The procedure of recovering messages and driecting them to the correct process is called demultiplexing.

Because multiple protocols can be supported for any given layer, multiplexing and demultiplexing can occur at many layers. Some examples of multiplexing include the following:

Transport of different Ethernet frame types over the same medium. (Data link layer)

Messages for mulitple trasnport protocols, such as TCP and UDP on TCP/IP systems (Transport layer)

Messages for mulitple application protocols such as telnet, FTP and SMTP on a unix host session and higher layers

The Session Layer

The session layer is responsible for dialog control between nodes. A dialog is a formal conversation in which two nodes agree to exchange data.

Communication can take place in three dialog modes:

Simplex One node transmit exclusively, while another receives exclusively.

Half dublex Only one node can send at any given time, and nodes take turns transmitting.

Full dublex Nodes can transmit and recieve simultaneously. Full communication typically requires some form of flow control to ensure that neither device sends faster than the other device can receive.

Sessions enable nodes communicate in an organized manner Each session has three phases:

1. Connection establishment. The nodes establish contact. They negotiate the rules of communications, including the protocols to be used and communication parameters.

2. Data Transfer The node engage in a dialog to exchange data.

3. Connection release When the nodes no longer need to communicate, they engage in an orderly release of the session.

The Presentation Layer

The presentation layer is responsible for presenting data to the application layer. The presentation layer directly translate the data from one format to another. IBM mainframe computers use a character encoding scheme called EBCDIC, all other computers use the ASCII encoding scheme. If data are being transmitted from an EBCDIC computer to an ASCII computer, the Presentation layer might be responsible for translating between the different character sets. Numeric data is also represented quite differently on different computer architectures and must be converted when transferred between different machine times.
A common techniqueused to improve data trasfer is to covert all data to a standard format before transmission. This standard format probaly is not the native data format of any computer. All computers can be configured to retrieve standard format data, however, and convert it ito their ative data formats. The OSI protocol standards define Abstract Syntax Represetatio, Revision (ASN.1) as a standard data syntax for use at the Presentation layer. Although the TCP/IP protocol suite does not formally define a Presentationlayer, a protocol that serves a similar function is External Data Representation (XDR), which is used with the Network File System (NFS). Other functions that may fall to the Presentation layer are data encryption/decryption and compression/decompression.

The Presentation layer is the least frequently implemented of the OSI layers. Few protocols have been formally defined this layer. In most cases, network applications perform the functions that might be associated with the Presentation layer.

The application Layer

The application layer provides the services user applications need to communicate through the network. Examples:

Electronic mail transport A protocol for handling electronic mail can be used by a variety of applications.

Remote file access Local applications can be given the capability to access files on remote nodes.

Remote job execution Local applications can be given the capabilitys to start and control processes on other nodes.

Directories The network can offer a directory of network resources, including logical node names. This directory enables applications to access network sources by logical names instead of by abstract numeric node ID's.

Network Management Nteowkr management protocols can enabl various applications to access network management information.

You frequently encounter the term application program interface (API) used in conjuction with Application layer services. An API is a set of rules that enables user-written applications to access the services of a software system. Developers of program products and protocols frequently provide APIs, which enable programmers to easily adapt their applications to use the services the products provide. Acommon UNIX API is Berkeley Sockets, which Microsoft has implemented as Windows sockets.

This is only brief this doesnt cover every aspect of it.