Digital Audio Interfacing and Networking 1507which of many sources it wishes to receive, and only the
source selected will transmit onto the network.39.12.10 Private BundlesIndividual CobraNet® transmitters®control their own
private bundles. Unlike multicast and unicast bundles,
there may be more than one private bundle with the
same bundle number on a network or VLAN at the
same time. This is because a private bundle is specified
using the transmitter’s unique MAC address in addition
to the bundle number.
Private bundles may be either unicast or multicast
addressed based on the transmitter’s reception of one or
more reverse reservation requests for its bundle number
and MAC address. The txUnicastMode variable is used
to control the transmitter’s ability to switch to multicast
on a private bundle number. The default setting of the
txUnicastMode variable disables the ability to transmit
multicast on a private bundle number. With the default
setting, if more than one receiver requests a given
private bundle number from the same transmitter, only
the first receiver to get its reverse reservation request in
will get that bundle. With the default setting, private
bundles can’t be used for point to multipoint routing,
instead multicast bundles must be used.
Some CobraNet® devices allow the same audio to
be transmitted on more than one bundle at a time. This
can provide an alternative way for a single CobraNet®
device to send private bundles to as many as four
receiving devices at the same time.
Private bundles are only transmitted if a receiver has
requested that bundle. This allows a receiver to select
which of many sources it wishes to receive, and only the
source selected will transmit onto the network.
39.12.11 Bundle Assignments
Over CobraNet®, all audio channels are packaged into
groups called bundles for transmission over the Ethernet
network. The usual assignment is eight audio channels
of 20 bit depth into one bundle. This is the maximum
size possible, although using less audio channels is
possible. In general for most efficient utilization of
network bandwidth, maximum-size bundles are
suggested. In the rest of this section we will be talking
about maximum-size bundles. If 24 bit audio channels
are used the maximum is seven audio channels packaged
into a single bundle due to the maximum allowable
Ethernet packet size.
A CobraNet® system is coordinated by one of the
devices called the conductor. When two or more
CobraNet® devices are interconnected properly, one of
the devices will be elected the network conductor based
on a priority scheme. The conductor indicator will light
on the CobraNet® device that is serving as the
conductor.
Each CobraNet® device has the ability to send and
receive a fixed number of bundles. The bundle number
tells the CobraNet® conductor which specific
CobraNet® device is trying to communicate with which
other CobraNet® device(s) over the network. Use of
bundle numbers removes the necessity of the user
having to tell the devices the Ethernet hardware (MAC)
addresses of the other devices with which it is trying to
communicate. As long as the CobraNet® devices are all
set to the same bundle number, the CobraNet® system
takes care of all the rest of the technical details of setting
up an audio path over Ethernet between the devices.
A given bundle may have only one transmitter that
places it onto the network. Unicast bundles may have
only a single receiver. Multicast bundles may have
multiple receivers.
In an ordinary Ethernet data network it is possible to
mix both repeater hubs and switches and have the
network continue to work. This is not the case with
CobraNet®! For a CobraNet® network, you must either
use all repeater hubs, or all switches in the network.
This is because the CobraNet® protocol changes
depending on which type of network it is operating over.
However, non-CobraNet® devices may be attached to a
switched CobraNet® network via repeater hubs.
On a repeater hub-based network, there is a fixed
maximum of eight bundles per network. Any bundle
may be placed onto the network from any port, and will
appear at every other port on the network. The bundles
usually used in a repeater hub network are numbered in
the range from 1 to 255 decimal, and are called multi-
cast bundles. Such bundles are always transmitted in a
multicast mode, and may be received by any of the
CobraNet® devices on the network.
As long as the limit of eight total bundles is not
exceeded, it does not matter which channel numbers in
the range of 1 to 65,279 are used.
It is not suggested to mix ordinary computer data on
a repeater network with CobraNet®, as this could result
in dropouts in the audio.
On a switched network, there is no fixed maximum
number of bundles possible. The number will be deter-
mined by the network design. Again, bundles from 1 to
255 decimal are multicast bundles and, since they are
multicast, will usually be sent to every port in the