1. COMMUNICATIONS and NETWORKING CLASS 6
COMP 421 /CMPET 401
COMMUNICATIONS and NETWORKING
CLASS 6

2. Encoding Techniques Digital data, digital signal
Easy encoding / Less Complex Less Expensive
Analog data, digital signal
Can transmit data over Digital Network
Digital data, analog signal
Modems / Fiber / Unguided Media
Analog data, analog signal
Cheap & Easy Baseband Transmission / FDM

3. Analog Data Choices easy conversion process Analog data, analog signal
Analog data, digital signal
permits use of modern digital switching and transmission equipment

4. Digital Data Choices Digital data, analog signal
equipment is less expensive than digital to analog conversion
some transmission media (e.g. optical fiber or unguided media) can only send analog signals

5. Transmission Choices Analog transmission Digital transmission
Only transmits analog signals, without regard for data content
Attenuation overcome with amplifiers
Digital transmission
Transmits analog or digital signals
Uses repeaters rather than amplifiers

6. Advantages of Digital Transmission
The signal is exact
Signals can be checked for errors
Noise/interference are easily filtered out
A variety of services can be offered over one line
Higher bandwidth is possible with data compression

7. Encoding schemes Analog data, Analog signal
Analog data, Digital signal
voice
analog
analog
digital
Telephone
CODEC
Digital data, Analog signal
Analog transmission means a transmitting analog signals without regard to their content; the signal may represent analog data (e.g. voice) or digital data (e.g. binary data). Digital transmission is concerned with a transmitting binary signal
Digital data, Digital signal
digital
Modem
analog
digital
Digital
transmitter
digital

8. Encoding and Modulation
x(t)
g(t)
x(t)
g(t)
Encoder
Decoder
digital or
analog
digital t
s(f)
m(t)
s(t)
Modulator
Demodulator
m(t)
For digital signal, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t).
Analog tranmission uses a continuous constant-frequency signal known as the carrier signal. The frequency of the carrier signal is choosen to be compatible with the transmission medium. Data is transmitted using a carrier signal by modulation, which is the process of encoding source data onto a carrier signal with frequency fc
digital or
analog
analog f fc fc

9. Why encoding?
Three factors determine successfulness of receiving a signal:
S/N (Signal to Noise Ratio)
Data rate
Bandwidth
With other factors held constant, the following statements are true.
An increase in data rate increases bit error rate
An increase S/N decreases bit error rate
An increase in bandwidth allows an increase in data rate [Stalling, p98]

10. Encoding Schemes' Evaluation Factors
Signal spectrum
Clocking
Error detection
Signal interference & noise immunity
Cost and complexity
Five factors are used to evaluate the various encoding scheme:
Signal spetrum A lack of high-frequency components means that less bandwidth is required for transmission. No dc component is desirable.
Clocking Suitable encoding provide some synchronization mechanism to determine the beginning and end of each bit position.
Error detection Some error detection can be built into the encoding scheme
Signal interference & noise immunity Some encoding scheme has superior performance in the presence of noise
Cost and complexity Higher signaling rate to achieve a greater data rate results expensive devices.

11. Digital Data, Digital Signal / Characteristics
Uses discrete, discontinuous, voltage pulses
Each pulse is a signal element
Binary data is encoded into signal elements

12. Terms (1) Unipolar Polar Data rate Duration or length of a bit
All signal elements have same sign
Polar
One logic state represented by positive voltage the other by negative voltage
Data rate
Rate of data transmission in bits per second
Duration or length of a bit
Time taken for transmitter to emit the bit

13. Terms (2) Modulation rate Mark and Space
Rate at which the signal level changes
Measured in baud = signal elements per second
Mark and Space
Binary 1 and Binary 0 respectively

14. Interpreting Signals Need to know
Timing of bits - when they start and end
Signal levels
Factors affecting successful interpretation of signals:
Signal to noise ratio
Data rate
Bandwidth

15. Comparison of Encoding Schemes (1)
Signal Spectrum
Lack of high frequencies reduces required bandwidth
Lack of dc component allows ac coupling via transformer, providing isolation
It is important to concentrate power in the middle of the bandwidth

16. Comparison of Encoding Schemes (2)
Clocking issues
Synchronizing transmitter and receiver is essential
External clock is one way used for synchronization
Synchronizing mechanism based on signal is also used & preferred (over using an external clock)

17. Spectral density 0.5 1 1.5 1.5 B8ZS,HDB3 NRZ-L,
NRZI 1
AMI, Pseudoternary
Mean square voltage per unit bandwidth
0.5
Manchester,
Differential Manchester
NRZ make efficient use of bandwidth. most of the frequency in NRZ and NRZI signals are between dc and half the bit rate.
Manchester& Different Manchesterh has the bulk of the energy between one-half and one times the bit rate. Thus the bandwidth is reasonably narrow and contain no dc component.
AMI make use of bandwidth less than the bandwidth of NRZ
[Stallings, p102]
0.5 1
1.5
-0.5
Normalized frequency (f/r)

18. Comparison of Encoding Schemes (3)
Error detection
Can be built into signal encoding
Signal interference and noise immunity
Some codes are better than others
Cost and complexity
Higher signal rate (& thus data rate) lead to higher costs
Some codes require signal rate greater than data rate

19. Encoding Schemes Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI (Alternate Mark Inversion)
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3

20. Digital Data, Digital Signal1 1 1 1 1
NRZ
NRZI
Bipolar -AMI
Pseudoternary
Definition of Digital Signal Encoding Formats
Nonreturn-to-Zero-Level (NRZ-L)
0 = hign level
1 = low leve l
Nonreturn to Zero Inverted (NRZI)
0 = no transition at beginning of interval (one bit time)
1 = transition at beginning of interval
Bipolar-AMI
0 = no line signal
1 = positive or negative level, alternating for successive ones
Pseudoternary
0 = positive or negative level, alternating for successive zeroes
1 = no line signal
Manchester
0 = transition from high to low in middle of interval
1 = transition from high to low in middle of interval
Differential Manchester
Always a transitiion in middle of interval
0 = no transition at beginning of interval
B8ZS
Same as bipolar AMI, except that anystring of eight zeros is replaced by a string with two code violations
HDB3
Same as bipolar AMI, except that any string of four zeros is replaced by a string with one code violation
[Stallings, p99,100]
Manchester
Differential
Manchester

21. Nonreturn to Zero-Level (NRZ-L)
Two different voltages:
0 - Low Level
1 - High Level
Voltage constant during bit interval
Most often, negative voltage for one value and positive for the other

22. Nonreturn to Zero Inverted
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding (Data represented by changes rather than levels)

23. NRZ

24. NRZ pros and cons Pros Cons Used for magnetic recording
Easy to engineer
Makes good use of bandwidth
Cons
dc component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission

25. Bipolar-AMI Zero represented by no line signal
One represented by positive or negative pulse
One pulses alternate in polarity
No loss of sync if a long string of ones happens (zeros still a problem)
No net dc component  Can use a transformer for isolating transmission line
Lower bandwidth
Easy error detection

26. Pseudoternary One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI

27. Bipolar-AMI and Pseudoternary

28. Trade Off for Multilevel Binary
Not as efficient as NRZ
With multi-level binary coding, the line signal may take on one of 3 levels, but each signal element, which could represent log23 = 1.58 bits of information, bears only one bit of information
Receiver must distinguish between three levels : (+A, -A, 0)
Requires approx. 3dB more signal power for same probability of bit error

29. Biphase Manchester Used by IEEE 802.3 (Ethernet)
Transition in middle of each bit period
Transition serves as clock and data
One is represented by a transition from low to high
Zero is represented by a transition from high to low
Used by IEEE (Ethernet)

30. Differential Manchester
Always a transition in the middle of the interval for clocking
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme used by
IEEE (Token Ring)

31. Biphase Pros and Cons Con
At least one transition per bit time and possibly two
Maximum modulation rate is twice NRZ
Requires more bandwidth
Pros
Synchronization on mid bit transition (self clocking)
No dc component
Error detection
Absence of expected transition points to error in transmission

32. Modulation Rate
The modulation Rate is at which signal elements are generated
In General the Modulation Rate D = R/b where
R=Data Rate=bits/sec
b=number of bits per signal element
Data Rate (bit Rate 1/Tb) where Tb is bit duration
For Manchester Encoding maximum Rate is: 2/Tb

33. Scrambling Techniques
Used to reduce signaling rate relative to the data rate by replacing sequences that would produce constant voltage for a priod of time with a filling sequence that accomplishes the following goals:
Must produce enough transitions to maintain synchronization
Must be recognized by receiver and replaced with original data sequence
is same length as original sequence

34. Scrambling Techniques
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
As an example, fax machines use the modified Huffman code to accomplish this.

35. B8ZS Same as Bipolar AMI with 8 Zeros Substitution
B8ZS: Abbreviation for bipolar with eight-zero substitution
Same as Bipolar AMI with 8 Zeros Substitution
Based on Bipolar-AMI
If octet of all zeros and last voltage pulse preceding was positive, encode as
If octet of all zeros and last voltage pulse preceding was negative, encode as
Causes two violations of AMI code
This is unlikely to occur as a result of noise
Receiver detects and interprets the sequence as octet of all zeros

36. B8ZS
A one is sent on a T1 by sending a pulse, as opposed to not sending a pulse.
The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going.
If a T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred.
In B8ZS a specific combination of valid pulses and bipolar violations is used to represent a string of eight zeroes, whenever the user data contains eight zeroes in a row

37. B8ZS
Since a T1 uses a single pair of wires in each direction and the only signals on those wires are the pulses which represent data; the only way to recover clock and retain synchronization on a T1 is by detecting the rate at which pulses are being received. All of the equipment in a T1 circuit must operate at the same rate because all of the equipment must sense the T1 at the correct time in order to determine if a pulse (1) or no pulse (0) has been received at each bit time.
Since only ones are sent as pulses and zeroes are represented by doing nothing, if too many zeroes are sent at a time there will be no pulses on the T1 at all and the clock circuitry in all of the hardware will rapidly fall out of synchronization. Thus the design of AMI requires that a certain ONES DENSITY be maintained, that a certain minimum of the bits over a certain period of time be guaranteed to be a ONE (pulse). This is why AMI circuits require DENSITY enforcement

38. B8ZS
Briefly stated; on average one bit in eight must be a one and no more than (varies according to specific standard) so many zeroes may be sent in a row.
In order to be able to satisfy the ones density requirement on an AMI T1 one bit out of every eight is taken away from the user, not available for voice or data traffic, and that 1 bit in 8 is always sent as a one. Once this has been done the requirement for ones density is satisfied and the user is free to send any data pattern in the remaining bandwidth.

39. B8ZS
The rate of a T1 is megabits per second. 8K is used for framing leaving 1.536MBPS. The is usually divided into 24 timeslots (DS0s) or "channels" each being inherently 64KBPS. By taking the 1 bit in 8 that is reserved to satisfy ones density the user is left with 56K per timeslot.

40. AMI
AMI = Alternate Mark Inversion. This is the original method of formatting T1 data streams. In AMI a zero is always sent by doing nothing, at the time when a pulse might otherwise be sent, a pulse is not sent to represent a zero.
A one is sent on an AMI T1 by sending a pulse, as opposed to not sending a pulse.
The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going.
If an AMI T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred.
Thus AMI has a rudimentary error checking capability with a 50% probability of detecting altered, inserted or lost bits end to end.

41. ESF Extended Super Frame
A DS level and framing specification for synchronous digital streams over circuits in the North America. A DS1 "frame" is composed of 24 eight-bit bytes plus one framing bit (193 bits) bytes per second come from each source, and thus 8000 frames per second are transported by the DS1 signal. The result is 193*8000 = 1,544,000 bits per second.
In the original standard, the framing bits continuously repeated the sequence , and such a 12-frame unit is called a super-frame. In voice telephony, errors are acceptable (early standards allowed as much as one frame in six to be missing entirely), so the least significant bit in two of the 24 streams was used for signaling between network equipments. This is called robbed bit signaling

42. ESF
To promote error-free transmission, an alternative called the extended super-frame (ESF) of 24 frames was developed. In this standard, six of the 24 framing bits provide a six bit cyclic redundancy check(CRC-6), and six provide the actual framing. The other 12 form a virtual circuit of 4000 bits per second for use by the transmission equipment, for call progress signals such as busy, idle and ringing. DS1 signals using ESF equipment are nearly error-free, because the CRC detects errors and allows automatic re-routing of connections.

43. HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI
String of four zeros replaced with one or two pulses
Note: The following is the explanation for the HDB3 code example on the next slide (see rules in Table 5.4, page 142):
Assuming that an odd number of 1's have occurred since the last substitution, since the polarity of the preceding pulse is "-", then the first 4 zeros are replaced by "000-". For the next 4 zeros, since there have been no Bipolar pulses since the 1st substitution, then they are replaced by"+00+" since the preceding pulse is a "-". For the 3rd case where 4 zeros happen, 2 (even) Bipolar pulses have happened since the last substitution and the polarity of the preceding pulse is "+", so "-00-" is substituted for the zeros.

44. B8ZS and HDB3 See Table 5.4 for HDB3 Substitution Rules
(Assume odd number of 1s
since last substitution)
See Table 5.4 for HDB3 Substitution Rules

45. Digital Data, Analog Signal
Transmitting digital data through PSTN (Public telephone system)
300Hz to 3400Hz bandwidth
modem (modulator-demodulator) is used to convert digital data to analog signal and vice versa
Three basic modulation techniques are used:
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PSK)

46. Modulation Techniques

47. Amplitude Shift Keying
Values represented by different amplitudes of carrier
Usually, one amplitude is zero
i.e. presence and absence of carrier is used
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber

48. ASK Vd(t) Vc(t) VASK(t) Signal power frequency spectrum Frequency
In ASK, the amplitude of a single-frequency known as the carrier frequency is switched between two levels at a rate determined by the bit rate of the transmitted binary data signal. Bandpass filter is used to limit the band of frequencies based on Nyquist’s theorem. [Halsall, p.59]
Signal
power
frequency spectrum
Frequency
fc-3f0
fc-f0 fc
fc+f0
fc+3f0

49. Frequency Shift Keying
Values represented by different frequencies (near carrier)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
High frequency radio (3-30 MHz)
Higher frequency on LANs using co-ax

50. FSK Data signal vd(t) Carrier 1 v1(t) v2(t) Carrier 2 FSK(t) Signal
In FSK, two fixed amplitude carrier signals are used, one for a binary 0 and the other for a binary 1. The different between the two carriers is known as the frequency shift. The modulation operation is equivalent to summing together the outputs of two separate ASK modulators.
FSK is the modulation method that was used in all early low bit rate modems. [Halsall p.62]
Signal
power
frequency spectrum
Frequency f1 f2

51. FSK in modem (on Voice Grade Line)
400
980
(1070)
1850
(2225)
1180
(1270)
1650
(2025)
3400
Amplitude
Frequency(Hz)
PSTN bandwidth
frequency spectrum

52. Phase Shift Keying
Phase of carrier signal is shifted to represent data
Differential PSK
Phase shifted relative to previous transmission rather than some reference signal

53. PSK bit rate = signaling rate
Data
Signal
vc(t)
Carrier
vc(t)
Phase
coherent
vPSK(t)
Differential
v’PSK(t)
In PSK, the phase of the carrier signal is shift to represent data. Two type of PSK are used.
Phase coherent PSK : Used two fixed carrier signals to represent a binary 0 and 1 with a 180° phase different, and is known asst used two fixed carrier signals to represent a binary 0. the disanvantage of this scheme us that a reference carrirer signal is required at the receiver against with the phase of the received signal is compared, this required more complex demodulation circuit.
Differential PSK : A phase shift of 90° relative to the current signal indicates a binary 0 is the next bit while a phase shift 270° indicates a binary 0.[Halsall, p.65]
bit rate = signaling rate
180=0
0=1
Differential example: for every logic 1, 180 degree phase shift
phase diagram

54. Quadrature PSK
More efficient use by each signal element representing more than one bit
e.g. shifts of /2 (90o)
Each element represents two bits
Can use 8 phase angles and have more than one amplitude
9600bps modems use 12 angles , four of which have two amplitudes

55. Multilevel Modulation Method00 01 10 11
+90°=01
+180°=10
0°=00
+270°=11 0°
+90°
+180°
+270°
4-PSK phase diagram
More sophisticated modulation methods are used which involve either multiple signal levels or a mix of the basic scheme, particularly amplitude and phase.
More bit rate can be achieved if signaling element represent more than one bit.
QPSK ( Quadrature PSk or 4-PSK) : Used four different phase change( 0°, 90°, 180°, 270°) to enables each phase change to convey 2 bits (bitrate=2*signaling rate).
QAM (Quadrature Amplitude Modulation or 16-QAM) : Phase and amplitude changes, 16 levels persignal element and hence 4 bit symbols (bitrate=4*signaling rate). [Halsall, p.67]
bit rate = n x signaling rate

56. Performance of Digital to Analog Modulation Schemes
Bandwidth
ASK and PSK bandwidth directly related to bit rate
FSK bandwidth related to data rate for lower frequencies
Requires more analog bandwidth than ASK
In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK

57. Analog Data, Digital Signal
Digitization
Conversion of analog data into digital data
Digital data can then be transmitted using NRZ-L or using other codes
Digital data can then be converted to analog signal
Analog to digital conversion done using a CODEC
Pulse code modulation
Delta modulation

58. Analog data, Digital signal
Two principle techniques used
PCM (Pulse Code Modulation)
DM (Delta Modulation)
Sampling clock
PAM signal
PCM signal
Digitization is a process of convering analog data into digital data. The digital signal is converted back into analog signal at the receiver.
The device used for converting analog data into digital form, and recovering the original analog data is known as CODEC (Coder-Decoder).
Sampling
Circuit
Quantizer
and compander
Analog
voice signal
Digitized
voice signal

59. Pulse Code Modulation(PCM) (1)
If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal
(Proof - Stallings appendix 4A)
Voice data limited to below 4000Hz
Require 8000 sample per second
Analog samples (Pulse Amplitude Modulation, PAM)
Each sample assigned digital value

60. Pulse Code Modulation(PCM) (2)
4 bit system gives 16 levels
Quantized
Quantizing error or noise
Approximations mean it is impossible to recover original exactly
8 bit sample gives 256 levels
Quality comparable with analog transmission
8000 samples per second of 8 bits each gives 64kbps

61. Pulse Code Modulation(PCM) (3)
The process starts with an analog signal, which is sampled by PAM sample. the resulting pulse are quantized to produced PCM pulses and then encoded to produce bit stream. At the receiver end, the process is reversed to reproduce the analog signal.

62. PCM 011100011011001100 Sampling signal based on Nyquist theorem
Original signal
3.9
4.2
3.4
3.2
2.8
PAM pulse
1.2
PCM is based on the sampling theorem. The original signal is assumed to be bandlimited with a bandwidth of B. Signal is sampled at a rate 2B. Samples signal are represents as narrow pulse whose amplitude is proportional to the value of the original signal and is known as PAM (Pulse Amplitude Modulation).
The amplitude of each PAM pulse is approximated by an n-bit interger,. In the sample above, n=3. Thus 8=23 levels are used for approximating the PAM pulses.
PCM pulse
with quantized error 3 4 3 3 4 1
011
100
011
011
001
100
PCM output

63. Nonlinear Encoding
Quantization levels are not necessarily equally spaced. The problem with equal spacing is that the mean absolute error for each sample is the same, regardless the signal level. Lower amplitude values are relatively more distorted.
Nonlinear encoding reduces overall signal distortion
Can also be done by companding

64. Without nonlinear encoding With nonlinear encoding
Quantizing level 15 15 14
Strong signal 14 13 13 12 12 11 11 10 10 9 8
Weak signal 9 8 7 7 6 6 5
Quantization levels are not necessary equally spaced. The problem with equal spacing is that the mean absolute error for each sample is the same, regardless the signsl level. Lower amplitude values are relatively more distorted. [Stallings, p.118] 4 5 4 3 3 2 2 1 1
Without nonlinear encoding
With nonlinear encoding

65. Nonlinear Encoding
Prior to the input signal being sampled and converted by ADC into a digital form, it is passed through a circuit known as a compressor. Similarly, at the destination, the reverse operation is perform on the output of the DAC by a circuit known as expander.

66. Delta Modulation Analog input is approximated by a staircase function
Move up or down one level () at each sample interval
Binary behavior
Function moves up or down at each sample interval

67. Delta Modulation - example

68. Delta Modulation - Performance
Good voice reproduction
PCM levels (7 bit)
Voice bandwidth 4khz
Should be 8000 x 7 = 56kbps for PCM
Data compression can improve on this
e.g. Interframe coding techniques for video

69. Analog Data, Analog Signals
Why modulate analog signals?
Higher frequency can give more efficient transmission
Permits frequency division multiplexing
Types of modulation
Amplitude
Frequency
Phase

70. Multilevel Modulation Method
Quadrature Amplitude Modulation (QAM)
Combines differential phase and amplitude shifts to achieve 16 distinct states, thereby allowing 4 bits to be represented by a single signal
More sophisticated modulation methods are used which involve either multiple signal levels or a mix of the basic scheme, particularly amplitude and phase.
QPSK ( Quadrature PSk or 4-PSK) : Four different phase changes (0°, 90°, 180°, 270°) to enables each phase change to convey 2 bits (bitrate=2*signaling rate).
QAM (Quadrature Amplitude Modulation or 16-QAM) : Phase and amplitude changes, 16 levels per signal element and hence 4 bit symbols (bitrate=4*signaling rate).
16-QAM phase diagram

71. V.34 Modulation
V Also known as V.FAST. It will allow modems to operate at 28Kb/s. Uses
multidimensional trellis coding and line probing equalization, power control
and framing.
Adaptive Pre-Emphasis or Precoding is a new form of adaptive equalization
that modifies the transmitted signal as well as the receiver.
Trellis Coding in more complex forms (64-state 4D, 32-state 4D, etc.) make
more efficient use of constellation space. Non-linear encoding wraps the
constellation space to bring the inner points closer and increase the distance
between the outer points.
Shell Mapping forms circular constellations which are optimum shape.
Shaping distributes consolation points nearer the center, which is less sensitive
to noise.
Adaptive Power Control changes the levels to produce the best performance
over impaired channels. This capability may also improve performance over
analog cellular services.
Scaling maintains the best transmit power levels when different modulation
technologies are employed.
Framing encodes bits over multiple symbols. This increases the systems ability
to support different combinations of symbol and data rates and makes it possible
to integrate a secondary channel.
V.FC V.FAST Class developed by Rockwell International. It is based on the V.34 proposed
design, but it is an interim solution. It does not support the V.8 handshaking
mechanism for full V.34 compatibility (it will require a software modification)
V.8 negotiation using a modulated calling tone and answer tone transfers information
about two modem’s functional capabilities in 5 seconds or less.

72. The 56K Modem
The V.90 modulation uses PAM. Each symbol is a different voltage level. 128 symbols multiplied by 8000 symbols per second, gives a 56,000 bits per second downstream rate.
If the environment is noisy, less voltage levels are used. For example, if 64 are in use, then the speed will be 48,000 bits per second in a 56Kbps connection, the server is a digital modem. The PAM modulation requires at least 45dB SNR. The minimum RX level a receiver can pull in is 34db below TX.
For upstream transmission, the information is transmitted in the old way, analog, using QAM, A2D, through the PSTN, D2A and analog again. The upstream rate is limited to 31.2Kbps

73. END Class