Controlled Access Protocols in Computer Network

In computer networks, Controlled Access Protocols (CAPs) manage how multiple devices share a common communication medium. These protocols control the right to transmit in such a way that collisions are avoided, efficiency is maintained, and fair access is ensured.

Note: Unlike random access methods (e.g., ALOHA, CSMA), in controlled access, stations do not transmit at will. Instead, they follow a systematic method to determine which station has the right to send at a given time.

What is Controlled Access?

In Controlled Access, before transmitting data, a station must first obtain permission to access the channel. At any given time, only one node can send data, preventing collisions. This coordination is achieved using one of three methods:

• Reservation
• Polling
• Token Passing

Reservation

In this method, stations reserve slots in advance before transmission. The timeline has two parts:

• Reservation Interval (fixed length): divided into slots, one per station.
• Data Transmission Period (variable length): reserved stations send data in order.

Example: If there are M stations, the reservation interval is divided into M slots. Each station signals intent by inserting a bit into its slot. Then, only the reserved stations transmit in the data phase.

Advantages

• Predictable access time and throughput.
• Priorities can be set for faster access.
• Supports QoS (Quality of Service) for applications like real-time audio/video.
• Efficient bandwidth utilization.

Disadvantages

• Performance decreases under light loads (wasted slots).
• High dependency on controlled synchronization.

Polling

Here, a controller (primary station) polls each node (secondary station) in sequence, granting permission to send. The controller sends a message with the address of a station. The addressed station responds:

• Sends data (if available)

• Sends a NAK (poll reject) if no data is pending.

 Advantages

• Predictable access times and bandwidth.
• High efficiency, since no slot is wasted.
• Priorities can be assigned.

Disadvantages

• Extra overhead due to polling messages.
• High dependence on controller reliability.
• Turnaround time increases under light loads.

Efficiency formula

\eta = \frac{T_t}{T_t + T_{poll}}

where:

• T_t =data transmission time
• T_{poll} = polling overhead time

Token Passing

In Token Passing, stations are logically connected (ring or bus). A special frame called a token circulates among stations:

• The token represents permission to transmit.
• A station can send data only when it holds the token.
• If no data is ready, the token is simply passed on.

Common Implementations

• Token Ring – token passed in a ring topology.
• Token Bus – stations use a bus medium but pass the token in a predefined logical order.

Advantages

• Excellent performance under high loads.
• Built-in error recovery and debugging features.
• Predictable throughput.

Disadvantages

• Expensive setup and hardware.
• Token management issues (lost/duplicate tokens, station failures).
• Complex implementation.

Performance Parameters

• Delay: Average delay = a/N, where a = T_p/T_t.
• Throughput:

1. For a<1a < 1a<1: S = \frac{1}{1 + a/N}
2. For a>1a > 1a>1: S = \frac{1}{a(1 + 1/N)}

where:

• N = number of stations
• T_p = propagation delay
• T_t = transmission delay

Comparison of Controlled Access Methods