Signal Modulation and Multiplexing Techniques
Signal: An electromagnetic field whose variations convey information. Signals can be analog (continuous) or digital (discrete sequences). Modulation: The process of altering a periodic carrier signal (constant amplitude and frequency) to transmit information. Modulation types include Amplitude Modulation (carrier amplitude changes with the modulating signal), Frequency Modulation (carrier frequency changes with the modulating signal), and Phase Modulation (carrier phase changes with the modulating signal). The modulated signal occupies a frequency band around the carrier frequency. Modulation is used to prevent signal mixing, achieve long communication ranges, and reduce antenna size. Attenuation: The decrease in signal amplitude (energy) as it travels through a transmission medium. Dispersion: Distortion of the transmitted signal due to variations in propagation delay with frequency.
Noise: Unwanted signals not originating from the transmitter. Delay: Time required for a signal to travel between two points. This includes Propagation Delay (time to travel through the medium) and Transmission Delay (time to push all bits into the medium). Jitter: Delay variation that can distort real-time services like VoIP. Bandwidth: The range of frequencies used by a signal/channel, calculated as the difference between the highest and lowest frequencies. Channel Capacity: The maximum data rate a channel can support. Nyquist’s theorem applies to noiseless channels, while Shannon-Hartley’s theorem applies to noisy channels. Spectral Efficiency: Net bit rate divided by bandwidth (bps/Hz). Modulation Efficiency: Gross bit rate divided by bandwidth (bps/Hz). Error correction mechanisms can improve reliability but reduce bandwidth.
Multiplexing: Allows multiple users to share a medium. Signals can be multiplexed using different frequency bands, time slots, or orthogonal codes. Multiple Access: Allows multiple users to transmit directly to the same medium from different points using different frequency bands, time slots, or orthogonal codes. Link Budget: Considers the received signal power, receiver sensitivity, attenuation, and gain. The received power must exceed the receiver sensitivity. Logarithmic units are used. Increasing bandwidth and signal power can maintain capacity with increased noise. Multiple access techniques assign time slots, frequency ranges, or orthogonal codes to transmitters.
There is no single element to multiplex signals from different sources. Temporary distortions include delay and jitter. The real data rate is less than the theoretical maximum capacity given by Shannon-Hartley and Nyquist. A 3dB difference between signal power and noise means the signal power is approximately twice the noise power. Twisted Cable: Attenuates with distance, with higher frequencies experiencing more attenuation. Twisting reduces interference, but excessive twisting increases weight and cost. Shielded cable is more expensive and heavier. xDSL Reference Model: Each user modem connects to a central modem (DSLAM). A splitter separates telephone and data services using low-pass and high-pass filters. The central office uses splitters to route low frequencies to the telephone network and high frequencies to the DSLAM. MDF organizes cables.
xDSL Issues: FDM allows sharing twisted cables with POTS, but crosstalk can occur. Crosstalk: Undesirable signal coupling between adjacent twisted pairs. Far-End Crosstalk (FEXT) occurs at the far end of a link. Near-End Crosstalk (NEXT) occurs between the receiver and transmitter and is more harmful. NEXT does not depend on cable length but increases with frequency and the number of active xDSL loops. xDSL Signal: Uses advanced digital modulation and error control. Full Duplex Transmission: Achieved using Frequency Division Duplexing (FDD) or Echo Cancellation. Discrete MultiTone (DMT): A modulation technique using multiple orthogonal subcarriers. Quadrature Amplitude Modulation (QAM): Combines amplitude and phase modulation. xDSL Error Control: Forward Error Correction (FEC) adds redundancy. Interleaving spreads data to mitigate burst errors but increases latency.
xDSL Standards: ADSL: Uses Trellis coding, Reed-Solomon codes, interleaving, and DMT modulation. Supports FDD and echo cancellation. Downlink: up to 8 Mbps, Uplink: up to 800 kbps. ADSL2+: Doubles downstream bandwidth to 2.2 MHz. Downlink: up to 24 Mbps, Uplink: up to 1 Mbps. VDSL2: Uses DMT modulation and is interoperable with ADSL. Supports various frequency band plans and profiles. Introduces mechanisms for QoS. Downlink/Uplink: up to 200 Mbps. Twisted pairs longer than 4 km do not support high frequencies. VPI and VCI direct cells through virtual circuits, similar to MAC and IP or VLAN tags in Ethernet. Ethernet uses 3 bits for frame prioritization. ATM uses parameterized traffic descriptors. ATM uses VPI/VCI for switching, while Ethernet uses VLAN identifiers.
Optical Fiber Advantages: Lower attenuation, longer distances between regenerators, high capacity, better noise tolerance, smaller size and weight, better security. Disadvantages: Cable joining requires fusion splicing, bending can cause light leakage, dispersion can distort signals. Fiber Cable: Composed of a core and cladding with different refractive indices. Types: Multimode fiber (carries several modes) includes Step-Index and Graded-Index. Modal dispersion is a problem in multimode fiber. Single-mode fiber avoids modal dispersion but suffers from chromatic dispersion. Other fiber types are based on material (e.g., plastic fiber). Elements include transmitter, receiver, modulator, amplifier, and detector.
Laser: Provides coherent light with narrow bandwidth, high power, and precise modulation. Suitable for single-mode fiber and high speeds. LED: Lower power, slower data rates, wider spectral width. Suitable for multimode fiber. Direct Modulation: Input signal directly modulates the light source. Indirect Modulation: Input signal modulates the optical output via an external modulator (used in lasers). PIN Photodiodes: Convert photons to electrons. Avalanche Photodiodes: Produce an avalanche of electrons for better responsivity. Receiver Overview: Photodiode, amplifier, filter, decision circuit. Power Splitters: Divide light into multiple channels. Optical Filters and Multiplexers: Filter and (de)multiplex wavelengths. Amplifier: Amplifies the optical signal. Regenerators/Repeaters: Perform OEO conversion. TDM and WDM: Used for multiplexing. TDM uses time slots, WDM uses different wavelengths. DWDM (Dense WDM) and CWDM (Coarse WDM) are variations of WDM.
Optical Access Systems (FTTX): Various fiber deployments in the access network. Types include FTTH (Fiber to the Home), FTTB (Fiber to the Building), FTTC (Fiber to the Curb/Cabinet), FTTN (Fiber to the Node), and FTTEx (Fiber to the Exchange). PtP vs. PtMP: Point-to-Point (PtP) uses one fiber per customer. Point-to-Multipoint (PtMP) uses power splitters to share a single fiber. Active Optical Networks (AONs): Use amplifiers and regenerators. Passive Optical Networks (PONs): Do not use active equipment outside the CO. TDMA-PON: Uses TDM for downstream and TDMA for upstream. Standards include EPON and GPON. GPON: Downstream: 2.5 Gbps, Upstream: 1.25 Gbps. Supports QoS. EPON: Downstream/Upstream: 1.25 Gbps. Transports Ethernet frames. WDMA-PON: Each ONU receives all wavelengths and filters its own. WDM-PON: Uses an AWG (Arrayed Waveguide Grating) for filtering. EDFAs are not used in GPON. Optical carriers and TDM can achieve 40 Gbps. WDM(A)-PON can achieve 1 Gbps per user. DWDM can transmit terabits per second, while CWDM is cheaper.
DOCSIS: Defines IP transport over HFC networks. Specifies PHY, MAC, and provisioning protocols. EuroDOCSIS: Adapted for European PAL systems. Differences: Different upstream and downstream bandwidths, 8 MHz carrier bandwidth. DOCSIS Standards: Various versions with increasing capabilities, including higher modulation orders, channel bonding, and IPv6 support.
Exercise: POTS ADSL Annex A… 4 25 138 1104KHz 2208KHz.
A) Calculate max modulation of ADSL(2+) for Downstream and Upstream
Modulation efficiency = bps/Hz
ADSL Annex A: Upstream = (0.8 Mbps) / (113 kHz) = 7.08 bps/Hz
Downstream = (8 Mbps) / (966 kHz) = 8.28 bps/Hz
ADSL2+ Annex A: Upstream = (1 Mbps) / (113 kHz) = 8.85 bps/Hz
Downstream = (24 Mbps) / (1890 kHz) = 12.7 bps/Hz
Explanation of ADSL(2+) Downstream modulation efficiency: Increased modulation order.
DSLAM: Central office equipment connecting telephone lines to the internet.