Wireless LANs (IEEE 802.11 / Wi-Fi)

1. IEEE 802.11 Architecture

The IEEE 802.11 standard defines the physical layer and MAC sublayer for Wireless Local Area Networks (WLANs), commercially branded as Wi-Fi.

                           [ Distribution System (DS) ] (Ethernet Backbone)
                                    /        \
                                   /          \
                            [ Access Point ]  [ Access Point ]
                               (AP 1)            (AP 2)
                               /    \            /    \
                            [STA1] [STA2]     [STA3] [STA4]
                            \___________/     \___________/
                                BSS 1             BSS 2
                            <----------------------------->
                               Extended Service Set (ESS)

Core Architectural Building Blocks:

BSS (Basic Service Set): The fundamental building block of a WLAN:
- Infrastructure BSS: Stations communicate only through a centralized Access Point (AP) connected to a wired backbone.
- Ad-hoc BSS (Independent BSS - IBSS): Peer-to-peer network where wireless stations communicate directly with one another without an AP.
ESS (Extended Service Set):
- Created by interconnecting two or more infrastructure BSSs through a Distribution System (DS) (typically wired switched Ethernet).
- Allows mobile wireless clients to roam seamlessly between APs without dropping active TCP connections.
Station Types (Mobility):
- No-transition mobility: Stationary or operates only within a single BSS.
- BSS-transition mobility: Moves from one BSS to another within the same ESS.
- ESS-transition mobility: Moves from one ESS to another (may require IP address re-assignment).

2. Why CSMA/CD Fails in Wireless: Hidden & Exposed Stations

Wired Ethernet uses CSMA/CD (Collision Detection), but wireless networks cannot use CD for two fundamental reasons:

Dynamic Range / Signal Attenuation: A transmitting radio's signal is thousands of times stronger than incoming received signals. The transmitter's own signal drowns out any colliding signal at its antenna.
Hidden Terminal Problem: Not all stations can hear each other.

A. HIDDEN TERMINAL PROBLEM
   [ Station A ] <==== Range ====> [ Access Point ] <==== Range ====> [ Station C ]
         |                                                                   |
         +----- A cannot hear C; both transmit to AP simultaneously --------+
                ===> DISASTROUS COLLISION AT THE ACCESS POINT!

B. EXPOSED TERMINAL PROBLEM
   [ Station A ] <--- Transmits to ---> [ Station B ]
                                              [ Station C ] <--- Wants to send to ---> [ Station D ]
         Station C hears B's transmission and mistakenly delays sending to D,
         even though transmitting to D would NOT interfere with B's reception!

Hidden Station Problem: Station A and Station C are out of radio range of each other, but both are in range of AP B. Both sense the channel idle and transmit simultaneously, resulting in a destructive collision at AP B.
Exposed Station Problem: Station C senses transmission between A and B and unnecessary defers transmitting to Station D, unnecessarily wasting channel throughput.

3. The MAC Solution: CSMA/CA and RTS/CTS

To prevent collisions, 802.11 implements CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) using Distributed Coordination Function (DCF):

Sender A                         Access Point                      Other Stations
   |                                  |                                  |
   |-- Waits DIFS + Backoff --------->|                                  |
   |                                  |                                  |
   |-- RTS (Request to Send) -------->|                                  |
   |                                  |-- RTS heard: Set NAV timer ----->|
   |                                  |   (Do not transmit)              |
   |<- CTS (Clear to Send) -----------|                                  |
   |                                  |-- CTS heard: Confirm NAV ------->|
   |-- DATA Frame ------------------->|                                  |
   |                                  |                                  |
   |<- ACK (Acknowledgment) ----------|                                  |

The RTS/CTS Handshake Protocol:

RTS (Request to Send): Sender sends a short RTS control frame containing the NAV (Network Allocation Vector), declaring the duration needed to transmit the data frame and receive the ACK.
CTS (Clear to Send): AP broadcasts a CTS frame granting permission and reiterating the NAV duration.
NAV (Virtual Carrier Sensing): All hidden stations hearing the CTS set an internal timer (NAV) and refrain from transmitting until the transaction completes, completely eliminating the hidden terminal collision.
SIFS / DIFS (Interframe Spaces):
- SIFS (Short Interframe Space): Shortest waiting period, reserved for immediate priority frames (CTS, ACK).
- DIFS (DCF Interframe Space): Longer waiting period required before a station can attempt to send an RTS or initial data frame.

\text{SIFS} < \text{PIFS} < \text{DIFS} < \text{EIFS}

4. IEEE 802.11 Standards Comparison

Standard	Release	Frequency Band	Max Data Rate	Modulation / Channel Bandwidth
802.11b	1999	$2.4\text{ GHz}$	$11\text{ Mbps}$	DSSS / HR-DSSS ( $22\text{ MHz}$ )
802.11a	1999	$5\text{ GHz}$	$54\text{ Mbps}$	OFDM ( $20\text{ MHz}$ )
802.11g	2003	$2.4\text{ GHz}$	$54\text{ Mbps}$	OFDM ( $20\text{ MHz}$ )
802.11n (Wi-Fi 4)	2009	$2.4\text{ / }5\text{ GHz}$	$600\text{ Mbps}$	MIMO ( $4\times4$ ), OFDM ( $20/40\text{ MHz}$ )
802.11ac (Wi-Fi 5)	2013	$5\text{ GHz}$	$6.9\text{ Gbps}$	MU-MIMO, 256-QAM ( $80/160\text{ MHz}$ )
802.11ax (Wi-Fi 6/6E)	2021	$2.4\text{ / }5\text{ / }6\text{ GHz}$	$9.6\text{ Gbps}$	OFDMA, 1024-QAM, Target Wake Time

5. IEEE 802.11 Frame Format

+----+----------+--------+--------+--------+-------+--------+---------------+---------+
| FC | Duration | Addr 1 | Addr 2 | Addr 3 | SeqCtl| Addr 4 | Data Payload  |   FCS   |
| 2B |   2 B    |  6 B   |  6 B   |  6 B   |  2 B  |  6 B   | 0-2312 bytes  |   4 B   |
+----+----------+--------+--------+--------+-------+--------+---------------+---------+

Frame Control (FC - 2 bytes): Contains Protocol version, Type (Management, Control, Data), Subtype, To DS, From DS, Retry, Power Management, and WEP/security flags.
Duration / ID (2 bytes): Encodes the NAV duration in microseconds.
Four Address Fields (6 bytes each): Depending on the values of To DS and From DS flags:
- Destination Address (DA).
- Source Address (SA).
- BSSID (MAC of the Access Point).
- Receiver Address (RA) / Transmitter Address (TA) for wireless distribution systems (mesh/bridge).

6. Key Exam Questions & Takeaways

Why is CSMA/CD unsuitable for wireless networks?
- Transmitters cannot reliably detect collisions while transmitting because local transmit power overwhelms faint incoming collisions from distant stations. Furthermore, hidden stations cannot hear each other.
Explain the Hidden Terminal Problem and how RTS/CTS resolves it.
- Two stations outside each other's radio coverage transmit to the same central access point at the same time, causing collisions. The RTS/CTS exchange reserves the channel: the AP sends a CTS heard by all stations in its range, setting their NAV timers to silence them.
What is the Network Allocation Vector (NAV)?
- A timer maintained in software by wireless stations representing virtual carrier sensing. A station must not transmit while its NAV is non-zero.
Order the 802.11 Interframe Spaces by increasing duration.
- $\text{SIFS} < \text{PIFS} < \text{DIFS} < \text{EIFS}$ .

Wireless LANs