The Plane Truth

The Plane Truth

By CWNP On 11/30/2009 - 6 Comments

As I continue to study and learn about the nuances of WLANs, I’ve found that there are some conceptual frameworks that help in the organization of the unwieldy quantities of information.  If you’re at all like me and have a lot of disparate pieces of information floating around in your head, organization is massively helpful.  Understanding how a technology works is usually a minor undertaking as compared to understanding how a set of technologies work in cooperation.  That is often my struggle, so I am hoping to share—at a high-level—about a framework (and terminology) that helps to organize some of the processes related to WLAN functionality.  This organizational taxonomy breaks down the basic WLAN functions into three planes: management, control, and data.  Each of these planes includes a set of functions.    

As vendor solutions become increasingly differentiated, it is important to realize if, how, and by which devices these functions are being performed.  As you read vendor documentation or compare vendor architectures, you may begin to realize that each vendor puts a unique twist on the structure of these planes.  Often times, some amount of functionality from each plane is present in each device, but some vendors offer architectures with more clearly defined roles.  Neither approach is necessarily better for all situations, which paves the way for each solution to be more compelling for specific vertical markets. Undoubtedly, a more exhaustive paper on this topic is warranted as the industry changes, but here are the nuts and bolts.    

Management Plane – This operation set addresses network configuration, monitoring, and administration.    In the early days of autonomous APs, management was performed uniquely on an AP-by-AP basis, and this was a major scalability drawback.  Initially, WLANs did not have a shared management plane, which meant that admins had to login to and manage each device independently.  The Wireless Network Management System (WNMS) came into play for management and monitoring of autonomous APs and were mostly usurped when WLAN controllers were introduced.  WLAN controllers were ushered in to centralize network management as well as to take on other roles that are part of the control and data planes.  At some point, multiple WLAN controllers become unwieldy, so a management solution is needed for them as well.  The WNMS comes back into play for that purpose.  Some devices exist solely to perform management functions.  Example functions of the management plane include firmware upgrades, device configuration, and network and status reporting and monitoring.

Control Plane – This plane includes the “control” functions related to effective cooperation and interaction between devices within a network.  Similar to the management functions, early networks with autonomous APs didn’t share a control plane.  They shared an Ethernet network for connectivity, but the APs did not communicate with each other to coordinate network control operations.  WLAN ‘control’lers are now the de facto solution to address the needs of the control plane, where many of these operations are centralized into one device (a controller) that communicates with all of the APs.  Again, similar to the management plane, multiple controllers pose new challenges, because controllers need a protocol for communications between one another.  In any case, graceful control of a WLAN is necessary for scalability of any kind.  Example functions include RRM (channel and power settings for automated networks) coordination, mobility management (such as fast secure roaming and uninterrupted policy and security management during transitions), and load balancing.  These operations are usually performed within a WLAN controller, though protocols may be used between APs to perform the same.   

Data Plane – This plane includes the handling of data within a network.  The two devices that usually participate in the data plane are the AP and the WLAN controller.  Autonomous APs obviously handle all data forwarding operations locally, but controller-based APs may have some variation of data handling.  Centralized data forwarding, where all data is forwarded from the AP to the WLAN controller for processing, may be used in many cases, especially when the WLAN controller manages encryption/decryption or applies security policies.  Distributed forwarding, where the AP performs data forwarding locally, may be used in situations where it is advantageous to perform forwarding at the edge and to avoid a central location in the network for all data, which may require significant Ethernet capacity.  As with the management and control planes, each vendor has a unique method for handling data forwarding, with pros and cons for each.  Other functions that are a part of the data plane are VLAN tagging, QoS classification and queuing, and policy enforcement.    

The combination of these three operational groups comprises a modern WLAN’s functionality.  To make the most sense of a new (or old) WLAN architecture, it may help to consider organizing features according to these planes.  Proper system planning and architecting demands that you understand how and where these processes are performed. 

 


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