Question

(a) What are the three basic units in communication systems?
Write briefly the function of each of these,
(b) Write any three applications of the Internet used in communication systems.

Answer 1

Answer:

Explanation:Communication system.

This system consists three basic components: transmitter, channel, and receiver.

he Internet has many important applications. Of the various services available via the Internet, the three most important are e-mail, web browsing, and peer-to-peer services . E-mail, also known as electronic mail, is the most widely used and successful of Internet applications. Web browsing is the application that had the greatest influence in dramatic expansion of the Internet and its use during the 1990s. Peer-to-peer networking is the newest of these three Internet applications, and also the most controversial, because its uses have created problems related to the access and use of copyrighted materials.

E-Mail

Whether judged by volume, popularity, or impact, e-mail has been and continues to be the principal Internet application. This is despite the fact that the underlying technologies have not been altered significantly since the early 1980s. In recent years, the continuing rapid growth in the use and volume of e-mail has been fueled by two factors. The first is the increasing numbers of Internet Service Providers (ISPs) offering this service, and secondly, because the number of physical devices capable of supporting e-mail has grown to include highly portable devices such as personal digital assistants (PDAs) and cellular telephones.

he Internet has many important applications. Of the various services available via the Internet, the three most important are e-mail, web browsing, and peer-to-peer services . E-mail, also known as electronic mail, is the most widely used and successful of Internet applications. Web browsing is the application that had the greatest influence in dramatic expansion of the Internet and its use during the 1990s. Peer-to-peer networking is the newest of these three Internet applications, and also the most controversial, because its uses have created problems related to the access and use of copyrighted materials.

Once packets get to the right machine, they have to get to the right program running on that machine. The abstraction here is the port number. Port numbers are in the range 0..65535.

On the Internet, port numbers are partitioned as follows:

0...1023 are the System Ports, assigned by the IETF Review or IESG Approval procedures described in RFC 8126.

1024...49151 are the User Ports, assigned by IANA using the IETF Review process, the "IESG Approval" process, or the "Expert Review" process, as per RFC 6335.

49152...65535 are the Dynamic Ports, which are unrestricted, do with them whatever you want.

Sockets

So hosts have IP addresses and applications run on specific ports. But how to we program with that info. Generally, there is an O.S.-level data structure called a socket consisting of:

The remote IP address

The remote port number

The local IP addresses

The local port number

A message queue

It’s likely that the O.S. provides a blocking read_socket call, which blocks until a message is available. It looks something like:

 read_socket(socket_descriptor, &buffer, number_of_bytes_to_read)

When a datagram arrives form the network, the link layer passes it to the network layer which passes it to the transport layer, where the port number is extracted. The O.S. uses the port number to send the data to the right application. If a thread is waiting there, the correct amount of data is copied into the buffer and the thread gets unblocked.