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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: Bmin = 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): QuickTime™ and a decompressor are needed to see this picture. 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 QuickTime™ and a decompressor are needed to see this picture. • 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) QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. 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 QuickTime™ and a decompressor are needed to see this picture. 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 QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. • 8B6T (8 Bits, 6 Ternary): Groups of 8 bits are represented as sets of six signal elements with three possible levels. • 100Base-4T cable. QuickTime™ and a decompressor are needed to see this picture. • 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 QuickTime™ and a decompressor are needed to see this picture. 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 nonzero, 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. QuickTime™ and a decompressor are needed to see this picture. 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 QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture.