A typical local area network (LAN) provides a communication facility for data exchange among devices located within a moderate sized geographical area. Such a network may include a number of workstations and other LAN compatible devices arranged in the proximity of each other for use in office automation, distributed data processing and other situations requiring connection to a local communication medium. It is desired that the network operate in an economical fashion with the ability for carrying bursty traffic at high peak data rates. The Ethernet.TM. is an example of such a network.
Pursuant to the 10 BASET standard soon to be promulgated by the Institute of Electrical and Electronic Engineers (IEEE), transmission of Ethernet.TM. data may occur over a twisted pair media as well as coaxial cable. This standard requires workstations or other devices to be placed in a star configuration in a point to point arrangement about a repeater, a device used to extend the length and topology of the Ethernet.TM.. The physical configuration envisions the bundling of telephone lines with the Ethernet.TM. twisted pair bundles or in close proximity to the twisted pair bundles.
In this arrangement, disturbances to the local area network effect the performance of the network. These disturbances may result from noise impulses caused by telephone operation on adjacent wiring In particular, if a telephone rings or the hand set is picked up, a signal induced by cross-talk coupling appears as a damped sinusoidal waveform on the twisted pair media. In a busy office, such noise impulses frequently cause the start of an Ethernet.TM. packet frame, and cause the transmission of packet fragments generated by the repeater to the entire network.
Another frequent disturbance on the network is caused by a condition known in the art as self cross-talk. The transceiver or Media Access Unit (MAU) of one workstation may, for example, transmit data to the repeater with the same data retransmitted to all of the other segments attached to the repeater. Adjacent transmit pairs may couple and provide a crosstalk signal to the receiving pairs of the repeater. The repeater may thereby interpret the crosstalk signal as a valid packet of data and provide the network with false collision activity.
These disturbances create extra traffic on the network, and will create collisions on real packets of data, resulting in reduced bandwidth on the entire network and to all of the attached nodes to the network. Further, when one workstation appears to be transmitting data, the system provides a wait or deference interval to other workstations attempting to transmit data until the first is finished with the transmission. Thus, the erroneous reception of a transmit signal significantly decreases throughput of the system by increasing the deference interval.
Prior art types of squelch circuits for local area networks commonly operate by detecting energy levels of the incoming data signal. Other types of squelch circuits operate by detecting discreet signal transitions in a known sequence from prior knowledge of the preamble of the packet of data. Such systems, however, are unreliable in discriminating crosstalk noise from a valid packet preamble.