r of terminals necessary and capital investment (see Fig. 3-7). Consider an organ- ization with many ASCII-code eight-level Teletype models 33 and 35 used on the TWX network and in timesharing and with some older, five-level Baudot Teletype models 28 and 32 used on Telex and international-record-carrier networks. It is desirable to stan- dardize on the models 33 and 35 because they are more widely used and more readily available. A combination of speed and code con- verters will make them usable on Telex-and TWX-like networks as well as in timesharing. The ASCII even-parity serial code of 11 bits (100 words per minute) is converted to a Baudot serial code of 7.5 bits (66 words per minute) and vice versa. Since most ASCII ter- minals can be interfaced by a converter, the more versatile CRT ter- minals can send messages through Telex or TWX-like networks. 3.12 TREATMENT OF ERRORS Many data transmission codes use an extra bit, called a parity bit, in each character for checking purposes. This is added so that the total number of 1 bits in the character transmitted will be an odd (or in some codes, even) number. The receiving machine detects whether there is an odd number of 1's. If not, it knows that noise or distortion on the line has lost or added a bit. It either notes this error or calls for a retransmission of the data. This is sometimes Speed Teletype and code 33 or 35 converter TWX-like To TELEX-LIKE network Baudot Network and ASCII application applications Speed timesharing and code converter Fig. 3-7. Combining speed and code conversion. 46 TELECOMMUNICATIONS called vertical redundancy checking. Unfortunately, noise on the communication line sometimes changes more than one bit, and this lessens the effectiveness of parity checking. If two 0 bits are both changed to 1's, the parity check will not detect this. Vertical parity, in which one extra bit is added to each character, will detect or correct about 85 percent of all errors transmitted, and will result in an average of one undetected error for every 700