DATA COLLISIONS

A data collision is the simultaneous presence of signals from two nodes on the network. A collision can occur when two nodes each think the network is idle and both start transmitting at the same time. Both packets involved in a collision are broken into fragments and must be retransmitted.

Collision Detection

To detect for a collision, nodes check the dc voltage on the line. A voltage level of two or more times higher

than expected indicates a collision, since this means there are multiple signals traveling along the backbone at the same time.

In a CSMA/CD (carrier sense multiple access with collision detection) systems, all workstations or nodes attached to the network monitor the transmission medium at all times. When a node needs to send data, it waits until the line is quiet and then transmits. If two or more nodes happen to transmit data at the same instant, a collision occurs. Each node detects the collision and then waits for a variable amount of time (as programmed in the NIC's microprocessor) before testing the bus again and retransmitting. Since each node waits for a different amount of time, say 10/1000 and 20/1000 of a second, it is very unlikely that the collision will occur a second time. The CSMA/CD detection method is further illustrated in figure 3-1.

Collision Avoidance

To avoid collisions, nodes can send special signals that indicate a line is being used for a transmission. In a

Figure 3-1.\A bus network using the CSMA/CD access method.

CSMA/CA (carrier sense multiple access with collision avoidance) system, the media-access method uses RTS (ready to send) and CTS (clear to send) signals before sending a frame onto the network. A node transmits only after the node has requested access to the line and access has been granted. Other nodes will be aware of the RTS/CTS transmission and will not try to transmit at the same time.

RTS.\ A hardware signal sent from a potential transmitter to a destination to indicate that the transmitter wishes to begin a transmission. If the receiver is ready, it sends a CTS signal in return.

CTS.\ A hardware signal sent from a receiver to a transmitter to indicate that the transmitter can begin sending. ACTS signal is generally sent in response to an RTS signal from the transmitter.

NETWORK SYSTEM CONNECTIONS

The testing and evaluation of network connections is accomplished with the same test equipment that is used to test network components. This equipment includes voltmeters, ammeters, volt-ohmmilliammeters, and line scanners. All of this test equipment checks the voltage, resistance, and current that passes through the cable and the connectors between the network nodes. Any increase or decrease in voltage or current or an increase in the resistance will cause communications problems for the users.

Whether the cable is pre-made or you make it, you should always test the cable before it installed into the network. This will alleviate the possibility of installing a bad cable or connector to the system. Any time that you can detect a bad connector will be to your advantage, since each connector has a limited number of connections before it has to be replaced.