Controlled Access Protocols (CAPs) in
Computer Networks
Controlled Access Protocols (CAPs) control how data packets are sent over a common
communication medium.
These protocols ensure that data is transmitted efficiently, without collisions, and with
minimal interference from other data transmissions.
In controlled access, stations seek permission from one another to find which station has
the right to send data.
Only one node is allowed to send at a time, preventing collisions on a shared medium. The
three main controlled-access methods are:
1. Reservation
2. Polling
3. Token Passing
1. Reservation
### Overview:
In the reservation method, a station needs to make a reservation before sending data. The
timeline is divided into two types of periods:
- Reservation Interval: A fixed time length.
- Data Transmission Period: Consists of variable frames.
If there are M stations, the reservation interval is divided into M slots, and each station gets
one slot.
For instance, if Station 1 has data to send, it transmits 1 bit during its designated slot. No
other station can transmit during this slot.
After all stations have reserved their slots, data transmission takes place in that order. This
avoids any collisions, as every station knows which other stations wish to transmit.
Advantages of Reservation:
- Predictable Performance: Network performance is predictable, making it ideal for real-
time applications such as audio or video streaming.
- Efficient Resource Use: Reduces contention and ensures efficient bandwidth use by pre-
allocating access to the network.
- Support for QoS: Can handle different traffic types (e.g., voice, video, data) by assigning
reservations to prioritize traffic.
- High Throughput: Ensures high rates and low access time.
Disadvantages of Reservation:
- Dependence on Reliability: Heavily depends on the controlled reliability of the reservation
mechanism.
- Low Capacity in Light Loads: Data rate and capacity decrease under light network loads.
- Turnaround Time: Increased turnaround time under low-load conditions.
2. Polling
### Overview:
In polling, a primary station (controller) sends messages to each secondary station in turn.
The controller asks each station if it has data to send. If a station has data, it responds and
sends the data. If not, a “poll reject” (NAK) message is sent.
Polling is similar to a roll-call performed in a classroom, where the teacher (controller) asks
each student (station) in sequence.
Advantages of Polling:
- Predictable Access Time: Maximum and minimum access times and data rates are fixed.
- Efficient Use of Bandwidth: Maximum bandwidth utilization, as no slot is wasted during
polling.
- Priority Assignment: Priority can be assigned to stations for faster access.
Disadvantages of Polling:
- High Overhead: The polling process incurs high overhead due to the control messages
exchanged.
- Link Sharing Bias: Not all stations may always have data to send, leading to inefficiency in
channel use.
- Increased Turnaround Time: Can lead to longer delays in data transmission, especially
under low load.
Efficiency Formula:
Let Tpoll be the polling time and Tt be the time required for data transmission. The
efficiency is given by:
Efficiency = Tt/(Tt + Tpoll)
3. Token Passing
### Overview:
In token passing, stations are connected logically in a ring or bus topology. A special bit
pattern called a "token" is passed from one station to another in a predefined order. The
token represents permission to send data. If a station has data queued for transmission, it
can send the data upon receiving the token. If there is no data, the station simply passes the
token to the next station.
Advantages of Token Passing:
- Good Throughput: Provides good throughput, especially in high load conditions.
- Fault Tolerance: Built-in debugging features like protective relay and auto-reconfiguration.
- Efficiency in High Load: Performs well in environments with high traffic.
Disadvantages of Token Passing:
- Cost: Token passing systems can be expensive due to hardware requirements.
- Complex Topology: The topology components and hardware are more complicated and
costly than other protocols.
- Reliability Issues: Token loss or duplication can disrupt the network, and the system needs
mechanisms to recover.
Performance Metrics:
- Delay: The time between when a packet is ready and when it is delivered.
- Throughput: Measures successful data transfer.
For Token Ring, the throughput S is given by:
S = 1/(1 + a/N)
Where:
- N = Number of stations
- a = Tp/Tt (Propogation delay over transmission delay)
Comparison Table of Controlled Access Methods
Feature Reservation Polling Token Passing
Access Method Pre-reserved slots Primary station Token circulates in
polls secondary predefined order
stations
Network Topology Fixed slots for each Star topology with a Ring or bus
station primary station topology
Efficiency High due to Maximum Good in high-load
predictable access bandwidth conditions
times utilization
Control Pre-allocated access Centralized Distributed control
controller (polling) via token
Overhead Low in heavy traffic, High due to polling Low overhead in
increases with light messages high-load, higher in
load low-load
Fault Tolerance Limited, depends on Depends on the Moderate, but token
reservation controller’s loss can disrupt
reliability reliability network
Cost Moderate, Low to moderate High due to
depending on the specialized
system size hardware
