History of Internetworking
The first networks were time-sharing networks that used mainframes and attached terminals. Such environments were implemented by both IBM's System Network Architecture (SNA) and Digital's network architecture.
Local area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers.
Wide- area networks (WANs) interconnect LANs across normal telephone lines (and other media), thereby interconnecting geographically dispersed users.
Today, high-speed LANs and switched internetworks are becoming widely used, largely because they operate at very high speeds and support such high-bandwidth applications as voice and videoconferencing.
Internetworking evolved as a solution to three key problems: isolated LANs, duplication of resources, and a lack of network management. Isolated LANS made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did a separate support staff. This lack of network management meant that no centralized method of managing and troubleshooting networks existed.
Internetworking Challenges
Implementing a functional internetwork is no simple task. Many challenges must be faced, especially in the areas of connectivity, reliability, network management, and flexibility. Each area is key in establishing an efficient and effective internetwork.
The challenge when connecting various systems is to support communication between disparate technologies. Different sites, for example, may use different types of media, or they might operate at varying speeds.
Another essential consideration, reliable service, must be maintained in any internetwork. Individual users and entire organizations depend on consistent, reliable access to network resources.
Furthermore, network management must provide centralized support and troubleshooting capabilities in an internetwork. Configuration, security, performance, and other issues must be adequately addressed for the internetwork to function smoothly.
Flexibility, the final concern, is necessary for network expansion and new applications and services, among other factors.
Open Systems Interconnection (OSI) Reference Model
The Open Systems Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for intercomputer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained, so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.
The following list details the seven layers of the Open System Interconnection (OSI) reference model:
Layer 7---Application layer
Layer 6---Presentation layer
Layer 5---Session layer
Layer 4---Transport layer
Layer 3---Network layer
Layer 2---Data Link layer
Layer 1---Physical layer
Standards Organizations
A wide variety of organizations contribute to internetworking standards by providing forums for discussion, turning informal discussion into formal specifications, and proliferating specifications after they are standardized.
Most standards organizations create formal standards by using specific processes: organizing ideas, discussing the approach, developing draft standards, voting on all or certain aspects of the standards, and then formally releasing the completed standard to the public.
Some of the best-known standards organizations that contribute to internetworking standards include:
International Organization for Standardization (ISO)---ISO is an international standards organization responsible for a wide range of standards, including many that are relevant to networking. Their best-known contribution is the development of the OSI reference model and the OSI protocol suite.
American National Standards Institute (ANSI)---ANSI, which is also a member of the ISO, is the coordinating body for voluntary standards groups within the United States. ANSI developed the Fiber Distributed Data Interface (FDDI) and other communications standards.
Electronic Industries Association (EIA)---EIA specifies electrical transmission standards, including those used in networking. The EIA developed the widely used EIA/TIA-232 standard (formerly known as RS-232).
Institute of Electrical and Electronic Engineers (IEEE)---IEEE is a professional organization that defines networking and other standards. The IEEE developed the widely used LAN standards IEEE 802.3 and IEEE 802.5.
International Telecommunication Union Telecommunication Standardization Sector (ITU-T)---Formerly called the Committee for International Telegraph and Telephone (CCITT), ITU-T is now an international organization that develops communication standards. The ITU-T developed X.25 and other communications standards.
Internet Architecture Board (IAB)---IAB is a group of internetwork researchers who discuss issues pertinent to the Internet and set Internet policies through decisions and task forces. The IAB designates some Request For Comments (RFC) documents as Internet standards, including Transmission Control Protocol/Internet Protocol (TCP/IP) and the Simple Network Management Protocol (SNMP).
