Physical Layer and Data Transmission Notes

The Physical Layer

• Provides the mechanical, electrical, functional, and procedural means to activate, maintain, and deactivate physical connections for bit transmission between data link entities.

• Defines hardware characteristics for physically connecting network equipment.

• Covers:

  • Topology: Bus, Ring, Star.

  • Transmission media: Coaxial cable, twisted pair, fiber optics, infrared, microwave.

  • Physical interface hardware between transmission medium and connected station.

• Purpose: Carry binary elements to their destination, minimizing communication costs.

• Includes hardware and software for transporting binary elements correctly:

  • Computer equipment connection interfaces (junctions).

  • MODEMs (MODulator-DEModulators): Transform binary signals into sinusoidal signals.

  • Multiplexers: Concentrate broadcasts from different machines onto a single line; demultiplexers do the reverse.

  • Switching nodes: Intermediate hardware between transmitter and receiver, directing information blocks.

  • Network-specific equipment: Such as satellites for wireless communication.

Network Cabling

Twisted-Pair (Copper) Cables

• Resemble telephone cables.

• Two insulated copper conductors helically wound to reduce electromagnetic interference.

• Used for public telephone service and private network users.

• Signals can travel several tens of kilometers without amplification.

• Crosstalk: Interference between signals on grouped twisted pairs.

• Shielding is used to limit interference.

• Types:

  • Unshielded Twisted Pair (UTP) (U/UTP): No protective shield; used for telephone and home computer networks.

  • Shielded Twisted Pair (STP) (U/FTP): Each pair has a conductive shield; better interference protection; used in token ring networks.

  • Screened Twisted Pair (FTP) (F/UTP): Common shielding around all pairs using aluminum foil; used for telephone and computer networks.

  • Shielded Twisted Pair (SFTP) (S/FTP): Double screen common to all pairs.

  • Super Shielded Twisted Pair (SSTP) (S/FTP): STP with a common screen between the outer sheath and pairs.

• Bandwidth depends on component quality, insulation, and cable length.

• UTP Categories (ANSI/TIA/EIA):

  • Category 1: Telephone communications; obsolete.

  • Category 2: 4 Mbit/s data transmission with 2 MHz bandwidth; used for token ring networks.

  • Category 3: 16 MHz bandwidth; used for analog/digital telephony and Fast Ethernet (100Mbps); being abandoned for Category 5e.

  • Category 4: 20 MHz bandwidth; used in token ring networks at 16 Mbps.

  • Category 5: 100 MHz bandwidth; up to 100 Mbps data rate.

  • Category 5e: 100 MHz bandwidth; up to 1000 Mbps data rates (TIA/EIA-568B standard).

  • Category 6: 250 MHz and more bandwidth.

  • Category 6a: 500 MHz bandwidth, enables 10 GBASE-T operation over 90 meters.

  • Category 7: 600 MHz bandwidth.

  • Category 7a: 1 GHz bandwidth, up to 10 Gbps speeds.

• Uses RJ45 connectors.

• Most widely used physical medium due to universal cabling, low cost, and wide range of use. (Figures II.1, II.2, II.3)

Fiber Optic Cables

• Transmit information by modulating a light beam.

• Consist of transmitting and receiving fibers supported by plastic reinforcement (e.g., Kevlar).

• Light is guided into the core (silicon dioxide/silica) surrounded by optical cladding (silica with lower refractive index for reflection).

• Cladding is protected by a plastic envelope.

• Mode: Path taken by a ray.

• Single-mode fiber: Transmits a single ray.

• Multimode fiber: Transmits several rays; larger core than single-mode.

• Light Sources:

  • Multimode fiber uses LEDs (Light Emitting Diodes).

  • Single-mode fiber uses lasers (more expensive).

• Maximum Lengths:

  • Multimode fiber: 2000 m.

  • Single-mode fiber: 3000 m.

• Single-mode fibers are more expensive and used for WAN links.

• Multimode fibers are less expensive and used in enterprises.

• Advantages:

  • Extremely fast (high bandwidth).

  • Insensitive to electromagnetic interference.

  • Very little signal attenuation, enabling long segments.

  • Small size and lightweight compared to copper.

  • Greater data confidentiality.

• High overall cost (adapter, cable, installation) limits use in local networks to backbones or high-bandwidth applications (multimedia, videophony, large files).

Wireless Networks

• Essential for mobile working; enables access to applications and information from various locations.

• Advantages:

  • Affordability: Lower cabling costs.

  • Convenient access: Network resources accessible within coverage area.

  • Simple installation and expansion: No need to run cables.

• WiFi (Wireless Fidelity): Wireless broadband transmission technology using radio waves; based on IEEE 802.11 standard (WLAN).

• Protocols and Data Rates:

  • 802.11b: 11-22 Mbits/second.

  • 802.11g: Up to 54 Mbits/s.

• Hardware: Router modems compatible with 108 Mbps.

• Requirements: WiFi access point and WiFi adapter on the computer (USB key, PCI/PCMIA expansion card, network card).

Access Points

• Wireless device used as a hub for wireless clients.

• Clients communicate via the access point.

• Multiple access points can cover a geographical area (home, business, park).

• Have wireless network card and Ethernet network cards for connections.

• Configuration Modes:

  • Infrastructure Mode:

    • Wireless access points connected to a wired network.

    • Each network has an SSID (network name).

    • Wireless clients connect to access points.

    • IEEE 802.11 standard defines connection protocol.

    • Clients can attach to a network by specifying an SSID or can attach to any network by not specifying an SSID.

  • Ad-hoc Mode:

    • Point-to-point connections.

    • IBSS mode (IEEE ad-hoc mode) defined by IEEE 802.11 standards.

    • Ad-hoc demo or Lucent ad-hoc mode (pre-standard 802.11 ad-hoc mode) for older installations.

WiFi Network Security

• Digital data transmitted can be intercepted.

• Security Techniques:

  • WEP (Wired Equivalent Privacy):

    • Encryption method using a secret key (64 or 128 bits).

    • Key declared on access point and wireless adapters.

    • Not very effective against serious hacking attempts.

  • MAC Address Filtering:

    • Unique identifier for each network card.

    • Access point verifies the identity of connecting computers.

    • Combine with WEP or WPA keys.

  • WPA (Wi-Fi Protected Access):

    • Better security than WEP.

    • Uses dynamic TKIP (Temporal Key Integrity Protocol) keys to authenticate devices.

    • Generates a single key for the entire network (unlike WEP).

Power Line Communication (PLC)

• Transports high-frequency signal by superimposing it on the 50 Hz electrical current.

• Transmits digital information over existing 220-volt electrical network.

• Creates a high-speed Internet local area network using a home's electrical network.

• Computers and equipment with RJ45 (Ethernet) sockets can be networked in a building.

• Theoretical data transfer rates of several hundred megabits per second.

• Factors Affecting Data Transmission: Age of electrical network, cable length, power strips, interfering devices, quality of powerline adapters.

• Alternative to cables and Wi-Fi due to ease of use.

Wiring Characteristics and Selection

• Performance described by:

  • Attenuation (dB/km): Influences maximum usable cable length.

  • Bandwidth: Attenuation values vary with frequency.

  • Velocity Coefficient: Speed at which a bit travels through the cable (percentage of the speed of light).

  • Communication Speed: (e.g., 10 Mbits/s or 100 Mbits/s).

  • Characteristic Impedance.

Radio Relay Systems

• Used for long-distance transmission as an alternative to coaxial cables.

• Transmitting and receiving dishes on pylons or towers.

• Hertzian beam established between antennas in direct view (tens of kilometers away).

• Used in telephone and television transmission systems.

• Short-distance radio links:

  • Connect two LAN to LAN network segments.

  • Links between a cable station and mobile computers.

  • Links using GSM cell phones equipped with modems.

• Microwave communications: Direct line from transmitting tower to receiving tower (line of sight).

• Allows multiplexing of communication channels, enabling very high data rates.

• Sensitive to natural terrain; transmission towers must be built to guarantee signal routing.

• Frequency bands around 1 GHz.

Satellites

• Radio-frequency wave relay with multiple receivers.

• Compared to tower-to-tower communications between ground and satellite.

• Receivers on the satellite amplify and retransmit signals to earth by transposing the frequency band.

• Geographical areas covered depend on the beams used.

• Geostationary satellites: Positioned at an altitude of 36,000 kilometers above the equator, with a rotation period of 24 hours.

• Used to set up fixed antennas on the ground, linking several ground stations.

• Communications with rotating satellites are limited in duration and require motorized antennas.

• Special protocols are needed to avoid premature error detection due to long communication distances.

Interconnection Equipment

• Infrastructure defines equipment and connections, establishes links, and ensures interconnection using communication protocols.

• Local area networks interconnect computers, and specific equipment links multiple local area networks.

• Main Types:

  • Repeaters:

    • Interconnect similar or different media, ensuring continuity of physical topology.

    • Lowest-level equipment.

    • Re-transmits frames bit by bit to other segments without interpreting them.

    • Regenerates signals to counter attenuation and distortion.

  • Concentrators (Hubs):

    • Broadcasts received frame to all connected equipment.

    • Simple data repeater used to create a local network.

    • Nodes connect to the same access point, sharing total bandwidth.

    • Physical structure is a star, but logical topology remains a bus.

  • Bridges:

    • Connect two local networks of different types.

    • Manage local or remote links, interconnect networks, and optimize communication flows.

    • Extends beyond limits imposed by the local network standard.

    • More powerful than repeaters, preventing fault propagation and filtering frames.

  • Network Switches:

    • Broadcasts received frame to the intended device.

    • Uses a MAC address table to determine the destination.

    • Operates at the data link layer of the OSI model.

    • Allocates entire bandwidth to stations or segments, unlike concentrators.

    • Increases overall bandwidth of a corporate network.

    • Does not implement security features, apart from improved availability.

  • Routers:

    • Routes packets between two networks, regardless of protocol.

    • Connected to two networks to route frames.

    • Acts at the network layer of the OSI model.

  • Gateways:

    • Connects two heterogeneous computer networks.

    • Acts between levels 4 and 7 of the OSI model.

    • Includes elements such as a firewall or a proxy server.

  • Firewalls:

    • Hardware or software element that enforces the network's security policy.

    • Defines authorized or prohibited communication types.

Theoretical Basis of Data Transmission

• Data is transmitted as signals by varying physical parameters of the signals.

• Example: $+V = 0$ and $-V = 1$

Bandwidth

• Each medium has a low and high cutoff frequency.

• Bandwidth is the difference between the high and low cutoff frequencies.

• Signals undergo distortion (attenuation) depending on frequency.

• Channel bandwidth is the range of frequencies transmitted without attenuation.

• Filters (low-pass, high-pass, band-pass) restrict the passband to a given frequency interval.

Relationship Between Bit Rate and Harmonics

• $D = 1/T$ = bit rate in bits per second (bps).

• $T$ = period of a bit.

• Limiting bandwidth limits maximum bit rate.

• Time $T$ to transmit a character depends on coding and signal transmission speed.

Transmission Speed

• $D = 1/T$ bps (bit rate).

• $R = 1/ \Delta$ bauds (transmission speed).

• $\Delta$ = elementary moment: smallest time interval during which the signal remains constant.

• $R$ = number of signal state changes per second.

• Example: $T = 1 \mu s$, $D = 1 Mbps$, $\Delta = 1 \mu s$, $R = 1 MBauds$

Sampling Theorem

• If a signal is applied to a low-pass filter with bandwidth $H$, the filtered signal can be reconstructed by sampling it at a rate equal to $2H$ per second.

• $Rmax = 2H$

  • $R$: Sampling frequency = modulation speed.

  • $H$: Channel bandwidth.

  • $Rmax$: No need for higher sampling frequency.

Maximum Channel Throughput

• $Dmax = 2 H log_2(V)$

  • $H$ = Signal bandwidth.

  • $V$: Signal value (number of significant levels) also known as valence.

  • $2H = R$: Sampling frequency.

  • $log_2(V)$: Number of bits per sample.

• Example: Channel with a bandwidth of 3000Hz:

  • Bivalent signal ($V = 2$).

  • $Dmax = 600$ bps ($log_2(2) = 1$).

  • Quadrivalent signal ($V = 4$) can be used to obtain higher data rates.

  • $Dmax = 1200$ bps ($log_2(4) = 2$).

• Flow cannot be increased indefinitely due to physical limitations and noise.

Types of Transmission

Baseband Transmission

• Reserved for local networks.

• Baseband encoder transforms data bits into a digital electrical signal.

• Signal takes the form of a sequence of voltage levels.

• Consists of transmitting currents on the line reflecting the bits of the character to be transmitted.

• Network card substitutes the initial signal (generally NRZ) with another signal adapted to the line.

• Transformation is of the digital/numeric type.

• Square-wave signals require at least 6 or 7 spectral components for proper signal reconstruction.

• High frequencies result in high bandwidth consumption and short distances.

• Repeaters reshape the signal to increase distances.

• Valence is the number of possible levels.

NRZ (No Return to Zero) Coding

• Valence of 2.

• A 1 bit is translated by a v level and a 0 bit by the -v level.
  (Figure II.10)

NRZI (No Return to Zero Inverted) Coding

• Valence of 2.

• Changes level when transmitting a bit 1; retains previous level when transmitting a 0 bit.
  (Figure II.11)

Manchester Code

• Two-phase code combining physical and logical coding.

• Logical coding transforms 1 bit into 2 bits (1b/2b).

• Valence of 2.

• A bit 1 is transformed into 10, and a bit 0 is transformed into 01.

• NRZ physical coding is used after logical coding.
  (Figure II.12, II.13)

Manchester Differential Code

• Valence of 2; uses NRZ coding as its physical coding.

• Logical coding depends on the last bit generated.

• Logic level '0' copies the signal of elementary moment t-1.

• Logic level '1' inverts the signal of elementary moment t-1.
  (Figure II.14)

Coding Selection Criteria

• Chosen according to known substrate parameters.

• Most transmission media cut off frequency abruptly close to zero, making NRZ unsuitable.

• Two-phase coding requires wide bands.

• Coded according to noise resistance; bipolar level 3 codes are more sensitive than 2-level codes.

• Coded for clock problems; data decoding becomes impossible with clock errors in the signal.

Broadband Transmission

• Used when the link exceeds a few hundred meters.

• Digital signal transformed into an analog signal by modulating a sinusoidal carrier wave.

• Coding is modulation; decoding is demodulation.

• Broadband transmission carries a non-square wave with few spectral components.

• Smaller frequency band and less significant signal distortion.

Modulation

• Signals degrade rapidly over distance in baseband transmission.

• Reducing bandwidth requires transforming the digital signal into an analog signal by modulating a carrier wave.

• Modulation (transmit side) and demodulation (receive side) performed by a modem (modulator-demodulator).

• Three main types of modulation: amplitude, phase, and frequency.

• MODEM transforms a baseband signal into a particular analog form.

• Amplitude Modulation (AM or ASK):
  * Sets (at least) 2 logical levels to the carrier amplitude: A0 and A1.
  * Generally used in conjunction with other modulation methods.
  * The figure below shows an example of amplitude modulation. (Figure II.15)

• Frequency Modulation (FM or FSK):
  * Frequency carrier F0 is modulated by two opposite frequency values, enabling two logic levels to be represented.
  * (Figure II.16)

• Phase Modulation (MP or PM):
  * A modulation mode that consists of transmitting information (sound, data, etc.) by modulating the phase of a carrier signal.
  * This modulation is non-linear.
  * Phase key shifting (PSK) enables higher transmission speeds than FSK modulation on the same carrier, for a similar bandwidth.

Multiplexing

• Transmitting communications belonging to several pairs of transmitting and receiving equipment on a single link line (high-speed channel).

• Each transmitter (resp. receiver) is connected to a multiplexer (resp. demultiplexer) via a link known as a low-speed channel.

• Techniques:

  • Frequency Multiplexing:

    • Assigns each low-speed channel a specific bandwidth on the high-speed channel, ensuring no overlap.

    • Multiplexer retransmits each low-speed channel signal on the high-speed channel in the specified frequency range.
      Demultiplexer can discriminate each signal from the high-speed channel.

  • Time-Division Multiplexing:

    • Distributes the use of the high-speed channel over time, allocating it to various low-speed channels.

    • Each time slot enables transmission of 1 or more bits.

  • Statistical Multiplexing:

    • Allocates the high-speed channel only to low-speed channels that have something to transmit.

    • Improves transmission throughput by not transmitting silences.

    • Requires higher-level protocols and is based on statistical averages of throughput.

ADSL Example

• ADSL (Asymmetric bit-rate Digital Subscriber Line):
  * Enables conventional telephone lines to be used over short distances with higher data rates.
  * Lite version enables connection to the Internet using a telephone line.
  * Maximum downstream speed of 8.2 M bit/sec and maximum upstream speed of 640 K bit/sec (theoretically).
  Performances cannot be obtained over long distances (more than 5 km).

• Technical points of view:
  ADSL operates on a full-duplex basis, using frequency multiplexing to transmit uplink and downlink signals simultaneously, along with the telephone voice signals. see (Figure 1.13)
  * Full-duplex, using frequency multiplexing to transmit uplink and downlink signals simultaneously, along with telephone voice signals.(figure II.17)
  * Uses DMT (Discrete MultiTone) technology, which divides the entire bandwidth into 256 sub-channels, each 4.3 kHz wide.
  * Channel 1 is reserved for telephony; channels 2-6 separate voice from digital data.
  Uplink stream
  * Upstream : 32 channels.
  downlink stream
  * Downstream: Remaining channels. QAM used
  * Each sub-channel is independently modulated using QAM (quadrature amplitude modulation), a method of modulating the amplitude of two carriers in quadrature (4 amplitude levels).
  * Handshake procedure measures transmission quality and adapts to the line (adaptive rate).
  (Figure II.18)