LWAPP's Split Mac Design

The Cisco Wireless Solutions utilizes a concept called "Split Mac" in which the AP and the Wireless Controller Splits responsibilities. This post explains who does what in this solution.

One of the key concepts of LWAPP is the concept of split MAC. Recently I was asked if there was a document that described this concept. The answer is yes, but it gives me an opportunity to share a bit about it here since its a concept that the CCNA Wireless covers.

In split Mac part of the 802.11 protocol operation is managed by the LWAPP AP, and other parts of the 802.11 protocol are managed by the Controller. If you are doing wireless with a single AP and no Controllers then this doesn't pertain to you. You only see this in the lightweight solution where you have an LWAPP AP with and a Wireless LAN Controller (WLC). In this post I refer to the WLC as a controller.

To keep it in simple terms we'll say this: In the split MAC concept, what an Autonomous AP traditionally would do by itself is broken into two parts. The simple timing-dependent operations are generally managed by the LWAPP AP, and less time-dependent operations are managed by the controller. The controller and AP communicate information with each other using LWAPP. User data is generally tunneled from the AP to the Controller inside of LWAPP and the controller transfers the data to the wired network VLAN that the traffic belongs to.

An LWAPP AP handles the following time-dependant functions:

    The frame exchange handshake between a wireless client and AP
    The transmission of beacon frames every 100MS by default.
    The buffering and transmission of frames for clients in power save mode
    The response to probe request frames from wireless clients. (these are also sent to the WLC for processing)
    Forwarding notification of received probe requests to the controller
    The provision of real-time signal quality information to the switch with every received frame
    The monitoring or each of the radio channels for noise, interference, and other WLANs.
    Monitoring for the presence of other APs
    Encryption and decryption of 802.11 frames

On the other hand some of the non-time-dependant functions provided by the controller include:

    802.11 authentication
    802.11 association and reassociation (mobility)
    802.11 frame translation and bridging
    802.1x/EAP/RADIUS processing
    Termination of 802.11 traffic on a wired interface, except for the REAP and H-REAP

While this list isn't necessarily complete and the definition of Split Mac is high Level, it should give you a better idea of how the solution is designed. Take some time to search the Cisco web site for "Split Mac" and you should get plenty of documents that can explain in further detail.

Copyright © 2009 IDG Communications, Inc.

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