1488 Chapter 39
Crestron have embraced it for audio/video system
control.
Yet with all these advantages, Ethernet per se is
poorly suited to carry real-time audio since it is by
nature an asynchronous system. Kevin Gross of Peak
Audio decided that there had to be a way to overcome
the limitations of Ethernet for transmission of real-time
information such as audio and video. His solution,
called CobraNet®, was granted a patent, and has been
licensed by many major professional audio companies
for inclusion in their products including: Biamp,
Creative Audio, Crest Audio, Crown, Digigram, EAW,
LCS, Peavey, QSC, Rane, and Whirlwind.
More recent entrants into digital audio networking
include Aviom, EtherSound, and Dante. All of the
above make use of at least some portion of Ethernet
technology.
Before we can examine these audio networking tech-
nologies, we need to get a good understanding of
Ethernet.
39.11.1 Ethernet History
In 1972 Robert Metcalf and his colleagues at the Xerox
Palo Alto Research Center (PARC) developed a
networking system called the Alto Aloha Network to
interconnect Xerox Altos computers. Metcalf changed
the name to Ethernet in 1973. While the Altos is long
gone, Ethernet has gone on to become the most popular
networking system in the world, Fig. 39-22.
This system had a number of key attributes. It used a
shared media, in this case a common coaxial cable. This
meant that the available bandwidth was shared among
all the stations on the cable. If any one station trans-
mitted, all the other stations would receive the signal.
Only one station could transmit at any instant in time
and get the data through uncorrupted. If more than one
station attempted to transmit at the same time, this was
called a collision, and resulted in garbled data.
In a shared media Ethernet network there will be
collisions as a normal part of operation. As a result a
mechanism for preventing collisions, detecting those it
could not prevent, and recovering from them was
required.
This mechanism is called carrier-sense multiple
access with collision detection (CSMA/CD). In other
words, while any station can transmit at any time
(multiple access), before any station can transmit it has
to make sure no other station is transmitting
(carrier-sense). If no other station is transmitting, it can
start to transmit. However, since it is possible for two or
more stations to attempt to transmit at the same time,
each transmitting station must listen for another
attempted transmission at the same time (a collision). If
a station transmitting detects a collision, the station
transmits a bit sequence called a jam to insure all trans-
mitting stations detect that a collision has occurred, then
is silent for a random time before attempting to transmit
again. Of course no retransmission can be attempted if
another station is transmitting. If a second collision is
detected, the delay before retransmission is attempted
again increases. After a number of tries the attempt to
transmit fails.
Now since the signals travel at the speed of light
(approximately) down the coax cable, and since a
station at one end of the cable has to be able to detect a
collision with a transmission from a station at the other
end of the cable, two requirements had to be imposed.
First, there was a limitation on how long the cable could
be. This was imposed to limit the time it could take for a
transmission from a station at one end of the cable to
reach the most distant station. Second, there was a
minimum length imposed on the data packet trans-
mitted. This made sure that stations that were distant
from each other would have time to realize that a colli-
sion had occurred. If the cable were too long, or the
packet were too short, and the stations at the ends of the
cable were to both transmit at the same time, it would
be possible for them to both finish transmitting before
they saw the packet from the other station, and never
realize that a collision had occurred, Fig. 39-23.39.11.2 Ethernet Packet FormatEvery Ethernet device in the world has a globally
unique media access control or MAC address. Manufac-
turers apply to the Institute of Electrical and Elec-Figure 39-22. Robert Metcalf’s first drawing of what
became known as Ethernet.
I CStationInterfaceTerminatorThe etherTap XTransceiverController