A repeater network is a network built with one or more repeater hubs, typically wired in a star configuration with a hub at the center. Repeater networks share the same bandwidth among all connected network devices. In a multi-port repeater, a data signal arriving in any port is electrically regenerated and reproduced out all other ports on the hub. A repeater hub does not buffer or interpret the data passing through it. The use of hubs require that all devices on the network run in half duplex mode.
Although CobraNet continues to support repeater networks, it is highly recommended that you use switches in your network designs. Even simple, unmanaged switches provide a number of advantages over repeater hubs, and due to the insignificant price difference, we strongly suggest their use instead of repeaters. Be aware that operation on repeater networks differs from operation on switched networks. As such, CobraNet devices automatically detect the type of network to which they are attached and operate accordingly.
The following figures show some examples of typical CobraNet designs for repeater networks (i.e., a network utilizing only repeater hubs - no switches). When you’ve finished reviewing the examples, be sure to perform the compliancy test to determine if the CobraNet repeater network you’ve designed is CobraNet compliant.
Using Category 5 cable, the maximum distance between two CobraNet devices is 100 meters…
To extend the distance between devices slightly, add a hub. Now the maximum distance between devices is increased to 200 meters…
If longer runs are required, fiber optic cable may be used. In its simplest point-to-point form, a CobraNet device can connect to another device located a maximum of 2 kilometers away. To accomplish this, simply connect the network devices to media converters. A hub must be present between one of the CobraNet devices and media converters for proper operation.
The real advantage of using a hub in a CobraNet network is not to extend the distance between devices, but to allow multiple CobraNet devices to connect to the network. The example below utilizes Category 5 cable to connect eight CobraNet devices to the network. If the distance between the hubs is more than 100 meters, simply add media converters at both ends and use fiber…
More Complex Configurations
The following example illustrates a CobraNet system for a stadium with multiple equipment rooms. All audio sources are sent via fiber to the main signal processor located in a remote control room. Here, the signals are processed and sent to another network which delivers the audio signals to Amp Rooms “A”, “B”, “C” and “D”.
Half-time Show Example
The following is an example of a live sound event system, similar to the system used at two recent Super Bowl half-time shows. Each of the A/D and D/A boxes are CobraNet devices. A remote truck supplies 32 channels of playback through a fiber to the front of house mixer. The mix is sent to crossovers that feed another network to three remote amplifier locations.
Is Your Repeater Network CobraNet Compliant?
There are some basic design principles you need to be familiar with in order to build a “CobraNet-friendly” network. To determine whether or not your network is CobraNet compliant, read through each topic below and perform the subsequent test:
Hubs cannot function in a ring. Here’s why: Data arriving via the receive pair of one connection is regenerated and sent out on the transmit pair to all other connected devices. The result in a ring configuration would be one giant never ending loop!
- Make sure your design uses a star topology.
With some exceptions, reliable operation of CobraNet requires a dedicated Ethernet network. Traffic generated by non-CobraNet Ethernet devices such as PCs, can cause audio dropouts on the network. Furthermore, CobraNet is capable of using over 90% of a network’s bandwidth leaving little for other Ethernet applications.
- Are there PCs or other non-CobraNet devices on your network? If so, an audio signal free of drop-outs cannot be guaranteed. Think about adding a separate network for your PCs and other non-CobraNet peripherals.
CobraNet uses a media access timing scheme to avoid collisions and to allow full utilization of the Fast Ethernet bandwidth. Switches buffer the data passing through them, thereby adding a delay and defeating CobraNet’s media access scheme. The result is reduced throughput and the possibility of dropouts.
- If you’ve designed a repeater network, make sure that you only have hubs on the network - no switches!
CAT5 cable runs over 100 meters are not allowed. Multimode fiber runs over 2 kilometers are not allowed. Maximum single mode fiber run lengths exceed 2 kilometers. The actual limit will depend on the fiber transceivers used. (Note that with repeater networks, there really is no need to use single mode fiber. Its ability to carry signals over cable runs longer than 2km exceeds the network diameter limitations listed below.)
Using Fiber and Media Converters
A CobraNet node goes through a process called auto-negotiation before it establishes a connection to another network device. This is a low bit rate (10Mb) form of communication where one device tells another device two main bits of information:
- full-duplex or half-duplex support
- 10MB or 100MB bit rate
Some media converters do not support auto-negotiation, but all Fast Ethernet hubs do. To ensure that your CobraNet devices know “who they are talking to,” either use an auto-negotiating media converter or insert a hub between a CobraNet device and a media converter at one end of the fiber run. Let the hub do the talking!
CobraNet supports up to 64 channels of 48KHz, 20-bit audio transmitted onto the network in blocks of 8 audio channels. More channels can be accommodated on a network carrying 16-bit audio, fewer on a network carrying 24-bit audio. Greater routing flexibility, at the expense of channel count, is gained by transmitting audio in blocks of 1, 2 or 4 channels. The number of audio receivers is unlimited.
- Count the number of active transmit blocks in your design. No more than 8 active transmit blocks from audio sources (8 blocks x 8 channels / block = 64 channels) should be fed onto the network.
Network Diameter/Propagation Time
The diameter of a network is defined by the longest cable distance between any two DTEs. Fast Ethernet requires that a specified maximum network diameter (about 200 meters) not be exceeded in an installation to ensure that collisions are reliably detected and resolved. Although CobraNet has increased the network diameter maximum from 200m to about 2km, CobraNet requires that the specified maximum not be exceeded to insure that collisions are reliably avoided.
Cable distance is an issue on an Ethernet network because of the time required for a packet to propagate from one end of a cable to the other. In order for the network to operate properly, a packet sent by any DTE on the network must reach all other DTEs within a certain time window.
To compute the propagation time from one DTE to another, one must figure not only the propagation time through the intervening cables, but also propagation delays through intervening network components (hubs and media converters).
Propagation times on Ethernet networks are measured as a round trip delay and specified, for convenience, in bit period units. A bit period on Fast Ethernet is 1/100MHz = 10 nano-seconds (10 billionths of a second).
- Identify the longest DTE to DTE path in the network – Trace the route from each DTE to all other DTEs to determine which is longest. In most designs, the longest path is easily identified without performing detailed computations.
- Compute the distance, in round trip bit periods for this path, including the propagation delays through network devices. The longest path should be no longer than 2560 bit periods.
- For examples of bit period calculations, a list of propagation delays for common network devices (hubs, media converters, etc.) and some helpful hints, refer to the Ethernet Networks document.