Practical TCP/IP and Ethernet Networking- P33: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 142 Practical TCP IP and Ethernet Networking 2 Reconnection 3 Suppress go ahead - . no longer send go-ahead signal after data 4 Approximate message size negotiation 5 Status request - used to obtain the status of a TELNET option from the remote machine. 6 Request timing mark - used to synchronize the two ends of a connection 7 Remote controlled transmission and echo 8 Output line width 9 Output page length 10 Output carriage-return action 11 Output horizontal tab stop setting 12 Output horizontal tab stop action 13 Output form feed action 14 Output vertical tab stop setting 15 Output vertical tab stop action 16 Output line feed action 17 Extend ASCII characters 18 Logout 24 Terminal type - used to exchange information about the make and model of a terminal being used 25 End of record - sent at end of data 28 Terminal location number 31 Window size 34 Line-mode - uses local editing and sends complete lines instead of individual characters. The two-octet sequence may be followed by a third octet containing optional parameters. An optional code of 1 indicates ECHO therefore the three octets sequence 255-251-1 means WILL ECHO and instructs the other end to begin echoing back the characters that it receives. A command sequence of 255-252-1 indicates that the sender either will not echo back characters or wants to stop echoing. The negotiation of options allows clients and servers to optimize their interaction. It is also possible for newer versions of TELNET software that provide more options to work with older versions as only the options that are recognized by both ends are negotiated. If the server application malfunctions and stops reading data from the TCP connection the operating system buffers will fill up until TCP eventually indicates to the client system a window size of zero thus preventing further data flow from the client. In such a situation TELNET control codes will not be read and therefore will have no effect. To bypass the normal flow control .
