The OSI Layers
The following sections briefly describe each of the seven layers of the OSI model, starting at the lowest layer. Appendix C, "Open System Interconnection (OSI) Reference Model," delves deeper into the details of the OSI model.
Physical Layer—Layer 1
The OSI physical layer defines specifications such as the electrical and mechanical conditions necessary for activating, maintaining, and deactivating the physical link between devices. Specifications include voltage levels, maximum cable lengths, connector types, and maximum data rates. The physical layer is concerned with the binary transmission of data. This binary data is represented as bits (which is short for binary digits). A bit has a single binary value, either 0 or 1.
Data Link Layer—Layer 2
Layer 2, the data link layer, defines the format of data that is to be transmitted across the physical network. It indicates how the physical medium is accessed, including physical addressing, error handling, and flow control. The data link layer sends frames of data; different media have different types of frames.
KEY POINT
A frame is a defined set of data that includes addressing and control information and is transmitted between network devices. A frame can contain a header field (in front of the data) and a trailer field (after the data); these two fields are said to "frame" the data.
For LANs, the Institute of Electrical and Electronics Engineers (IEEE) split Layer 2 into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
The LLC sublayer (defined by the IEEE 802.2 specification) allows multiple network layer (Layer 3) protocols to communicate over the same physical data link by allowing the Layer 3 protocol to be specified in the LLC portion of the frame.
Some examples of MAC sublayer protocols are IEEE 802.3 Ethernet and IEEE 802.5 Token Ring. The MAC sublayer specifies the physical MAC address that uniquely identifies a device on a network. Each frame that is sent specifies a destination MAC address; only the device with that MAC address should receive and process the frame. Each frame also includes the MAC address of the frame's source.
NOTE You might be interested in some IEEE trivia: The IEEE 802 committee was formed in February (the second month) of 1980, and thus was called "802." The IEEE 802.3 standard, for example, was ratified in the IEEE annex building 3 in Geneva at that time.
Network Layer—Layer 3
The network layer is responsible for routing, which allows data to be properly forwarded across a logical internetwork (consisting of multiple physical networks). Logical network addresses (as opposed to physical MAC addresses) are specified at Layer 3. Layer 3 protocols include routed and routing protocols. The routing protocols determine the best path that should be used to forward the routed data through the internetwork to its destination.
The network layer sends datagrams (orpackets); different routed protocols have different types of datagrams.
KEY POINT
A datagram is a defined set of data that includes addressing and control information and is routed between the data's source and destination.
If a datagram needs to be sent across a network that can handle only a certain amount of data at a time, the datagram can be fragmented into multiple packets and then reassembled at the destination. Therefore, a datagram is a unit of data, whereas a packet is what physically goes on the network. If no fragmentation is required, a packet is a datagram; the two terms are often used interchangeably.
Transport Layer—Layer 4
Layer 4, the transport layer, is concerned with end-to-end connections between the source and the destination. The transport layer provides network services to the upper layers.
Connection-oriented reliable transport establishes a logical connection and uses sequence numbers to ensure that all data is received at the destination. Connectionless best-effort transport just sends the data and relies on upper-layer error detection mechanisms to report and correct problems. Reliable transport has more overhead than best-effort transport.
KEY POINT
Best-effort delivery means that the protocol will not check to see whether the data was delivered intact; a higher-level protocol, or the end user, must confirm that the data was delivered correctly
Multiplexing allows many applications to use the same physical connection. For example, data is tagged with a number that identifies the application from which it came. Both sides of the connection then can interpret the data in the same way.
The transport layer sends segments.
KEY POINT
A segment is a defined set of data that includes control information and is sent between the transport layers of the sender and receiver of the data.
Upper Layers—Layers 5 Through 7
From the lower layers' perspective, the three upper layers represent the data that must be transmitted from the source to the destination; the network typically neither knows nor cares about the contents of these layers. For completeness, the following briefly describes the functions of these layers:
■ The session layer, Layer 5, is responsible for establishing, maintaining, and terminating communication sessions between applications running on different hosts.
■ The presentation layer, Layer 6, specifies the format, data structure, coding, compression, and other ways of representing the data to ensure that information sent from one host's application layer can be read by the destination host.
■ Finally, the application layer, Layer 7, is the closest to the end user; it interacts directly with software applications that need to communicate over the network.
KEY POINT
The OSI application layer is not the application itself; rather, the OSI application layer provides the communication services to the application.
For example, your e-mail application might use two OSI application layer protocols— Simple Mail Transfer Protocol (SMTP) and Post Office Protocol version 3 (POP3)—to send and retrieve e-mail messages.
Continue reading here: Communication Among OSI Layers
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