• Table of Contents

Multiple Access Protocol on Ethernet

In the data link layer, the multiple access problem occurs when several devices share a communication link and may experience collisions.

Figure 7.1 Multiple devices on a router.

Collisions happen when two or more stations attempt to send data simultaneously, causing signal interference. This is a random access issue because there's no central authority regulating access. Protocols like CSMA/CD are used to manage access and reduce collisions in shared networks.

Carrier-sense Multiple Access

The CSMA/CD (Carrier-sense multiple access with collision detection) protocol is used in Ethernet networks. The CSMA (Carrier-Sense Multiple Access) ensures that stations can transmit only if the channel is idle, meaning there's no ongoing communication.

Figure 7.2 Channel with multiple devices.

• A station listens to the channel to check if it's idle. If it's idle, the station can send a data frame. If it's busy, the station waits until the channel becomes idle.
• Stations use carrier-sense mechanisms to detect ongoing transmissions before initiating their own. They sense the presence of carrier signals and wait for ongoing transmissions to end.
• Frames are broadcast to all machines on the LAN, and any machine can use a transceiver's sense circuit to detect data on the link. If data is present, the transceiver quickly senses it, determining whether there's a frame on the media.

Even if every node listens before sending, collisions can still occur in certain scenarios, especially in the initial phase of transmission.

Figure 6.1 Message latency model.

Recall there’s delay in sending a message across a channel, comprising transmission $T_f$ and propagation delay $T_c$.

• For $t < T_c$, some stations may sense an idle channel and start transmitting.

  <aside> <img src="/icons/map-pin_gray.svg" alt="/icons/map-pin_gray.svg" width="40px" /> For example, let's consider a segment where stations $A$ and $B$ are trying to transmit data.

  1. If station $A$ begins transmitting data, it takes some time ($T_c$) for that data to propagate to the other end of the medium where station $B$ is located.
  2. During this initial time $t < T_c$, station $B$ may not yet detect the transmission by station $A$ because the signal hasn't reached station $B$'s location.
  3. If station $B$ senses the channel and finds it idle during this very early time frame $t$, it may mistakenly assume that it can also transmit. </aside>

• For $t > T_c$, all stations detect that the channel is already in use.

Thus, collision can still occur even if stations attempt to avoid collisions by listening before transmitting. This is why collision detection and avoidance methods are crucial in shared networks.

Collision Detection

The CD (Collision Detection) reduces the collision period to minimize the impact of collisions and ensure a more efficient and reliable data transmission. After sending a frame, the sender remains vigilant, watching for signs of a collision. If a collision is detected, the sender takes appropriate action.