In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. The bit representation is defined by the inversion or noninversion at the beginning of the bit. Note:
In bipolar encoding, we use three levels: positive, zero, and negative. Note:
4.2 Block Coding Steps in Transformation Some Common Block Codes
Block Coding <ul><li>Redundancy is needed to ensure synchronization and to provide error detecting </li></ul><ul><li>Block coding is normally referred to as mB/nB coding </li></ul><ul><li>it replaces each m-bit group with an n-bit group </li></ul><ul><li>m < n </li></ul>
<ul><li>4B5B Example </li></ul><ul><ul><li>In order to send information using 4B5B encoding, the data byte to be sent is first broken into two nibbles. If the byte is 0E, the first nibble is 0 and the second nibble is E. Next each nibble is remapped according to the 4B5B table. Hex 0 is remapped to the 4B5B code 11110. Hex E is remapped to the 4B5B code 11100. Other information remapping types are 5B6B and 8B10B. </li></ul></ul>11101 1111 F 11100 1110 E 11011 1101 D ... ... ... 10100 0010 2 01001 0001 1 11110 0000 0 4B5B Code (Binary) Data (Hex) 4B5B Encoding Table
Table 4.1 4B/5B encoding (Continued) 10001 K (start delimiter) 01101 T (end delimiter) 11001 S (Set) 00111 R (Reset) 11000 J (start delimiter) 00100 11111 00000 Code H (Halt) I (Idle) Q (Quiet) Data
Figure 4.15 Using block coding 4B/5B with NRZ-I line coding scheme
Scrambling <ul><li>It modifies the bipolar AMI encoding (no DC component, but having the problem of synchronization) </li></ul><ul><li>It does not increase the number of bits </li></ul><ul><li>It provides synchronization </li></ul><ul><li>It uses some specific form of bits to replace a sequence of 0s </li></ul>
B8ZS Bipolar with 8 zero substitution4 Eight consecutive zero level voltages are replaced by the sequence 000VB0VB: V stands for voilation breaks AMI rule. Oppsite polarity from the previous. B denotes bipolar. Whch is in accordance with AMI rule.
Figure 4.19 Two cases of B8ZS scrambling technique B8ZS substitutes eight consecutive zeros with 000VB0VB
Figure 4.20 Different situations in HDB3 scrambling technique HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution.
4.3 Sampling Pulse Amplitude Modulation Pulse Code Modulation Sampling Rate: Nyquist Theorem How Many Bits per Sample? Bit Rate
Pulse amplitude modulation has some applications, but it is not used by itself in data communication. However, it is the first step in another very popular conversion method called pulse code modulation. Note:
Example 4 What sampling rate is needed for a signal with a bandwidth of 10,000 Hz (1000 to 11,000 Hz)? Solution The sampling rate must be twice the highest frequency in the signal: Sampling rate = 2 x (11,000) = 22,000 samples/s
Example 5 A signal is sampled. Each sample requires at least 12 levels of precision (+0 to +5 and -0 to -5). How many bits should be sent for each sample? Solution We need 4 bits; 1 bit for the sign and 3 bits for the value. A 3-bit value can represent 2 3 = 8 levels (000 to 111), which is more than what we need. A 2-bit value is not enough since 2 2 = 4. A 4-bit value is too much because 2 4 = 16.
Example 6 We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. Sampling rate = 4000 x 2 = 8000 samples/s Bit rate = sampling rate x number of bits per sample = 8000 x 8 = 64,000 bps = 64 Kbps
Note that we can always change a band-pass signal to a low-pass signal before sampling. In this case, the sampling rate is twice the bandwidth. Note:
4.4 Transmission Mode Parallel Transmission Serial Transmission