Information Technology Reference
In-Depth Information
implement for non-typical applications. Thus, the code used tends to reflect the type of appli-
cation. In many cases Visual Basic gives an easy-to-implement package, with the required
functionality. I've seen many a student wilt at the prospect of implementing a Microsoft Win-
dows program in C++. 'Where do I start', is always the first comment, and then 'How do I
do text input', and so on. Visual Basic, on the other hand, has matured into an excellent de-
velopment system which hides much of the complexity of Microsoft Windows away from the
developer. So, don't worry about computer language snobbery. Pick the best language to
implement the specification.
UDP transmission can be likened to sending electronic mail. In most electronic mail
packages the user can request that a receipt is sent back to the originator when the elec-
tronic mail has been opened. This is equivalent to TCP, where data is acknowledged after a
certain amount of data has been sent. If the user does not receive a receipt for their elec-
tronic mail then they will send another one, until it is receipted or until there is a reply. UDP
is equivalent to a user sending an electronic mail without asking for a receipt, thus the origi-
nator has no idea if the data has been received, or not.
TCP/IP is an excellent method for networked communications, as IP provides the routing
of the data, and TCP allows acknowledgements for the data. Thus, the data can always be
guaranteed to be correct. Unfortunately there is an overhead in the connection of the TCP
socket, where the two communicating stations must exchange parameters before the connec-
tion is made, then they must maintain and acknowledge received TCP packets. UDP has the
advantage that it is connectionless. So there is no need for a connection to be made, and data
is simply thrown in the network, without the requirement for acknowledgments. Thus UDP
packets are much less reliable in their operation, and a sending station cannot guarantee
that the data is going to be received. UDP is thus useful for remote data acquisition where
data can be simply transmitted without it being requested or without a TCP/IP connection
being made.
The concept of ports and sockets is important in TCP/IP. Servers wait and listen on a
given port number. They only read packets which have the correct port number. For exam-
ple, a WWW server listens for data on port 80, and an FTP server listens for port 21. Thus a
properly set up communication network requires a knowledge of the ports which are ac-
cessed. An excellent method for virus writers and hackers to get into a network is to install a
program which responds to a given port which the hacker uses to connect to. Once into the
system they can do a great deal of damage. Programming languages such as Java have
built-in security to reduce this problem.
