---
title: "Reservation protocols"
subject: "Computer Networks and Security"
chapter: 3
key: "semester_7/computer_networks_and_security:reservation-protocols"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/reservation-protocols
site: BCT Engineering
---

# Reservation protocols

Computer Networks and Security · Chapter 3: Data Link Layer

- Asked in: 2070 Asar, 5 marks
- Video: [Controlled Access Protocol – Reservation](https://www.youtube.com/watch?v=baaPXiQ44vs) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/reservation-protocols
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-3

## Question

Explain reservation protocols in detail.

## Short answer

Reservation protocols divide time into a contention period for reservations (e.g., bit-map: N mini-slots, stations set a bit if they have data, then transmit in order) and a data period. They are collision-free, efficient at high load, but add overhead at low load. Variants include binary countdown and R-ALOHA.

## Answer

**Reservation protocols** are controlled-access MAC protocols where a station must **reserve** the channel before sending data. This prevents collisions, so they work well when the channel is heavily loaded.

#### Basic bit-map protocol
Time is divided into a **contention period** (reservation interval) and a data period. For N stations (numbered 0 to N − 1), the contention period has N mini-slots, one per station. A station with data sets its bit to 1 in its own slot. All stations see every reservation, so they know the order. Then stations transmit frames in numeric order.

```text
 Reservation: | 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 |   (stations 0-7; 1, 3, 4 ready)
 Data:        |  frame 1 | frame 3 | frame 4 |
```

- Low load: efficiency is low, d/(d + N), because the N reservation bits are spent on only one frame (d = frame length in mini-slot units).
- High load: efficiency approaches 100%, d/(d + 1), because the N bits are shared by N frames.
- The delay is unequal: a low-numbered station that becomes ready just after its slot waits on average about 1.5N slots, while a high-numbered station waits about 0.5N slots.

#### Other variants
- **Binary countdown:** each station sends its binary address bit by bit; the bitwise OR on the channel decides; a station that sees a 1 where it sent a 0 gives up. Higher address wins; efficiency is d/(d + log₂N).
- **Reservation in satellite networks (R-ALOHA):** slots are reserved by short requests and then used without contention.

Reservation is a **controlled-access** method, like polling and token passing. It is collision-free, but it adds delay under low load and needs N reservation slots per round.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the key idea of reservation protocols?
   A. Stations transmit randomly
   B. A station reserves the channel before sending, preventing collisions
   C. Stations sense the carrier
   D. The channel is divided by frequency
   Answer: B. Reservation is controlled access: stations first reserve, then send in order. (Key point: Reservation idea)

2. In the basic bit-map protocol with N stations, how many mini-slots are in the contention period?
   A. 1
   B. N, one per station
   C. log2 N
   D. 2N
   Answer: B. Each station has its own mini-slot where it sets 1 if it has data. (Key point: Bit-map protocol)

3. How does bit-map efficiency behave with load?
   A. Low at low load, near 100% at high load
   B. High at low load, low at high load
   C. Always 50%
   D. Always 100%
   Answer: A. At low load waiting and overhead dominate; at high load efficiency approaches d/(d+1). (Key point: Load behaviour)

4. In binary countdown, which station wins?
   A. The one with the lowest address
   B. The one with the highest address, because the OR of bits eliminates others
   C. The first to send
   D. A random one
   Answer: B. Stations drop out if they see a 1 where they sent 0; the highest address wins. (Key point: Binary countdown)

## Related in other subjects

- [Passive components behavior and application protocols](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-components-behavior-protocols) — RF and Microwave Engineering, chapter 1
- [Microwave vs low frequency and five system protocols](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-differences-five-protocols) — RF and Microwave Engineering, chapter 1
- [Lumped vs distributed analysis, applications, protocols](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Alumped-distributed-protocols) — RF and Microwave Engineering, chapter 1
- [Atomic commitment protocols (2PC, 3PC)](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Aacp-two-three-phase) — Distributed System, chapter 8