Chapter 12 Mmultiple access

Chapter 12 of Data Communications and Networking by Behrouz A. Forouzan, titled "Multiple Access", discusses the methods used to allow multiple devices to share the same transmission medium efficiently. Here are detailed notes:

1. Introduction to Multiple Access

• When multiple nodes or stations are connected and share a common link, known as a multipoint or broadcast link, a multiple-access protocol is required to coordinate access.

• The data link layer can be divided into two sublayers:

  1. Data Link Control: Responsible for flow and error control.

  2. Multiple-Access Resolution: Manages access to the shared medium.

2. Categories of Multiple Access Protocols

• Multiple access protocols are categorized into three groups:

  1. Random Access: Stations transmit data randomly and contend for access to the medium.

  2. Controlled Access: Access to the medium is managed in a coordinated manner to avoid conflicts.

  3. Channelization: The available bandwidth is divided into separate channels that stations can use.

3. Random Access Protocols

• ALOHA:

  • Developed for wireless networks at the University of Hawaii.

  • Pure ALOHA: Stations send data whenever they have it, leading to a high chance of collisions.

  • Slotted ALOHA: Time is divided into slots, and stations send data only at the beginning of a slot, reducing the chances of collisions.

• Carrier Sense Multiple Access (CSMA):

  • CSMA: Before transmitting, stations sense the medium to check if it is idle.

  • Persistent CSMA: The station keeps sensing the channel and transmits as soon as it is idle.

  • Nonpersistent CSMA: If the medium is busy, the station waits for a random amount of time before trying again.

  • P-persistent CSMA: The station senses the medium and transmits with a probability p if the medium is idle.

• CSMA with Collision Detection (CSMA/CD):

  • An enhancement to CSMA, where stations listen to the channel while transmitting. If a collision is detected, they stop and retransmit after a random delay.

• CSMA with Collision Avoidance (CSMA/CA):

  • Commonly used in wireless networks to avoid collisions through techniques like interframe space (IFS), contention windows, and acknowledgments.

4. Controlled Access Protocols

• Reservation: Stations reserve the medium by sending a reservation frame. Time is divided into intervals, and within each, a reservation frame is sent before the data.

• Polling: A primary station controls the medium, and secondary stations can only transmit data when polled.

• Token Passing: Stations are organized in a logical ring, and a special token circulates. A station can transmit data only when it holds the token.

5. Channelization Protocols

• Frequency-Division Multiple Access (FDMA):

  • The bandwidth is divided into frequency bands, and each station is assigned a different band.

  • Guard bands are used between frequencies to prevent interference.

• Time-Division Multiple Access (TDMA):

  • Stations share the bandwidth by taking turns, each having a specific time slot to transmit data.

• Code-Division Multiple Access (CDMA):

  • Stations share the same bandwidth but transmit data using unique codes, which are orthogonal and allow the data to be separated at the receiver.

6. Summary

• Multiple access protocols are crucial for managing communication in shared media environments.

• Random Access protocols are more flexible but prone to collisions, whereas Controlled Access and Channelization methods aim for more structured and collision-free communication.

These detailed notes summarize the key concepts and techniques of Chapter 12 .Give