QSC Audio S series - Manual (Page 8)

8
Ethernet networks are used most often for data communica-
tions, such as with file or print sharing on an office LAN (Lo-
cal Area Network). A typical application might include a num-
ber of PCs or workstations, servers and shared printers all
connected to common Ethernet hardware. Messages (Ether-
net frames) are communicated between devices on the LAN
in a random and non-deterministic manor. Network response
to print messages or file access is usually noncritical so long
as the intended outcome occurs in a reasonable amount of
time. For example, when multiple users are attempting to
share a common printer at the same time, some users will
likely experience delays in output. Collisions or failed attempts
to access the network may also be acceptable so long as the
intended messages are retransmitted. Most often, a limited
amount of collisions are expected and are usually transpar-
ent to the user.
Audio networks are different because late arrival or failed
attempts to transmit audio messages are immediately per-
ceived by the listener. Therefore, audio network transmission
must be error free with low latency and delivery must be pre-
cisely defined. To do this, RAVE incorporates the CobraNet
protocol. CobraNet is the industry’s most reliable audio deliv-
ery mechanism. CobraNet provides low latency, determinis-
tic delivery of audio over Ethernet on either network repeat-
ers or switches. Collisions are prevented on shared media
links, such as repeater hubs, by CobraNet’s proprietary “order
persistent” media access scheme. On dedicated media links,
such as with network switches, collisions are prevented due
to separate transmit and receive paths in a full-duplex con-
Introduction:
How It Works
nection and/or an abundance of bandwidth dedicated to a
single device on each switch port. On network switches, RAVE
establishes a half-duplex link to a dedicated port. The 100
Mbps of bandwidth available at a switched port is more than
enough to support all of the typical communications require-
ments of RAVE while providing for an ample amount of con-
trol and monitoring through the MI via SNMP messages.
A network of CobraNet devices can be thought of as a syn-
chronized orchestra. The unit acting in the “conductor” role
provides the system clock and grants network permissions to
the “performer” units. Each RAVE has a local internal clock so
that any RAVE may arbitrate for the role of network “conduc-
tor”. RAVE devices may also be synchronized to an external
clock source, which is attached to a rear-panel BNC connec-
tor. External synchronization requires configuration setup
through the front-panel interface or via SNMP. The permis-
sions list is a message sent with the system clock that allow
individual units access to the network and reserve bundle as-
signments.
RAVE routes audio in bundles that are populated with a de-
fault value of 8 channels per bundle. The actual number of
channels per bundle may be altered, from 0 to 8, from the
management interface using SNMP. Reducing channel count
reduces system bandwidth requirements. On the models sup-
porting CobraNet transmission (81/88/161/188) audio is
brought into the RAVE using the rear panel connectors.