Controlled Access Protocols || Reservation || Polling || Token Passing || Computer Networks

Key Concepts

• Controlled Access Protocols: Reservation, Polling, Token Passing
• Reservation Frame: Divided into slots, each representing a station.
• Primary Device & Secondary Devices: Roles in the Polling protocol.
• Select Function: Used by the primary device to check the availability of a secondary device.
• Poll Function: Used by the primary device to request data from a secondary device.
• Token: A special frame in Token Passing, granting transmission rights.
• Predater & Successor Stations: Neighboring stations in Token Passing.

Reservation Protocol

• Main Idea: Stations reserve slots in a reservation frame before transmitting data to avoid collisions.
• Reservation Frame: The frame is divided into 'n' slots, where 'n' is the number of stations. Each slot corresponds to a specific station.
• Reservation Process:
  1. A station wanting to transmit places a '1' bit in its assigned slot within the reservation frame.
  2. Stations not wanting to transmit leave their slots as '0'.
  3. All stations monitor the reservation frame to see which stations have reserved slots.
• Example: A reservation frame with five slots. Station 1 and Station 3 place '1's in their respective slots, indicating their intent to transmit. Stations 2, 4, and 5 have '0's.
• Collision Avoidance: Since each station knows which other stations intend to transmit, collisions are avoided.
• Data Transmission: Stations transmit their data in the order of their slot number after the reservation frame. Station 1 transmits first, then Station 3.
• Controlled Access: The protocol controls which station accesses the medium based on the reservation frame.

Polling Protocol

• Main Idea: A primary device controls which secondary device can access the shared medium.
• Device Roles:
  • Primary Device: Initiates and controls the communication.
  • Secondary Devices: Respond to the primary device's requests.
• Functions Used by Primary Device:
  • Select Function: Used to determine if a secondary device is free to receive data.
  • Poll Function: Used to ask a secondary device if it has data to send.
• Select Function Process:
  1. The primary device sends a "Select" frame to a secondary device (e.g., "Select A"). The frame contains the address of the intended secondary device.
  2. If the secondary device is free, it sends an acknowledgment (ACK) to the primary device.
  3. The primary device then sends the data to the secondary device.
  4. The secondary device acknowledges the received data.
  5. If the secondary device is busy, it sends a negative acknowledgment (NACK). The primary device then tries another secondary device.
• Poll Function Process:
  1. The primary device sends a "Poll" request to a secondary device (e.g., "Poll A").
  2. If the secondary device has data to send, it sends the data to the primary device.
  3. The primary device acknowledges the received data.
  4. If the secondary device has no data to send, it sends a negative acknowledgment (NACK).
• Controlled Access: The primary device decides which secondary device can use the channel, controlling access to the shared medium.

Token Passing Protocol

• Main Idea: A special frame called a "token" circulates among stations. Only the station possessing the token can transmit data.
• Ring Topology: Stations are logically arranged in a ring.
• Station Relationships: Each station has a "predater" (previous station) and a "successor" (next station).
• Token Circulation: The token is passed from station to station in a circular fashion.
• Transmission Process:
  1. A station must possess the token to transmit data.
  2. If a station has data to send and holds the token, it transmits the data.
  3. After transmitting, the station passes the token to its successor.
  4. If a station has the token but no data to send, it immediately passes the token to its successor.
• Collision Avoidance: Only one station can transmit at a time, eliminating collisions.
• Waiting for Token: If a station has data to send but does not have the token, it must wait until it receives the token from its predater.
• Example: Station 1 has the token and sends data. After sending, it passes the token to Station 2. If Station 3 has no data, it immediately passes the token to Station 4.
• Controlled Access: The token controls which station can access the medium, ensuring orderly transmission.

Synthesis/Conclusion

The video explains three controlled access protocols: Reservation, Polling, and Token Passing. Reservation uses a reservation frame to allocate slots for transmission. Polling uses a primary device to control access for secondary devices via "Select" and "Poll" functions. Token Passing uses a token that circulates among stations, granting transmission rights to the token holder. All three protocols aim to control which station accesses the shared medium, preventing collisions and ensuring orderly communication.