Chapter 20: Networks and Communications^531
Dial-up Connections
Windows PCs control a modem through the Dial-u pNetworking (DUN) utility. DUN
has a built-in dialer applet that is invoked whenever an application, such as a browser or
an e-mail client, is opened. The dialers send to the modem the commands needed to dial
up a remote modem and make a connection. The speed of a dial-up connection is typi-
cally between 28.8 Kbps and 56 Kbps.
When two PCs directly connect over a modem-to-modem connection, a process
called a handshake must take place to set up the connection for the length of the session.
The handshake process includes a series of signals that are passed between the two
modems. See Chapter 19 for more information on the handshaking process.
When you call your ISP, you are assigned an IP address through its NAS (network ac-
cess services) or modem banks and the ISP’s RADIUS (Remote Authentication Dial-In
User Service) services. Once you have been authenticated through a username and pass-
word combination, an IP address is assigned to your PC and you are able to communicate
over the Internet WAN.
Messages are sent from one PC to another over a network in the form of network
packets, regardless of whether the network is a LAN, WAN, or another type of network.
A packet holds one portion of the whole message along with the IP addresses of the
sender and the destination PC. The TCP/IP protocols break up the original message and
create the packets so that the message can be transmitted over the network media. At the
receiving end, the protocols reassemble the message from the packets and send it to the
destination PC.
Dial-upnetworkingusesthePoint-to-PointProtocol(PPP)tosendpacketsoverPSTN
lines. PPP inserts the packet created by the sending protocols into a PPP packet and car-
ries it over the transmission. At the receiving end, the original packet is removed from the
PPP packet and passed to the processing protocols. PPP is merely the intermediary that
carries the data packet over the telephone line. If the packet begins the journey as a packet
from TCP/IP, IPX (Internetwork Packet Exchange), or another protocol, it arrives at its
destination as a packet from that protocol.Troubleshooting Modem Connections
Adial-upconnectionthatwillnotconnecthasseveralareasthatshouldbeincludedinthe
troubleshooting: Phone connection Nearly all modems use sound to allow the user to track
the action of the connection (handshake) as it is being made. The first of these
sounds is the dial tone from the phone line. If the modem is not connecting and
you do not hear a dial tone, chances are there is a problem with the wall jack
connection or the phone line itself. You will probably get an error message to
the effect that you have no dial tone.
Modem problems If the modem cannot complete the handshake with the
other end, it could be that the modem is configured incorrectly in terms of its
character length, start and stop bits, and speed.