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Open Shortest Path First (OSPF) Protocol

A link state routing protocol that is used to exchange routing information between dynamic routers. Open Shortest Path First (OSPF) is an Interior Gateway Protocol (IGP) that is used mainly in larger TCP/IP internetworks and within autonomous systems of the Internet. OSPF is more efficient in terms of network overhead than the Routing Information Protocol (RIP), but it is considerably more complex to plan and implement in an enterprise.

Open Systems Interconnection (OSI) reference model

An architectural model for open networking systems that was developed by the International Organization for Standardization (ISO) in Europe in 1974. The Open Systems Interconnection (OSI) reference model was intended as a basis for developing universally accepted networking protocols, but this initiative essentially failed for the following reasons:

The standards process was relatively closed compared with the open standards process used by the Internet Engineering Task Force (IETF) to develop the TCP/IP protocol suite.
The model was overly complex. Some functions (such as connectionless communication) were neglected, while others (such as error correction and flow control) were repeated at several layers.
The growth of the Internet and TCP/IP—a simpler, real-world protocol model—pushed the OSI reference model out.

The U.S government tried to require compliance with the OSI reference model for U.S. government networking solutions in the late 1980s by implementing standards called Government Open Systems Interconnection Profiles (GOSIPs). This effort was abandoned in 1995, however, and now few real-world implementations of OSI networking protocols exist outside of Europe.

The OSI reference model is best seen as an idealized model of the logical connections that must occur in order for network communication to take place. Most protocol suites used in the real world, such as TCP/IP, DECnet, and Systems Network Architecture (SNA), map somewhat loosely to the OSI reference model. The OSI model is a good starting point for understanding how various protocols within a protocol suite function and interact.

The OSI reference model has seven logical layers, as shown in the following table.

The OSI Reference Model

Layer Number	Layer Name	Description
7	Application layer	Interfaces user applications with network functionality, controls how applications access the network, and generates error messages. Protocols at this level include HTTP, FTP, SMTP, and NFS.
6	Presentation layer	Translates data to be transmitted by applications into a format suitable for transport over the network. Redirector software, such as the Workstation service for Microsoft Windows NT, is located at this level. Network shells are also defined at this layer.
5	Session layer	Defines how connections can be established, maintained, and terminated. Also performs name resolution functions.
4	Transport layer	Sequences packets so that they can be reassembled at the destination in the proper order. Generates acknowledgments and retransmits packets. Assembles packets after they are received.
3	Network layer	Defines logical host addresses such as IP addresses, creates packet headers, and routes packets across an internetwork using routers and Layer 3 switches. Strips the headers from the packets at the receiving end.
2	Data-link layer	Specifies how data bits are grouped into frames, and specifies frame formats. Responsible for error correction, flow control, hardware addressing (such as MAC addresses), and how devices such as hubs, bridges, repeaters, and Layer 2 switches operate. The Project 802 specifications divide this layer into two sublayers, the logical link control (LLC) layer and the media access control (MAC) layer.
1	Physical layer	Defines network transmission media, signaling methods, bit synchronization, architecture (such as Ethernet or Token Ring), and cabling topologies. Defines how network interface cards (NICs) interact with the media (cabling).

You can think of each layer as being logically connected to the same layer on a different computer on the network. For example, the application layer on one machine communicates with the application layer on another machine. But this communication is logical only; physical communication occurs when packets of data are sent down from the application layer of the transmitting computer, encapsulated with header information by each lower layer, and then put on the wire at the physical layer of the transmitting computer. After traveling along the wire, the packets are picked up by the physical layer of the receiving computer, passed up the seven layers while each layer strips off its associated header information, and then passed to the application layer of the receiving computer, where the receiving application can process the data.

Out-of-band management (OBM)

A method of remotely managing the wide area network (WAN) telecommunications components of your network by using a secondary serial communication link. These devices are typically routers, switches, or Channel Service Unit/Data Service Units (CSU/DSUs) that establish and maintain WAN links to the corporate network. Devices that can be managed out-of-band usually have an RS-232 port or some other serial port for remote control of their functions.

Out-of-band signaling

Any transmission technology in which signaling is separate from the data being transmitted. Out-of-band signaling uses one or more channels for transmitting data or voice information and one special out-of-band channel for performing signaling functions such as establishing and terminating the communication link, controlling flow, or transmitting error information. The out-of-band channel can be

A physically separate set of wires (such as pins 4 and 5 of an RS-232 cable, which perform flow control functions and do not carry data)
A multiplexed system in which bandwidth is divided into two or more channels within the same set of wires (such as Integrated Services Digital Network, in which the two B channels and one D channel are multiplexed onto the same set of wires)

The opposite of out-of-band is in-band, in which signaling information is sent over the same channel as the data transmission. Out-of-band transmission is usually considered a better choice than in-band transmission for the following reasons:

None of the valuable data bandwidth is used for signaling.
The data stream is not interrupted with signaling information.
The signaling information cannot be disrupted by the noise created by the data transmission.
Data transmission characters cannot accidentally (or purposefully) initiate control actions.

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