In general two types of local area networks (LANS) are presently available: collision based and token-ring.
Collision based LANs are wired in "parallel" so that each station on the LAN is logically connected to the same cable as every other station. As a result when any one station transmits, its signal reaches all the other stations at roughly the same time, allowing for propagation delays. Each station on the LAN watches the destination address of the transmitted frame and determines if it is the intended receiver, if it is, the station copies the frame. If two or more stations transmissions overlap in time they will interfere with each other additively and the data will be lost. This is called a collision.
Stations attempt to avoid collisions by looking for an idle state on the line before transmitting. However, due to speed of light delays, a station will not always be able to immediately detect if another station is transmitting because that station's signal might not have reached it across the cable yet. The extreme case of this is when each station begins transmitting simultaneously. When any station detects a collision it will stop transmitting, pick a random time interval, wait that interval, and then attempt to retransmit.
In a network where a high volume of frames are being sent collisions can significantly limit the usable data bandwidth on a network.
Token ring LANs avoid this limitation by allowing only one station on the network to transmit at any one time. Token ring LANs are wired in a logical ring with a transmit output from each station connected to a receive input on the next station. As long as no station has a frame to transmit a special frame called a token is circulated around the ring passed from station to station. When a station needs to transmit a frame it must wait until it receives the token. The station then removes the token from the ring, transmits its frame, then releases a new token. Thus, there can be only one token on the ring at a time and therefore only one station can be transmitting at a time.
While logically connected in a ring, the physical wiring of token ring LAN forms a star with the transmit and receive pairs for each station, combined in a single cable, running to a central point where they are connected to a media access unit (MAU). The MAU merely provides the connection of one station's transmitter to the next station's receiver to complete the logical ring. A station that is not currently part of the ring (also referred to as being DEINSERTED from the ring) will be bypassed by relays in the MAU. When that station wishes to become part of the ring (when it INSERTS) it will provide a steady DC voltage signal to cause the relays in the MAU to switch and insert it into the ring.
The protocol that describes the operation of the particular token ring LANs of interest is defined by the IEEE standard 802.5. Each station on the ring implements the Media Access Control (MAC) level of the 802.5 standard in a combination of hardware and software. Acquiring the token before transmitting as described above is one of the basic operations defined by the MAC protocol. Once a station acquires the token, it may transmit data addressed to one or more of the other stations on the ring.
Each station examines data on its receiver to determine if the data is addressed to it. If not, the data is merely buffered and regenerated through its transmitter to the next station downstream on the ring. In this way, each station acts as a repeater. If a station determines that data is addressed to itself, it will copy the data and change a "frame copied" bit in the data frame to indicate that the data has been copied. The data frame with the copied bit set is then sent to the next station downstream on the ring. Thus, when the station which originally sent the data eventually receives its own data frame back, it is able to determine that the data was successfully copied. At that point the originating station strips its transmitted frame from the ring and releases a new token. Now another station which has data to transmit may acquire the token and send data over the ring.
Part of MAC protocol requires that one station assume a leadership role of "active monitor." The active monitor can be any of the active stations on the ring and can change if the current station acting as active monitor deinserts itself from the ring or is otherwise unable to perform the duties of the active monitor. The active monitor performs several key functions in maintaining the MAC protocol. For instance, it watches for certain protocol violations and initiates recovery procedures if they are necessary. It also initiates a "neighbor notification" process by periodically sending (default of every seven seconds) an "active monitor present" frame which causes each station in turn to identify itself to its next downstream station. Neighbor notification allows network management software to obtain a map of the ring topology and inform each station of its nearest upstream active neighbor (NAUN) which is useful information in error recovery and trouble shooting. Most importantly, the active monitor provides the master or reference clock with which every station on the network must synchronize.
A typical station on a token ring network is illustrated in prior art FIG. 1. A station 50 normally comprises a processor 52, monitor 54, and a network interface card (NIC) 56. It is critical that each station include a NIC 56 since the MAC code as well as the hardware which allows a station to insert into the network are normally stored on the NIC. Each NIC is connected to the MAU by a receive pair and transmit pair, in this case 60 and 64, respectively.
As shown in prior art FIG. 2, MAU 68 includes a plurality of relays and ports to connect stations on the token ring. For example, the transmit pair from port 72 is connected to the receive pair of port 76, and the transmit pair of port 76 is connected to the receive pair of port 80 and the transmit pair from port 80 is connected to the receive pair of port 82. Each port also contains relays including relays 84, 86, which connect to incoming transmit and receive pairs from a station's NIC. Relays 84, 86 remain connected to each other, thereby looping the transmit and receive pair from a station back on each other, until the station 50 applies a "phantom voltage" of +5 v to open the relays 84, 86 and insert the station 50 into the network. Until the phantom voltage is applied, the transmit and receive pairs of a port are electrically connected to each other.
Occasionally, problems arise in the operation of a token ring. For instance, sometimes the ring will enter a "beacon state" in which the entire operation of the ring will be shut down until the problem is solved. One example of an error which would cause a beaconing ring is if a station on the ring has a bad NIC and is not correctly repeating frames. This could cause the token to be lost, and no station could then send data. Another problem that occasionally arises relates to the inability of a station to insert into the ring. In order to trouble-shoot problems such as these, testers have been developed to try to focus in on the cause or source of the error(s).
A variety of tests are currently performed during a trouble-shooting procedure. Testers can currently measure phase jitter over a ring and test the continuity of cables used to connect stations in the ring. Prior art testers can also determine the speed at which the network is operating and not insert if the station is not at the correct speed. Prior art testers are also able to detect if a station, which has been unable to insert into the network, is trying to use an address that is already taken by a station on the ring. However, in order to perform these tests with prior art testers one or all of the users on the network are sometimes inconvenienced and/or the tests are not always conclusive.
For instance, in the case where it is desired perform tests on the network when all of the ports in the MAU are full, one of the stations must be disconnected to allow for access to the network by the tester. This situation is illustrated in prior art FIG. 3. Station 50 had to be disconnected from MAU 68 in order for prior art tester 90 to have access to the network. Depending on the tests being performed, the user of station 50 might be off the network for a substantial period of time. Also, if station 50 had been the cause of the error and it was removed from the network to make room for a prior art tester, it would not be clear to the user of the prior art tester 90 whether the problem was merely intermittent or if the removal of station 50 rectified the error.
Prior art testers which measure phase jitter on the token ring require that the ring be rendered inoperable for the duration of the test. Depending on the size and usage of the ring, a delay of such a duration could be very undesirable if not intolerable.
Prior art testers are also deficient in a number of other respects. In testing for a fault at the end of a conductor cable adjacent to the tester, an intermediate cable of several feet must be inserted between the tester and the cable conductor end when using time delay reflectory (TDR) to determine whether a fault exists in the cable connector end. In using TDR to identify faults along the cable conductor, prior art testers typically require a number of circuit elements to be changed in matching the tester's input impedance to the cable conductor impedance in order to avoid unwanted multiple return or reflected signals. Prior art testers also lack the ability to check and modify the speed at which they transmit frames into the token ring so that this rate conforms to the operating speed of the token ring. Prior art testers also lack certain information gathering techniques that are useful in isolating faults on the token ring. In addition, in order to perform the necessary token ring tests, prior art testers are required to be configured in a variety of ways. Prior art testers do not have the capability to perform all of these tests from one location in the network.