---

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.

Core Architectural Building Blocks:

1. 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.
2. 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.
3. 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:

1. 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.
2. Hidden Terminal Problem: Not all stations can hear each other.

• 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):

The RTS/CTS Handshake Protocol:

1. 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.
2. CTS (Clear to Send): AP broadcasts a CTS frame granting permission and reiterating the NAV duration.
3. 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.
4. 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.

---

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

• 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).

---