Match the terms in Table A with the statements in Table B. I Transmission mode in which each computer takes turns sending and receiving

Table A

a Bracketing

b Half-duplex

с Full-duplex

d Checksum

Table В

i Transmission mode in which each computer takes turns sending and receiving

ii Mathematical calculations based on the contents of data

iii Set boundaries for the beginning and end of a message

iv Transmission mode in which both computers send and receive at the same time

2 Mark the following statements as True or False:

a Most of the work that an application does to prepare a message for sending over a network is not seen by the user.

b ASCII is always used to transmit data.

с The encryption layer compresses the message.

d The network layer keeps track of how many packets are in each message.

e The network layer keeps a copy of each packet until it arrives at the next node undamaged.

f Analogue signals are used on ordinary telephone lines.

g When a message arrives at its destination, it passes through the same seven network communications layers as when it was sent, but in reverse order.

Text 7

I. Find the answers to these questions in the following text.

1. What purpose does the Internet address have apart from identifying a node?

2. What data-delivery systems are mentioned in the text?

3. What do IP modules need to know about each other to communicate?

4. How many Internet addresses does a gateway have?

5. What does UDP software do?

6. When does the TCP part of TCP/IP come into operation?

7. What processes are performed by TCP software to provide reliable stream service?

8. What standard protocols are mentioned which are used to deal with the data after TCP brings it into the computer?

How TCP/IP Links Dissimilar Machines

At the heart of the Internet Protocol (IP) portion of TCP/IP is a concept called the Internet address. This 32-bit coding system assigns a number to every node on the network. There are various types of addresses designed for networks of different sizes, but you can write every address with a series of numbers that identify the major network and the sub-networks to which a node is attached. Besides identifying a node, the address provides a path that gateways can use to route information from one machine to another.

Although data-delivery systems like Ethernet or X.25 bring their packets to any machine electrically attached to the cable, the IP modules must know each other's Internet addresses if they are to communicate. A machine acting as a gateway connecting different TCP/IP networks will have a different Internet address on each network. Internal look-up tables and software based on another standard - called Resolution Protocol - are used to route the data through a gateway between networks.

Another piece of software works with the IP-layer programs to move information to the right application on the receiving system. This software follows a standard called the User Datagram Protocol (UDP). You can think of the UDP software as creating a data address in the TCP/IP message that states exactly what application the data block is supposed to contact at the address the IP software has described. The UDP software provides the final routing for the data within the receiving system.

The Transmission Control Protocol (TCP) part of TCP/IP comes into operation once the packet is delivered to the correct Internet address and application port. Software packages that follow the TCP standard run on each machine, establish a connection to each other, and manage the communication exchanges. A data-delivery system like Ethernet doesn't promise to deliver a packet successfully. Neither IP nor UDP knows anything about recovering packets that aren't successfully delivered, but TCP structures and buffers the data flow, looks for responses and takes action to replace missing data blocks. This concept of data management is called reliable stream service.

After TCP brings the data packet into a computer, other high-level programs handle it. Some are enshrined in official US government standards, like the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). If you use these standard protocols on different kinds of computers, you will at least have ways of easily transferring files and other kinds of data.

Conceptually, software that supports the TCP protocol stands alone. It can work with data received through a serial port, over a packet-switched network, or from a network system like Ethernet. TCP software doesn't need to use IP or UDP, it doesn't even have to know they exist. But in practice TCP is an integral part of the TCP/IP picture, and it is most frequently used with those two protocols.

II. Re-read the text to find the answers to these questions.