Elementos de Redes de Computadores

Origen

De acuerdo como lo expone el autor Charles E. Spurgeon en su libro "Ethernet: The Definitive Guide" (2000), capitulo 1, sección 1.1, la historia de la Internet podría decirse que comienza en el año 1973:

"On May 22, 1973, Bob Metcalfe (then at the Xerox Palo Alto Research Center, PARC, in California) wrote a memo describing the Ethernet network system he had invented for interconnecting advanced computer workstations, making it possible to send data to one another and to high-speed laser printers. Probably the best-known invention at Xerox PARC was the first personal computer workstation with graphical user interfaces and mouse pointing device, called the Xerox Alto. The PARC inventions also included the first laser printers for personal computers, and, with the creation of Ethernet, the first high-speed LAN technology to link everything together.

This was a remarkable computing environment for the time, since the early 1970s were an era in which computing was dominated by large and very expensive mainframe computers. Few places could afford to buy and support mainframes, and few people knew how to use them. The inventions at Xerox PARC helped bring about a revolutionary change in the world of computing.

A major part of this revolutionary change in the use of computers has been the use of Ethernet LANs to enable communication among computers. Combined with an explosive increase in the use of information sharing applications such as the World Wide Web, this new model of computing has brought an entire new world of communications technology into existence. These days, sharing information is most often done over an Ethernet; from the smallest office to the largest corporation, from the single schoolroom to the largest university campus, Ethernet is clearly the networking technology of choice."

Con respecto a la evolución  y estandarización de las redes,Spurgeon agrega lo siguiente:

"The original 10 Mbps Ethernet standard was first published in 1980 by the DEC-Intel-Xerox vendor consortium. Using the first initial of each company, this became known as the DIX Ethernet standard. This standard, entitled The Ethernet, A Local Area Network: Data Link Layer and Physical Layer Specifications, contained the specifications for the operation of Ethernet as well as the specs for a single media system based on thick coaxial cable. As is true for most standards, the DIX standard was revised to add some technical changes, corrections, and minor improvements. The last revision of this standard was DIX V2.0.

When the DIX standard was published, a new effort led by the Institute of Electrical and Electronics Engineers (IEEE) to develop open network standards was also getting underway.^[3] Consequently, the thick coaxial variety of Ethernet ended up being standardized twice—first by the DIX consortium and a second time by the IEEE. The IEEE standard was created under the direction of the IEEE Local and Metropolitan Networks (LAN/MAN) Standards Committee, which identifies all the standards it develops with the number 802. There have been a number of networking standards published in the 802 branch of the IEEE, including the 802.3^[4] Ethernet and 802.5 Token Ring standards.

[3] The IEEE is the world's largest technical professional society, with members in 150 countries. The IEEE provides technical publishing, holds conferences, and develops a range of technical standards, including computer and communications standards. The standards developed by the IEEE may also become national and international standards.

[4] Pronounced "eight oh two dot three."

The IEEE 802.3 committee took up the network system described in the original DIX standard and used it as the basis for an IEEE standard. The IEEE standard was first published in 1985 with the title IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications . The IEEE standard does not use "Ethernet" in the title, even though Xerox relinquished their trademark on the Ethernet name. That's because open standards committees are quite sensitive about using commercial names that might imply endorsement of a particular company. As a result, the IEEE calls this technology 802.3 CSMA/CD or just 802.3. However, most people still use the Ethernet name when referring to the network system described in the 802.3 standard.

The IEEE 802.3 standard is the official Ethernet standard. From time to time you may hear of other Ethernet technology "standards" developed by various groups or vendor consortiums. However, if the technology isn't specified within the IEEE 802.3 standard, it isn't an official Ethernet technology. Periodically, the latest IEEE 802.3 standards are presented to the American National Standards Institute (ANSI), which forwards them on, where they are adopted by the International Organization for Standardization (ISO). This organization is described in more detail later in this chapter. Adoption by the ISO means that the IEEE 802.3 Ethernet standard is also a worldwide standard, and that vendors from around the globe can build equipment that will work together on Ethernet systems.”

Modelo OSI 

El modelo de referencia OSI (Open System Interconnection) se compone de 7 "capas" ascendentes del hardware al usuario final:

• Capa física
• Capa de enlace de datos
• Capa de red
• Capa de transporte
• Capa de sesión
• Capa de presentación
• Capa de aplicación 

Es un modelo de referencia que no se implementó por estas razones:

•Mal momento

•Mala tecnología

•Mala implementación

•Malas políticas

A continuación se muestra un gráfico del modelo de referencia OSI:

  Tanenbaum, 4 ed. 2003

Modelo TCP /IP

Modelo de descripción de protocolos de red desarrollado en los años 70, implantado en la red  ARPANET  la primera  red de área amplia o WAN, desarrollada por encargo de DARPA, una agencia del  Departamento de Defensa de los Estados Unidos, y predecesora de la actual red Internet

Tanenbaum, 4 ed. 2003

 

Crítica al modelo TCP /IP

• El servicio, la interface y el protocolo no están diferenciados


• No es un modelo general


• La capa de cliente a red (host to network) no es en realidad una capa.


• No se mencionan las capas fìsica y de enlace de datos.


•Los protocolos menores son dìficiles de reemplazar.

Equipos Básicos de Comunicación

 

Topologías de Redes

Fuente: http://www.glogster.com/rodri17/topologia-de-redes/g-6kuo1qi4591ngs7f82vrba0

Fuentes: Wikipedia
Charles E. Spurgeon en su libro "Ethernet: The Definitive Guide" (2000). O'Reilly.