Traffic

CobraNet is an IEEE registered protocol that has a protocol number (Ethertype) of 0x8819.
There are four types of Ethernet frames used by CobraNet that will be seen on the network:
- Beat packet – only sent by a single Conductor device.
  Sent 750 times per second (1 1/3 mS being the isochronous cycle time).
  
  The size of the beat packet depends on the number of CobraNet devices and audio bundles in use on the network.
  
  It can be as small as approx 200 bytes or can be as large as a full frame
  
  The beat packet is always multicast
  
  Identified by 0x00 following the Ethertype field.
  
  Typically, any CobraNet device on the network can become the Conductor.
- Reservation packet – Sent by all CobraNet devices.
  Sent every 1 ½ to 8 seconds depending on network size.
  
  Usually small.
  
  The Reservation packet is always multicast.
  
  Identified by 0x01 following the Ethertype field.
- Audio bundle packet – Sent by any or all CobraNet devices.
  Can be large or small depending on configuration.
  
  Each device can send and receive multiple bundle packets every isochronous cycle.
  
  Can be multicast or unicast depending on configuration.
  
  Identified by 0x10 following the Ethertype field.
- Serial bridge packet – Sent by any or all CobraNet devices.
  Use is optional; will not be seen in all cases.
  
  Can be multicast or unicast; typically multicast
  
  Usually smaller frames.
  
  Use to bridge low data rate asynchronous (RS-232) traffic.
  
  Identified by 0x20 following the Ethertype field.
CobraNet also generates and uses other types of traffic:
- SNMP is used to monitor and configure the device.
- TFTP is used to update the device’s firmware.
- Packet bridge – The CobraNet device can be used as a NIC by an attached host processor.
  Some products or implementations will use this feature in which case any type of traffic may be seen sent or received by the CobraNet device.
  
  No more than one bridge packet per device per every 2 isochronous cycles (2 2/3 mS) should be seen sent or received.
From the above, it can be seen that a CobraNet network will usually contain a finite amount of traffic of known types.
This traffic can vary widely in makeup between multicast and unicast
The makeup of the traffic (multicast vs. unicast) will be consistent in an operational installation.
Total multicast CobraNet traffic on a network can range from less than 1 Mbit/s to all of the traffic that can be supported on the net.
In a small network, expect to see less than 1 Mbit of multicast beat packets.
As a general rule, more than four 8 channel multicast bundles on a network will not work. The contractor should be aware of this constraint. If you, as an IT professional, see more than four multicast bundles on the net point this out to the audio contractor and make sure this condition is known and intended. A multicast bundle can be identified by: destination MAC address is multicast, Ethertype = 0x8819, 1st data byte = 0x10.
In the largest network, expect to see as much as 10 Mbit of multicast beat packets.
A CobraNet device should not require more than 50 Mbits of bandwidth in each direction. In most cases, half that bandwidth is typical.
Links between switches can require far more than 50 Mbits of bandwidth. Use gigabit links between switches. The system contractor can relate what the bandwidth requirements of each link will be. Over-provision between switches to ensure good performance and to accommodate future changes.
Notes:
- Do not configure a switch to filter multicast traffic. CobraNet depends on multicast traffic.
- Some switches automatically configure links between two switches as trunking ports. It is common for trunk links to be processed by the CPU in a switch, rather than simply passing traffic through the hardware switch fabric. This can add to the variability of forwarding delay timing and can cause dropouts or loss of clock lock. Always check switch settings to be sure the switch is not using functionality that could add forwarding delay variability. Often, newer switches that use auto configuration, referred to as smart switches, can exhibit this problem.