Computer_Networking_-_Chapter_6.3

6.3 Multiple Access Links and Protocols

6.3.1 Overview of Multiple Access Protocols

Two primary network link types:

  • Point-to-Point Links:

    • Involves one sender and one receiver with a direct connection.

    • Examples:

      • Point-to-Point Protocol (PPP): Widely used for direct connections over serial links, providing framing, addressing, and error detection.

      • High-Level Data Link Control (HDLC): An ISO standard for point-to-point and point-to-multipoint connections that provides error detection and correction capabilities.

  • Broadcast Links:

    • Enable multiple nodes to send and receive frames over a shared channel, facilitating communication among several devices simultaneously.

    • Examples:

      • Ethernet: The dominant local area network (LAN) technology that uses a star or bus topology for frame transmission.

      • Wireless LANs (WLANs): Utilize radio frequencies for wireless communication, allowing mobile connections and ease of deployment within an area.

    • Essential Challenge: Coordinating multiple nodes to access a shared broadcast channel creates the multiple access problem, requiring efficient traffic management and collision handling.

    • Analogies:

      • Telephone: One speaker can talk at a time while many listeners can hear, necessitating protocols that prevent overlapping conversations.

      • Cocktail Party: Everyone speaks and listens simultaneously, demanding protocols to manage conversation flow and ensure that messages are effectively conveyed without confusion.

6.3.2 Characteristics of Multiple Access Protocols

A good multiple access protocol should:

  • Allow a single active node to transmit at a throughput of R bps to maintain suitable communication speed.

  • Allocate fair throughput of R/M bps when M nodes are active, avoiding network congestion.

  • Operate in a decentralized manner, eliminating reliance on a master node to manage access and improve resilience.

  • Be simple and cost-effective to implement, making it accessible for a wide range of applications and environments.

6.3.3 Types of Multiple Access Protocols

Channel Partitioning Protocols:

  • These protocols divide the channel into distinct segments to manage access among multiple users.

    • Time Division Multiplexing (TDM):

      • Divides time into slots assigned to each node, reducing the chance of collisions but potentially leading to inefficiencies if nodes have no data to send during their allocated times.

    • Frequency Division Multiplexing (FDM):

      • Assigns different frequency bands to each user, similar to TDM, but suffers from similar limitations regarding unused bandwidth during assigned times.

    • Code Division Multiple Access (CDMA):

      • Each node transmits using a unique code, enabling simultaneous transmission without interference, enhancing efficiency in high-traffic scenarios.

6.3.4 Random Access Protocols

  • Random Access Protocols:

    • Allow nodes to transmit at full channel rate R but necessitate retransmission after collisions.

    • Examples include:

      • ALOHA Protocols:

        • Simple protocol that operates without synchronization and is easy to implement.

        • Slotted ALOHA: Introduces time slots to reduce wasted time and increase efficiency, though it remains limited to 37% efficiency in high collision rates.

      • Carrier Sense Multiple Access (CSMA):

        • Nodes listen for existing traffic before transmitting to avoid immediate collisions.

        • Simple but can still experience collisions if multiple nodes detect silence and transmit at the same time.

6.3.5 Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

  • CSMA/CD:

    • Improvements on CSMA that allow nodes to monitor ongoing communications, enhancing performance and minimizing collisions.

    • Steps include:

      • Prepare frame for transmission.

      • Sense the channel; transmit if no other transmission is detected.

      • While transmitting, listen for collisions; if one is detected, cease transmission immediately.

      • Random Backoff Algorithm: Employs a Binary Exponential Backoff strategy, increasing the waiting time exponentially after each collision to reduce the likelihood of repeated collisions.

6.3.6 Taking Turns Protocols

  • Taking Turns Protocols:

    • Ensure fair access to the channel, preventing multiple transmissions at once but potentially increasing latency.

    • Polling Protocol:

      • A designated master node polls active nodes, granting them time to transmit data.

      • Although it removes collisions, it introduces delays and creates a single point of failure, impacting network reliability.

      • Example: Bluetooth protocol, which uses polling to coordinate connections among devices.

    • Token-Passing Protocol:

      • Nodes share a token that permits them to transmit, ensuring only the node holding the token can send data.

      • While decentralized, it can be affected by a failed node, causing disruption to overall network operations.

6.3.7 DOCSIS: Link Layer Protocol for Cable Internet Access

  • DOCSIS (Data Over Cable Service Interface Specification):

    • Utilizes a hybrid approach combining FDM, TDM, and random access protocols to manage upstream and downstream traffic effectively.

    • Each modem must request to access the shared channel, with the possibility of collisions if multiple requests occur simultaneously.

    • Minimizes collisions by allocating mini-slots based on control messages known as MAP (Media Access Protocol), efficiently organizing channel access.