wirelesstot

Vocera Multicast Flow

Initiation

  1. The user initiates a broadcast by pressing the button on the Vocera Badge and saying “Broadcast {{ groupname }}”.
  2. The Vocera Badge sends a unicast frame to the Vocera Server requesting a multicast group.

Initial Packet Handling

  1. The Access Point (AP) receives the packet, encapsulates it using CAPWAP (Control And Provisioning of Wireless Access Points), and forwards it to the wireless management IP address of the Wireless LAN Controller (WLC).
  2. The WLC strips the CAPWAP encapsulation and forwards the packet to the LAN for further processing.

Multicast Group Setup

  1. The Vocera Server receives the request and:
  • Sets up a multicast group (230.230.X.X)
  • Sends packets to all Vocera Badges that are members of the specified Vocera broadcast group ({{ groupname }})
  • Instructs these Badges to join the multicast group 230.230.X.X

Distribution to Badges

  1. These messages are sent over the LAN/WAN back to the WLC.
  2. The WLC encapsulates each packet using CAPWAP and forwards them as individual unicast packets to each AP with an associated Vocera Badge in the broadcast group {{ groupname }}.
  3. Each receiving AP:
  • Strips the CAPWAP encapsulation
  • Transmits the packet over-the-air to the Vocera Badge

Badge Join Process

  1. Each Vocera Badge that receives the request from the Vocera Server sends a unicast IGMP join request to join the multicast group (Vocera reserved multicast range: 230.230.0.1 – 230.230.15.254).
  2. Upon successful join, the Vocera Badge emits an audible “ding” to indicate it has joined the Vocera broadcast group.

Multicast Transmission

  1. Each multicast packet from the source badge is:
    • Sent over-the-air to the AP
    • CAPWAP encapsulated by the AP
    • Forwarded to the WLC’s wireless management IP address as a unicast packet

WLC Processing

  1. As the WLC receives the multicast packets, it strips the AP CAPWAP encapsulation and processes them based on the configured mode: a. Multicast-Over-Multicast (MoM) Mode (no IGMP Snooping on the WLC):
    • WLC re-encapsulates the packet with CAPWAP, setting the destination to the WLC multicast group
    • Forwards it to the LAN for further processing (LAN is responsible for replicating and forwarding the packets to the appropriate subscribers of the WLC multicast group)
    b. MoM Mode with IGMP Snooping:
    • WLC replicates the packet for each AP that has active clients subscribed to the specific multicast group (230.230.X.X)
    • CAPWAP encapsulates these packets
    • Forwards them to the LAN

Final AP Processing

  1. When the AP receives the unicast CAPWAP packet:
    • It strips the CAPWAP encapsulation
    • Without IGMP Snooping: Broadcasts the packet to the appropriate BSSID
    • With IGMP Snooping: Uses its IGMP Snooping table to determine which clients should receive the multicast traffic, and transmits the packets as multicast frames on the appropriate BSSID

Note on Directed Multicast Service (DMS)

If Directed Multicast Service is enabled on the WLC:

  • Clients can request the AP to send multicast traffic as unicast frames
  • This allows the AP to buffer multicast traffic for clients in power-save state
  • Provides L2 reliability (unicast requires an ACK)

Without DMS:

  • Clients wake up at every DTIM (Delivery Traffic Indication Message) interval to listen for broadcasted multicast traffic
  • May miss multicast traffic while in power-save state

References:

Taylor N. Thurston
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