Unit 6: PHYSICAL LAYER

1. Basics for Data Communications

The Physical Layer is the lowest layer of the OSI (Open Systems Interconnection) model. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits and is responsible for transmitting individual bits from one node to the next.

Key Components of Data Communication

• Message: The information (data) to be communicated (text, numbers, pictures, audio, video).
• Sender: The device that sends the data message.
• Receiver: The device that receives the message.
• Transmission Medium: The physical path by which a message travels from sender to receiver.
• Protocol: A set of rules that govern data communications. It represents an agreement between the communicating devices.

Direction of Data Flow

• Simplex: Communication is unidirectional, like on a one-way street. Only one of the two devices on a link can transmit; the other can only receive (e.g., Keyboards, traditional Monitors).
• Half-Duplex: Each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa (e.g., Walkie-talkies).
• Full-Duplex: Both stations can transmit and receive simultaneously. The link must contain two physically separate transmission paths or divide the capacity of the channel (e.g., Telephone networks).

Signals

Data must be transformed into electromagnetic signals to be transmitted.

• Analog Signals: Continuous waves that change smoothly over time. Characterized by Amplitude, Frequency, and Phase.
• Digital Signals: Discrete signals that have a limited number of defined values (usually 1s and 0s).

2. Transmission Media

Transmission media are essentially the physical layer pathways through which signals are propagated. They are broadly categorized into Guided and Unguided media.

2.1 Guided Media (Wired)

Guided media provide a physical conduit from one device to another.

• Twisted-Pair Cable: Consists of two conductors (normally copper), each with its own plastic insulation, twisted together. The twisting cancels out electromagnetic interference (EMI) from external sources and crosstalk from neighboring pairs.
  • Unshielded Twisted Pair (UTP): Most common, widely used in LANs (e.g., Cat 5, Cat 6).
  • Shielded Twisted Pair (STP): Includes a metal foil or braided mesh covering that encases each pair of insulated conductors, reducing noise further.
• Coaxial Cable (Coax): Carries signals of higher frequency ranges than twisted pair. It has a central core conductor of solid wire enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal braid, tube, or foil. (Used in Cable TV).
• Fiber-Optic Cable: Made of glass or plastic and transmits signals in the form of light. It uses the concept of total internal reflection.
  • Advantages: Extremely high bandwidth, immune to electromagnetic interference, lightweight, and long-distance transmission.
  • Modes: Multimode (Step-index and Graded-index) and Single-mode.

2.2 Unguided Media (Wireless)

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are broadcast through free space.

• Radio Waves: Frequencies between 3 kHz and 1 GHz. They are omnidirectional, meaning they travel in all directions from the source. Good for multicasting (e.g., AM/FM radio, television).
• Microwaves: Frequencies between 1 GHz and 300 GHz. They are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This requires line-of-sight propagation (e.g., Satellite communication, cellular networks).
• Infrared: Frequencies between 300 GHz and 400 THz. Used for short-range communication in a closed area using line-of-sight propagation. Cannot penetrate walls (e.g., TV remote controls, wireless keyboards).

3. Transmission Impairments and Performance

Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment, meaning the signal at the beginning of the medium is not the same as the signal at the end.

3.1 Transmission Impairments

• Attenuation: Loss of energy. When a signal travels through a medium, it loses some of its energy in overcoming the resistance of the medium. Amplifiers are used to compensate for this loss. Measured in decibels (dB).
• Distortion: Means that the signal changes its form or shape. Distortion occurs in a composite signal made of different frequencies. Each frequency component has its own propagation speed and therefore its own delay in arriving at the final destination.
• Noise: Thermal noise, induced noise, crosstalk, and impulse noise may corrupt the signal.
  • Thermal Noise: Random motion of electrons in a wire, creating an extra signal.
  • Induced Noise: Comes from sources such as motors and appliances.
  • Crosstalk: The effect of one wire on another (magnetic coupling).
  • Impulse Noise: A spike (a signal with high energy in a very short time) that comes from power lines, lightning, etc.

3.2 Performance Metrics

• Bandwidth: The range of frequencies contained in a composite signal or the range of frequencies a channel can pass. (Measured in Hertz or Bits per second).
• Throughput: A measure of how fast we can actually send data through a network. While bandwidth is the potential capacity, throughput is the actual speed.
• Latency (Delay): Defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out. Latency = Propagation time + Transmission time + Queuing time + Processing delay.
• Jitter: Variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets, causing a degraded user experience.

4. Wireless Networks: Introduction

Wireless networks utilize electromagnetic waves to transmit information between devices. They provide mobility, flexibility, and ease of installation compared to wired networks.

Basic Architecture of Wireless LANs

• Basic Service Set (BSS): The fundamental building block of an IEEE 802.11 LAN. Contains stationary or mobile wireless stations and an optional central base station known as the Access Point (AP).
  • Ad-hoc Network: A BSS without an AP. Stations communicate directly with one another.
  • Infrastructure Network: A BSS with an AP. All communication goes through the AP.
• Extended Service Set (ESS): Created by linking two or more BSSs with APs via a distribution system (usually a wired LAN like Ethernet).

5. IEEE 802.11 Standards (Wi-Fi)

The IEEE 802.11 standard defines the Physical and Data Link Layer (MAC sublayer) specifications for wireless LANs. Over time, several amendments have been made to improve speed and reliability.

802.11a

• Frequency Band: 5 GHz
• Maximum Data Rate: 54 Mbps
• Modulation Technique: OFDM (Orthogonal Frequency Division Multiplexing).
• Characteristics: Less interference since 5 GHz is less crowded. However, higher frequency means a shorter range and worse penetration through solid objects compared to 2.4 GHz.

802.11b

• Frequency Band: 2.4 GHz
• Maximum Data Rate: 11 Mbps
• Modulation Technique: HR-DSSS (High-Rate Direct Sequence Spread Spectrum).
• Characteristics: Longer range and better signal penetration than 802.11a, but heavily susceptible to interference from household appliances (microwaves, cordless phones) that also use the 2.4 GHz band.

802.11g

• Frequency Band: 2.4 GHz
• Maximum Data Rate: 54 Mbps
• Modulation Technique: OFDM (for speeds > 11 Mbps) and DSSS (for backward compatibility).
• Characteristics: Combines the best of both previous standards: the high speed of 802.11a and the better range of 802.11b. Fully backward compatible with 802.11b hardware.

802.11n (Wi-Fi 4)

• Frequency Band: 2.4 GHz and/or 5 GHz (Dual-band).
• Maximum Data Rate: Up to 600 Mbps (theoretically).
• Key Technology: MIMO (Multiple Input Multiple Output). Uses multiple antennas at both the transmitter and receiver to improve communication performance, utilizing multipath propagation.
• Characteristics: Drastically improved speed and range. Introduced channel bonding (using two 20 MHz channels to form a 40 MHz channel).

6. Bluetooth

Bluetooth is a wireless technology standard used for exchanging data between fixed and mobile devices over short distances using short-wavelength UHF radio waves in the ISM band (2.400 to 2.485 GHz).

6.1 Architecture

Bluetooth architecture defines two types of networks:

• Piconet: A Bluetooth network is called a piconet. It can have a maximum of eight active nodes (1 primary/master and up to 7 secondaries/slaves). All communication is between the master and a slave; slaves cannot communicate directly with each other.
• Scatternet: Formed by combining multiple piconets. A slave device in one piconet can act as the master in another piconet, creating a larger network.

6.2 Bluetooth Protocol Stack / Layers

The Bluetooth standard has its own protocol stack, which does not map exactly to the OSI model but roughly corresponds to the Physical and Data Link layers.

1. Radio Layer:

   • Equivalent to the physical layer of the OSI model.
   • Deals with the radio transmission and modulation.
   • Uses a 2.4 GHz ISM band divided into 79 channels of 1 MHz each.
   • Employs Frequency Hopping Spread Spectrum (FHSS) to avoid interference, hopping 1600 times per second.

2. Baseband Layer:

   • Roughly equivalent to the MAC sublayer in LANs.
   • Handles connection establishment within a piconet, addressing, packet format, timing, and power control.
   • Defines two types of links:
     • SCO (Synchronous Connection-Oriented): For real-time data like voice.
     • ACL (Asynchronous Connection-Less): For data transfer where integrity is more important than latency.

3. LMP (Link Manager Protocol) Layer:

   • Responsible for link setup, authentication, configuration, and other protocols related to security.
   • Manages power modes (Active, Sniff, Hold, and Park).

4. L2CAP (Logical Link Control and Adaptation Protocol) Layer:

   • Equivalent to the LLC sublayer in LANs.
   • Provides connection-oriented and connectionless data services to upper-layer protocols.
   • Responsible for multiplexing, segmentation and reassembly (SAR), and quality of service (QoS).
   • It only handles data coming from the ACL link, not SCO (voice bypasses L2CAP).