Transmitting digital signals How do we encode digital signals for transmission? How can we interpret those signals?

Baseband transmission Signal is sent without conversion to an analog signal. Requires a transmission channel with bandwidth that starts at 0Hz (a low-pass channel). For perfect preservation, requires a dedicated channel with infinite bandwidth. Usually, we just approximate Wide bandwidth channel: Ignore frequencies at the borders Narrow bandwidth: Use an analog signal and adjust frequency and phase to create an approximate match.

Figure 3.21 Rough approximation of a digital signal using the first harmonic for worst case

Figure 3.22 Simulating a digital signal with first three harmonics

Broadband transmission First convert digital signal to analog signal. Uses a bandpass channel - one whose bandwidth starts anywhere. The signal is modulated, which means we manipulate the frequency or amplitude to represent different components of digital data.

Transmission problems There are three major roadblocks that we need to be concerned about when exploring the physical level: –Attenuation: How rapidly does the signal fade as it propagates. –Distortion: How does the signal change as it is transmitted across the medium? –Noise: How do extraneous factors interfere with the signal being sent?

Converting digital data to a digital signal Data element: A bit is a data element - the shortest piece of data that can be sent –Data rate is the number of data elements sent in one second (bits per second) Signal element: A signal element is the shortest signal segment. –Signal rate is the number of signal elements sent in one second (baud). r is defined as the ratio of data elements to signal elements - i.e., how many bits are sent in each signal element.

Maximizing throughput High signal rates are obtained by using a broader range of frequencies, thus allowing for a broader range of possible signals Therefore, higher signal rates require higher bandwidths. Ideally, we want to maximize data rate and minimize signal rate. Increasing r, though, reduces reliability by requiring more sophisticated hardware on both the sending and receiving ends.

Bandwidth requirements The bandwidth required is related to the signal rate, since that determines how many frequencies are needed: B min = c  N  1/r

Issues to be aware of Baseline wandering Self-synchronization Error Detection Complexity

Line Coding Line coding is the primary way we encode digital data in digital signals. There are five primary groups of line coding schemes: –Unipolar –Polar –Bipolar –Multilevel –Multitransition

Unipolar line coding Either all signals levels are positive or they are all negative Non-Return-to-Zero (NRZ):

Polar line coding Voltages can be both positive and negative. Typically positive voltages are 0 and negative voltages are 1. NRZ-L: Level of voltage determines bit value NRZ-I: Inversion of signal determines bit value

Return to Zero (RZ): Use three values - positive, negative, and 0. Signal goes to 0 in the middle of each bit. Biphase: –Manchester: Transition in middle of bit - voltage value of start of bit determines bit value (like RZ and NRZ-L) –Differential Manchester: Transition in the middle of each bit whether there is a transition at beginning of bit determines value (like RZ and NRZ-I)

Bipolar Line Coding Always have three voltage levels - positive, 0, and negative. 0 volts is typically the 0 bit, while the 1 bit alternates between positive and negative. Long-distance lines

Multilevel line coding 2B1Q: (2 bit, 1 quaternary) 2-bit combinations are associated with specific changes in the input according to a transition table. DSL

8B6T (8 Bits, 6 Ternary): Groups of 8 bits are represented as sets of six signal elements with three possible levels. 100Base-4T cable.

4D-PAM5 (4 Dimensional five-level pulse amplitude modulation):For situations where multiple wires are available for simultaneous communication. –Uses four wires and four voltage levels –8 bits can be sent simultaneously using one signal element

Multitransition line coding MLT-3 (MultiLine Transmission, three level) –Similar to NRZ-I and differential Manchester in that transitions define bits. –Uses three voltage levels and three transition rules: 1.No transition means next bit is 0. 2.If the current level is non- zero, a transition to 0 means the next bit is 1. 3.If the current level is zero, a transition to the opposite of the last non-zero level is a 1.

Which is best? Well, none of them are great. Most don’t provide synchronization. The ones that do either require high bandwidth (Biphase, 8B6T) or multiple wires (4D-PAM5). None of these can inherently provide for any sort of error detection. To accomplish synchronization and, especially, error detection, there need to be redundant elements to the signal.

Block Coding Block coding tries to provide this redundancy. In block coding, we take a set of m bits and recode them as a longer group of n bits. –i.e., 4B/5B coding converts groups of four bits into groups of 5 bits. Isn’t this bad? Doesn’t it increase the size of the data stream we need to send?

The benefits of redundancy Representing 4 bits requires 16 data patterns. Having 5 bits available gives us 32 possible patterns. Among the benefits of this are: –Eliminate baseline drift by eliminating long strings of 0’s. –This also improves synchronization. –Some sequences can be used as control sequences for further synchronization. –What happens if the receiver gets a sequence that does not correspond to a valid one? Typically used with simpler line-coding schemes like NRZ

Long-distance transmission NRZ, even with block coding, still has the problem of DC components. Biphase schemes require too much bandwith for high-capacity long-distance transmission. Bipolar AMI coding avoids both these problems, but can generate long strings of 0’s, which affects synchronization.

Scrambling Scrambling those long strings of 0’s is the solution. Recall that bipolar encoding requires the voltage to alternate between high and low. With a long string of 0’s, we can use a violation signal (one that doesn’t alternate) as a substitute for a 0. B8ZS (Bipolar with 8 Zero Substitution) - North America HDB3 (High-Density Bipolar 3-zero) - outside NA