Now That I Know Whom to Call, What Do I Say?

Now That I Know Whom to Call, What Do I Say?

So far, you've learned a few of the concepts behind how two modems, on either end of a telephone circuit, can transmit and receive data. Those details mostly relate to OSI Layer 1, the physical layer.

Like all good networking devices, when a PC makes a phone call to an ISP, the PC typically needs to send and receive IP packets to and from the Internet. In most cases today, the PC and router use PPP as the data link protocol. (PPP was covered in Chapter 14.) PPP lets Fred send and receive packets over the dialed phone circuit.

For instance, in Figure 16-5, imagine that Fred hits the www.bowling.com website to check out the latest in bowling news. You know from Chapter 13 that Fred will need to send an IP packet to the DNS server to resolve the name www.bowling.com to an IP address. Then, he will need to send some IP packets to the web server, just to set up the TCP connection. After that, Fred can send a packet to the server asking for the home page of www.bowling.com. Routing takes place, just like it always did. Between the ISP router and Fred's PC, PPP is used to send and receive frames over the dialed circuit.