Nos últimos vinte anos, condições meteorológicas extremas mataram 530 mil pessoas e causaram prejuízos econômicos que chegam a 2,5 trilhões de dólares. Em 2012, os países mais afetados por desastres naturais foram o Paquistão, o Haiti e as Filipinas, segundo relatório da Organização das Nações Unidas (ONU). A passagem do supertufão Hayan serve de trágica ilustração para a 19.ª Conferência de Clima, na Polônia, e deveria servir de incentivo para que a comunidade internacional coloque em prática todas as promessas feitas de combate ao aquecimento global.

O Globo, 13/11/2013, p. 33 (com adaptações).

     O crescimento mais acelerado do rendimento das classes mais baixas, o aumento do emprego e as transferências de renda aos mais pobres nos últimos anos não foram suficientes para eliminar o fosso que ainda separa os moradores de favelas dos que vivem fora delas, segundo demonstra recente pesquisa realizada pelo Instituto Brasileiro de Geografia e Estatística (IBGE). Mas os dois universos se aproximam bastante quando a pesquisa se volta para alguns índices de consumo, como a posse de aparelhos de TV e geladeira, por exemplo.

Jornal de Brasília, 7/11/2013, p. 19 (com adaptações).

Repeaters and hubs

A repeater is an electronic device that receives a network signal, cleans it of unnecessary noise, and regenerates it. The signal is retransmitted at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. A repeater with multiple ports is known as a hub. Repeaters work on the physical layer of the OSI model. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network performance. As a result, many network architectures limit the number of repeaters that can be used in a row, e.g., the Ethernet 5-4-3 rule.
Hubs have been mostly obsoleted by modern switches; but repeaters are used for long distance links, notably undersea cabling.


Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model to form a single network. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do. Instead, bridges learn which MAC addresses are reachable through specific ports. Once the bridge associates a port with an address, it will send traffic for that address to that port only.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, the bridge assumes that the MAC address is associated with that port and stores its source address.
The first time a bridge sees a previously unknown destination address, the bridge will forward the frame to all ports other than the one on which the frame arrived.

Bridges come in three basic types:

Local bridges: Directly connect LANs

Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced with routers. Wireless bridges: Can be used to join LANs or connect remote devices to LANs.

Internet: http://en.wikipedia.org (adapted).

Types of operating systems

Real-time
A real-time operating system is a multitasking operating system that aims at executing real-time applications. Real-time operating systems often use specialized scheduling algorithms so that they can achieve a deterministic nature of behavior. The main objective of real-time operating systems is their quick and predictable response to events. They have an event-driven or time-sharing design and often aspects of both. An event-driven system switches between tasks based on their priorities or external events while time-sharing operating systems switch tasks based on clock interrupts.

Multi-user
A multi-user operating system allows multiple users to access a computer system at the same time. Time-sharing systems and Internet servers can be classified as multi-user systems as they enable multiple-user access to a computer through the sharing of time. Single-user operating systems have only one user but may allow multiple programs to run at the same time.

Multi-tasking vs. single-tasking
A multi-tasking operating system allows more than one program to be running at the same time, from the point of view of human time scales. A single-tasking system has only one running program. Multi-tasking can be of two types: pre-emptive and co-operative. In pre-emptive multitasking, the operating system slices the CPU time and dedicates one slot to each of the programs. Unix-like operating systems such as Solaris and Linux support pre-emptive multitasking, as does AmigaOS. Cooperative multitasking is achieved by relying on each process to give time to the other processes in a defined manner. 16-bit versions of Microsoft Windows used cooperative multi-tasking. 32-bit versions of both Windows NT and Win9x, used pre-emptive multi-tasking. Mac OS prior to OS X used to support cooperative multitasking.

Distributed
A distributed operating system manages a group of independent computers and makes them appear to be a single computer. The development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine. When computers in a group work in cooperation, they make a distributed system.


Embedded
Embedded operating systems are designed to be used in embedded computer systems. They are designed to operate on small machines like PDAs with less autonomy. They are able to operate with a limited number of resources. They are very compact and extremely efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems.

Internet: http://en.wikipedia.org (adapted)