Understanding Controlled-Access Protocols

In computer networking, controlling access to a shared medium is crucial for efficient communication. Controlled-access protocols are a category of network protocols designed to manage how devices access a shared communication channel. Unlike contention-based methods (where devices compete for access and might collide), controlled-access methods ensure that only one device transmits at a time or regulate the transmission opportunities in a structured way.

Analysis of Protocol Options

Let's examine each option provided in the question to determine if it fits the definition of a controlled-access protocol:

A. Reservation

In a Reservation system, stations must first make a reservation for a specific time slot or channel well in advance of sending data. This reservation process dictates who can transmit and when, making it a controlled-access method.

B. Polling

Polling involves a central controller (or master station) asking individual stations (or slave stations) sequentially if they have data to transmit. Only the station that is polled and grants permission can send data. This centralized supervision makes it a controlled-access method.

C. TDM (Time Division Multiplexing)

TDM assigns a specific, recurring time slot to each station. While this is a deterministic way to share a channel and prevents collisions by pre-allocating time, in some contexts, it might be considered a channel *partitioning* technique rather than a protocol managing dynamic access requests. Based on the provided answer options, TDM is not grouped with the other controlled-access protocols in this specific question.

D. Token Passing

The Token Passing protocol uses a special control frame, known as a 'token', that circulates around the network ring. A station can only transmit data if it possesses the token. Once transmission is complete, the station releases the token. This method strictly controls access, preventing collisions, and is a classic example of a controlled-access method.

E. CSMA/CA

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is a protocol used primarily in wireless networks (like Wi-Fi). While it aims to *avoid* collisions by having stations sense the channel and potentially wait (back off) before transmitting, it's generally considered distinct from strictly controlled-access methods like polling or token passing. It doesn't offer the same deterministic guarantees and is more accurately described as a contention-based method with collision avoidance mechanisms.

Conclusion on Controlled Access

Based on the standard definitions and the structure implied by the correct answer (A, B, and D), the protocols identified as controlled-access are:

• Reservation (A): Requires advance booking of transmission time.
• Polling (B): Access is granted by a central controller upon request.
• Token Passing (D): Access is regulated by possession of a circulating token.

Protocols like TDM (C) and CSMA/CA (E) fall into different classifications or are considered less strictly controlled in the context of this question.