Random Access Method – Introduction

 

Random Access Method – Introduction

Random access is a type of multiple-access protocol used when several stations share a common communication medium. In this method, no station has control or priority over another station. Each station can independently decide when to transmit, provided it follows the procedure defined by the protocol.

The method is called random access because there is no fixed or scheduled time for a station to transmit. A station transmits when it has data to send. It is also called a contention method because the stations compete with one another for access to the shared medium.

For example:

             Shared Medium
          ───────────────────
             │      │      │
           Station Station Station
              A      B      C
             │      │      │
             └── Compete ──┘

If only one station transmits, the transmission can proceed successfully. However, if two or more stations transmit at the same time, an access conflict or collision occurs. The transmitted frames may then be destroyed or modified.

Therefore, a random-access protocol must answer four basic questions:

  1. When can a station access the medium?
  2. What should it do if the medium is busy?
  3. How can it determine whether the transmission was successful?
  4. What should it do if a collision occurs?

Evolution of Random Access Methods

The random-access methods discussed here evolved progressively:

ALOHA
   │
   ▼
CSMA
   │
   ├──────────────┐
   ▼              ▼
CSMA/CD         CSMA/CA

ALOHA

Uses a simple multiple-access (MA) procedure.

CSMA

Improves ALOHA by requiring a station to sense the medium before transmitting.

CSMA/CD

Adds collision detection. The station determines what to do when a collision is detected.

CSMA/CA

Uses collision avoidance, attempting to avoid collisions rather than detecting them after they occur.

Thus, the main idea is:

Random access allows stations to compete for a shared medium without a central controller, while the protocol provides rules for accessing the medium and handling collisions.

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