How Does E-Mail Reach Its Destination?

1. Leia o texto abaixo.
To demonstrate the various components driving computer networks in general, and specifically - the Internet, let us assume that you are at home, writing an electronic mail message to your uncle, who works on the other side of the world. You know that once you send your mail, it will somehow make itself to you uncle's computer. How exactly does this happen?

Step 2: At the ISP's
The ISP's modem converts data from the phone line back to digital pulses, and its server interprets the information. The server figures out the nature of you data, by decoding the SLIP/PPP data (in this case, the data is your e-mail).
The ISP has a local area network (LAN) (or specifically, an Ethernet LAN). The LAN connects computer in the ISP's office space. Once the data is interpreted, the server first role is to figure out if the data you had just sent is destined for a computer in the ISP's LAN. In your particular, this isn't true, since your uncle's computer isn't even in the same country.

Data not destined for a computer connected to the ISP's LAN, is forwarded to the Wide Area Network (WAN) - the Internet. For security reasons, the ISP had installed a firewall, a device that keeps unwanted people from tampering (interfering) with data on its LAN. The firewall job is to receive all data going to/from the Internet, and filtering out their "bad" parts. The firewall, in this case, also serves as a router (we'll discuss routers below).

Notice that the firewall is connected at a great distance from the network bus. Under normal circumstances, electrical signals won't be able to travel such a long distance. But here we see that the ISP installed a repeater, a device that enhances the signals, thus allowing them to travel a longer distance.

Step 4: At your uncle's office
Your uncle's office has a local area network that's connected directly to the Internet through a router. A router is a device that serves as a kind of electronic "yellow-pages" of computers. Every computer on the internet has an IP "address", and routers are the devices that know how to get data to each computer using this address.

The LAN also has a special connection to the Integrated Services Digital Network (ISDN - digital phone lines). This is done by using a gateway.

Notice that the LAN in this office is arranged in a "star" topology (unlike the network we saw at the ISP). This is a Token Ring Network.

a. A number given to each host on a TCP/IP network.

Ethernet

This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not normally affect the speed of transmission on the network.

            Ethernet cards come in 3 or 4 basic types, 10Mb, 100Mb and 1,000Mb (aka 1Gb), 10,000Mb (10Gb) is also becoming more common. These refer to the maximum data transfer rate the card can work at per second. 10Mb is obviously the slowest. 100Mb, 1Gb and 10Gb are backwards compatible and can work at the lower speeds provided by the other cards.
            A network connection will work at the highest speed that both cards can manage. So if you mix a network with 10Mb, 100Mb and 1Gb cards, whenever any card talks to a 10Mb card, the connection will always be at 10Mb as the 10 Mb card cannot work any faster. If a 100Mb card talks to another 100Mb card, the data transfer rate will be 100Mb as both cards can do 100Mb. 1Gb cards can therefore only transfer data at 1Gb per second when the other card and all intermediate network hardware is 1Gb.
            For home use you can discount, 1Gb cards as they are very expensive as are other network components designed to work at the same speed. If you remember that to utilize the 1Gb capability, every other component to which the 1Gb card will communicate with must be 1Gb, it quickly becomes prohibitively expensive. 1Gb cards are only really used in network servers, or machines where fast data transfer is imperative and these are usually only required by people who can afford it.
            So your decision is basically between 10Mb and 100Mb and quite honestly, 10Mb is very outdated and as 100Mb are so cheap there is no real point to buying 10Mb unless you have a PC that only has old style ISA slots inside. Most modern PCs have PCI slots and can easily accommodate cards of either speed, though ISA slots are limited to 10MB cards (I think). So, if you have PCI slots you should get 100Mb cards. You can pick them up for as little as £10, maybe even £8. 10Mb ISA cards can be found for around £3 or 4 for £10 at computer fairs.
            So that's that. To connect 2 computers together that's all you need hardware wise. A network card in each machine and crossover cable.

The difference between a hub and a switch is that a switch is designed to detect network traffic on the line and time the transmission of data along the line so as to avoid data collisions which would require each machine to retransmit its data.