Data Communication NetworksLec 10

Multilevel Schemes

• Goal is to increase the number of bits per baud.• m data elements encoded into a pattern of n signal

elements.• Design are in coding of mBnL, m is the length of binary

pattern, B means binary data, n is the length of signal pattern, L is the number of levels in the signaling.

• A letter is often used in the place of L: B(binary) for L=2, T (ternary) for L=3 and Q(quaternary)for L=4.

• First two letters define the data pattern and second two define signal pattern.

4.3

In mBnL schemes, a pattern of m data elements is encoded as a pattern of n

signal elements in which 2m ≤ Ln.

Note

2B1Q(two binary, one quaternary)

• In this encoding, m=2, n=1 and L=4.• Have four different signal level, receiver has to

discern four different thresholds.• Used in DSL , to provide high speed

connection by using telephone lines.

4.5

Figure 4.10 Multilevel: 2B1Q scheme

8B6T(eight binary, six ternary)

• Used for 100BASE-4T cable.• Idea is to encode a pattern of 8 bits as a

pattern of 6 signal elements.• Each pattern has a weight of 0 or +1.

4.7

Figure 4.11 Multilevel: 8B6T scheme

4D-PAM5(four dimensional five-level pulse amplitude modulation )

• 4D means data is send over four wires at a time.

Multiline Transmission(MLT-3)

• Three levels(+V,0, and -V)and three transition rules to move between levels.– If next bit is 0, no transition.– If next bit is 1, and current level is not 0, the next

level is 0.– If next bit is 1, the current level is 0, the next level

is opposite of the last nonzero level.

Multiline Transmission(MLT-3)

• One wonder, the signal rate is same as that for NRZ-I, but greater complexity.

• Look at the worst –case.• A non-periodic signal as changed to periodic

signal.

4.11

Figure 4.13 Multitransition: MLT-3 scheme

4.12

Table 4.1 Summary of line coding schemes

Block Encoding

• Need redundancy , to ensure synchronization and to provide some kind of inherent error detecting.

• Block encoding changes a block of m bits into a block of n bits,where n is larger than m.– Division– Substitution– Combination

Block Encoding

• In division step, a sequence of bits is divided into group of m bits, e.g. 4B/5B , the original sequence is divided into 4-bit groups.

• In substitution , m-bit group for n-bit group, e.g. 4-bit code to 5-bit group.

• Finally , combine to form a stream.

4.15

Block coding is normally referred to as mB/nB coding;

it replaces each m-bit group with an n-bit group.

Note

4.16

Figure 4.14 Block coding concept

4B/5B(four binary/five binary)

• Designed to use in combination with NRZ-I.• NRZ-I is synchronization problem(long

sequence of 0s).• One solution is to change the bit stream , to

having no sequence of 0s.• 4B/5B achieve this goal.• A block encoded scheme does not have more

than three consecutive 0s.

4.18

Figure 4.15 Using block coding 4B/5B with NRZ-I line coding scheme

4.19

Table 4.2 4B/5B mapping codes

4.20

Figure 4.16 Substitution in 4B/5B block coding

4.21

We need to send data at a 1-Mbps rate. What is the minimum required bandwidth, using a combination of 4B/5B and NRZ-I or Manchester coding?

SolutionFirst 4B/5B block coding increases the bit rate to 1.25 Mbps. The minimum bandwidth using NRZ-I is N/2 or 625 kHz. The Manchester scheme needs a minimum bandwidth of 1 MHz. The first choice needs a lower bandwidth, but has a DC component problem; the second choice needs a higher bandwidth, but does not have a DC component problem.

Example 4.5

4.22

Figure 4.17 8B/10B block encoding

Disparity Controller

• To prevent a long run of consecutive Os or Is, the code uses a disparity controller which keeps track of excess Os over Is (or Is over Os).

• If the bits in the current block create a disparity that contributes to the previous disparity (either direction), then each bit in the code is complemented (a 0 is changed to a 1 and a 1 is changed to a 0).

4.24

4-3 TRANSMISSION MODES

The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

Parallel TransmissionSerial Transmission

Topics discussed in this section:

4.25

Figure 4.31 Data transmission and modes

4.26

Figure 4.32 Parallel transmission

4.27

Figure 4.33 Serial transmission

4.28

In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each

byte. There may be a gap between each byte.

Note

4.29

Asynchronous here means “asynchronous at the byte level,”

but the bits are still synchronized; their durations are the same.

Note

4.30

Figure 4.34 Asynchronous transmission

4.31

In synchronous transmission, we send bits one after another without start or

stop bits or gaps. It is the responsibility of the receiver to group the bits.

Note

4.32

Figure 4.35 Synchronous transmission