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Difficulty: HardWireless Principles and Architecture

An enterprise network engineer is designing a wireless deployment using a Centralized Wireless LAN Controller (WLC) architecture based on the Split-MAC protocol structure. To ensure low latency for time-sensitive radio frequency operations, certain MAC layer operations must occur on the physical Access Point (AP). Which function is executed directly by the Lightweight Access Point (LAP) hardware rather than being handled by the Centralized WLC?

  1. Transmission of beacon frames and immediate 802.11 layer acknowledgmentsAnswer
  2. B
    Client 802.1X EAP authentication state management and key exchange processing
  3. C
    Radio Resource Management (RRM) calculations for global channel allocation
  4. D
    Translation between 802.11 wireless frames and 802.3 Ethernet frames across all client traffic

Answer

Transmission of beacon frames and immediate 802.11 layer acknowledgments
Under the Cisco Split-MAC architecture (CAPWAP model), functions are divided based on timing requirements. Real-time 802.11 MAC operations—such as sending beacon frames, responding to probe requests, handling frame acknowledgments (ACKs), and executing MAC-layer encryption/decryption—must occur directly on the Lightweight Access Point (LAP) hardware to satisfy strict 802.11 timing constraints.

Step-by-Step Solution

1
Analyze the Split-MAC architecture division of responsibilities between LAPs and WLCs.
Identify that functions requiring microsecond-level timing must be processed locally at the AP radio level.
CAPWAP tunnel latency to the WLC would introduce unacceptable delays for real-time RF MAC operations.
2
Categorize real-time MAC functions versus non-real-time control/management functions.
Beacon frame generation, probe responses, frame queuing/prioritization, and 802.11 layer ACK responses belong to real-time MAC functions performed by the LAP.
These tasks must occur immediately in response to RF signals.
3
Evaluate the management and control functions offloaded to the WLC.
802.1X/EAP authentication, association requests, RRM channel selection, and centralized 802.11 to 802.3 frame translation are handled by the controller.
Centralizing these non-real-time operations allows uniform policy enforcement and global RF optimization across the enterprise network.

Key Concept

Split-MAC Architecture Division of Responsibilities
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Topics

Wireless Principles and Architecture

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