1. Chapter Nine: Data Transmission

2. Introduction
Binary data is transmitted by either by serial or parallel methods
Data transmission over long distances is usually done by serial means

3. Data Coding
Data coding techniques are used to convert alphanumeric characters into binary numbers
A data code is a standardized relationship between signaling elements and characters
Data codes are also called character sets or character codes

4. Binary and Text Data
Alphanumeric characters are not the only source of data
Digitized audio and video signals are examples of binary data that has nothing to do with the alphabet
Character codes are not needed for binary data; however, some communication systems divide all data into blocks that typically contain 7 or 8 bits, referred to as characters

5. Character Codes
Baudot code dates from 1874 and is still in use in telex and radioteletype services
It is designated as ITA2 by the CCITT
Baudot is a five-bit code giving a total of 32 possible combinations. Additional combinations are added by using two characters called LTRS and FIGS
LTRS includes only uppercase letters
The FIGS character set also includes control characters

6. ASCII Code
The American Standard Code for Information Interchange (ASCII) is the most common code for communications between microcomputers
ASCII is a seven-bit code, allowing for 128 possible combinations without shifting. Sometimes an eighth bit is added to allow for the transmission of graphic characters, mathematical symbols, and foreign language characters
ASCII has certain regularities that make programming easier; for example, bit 6 switches between upper- and lowercase letters and numbers are represented by their BCD (binary-coded decimal) equivalents

7. Asynchronous Transmission
Digital transmission formats must have the following elements:
Standard voltage ranges
Standard clock speeds
Transmitter and receiver clocks must be synchronized
Framing is necessary to determine the first bit of a character or PCM sample
Asynchronous systems use free-running clocks on the transmitting and receiving sides
A start bit is transmitted at the beginning of each character and at least one stop bit at the end of the character

8. Asynchronous Communication

9. Parallel-to-Serial Conversion
Internally, computers use parallel communication sending at least eight and usually 16 or 32 bits along on a bus
Most data communication over longer distances uses serial techniques
Therefore, a device is needed to convert parallel signals/data into serial signals/data
Typically, this device is an IC called a UART (universal asynchronous receiver-transmitter)

10. UART

11. Synchronous Transmission
In synchronous systems, the receive and transmitter clocks are synchronized to the same clock frequency
Synchronous communication is more efficient than asynchronous because start and stop bits are not necessary
Data is sent in blocks of specific data segments and have an initial identifying sequence that allows proper framing and often identifies the content of the block as well

12. Clock Recovery
Asynchronous digital systems can function with a receiver clock whose speed is only approximately the same as the transmitter clock because the clock is reset at the start of each character
The most economical way for the receiver to get clock-synchronizing information from the incoming data stream

13. Framing
Framing is necessary in data communications in order to determine which bit is the start bit of a character
In asynchronous systems, the use of a start and a stop bit solves this problem
In synchronous communications, it is only necessary to determine the start of a block of data

14. Synchronous Data-Link Protocols
Synchronous communication protocols are either bit- or character-oriented, with character-oriented protocols becoming obsolete
One of the most common protocols is the BISYNC protocol from IBM
These protocols use at least two synchronizing characters at the beginning of a block

15. Error Detection and Correction
Errors will occur with data whether sent by analog or digital means. Some method must be used to detect and correct these errors
Errors may be caused by:
High data rates
Thermal noise
RF interference
There are two stages in the control of errors:
Error detection
Error correction

16. Techniques for Detection and Correction of Errors
Some systems rely on the receiver being able to correct errors from redundant information
This technique is called forward error correction (FEC)
Systems that are full- or half-duplex that request the transmitter to repeat any data blocks that have errors are called automatic request for retransmission (ARQ) schemes

17. Parity
Parity is a simple means of error detection by using the addition of one extra bit to the bits that encode a character
The parity bit is set to make the transmitted character have either an even number or an odd number of ones
The parity is checked by the receiver and a request is made to retransmit the character

18. Longitudinal Redundancy Check
The longitudinal redundancy check (LRC) is an extension of the parity check that can provide some correction as well as detection
A parity character is made up from the block of data
By checking the parity of each character, the receiver can go to the LRC character to determine which bit is in error, allowing the correction of single-bit errors

19. Checksums Another method for error detection is the use of checksums
Checksums are developed by adding all the data words in a block and then dividing by some fixed number
The remainder that results is transmitted at the end of the block; the receiver performs the same division, and if the numbers are different, an error has occurred
XMODEM is one data-transfer method that uses this method

20. Cyclic Redundancy Checking
Cyclic redundancy checking (CRC) codes are used in many sophisticated digital systems such as the compact disk
This error-detecting and correcting scheme uses a form of feedback in which the state of each message bit depends on the state of the previous blocks in the message
CRC codes are generated by computing a polynomial from the message bits

21. Data Compression and Encryption
Data Compression can be done in several ways, depending upon the type of data to be processed
Huffman coding uses strings of from 4 to 11 bits to encode the alphabet instead of 7 or 8 bits for every character
Another important compression technique is called run-length encoding, taking advantage of the fact that information bits are often repeated

22. Encryption
Encryption is used to provide security for data transmissions
Two main types of encryption are:
Private key - a long binary number is combined with the message data according to an algorithm that is built into the system. At the receiver, the same key is used to decode the information
Public key - anyone can encode messages using a public key, but a private key is necessary to decode the information