Split MAC Architecture

Okay—yes. It sounds weird, but this odd-sounding name is actually a pretty cool feature. We’re basically splitting the processing of the 802.11 protocol between two devices, the AP and a centralized Cisco WLAN controller. Figure 1 shows how the “splitting” of processing occurs at each location.

Figure 1: Split-MAC Architecture

Although the 1520 AP and the 1020 AP appear to be directly connected to the controller in Figure 1, they can’t be—first, because they’ve go to connect with a switch to provide 10/100 to gigabit conversion, and second, because the controller only forwards LWAPP packets coming from an LWAPP-enabled port. This means you need a router if you want to take an LWAPP packet and forward it out as IP data to a non-LWAPP network. A high-end switch can handle the routing.

The AP handles the portions of the protocol that have real-time requirements:

1.The frame exchange handshake between a client and AP when transferring a frame over the air 2.Transmitting beacon frames
3.Buffering and transmitting frames for clients in power save operations
4.Responding to probe request frames from clients
5.Forwarding notification of received probe requests to the controller
6.Providing real-time signal quality information to the controller with every received frame 7.Monitoring each of the radio channels for noise, interference, and other WLANs
8.Monitoring for the presence of other APs
9.Encryption and decryption except in the case of VPN/IPSec clients

All remaining functionality is handled in the Cisco WLAN controller, so time sensitivity isn’t a concern but controller-wide visibility is certainly required. The following are some of the MAC-layer functions provided in the WLAN controller:

1.802.11 authentication
2.802.11 association and reassociation (mobility)
3.802.11 frame translation and bridging

And if a Cisco Wireless Controller in Appliance mode fails, its dropped Cisco APs will poll the network for another Cisco Wireless Controller. When an online Cisco Wireless Controller has any remaining AP ports, the management interface listens to the network for Cisco AP polling messages to auto-discover, associate, and communicate with as many Cisco APs as it can.

Note:Basically, the split-MAC architecture allows the splitting of 802.11 protocol packets between the Cisco LWAPP-based AP that handles real-time portions of the protocol and the WLAN controller that handles any items that are not time sensitive.