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Data Communication Networks. Lec 10. Multilevel Schemes. Goal is to increase the number of bits per baud. m data elements encoded into a pattern of n signal elements.

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Multilevel schemes
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.

Data communication networks


In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln.

2b1q two binary one quaternary
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.

Data communication networks

Figure 4.10 Multilevel: 2B1Q scheme

8b6t eight binary six ternary
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.

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Figure 4.11 Multilevel: 8B6T scheme

4d pam5 four dimensional five level pulse amplitude modulation
4D-PAM5(four dimensional five-level pulse amplitude modulation )

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

Multiline transmission mlt 3
Multiline Transmission(MLT-3) modulation )

  • 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 31
Multiline Transmission(MLT-3) modulation )

  • 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.

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Figure 4.13 modulation )Multitransition: MLT-3 scheme

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Table 4.1 modulation )Summary of line coding schemes

Block encoding
Block Encoding modulation )

  • 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 encoding1
Block Encoding modulation )

  • 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.

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Note modulation )

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

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

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Figure 4.14 modulation )Block coding concept

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

  • 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.

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

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Table 4.2 modulation )4B/5B mapping codes

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Figure 4.16 modulation )Substitution in 4B/5B block coding

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Example 4.5 modulation )

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?


First 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.

Data communication networks

Figure 4.17 modulation )8B/10B block encoding

Disparity controller
Disparity Controller modulation )

  • 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).

Data communication networks

4-3 TRANSMISSION MODES modulation )

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.

Topics discussed in this section:

Parallel TransmissionSerial Transmission

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Figure 4.31 modulation )Data transmission and modes

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Figure 4.32 modulation )Parallel transmission

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Figure 4.33 modulation )Serial transmission

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Note modulation )

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.

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Note modulation )

Asynchronous here means “asynchronous at the byte level,”

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

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Figure 4.34 modulation )Asynchronous transmission

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Note modulation )

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.

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Figure 4.35 modulation )Synchronous transmission