A local area network (LAN), such as an Ethernet network, allows the exchange of data among computers, workstations and other such data terminal equipment (DTE). The DTEs are linked to one another either directly or via repeater units for office automation, distributed processing and other applications requiring exchange of information.
In a typical network, at least some of the network link segments consist of two twisted-wire pairs. Each pair consists of two continuous insulated conductors twisted helically about one another. The ends of the twisted pair are coupled to a DTE or repeater unit by way of a medium attachment unit (MAU) which provides the means by which the signals to and from the DTE or repeater unit are coupled to and from the twisted pairs.
The Ethernet LAN uses the Manchester signal encoding scheme defined in the IEEE 802.3 CSMA/CD 10 Base T standard. With this type of binary-to-phase encoding, there is a phase transition in every bit cell center. A logical one is a transition from low to high; a logical zero is a transition from high to low. The transition every bit allows clocking to be combined with data transmission and allows the carrier to be detected by the presence of transitions on the twisted pair media.
In the Ethernet LAN the encoding-decoding function is carried out by a controller board in each DTE or repeater unit on the network. The same board is responsible for data encapsulation and decapsulation according to the 10 Base T standard. In particular, when a packet of data is to be transmitted from the DTE over the network, the controller formats it as a frame with a preamble, a starting delimiter, address fields, a data field and an end-of-frame delimiter (EOF) waveform, necessary for transmission in accordance with the network protocol. The MAU contains the logic required to send this frame over the twisted pair after determining the availability of the link. Conversely, when the MAU receives a transmission over the twisted pair, it couples the encoded data to the encoder/decoder of the DTE. That unit checks the incoming frame to verify that it should be accepted, strips off the frame preamble and the delimiters and passes the address and data fields to the DTE.
In addition to providing the transmit and receive functions just described, the MAU provides other functions specified in the above standard, namely collision detection, loopback, jabber, and link integrity test (LIT) functions. Of these, the LIT function is of relevance to the present invention, so we will amplify upon it at this point.
In order to protect the network from the consequences of a media failure or an installation error of the twisted pair wire, each MAU includes a LIT pulse generator which emits a positive-going LIT pulse periodically for transmission over the associated link segment when that MAU is not transmitting a frame. Each MAU also monitors the associated link segment for frame data and LIT pulses from the MAUs.
While the MAU is in its so-called LIT Fail State, its normal communication functions, e.g., transmit, receive, loopback, are disabled and it looks continually for a frame or for a pre-defined sequence of LIT pulses and/or receive frames in order to enter the LIT Pass State in which it is enabled to perform the normal data transfer functions. More particularly, when frame information or a selected number, LC Max=2-10, of consecutive LIT pulses is received, the MAU enters the LIT Pass State. The standard also specifies that only LIT pulses that occur between a time, LIT Max=25-150 ms, of each other will be considered consecutive and countable.
In addition, while the MAU is in the LIT Pass State, detected LIT pulses that occur within a time, LIT Min=2-7 ms, of a previous frame or LIT pulse are ignored. In the LIT Fail State, such pulses reset the counted number of consecutive LIT pulses to 0.
When the MAU is in the LIT Pass State, it operates to provide the normal communication services between the twisted-pair media and associated DTE or repeater unit.
If, while the MAU is in the LIT Pass State, the MAU receives neither frame data nor a LIT pulse for a defined time, LINK-loss=50-150 ms, the MAU enters the LIT Fail State and disables the MAU's normal communication functions.
Further in accordance with the above IEEE standard, each Manchester-encoded signal transmitted and received via the twisted pair media is a differential-mode voltage which constitutes the algebraic difference between the two signals on the twisted pair, with both signals being referred to a common reference. In order for the differential signaling arrangement to work properly, the drivers and receivers of the MAUs must agree about the polarity of the conductors in the twisted pair, i.e., the positive and negative inputs to the receiving MAU must agree with the positive and negative outputs from the transmitting MAU. If there is a polarity difference due, for example, to a twisted pair wire installation error, the link cannot establish proper communication between the devices at the opposite ends of the link.
It should be understood also that the above standard assumes that the twisted pair polarity for each network link is correct, i.e., it does not require detection or correction of wiring polarity errors.
Actually, wire installation errors are quite common because when a LAN is installed in a building or office complex, the people who install the physical medium or wires are usually not the same people who install the DTE or repeater units and/or the wires and those devices are installed at different times. Consequently, wiring polarity errors are typically not discovered until after the devices have been installed and tested. Needless to say, correction of such wiring errors in a large network installation can be a tedious and time consuming task.
Some devices do incorporate means which allow detection and correction of twisted pair polarity errors. Typically, a chip is incorporated into the equipment which allows detection of the polarity of the twisted pair based on the polarity of a received LIT pulse. Depending on the polarity of that received pulse, the MAU receiver input polarity is selected accordingly.
However, the prior polarity detection/selection scheme is quite sensitive to electrical disturbances or line noise which may result from cross-talk or from noise impulses caused by telephone operations on telephone wires bundled in close proximity to the network twisted pair bundles. Whatever the cause, such noise can produce unwanted switching of the receiver polarity, resulting in undesirable switching errors and packet frame loss. Even if the equipment is correctly wired to the twisted pairs at the outset, such noise can cause the equipment to falsely switch polarity, resulting in errors.
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