How Does E-Mail Reach It's Destination?
Step 1: Sending (uploading) mail
To connect to the Internet, you must have an account with an Internet Service Provider (ISP), a company that connects people to the Internet. Connection from your computer to the ISP's computers will probably be made through the phone, using a device called a modem. The modem enables your computer to transmit digital data over the phone lines, which are analog.The data passed between the two computers is decoded using computer protocols. In this case, various modem specific protocols (such as X.25) are used, as well as protocols from the Internet protocol suite (such as PPP and SLIP). Your mail program sends your mail through the use of yet another protocol - the Simple Mail Transfer Protocol (SMTP) .
Step 2: At the ISP's
The ISP's modem converts data from the phone line back to digital pulses, and it's 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 spefically, 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 with data on it's LAN. The firewall job is to receive all data going to/from the Internet, and filtering out it's "bad" part. 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 3: The WAN
Wide Area Network (WAN) is a general term that refers to a network that exceeds local boundries. A WAN usually includes many LANs in it, which together form a huge network (as represented by the cloud to the left). The world's most famous WAN is the Internet.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 "address", and routers are the devices that know how to get data to each computer using this address.The LAN is also has a special connection to the Integrated Services Digital Network (ISDN - digital phone lines). The is done by using a gateway - a device that "translates" between the "languages" of the LAN and ISDN.
Notice that the LAN in this office is arranged in a "star" topology (unlike the network we saw a the ISP). This is a Token Ring Network
