The idea
CSMA/CD works over a wire because a station can listen to its own cable while transmitting and hear a collision as a jump in energy level. None of that holds over a radio channel.
A collision in the air does not add enough energy for a station to notice it on its own transmission. Stations may sit far enough apart, or fade enough, that one end never hears a collision the other end would see. Received signal power is not something a wireless station can rely on the way a wired one can. And wireless has a problem a shared cable never does: a station can be hidden from another station entirely, in range of a shared destination but not of each other.
None of that lets a wireless network detect a collision after it happens. So instead of detecting collisions, wireless has to avoid them before they happen. That is CSMA/CA — Carrier Sense Multiple Access with Collision Avoidance.
The handshake
How it works
RTS, CTS, data, defer
Before sending data, a station sends a short request-to-send (RTS) packet to the base station, using ordinary carrier sensing. RTS packets can still collide with each other — two stations can key up at once — but because they are short, a collision there costs far less than losing a full data frame.
If the base station receives the RTS cleanly, it broadcasts a clear-to-send (CTS), heard by every station in range, not just the one that sent the RTS. That is what lets CSMA/CA work without collision detection: everyone who could interfere now knows a transmission is about to happen.
The original sender then transmits its data frame. Every other station that heard the CTS defers.
If the sender never receives a CTS, it assumes the RTS collided, falls back to its contention window, and backs off — the same idea as Ethernet’s backoff, just triggered by a missing reply instead of a sensed collision.
NAV — how everyone else knows to wait
Both the RTS and the CTS carry the amount of time the exchange needs to occupy the channel. Every station that hears either one starts a timer called the Network Allocation Vector (NAV), not because it is told directly to wait, but because it now knows exactly how long the channel will be busy.
NAV counts down the time that must pass before an affected station is even allowed to check whether the channel is idle again. Every time a station sends an RTS, every other station in range restarts its own NAV based on the new duration.
| Parameter | Duration | Purpose |
|---|---|---|
| DIFS (Distributed Inter Frame Space) | 50 µs | Channel must be idle this long before a new transmission can start |
| SIFS (Short Inter Frame Space) | 20 µs | Gap before an ACK — shorter than DIFS, so replies get priority over new transmissions |
| Time slot | 10 µs | The unit the contention window counts down in |
Sender and receiver each run a matching half of the same rule:
- Sender. If the channel is idle for a full DIFS, transmit the entire frame — there is no collision detection once transmission starts. If it is busy, start a random backoff timer that only counts down while the channel is idle, and transmit once it reaches zero. No ACK after transmitting means assume a collision, widen the backoff range, and try again.
- Receiver. If the frame arrives intact, reply with an ACK after a SIFS.
There is no collision detection once data starts moving, so CSMA/CA never confirms success the way a wired station does. The ACK is the only signal that a frame got through. Silence means retry.
Hidden node and exposed node
Hidden node. Stations A and B both want to reach the same access point, AP. Each is within range of AP, but they are outside each other’s range, so neither can hear the other’s handshake. Both can end up transmitting to AP at once, and the collision happens at AP — a place neither A nor B can sense.
Exposed node. Station A is transmitting to B. Station C is within range of A, so it hears A’s transmission and defers, even though C actually wants to send to a different node, D, and that transmission would never have collided with anything. C stays silent for no reason.
RTS/CTS fixes the first problem directly: the CTS from AP reaches both A and B, so each learns about the other’s pending transmission even though they cannot hear each other. It does nothing for the second — C still hears the RTS/CTS around A and B and still defers, whether or not its own transmission would actually interfere.
Where marks get lost
RTS/CTS solves one problem, not both
It is tempting to write “RTS/CTS solves the wireless collision problem” and stop there. An exam question phrased as “does RTS/CTS solve wireless collision problems” is checking whether you know it solves hidden node only. Exposed node is a real cost, wasted capacity rather than a corrupted frame, and the lecture gives no fix for it.
| CSMA/CD | CSMA/CA | |
|---|---|---|
| Medium | Wired | Wireless |
| Strategy | Detect a collision while transmitting, then abort | Avoid the collision before transmitting, using RTS/CTS |
| Confirms success how | Absence of a detected collision | An ACK — silence means failure |
| Handles hidden stations? | Doesn't need to — every station shares one wire | Yes, via RTS/CTS and NAV |
Check yourself
A and B are hidden from each other but both reachable by AP. B sends an RTS. What stops A from transmitting into the same slot?
AP’s CTS in response to B’s RTS is heard by A too, even though A never heard B’s RTS. A now knows a transmission is about to happen and defers — that is the whole trick.
In the exam
- List the reasons CSMA/CD does not work wirelessly. Not enough collision energy to detect, hidden node, distance and fading, unknown received power. Four separate reasons, not one.
- Walk through RTS → CTS → data → defer, in that order, and say who hears the CTS — everyone in range of the base station, not just the RTS sender.
- NAV is set by duration information in the RTS/CTS, not by a separate instruction. Every station restarts its NAV whenever it hears a new RTS.
- Hidden node vs exposed node. RTS/CTS fixes hidden node. It does not fix exposed node, and the lecture gives no alternative for that one.
- DIFS = 50 µs, SIFS = 20 µs, slot = 10 µs. SIFS is shorter than DIFS on purpose, so replies do not get overtaken by new transmissions.
- No collision detection once transmission starts. CSMA/CA relies entirely on the ACK to confirm success.