📡Systems Approach to Computer Networks Unit 14 – Multiple Access: Links and Protocols

Key Concepts

• Multiple access enables multiple devices to share a common communication medium (Ethernet, Wi-Fi)
• Efficient coordination among devices is crucial to avoid collisions and ensure fair access
• Medium Access Control (MAC) protocols govern how devices share the medium
  • Determine when a device can transmit data
  • Handle collision detection and resolution
• Throughput, latency, and fairness are key performance metrics for evaluating multiple access protocols
• Collision domains define the scope within which collisions can occur (shared Ethernet segments)
• Channel partitioning techniques (TDMA, FDMA) allocate dedicated time slots or frequency bands to devices
• Random access protocols (ALOHA, Slotted ALOHA) allow devices to transmit at any time, risking collisions

Types of Multiple Access

• Time Division Multiple Access (TDMA) divides the channel into time slots, each assigned to a specific device
  • Guarantees a fixed amount of bandwidth for each device
  • Commonly used in cellular networks (GSM)
• Frequency Division Multiple Access (FDMA) allocates different frequency bands to each device
  • Devices transmit simultaneously on separate frequencies without interfering
  • Applied in analog cellular systems and cable TV
• Code Division Multiple Access (CDMA) assigns unique codes to each device, allowing simultaneous transmission
  • Devices use spread spectrum techniques to spread their signal across a wide frequency band
  • Employed in 3G cellular networks (UMTS)
• Space Division Multiple Access (SDMA) utilizes spatial separation to enable multiple devices to communicate concurrently
  • Directional antennas or multiple antennas (MIMO) focus signals towards intended receivers
  • Increases capacity by reusing frequencies in different spatial regions

MAC Protocols

• MAC protocols coordinate access to the shared medium among multiple devices
• Contention-based protocols allow devices to compete for channel access
  • Examples include ALOHA, Slotted ALOHA, CSMA/CD, CSMA/CA
  • Devices listen to the channel before transmitting to avoid collisions
• Contention-free protocols allocate dedicated resources (time slots, frequencies) to each device
  • Eliminates collisions but may lead to underutilization if devices have no data to send
  • Examples include TDMA, FDMA, polling, token passing
• Hybrid protocols combine contention-based and contention-free approaches
  • Dynamically adapt to traffic demands and network conditions
  • IEEE 802.11 PCF (Point Coordination Function) is an example of a hybrid protocol

CSMA/CD and CSMA/CA

• Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is used in wired Ethernet networks
  • Devices listen to the channel before transmitting (carrier sense)
  • If a collision is detected during transmission, devices abort and wait a random backoff time before retrying
  • Exponential backoff increases the waiting time after each collision to reduce congestion
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is employed in wireless networks (Wi-Fi)
  • Devices perform carrier sense and exchange RTS/CTS (Request to Send/Clear to Send) frames to reserve the channel
  • RTS/CTS handshake helps mitigate the hidden node problem, where devices cannot hear each other's transmissions
  • If the channel is idle, devices wait for a random backoff time before transmitting to avoid simultaneous transmissions

Collision Detection and Avoidance

• Collision detection mechanisms allow devices to detect collisions during transmission
  • In CSMA/CD, devices monitor the channel while transmitting and abort if a collision is detected
  • Ethernet uses voltage levels on the wire to detect collisions
• Collision avoidance techniques aim to prevent collisions before they occur
  • RTS/CTS handshake in CSMA/CA reserves the channel and informs nearby devices to defer transmission
  • Backoff mechanisms introduce random waiting times to reduce the probability of simultaneous transmissions
  • Carrier sense helps devices determine if the channel is idle before attempting to transmit

Performance Metrics

• Throughput measures the effective data rate achieved by the network
  • Represents the amount of data successfully transmitted per unit time
  • Affected by factors such as collision rate, protocol overhead, and channel utilization
• Latency refers to the delay experienced by data packets from source to destination
  • Includes propagation delay, transmission delay, and queuing delay
  • MAC protocols impact latency through channel access delays and collision resolution mechanisms
• Fairness ensures that all devices have equal opportunities to access the shared medium
  • Prevents a single device from monopolizing the channel and starving others
  • MAC protocols employ fairness mechanisms (round-robin, proportional fairness) to allocate resources equitably
• Channel utilization indicates the percentage of time the channel is effectively used for data transmission
  • Higher utilization implies more efficient use of the available bandwidth
  • Collision avoidance and minimizing protocol overhead contribute to improved channel utilization

Real-World Applications

• Ethernet (IEEE 802.3) is the most widely used wired LAN technology
  • Employs CSMA/CD for multiple access in shared Ethernet segments
  • Switched Ethernet eliminates collisions by providing dedicated links between devices and switches
• Wi-Fi (IEEE 802.11) is the dominant wireless LAN technology
  • Uses CSMA/CA with RTS/CTS for multiple access in the unlicensed 2.4 GHz and 5 GHz bands
  • Supports various data rates and modulation schemes (802.11a/b/g/n/ac/ax)
• Cellular networks (2G, 3G, 4G, 5G) employ a combination of multiple access techniques
  • TDMA and FDMA in 2G (GSM), CDMA in 3G (UMTS), OFDMA in 4G (LTE) and 5G (NR)
  • Efficient multiple access enables high-speed mobile data services and voice communication
• Satellite communication systems use multiple access to share limited satellite resources among multiple users
  • TDMA, FDMA, and CDMA are commonly employed in satellite networks (Iridium, Globalstar)

Advanced Topics and Future Trends

• Multi-user MIMO (MU-MIMO) leverages spatial multiplexing to serve multiple users simultaneously
  • Base stations equipped with multiple antennas can transmit to multiple devices concurrently
  • Increases network capacity and spectral efficiency
• Cognitive radio networks dynamically adapt to the available spectrum and share it opportunistically
  • Devices sense the spectrum, identify unused channels, and adjust their transmission parameters accordingly
  • Enables efficient utilization of scarce spectrum resources
• Non-Orthogonal Multiple Access (NOMA) allows multiple devices to share the same time-frequency resources
  • Devices are assigned different power levels and use successive interference cancellation (SIC) to decode signals
  • Improves spectral efficiency and accommodates more devices compared to orthogonal multiple access schemes
• Machine-to-Machine (M2M) communication and Internet of Things (IoT) pose new challenges for multiple access
  • Massive number of devices with diverse requirements (low latency, energy efficiency)
  • Scalable and efficient multiple access protocols are needed to support M2M and IoT applications