• Can control/management/broadcast/multicast traffic cause wireless bandwidth wastage?

 
 
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Introduction

Wireless medium is shared between many clients and Wi-Fi/non-Wi-Fi interference could cause bandwidth wastage. Such traffic is classified as auxiliary Traffic and can be categorized into:

  • Control/management traffic
  • Broadcast/multicast traffic

Although Control/management traffic is essential for proper operation of the wireless network, its volume should not be excessive. Most typical applications sends unicast frame, but excessive broadcast/multicast traffic can indicate improper usage of the network.

This article provides the insights of these frames and how can network be optimized.

Prerequisites

  • Access to CloudVision Wi-Fi.

Understanding frames and consideration

Let’s understand the different types of frames that can be transmitted on the wireless medium by the wireless stations as per 802.11 MAC standard. These frames are categorized into three types: Control, Management, and Data.

 

Control Frames

These frames contain control messages for coordinating access to the wireless medium and data exchange over the wireless link. Examples of control frames are:

  • RTS/CTS: Used by wireless stations to reserve the wireless medium for a specified time for their data transmission.
  • PS Poll: Used by wireless stations in power saving mode to fetch their data from the Access Point (AP).
  • ACK: Sent by the receiver to indicate to the transmitter that the preceding data frame has been successfully received. (Since ACK frames are generated for every data packet, they are not counted when generating this alert).

 

Management Frames

These frames carry messages required to manage the relationship between different wireless devices in the WLAN. Examples of management frames are:

  • Beacon: Periodically transmitted by APs to advertise their presence in the WLAN.
  • Probe Request/Response: Used by wireless Clients to discover the APs in their vicinity.
  • Association Request/Response: Used by wireless Clients to establish wireless connectivity with the AP.
  • Authentication Request/Response: Used by wireless Clients to establish wireless connectivity with the AP .
  • De-authentication/Disassociation: Used for tearing down the wireless link.

 

Data Frames

These frames carry actual data (upper layer protocol packets) over the wireless link.

During normal network operation, wireless bandwidth consumed by control and management traffic should be small. This keeps a larger fraction of the wireless bandwidth available for data, thereby increasing network throughput. The System keeps track of frames transmitted on the wireless medium and warns the administrator if high levels of control and/or management traffic are detected.

 

Types of Wireless Traffic:

 

Traffic transmitted over the wireless medium can be unicast, broadcast, or multicast.

Unicast Frame: A Unicast frame is destined to a unique wireless station (an AP or a wireless Client). The unique destination/receiver address in the unicast frame is the MAC address of the intended destination wireless station. Most popular Internet applications such as WWW, email, database access, FTP, VoIP etc. use unicast frames in their operation.

Broadcast Frame: A Broadcast frame is destined to all wireless stations that are connected to a given AP. The destination/receiver MAC address in the broadcast packet is FF:FF:FF:FF:FF:FF. Many network protocols (e.g. Address Resolution Protocol (ARP), routing protocols) and application control protocols (e.g. NETBIOS) use broadcast frames in their operation.

Multicast Frame: A Multicast frame is destined to a multicast address. A wireless station receives (i.e. decodes and passes to higher layers) a multicast packet only if it has subscribed to the specific multicast address found in the destination/receiver MAC address field of the packet. A number of applications such as conferencing, webinars, and file download to bulk of devices, etc. can be using multicast packets in their operation. Certain network protocols such are routing protocols also use multicast packets in their operation.

Figure 1: Unicast, Multicast and Broadcast Traffic

During normal operation of wireless network, the volume of unicast traffic is much larger than broadcast and multicast traffic.

Excessive volume of broadcast/multicast traffic may occur due to mis-configured devices, mis-configured protocols, device implementation issues, malicious activity (e.g. malicious ping), or legitimate activity (e.g. authorized webinar application). In the first four cases, broadcast/multicast traffic causes wastage of wireless bandwidth. This can slow down legitimate unicast applications. The System tracks the volume of broadcast/multicast traffic on the wireless medium and warns the administrator if high levels of such traffic are detected.

 

Consideration:

  • Excessive control/management traffic causes reduction in bandwidth available for data traffic.
  • Excessive control/management traffic can occur due to the AP transmitting beacons too frequently (recommended beacon interval is about 100 ms).
  • It can also occur if the AP is sending out too many Probe Response messages in response to the Client which is sending out frequent Probe Requests.
  • If the Clients frequently disconnect from the AP, it may also cause increased management traffic (disconnection messages, connection messages, etc.).
  • A client with poor RSSI value can lead to increase in excessive control/management traffic.
  • Excessive Control/management traffic can also result from device implementation issues and configuration specific behaviour.
  • Excessive Control/Management traffic can also be noticed in high utilization or dense deployments.
  • To improve high utilization or dense deployments it is advised to remove or limit lower data rates.  Management frames are always transmitted at the lowest data rate.  Management frames at the rates of 1, 5, 6, 9, 11 transmit further that data rates of 12 and above, and they also consume more airtime.
  • To remove the lower data rates navigate to Configure>SSID>Traffic Shaping & QoS.  Enable “Set the data rate for multicast, broadcast and management traffic to *”.  Add the lowest data rate for the SSID (1 – 54). Then Save.

Lower data rates need to be set per SSID. Legacy devices may have issues with the lower data rates removed.

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