Reservation Protocols in Computer Network

Reservation protocols are a class of protocols where stations wishing to transmit data announce their transmission intent before actual data transfer. These protocols operate in the medium access control (MAC) layer and transport layer of the OSI model to ensure collision-free communication.

In these protocols, there is a contention period prior to transmission. During this period, each station broadcasts its desire for transmission. Once all stations announce themselves, one or more of them get the desired network resources based upon agreed criteria. Since each station has complete knowledge of whether every other station wants to transmit, all possibilities of collisions are eliminated.

How Reservation Protocols Work

The reservation process follows a structured approach:

• Announcement Phase − Stations broadcast their transmission requests

• Reservation Phase − Network resources are allocated based on predetermined criteria

• Data Transmission Phase − Stations transmit data without collision risk

Examples of Reservation Protocols

The two prominent reservation protocols are:

• Bit-Map Protocol − Operates in the MAC layer for local network access control

• Resource Reservation Protocol (RSVP) − Operates in the transport layer for quality of service reservations

Bit-Map Protocol

In this protocol, the contention period is divided into N slots, where N is the total number of stations sharing the channel. If a station has a frame to send, it sets the corresponding bit in its designated slot to 1.

For example, if there are 8 stations and stations 2, 3, 6, and 7 wish to transmit, they will set their corresponding slots to 1. Transmission typically occurs in ascending order of slot numbers, ensuring orderly and collision-free access.

Contention Slots: [0] [1] [2] [3] [4] [5] [6] [7]
Bit Map Result:    0   0   1   1   0   0   1   1
Transmission Order: Station 2 ? Station 3 ? Station 6 ? Station 7

Resource Reservation Protocol (RSVP)

RSVP is a transport layer protocol used to reserve resources in a computer network to achieve different quality of service (QoS) levels while accessing Internet applications. It operates over Internet Protocol (IP) and initiates resource reservations from the receiver's end.

RSVP supports both unicasting (one-to-one communication) and multicasting (one-to-many communication). The protocol uses PATH and RESV messages to establish and maintain resource reservations along the data flow path.

Advantages and Disadvantages

Conclusion

Reservation protocols provide collision-free communication by requiring stations to announce their transmission intent before data transfer. While they introduce overhead through the reservation process, they are highly effective for networks requiring predictable access and quality of service guarantees.