In a half duplex Ethernet network, a collision is the result of two devices on the same Ethernet network attempting to transmit data at exactly the same time.
A Keepalive signal is a message sent by one device (such as a network interface card) to another device (such as a switch) to check that the link between the two is operating. A keepalive signal is sometimes referred to as a heartbeat.
When troubleshooting physical problems, it helps to identify the fault domain. The fault domain is the location of a physical problem. It's often manifested when you identify the boundary between communicating devices. For example, if a cable break occurs, a given host might be able to communicate with some devices, but not others. When you identify the fault domain, you identify the boundaries of communication and the most probable location of the physical problem.
The following table compares how a single break in the network affects device-to-device communication for specific topologies:
A break in the network bus means that the end of the network bus is no longer terminated. For this reason, a break in the bus typically means that no devices can communicate. Consider the following examples:
• When a cable on the network breaks, each end of the cable on either side of the break loses its termination.
• When a cable becomes loose or is disconnected, the computer is not connected to the network; this also creates an end that is not terminated. Likewise, when a terminator becomes loose, there is an end that is not terminated.
It is difficult to identify the location of a break on a true bus network.
A break in a star means that the device connected to the central device through that cable can no longer communicate on the network. All other hosts will be able to communicate with all other devices.
A break in the ring means that messages can only travel in one direction (downstream) to the break. Computers can send messages downstream to other devices, but cannot receive any responses because of the break.
A break in a single link in a mesh topology has no effect on communications. Data can be routed to the destination device by taking a different (though sometimes longer) path through the mesh topology.
If a single device is unable to communicate on the network, begin troubleshooting by verifying the physical network connection. Most network cards include link and status lights that can help you verify physical connectivity. The following troubleshooting table describes various light combinations and their meanings:
The network card does not have a connection to the network. The link light only activates when the computer detects a connection to another device. If the link light is off, possible causes include the following:
• Bad NIC
• Faulty cable
• Missing device on the other end (unplugged cable)
• Switch or hub port turned off or bad
If the link light comes on and is not green, then the NIC has detected a signal, but the signal is not what was expected. Possible causes include:
• Faulty transceiver on the NIC or remote device
• Incorrectly configured network cabling
• Incompatible networking standards
On some switches, an amber link light indicates a slower connection (such as a 100 Mbps connection instead of a a 1000 Mbps connection, which might show a green light).
Link:Red- Solid Green
A solid green link light indicates a valid network connection. However, an Activity light that never lights up means that no data is being received. To resolve this issue, check all components and connections.
This is a normal condition that indicates a valid active connection. The Activity light flashes periodically even if you are not currently sending data over the link (this is known as a heartbeat or keepalive signal that lets the NIC know it has an active connection).
Collision- Flashing/Lit Occasionally
This is a normal condition that indicates collisions. A small number of collisions are to be expected on an Ethernet network that uses a hub.
If your network uses full-duplex switches, there should be no collisions on the network.
Collision- Flashing/Lit Occasionally
An Activity light that is continuously flashing indicates that there is constant traffic being sent or received on the link. This could be caused by a device that is very busy (such as a server). In most cases, there should be at least some periods of little or no activity. A constantly flashing activity light could be caused by a faulty NIC on the link that is constantly sending out data. This condition is known as chattering or jabbering.
Collision- Flashing/Lit Constantly
If the collision light is constantly flashing, then there are too many collisions on the network. Possible causes include the following:
• Too many devices on the segment. As the number of devices increases, so does the number of collisions. Reducing the number of devices or using switches, bridges, or routers to divide the network into multiple collision domains reduces or even eliminates the number of collisions.
• Faulty cabling or cable runs that are too long.
• A faulty NIC that does not properly sense the medium before transmitting.