Interpretation of results of the 'cable test' Denis de Castro, Dunedin 22 September 2021 Contents Introduction Background information Interpretation of 'cable-test' report 1 Ethernet port with a cable connected and an active IP connection 2 Ethernet port with no cable connected to that port 3 Ethernet port with a cable connected to that port but its other end free 4 Ethernet port with a defective cable connected to it 5 SPF port with no cable connected to that port Experimental results References ------------------------------------ Introduction When a cable is plugged into a Mikrotik router, testing can be done with the button Cable-test in webfig at Interfaces>Interface>[choose a line] or in Interfaces>Ethernet>, or or in the Terminal with /interface ethernet cable-test . A cable can be tested with the far end free or plugged into another device. For convenience, a cable can also be tested by plugging in both its ends to the same router. This will also show what connection-rate the cable is capable of. See Ref. 1 for a list of MT devices which support cable-test. The following results come mainly from RouterOS v6.48.4 (stable) Denis de Castro 8 Sep 2021 Corrections and additions please to BMCP@pm.me. ------------------------------------ Background information. RJ45 hardware layout. The following assumes that the colours of the insulation of the cores within the cable are arranged according to the standard T568B layout and does not apply where the alternate T568A wiring colour standard has been used. In the standard T568B layout at the 8 position, 8 contact (8P8C) plug: Holding the plug pointing away from you with the flat side visible, pair 1 are blue/light blue in the midline of the plug pair 2 are light orange/orange on the left pair 3 are light green/green around pair 1 pair 4 are light brown/brown on the right So looking down on the face of the plug in the above position, the order of the cores from pin 1 to pin 8 is 1 light orange 2 orange 3 light green 4 blue 5 light blue 6 green 7 light brown 8 brown. 1OOMbps ethernet (100BASE-T) only uses pair 2 (orange and light orange) and pair 3 (light green and green). Where TX = transmit and RX= receive; pin 1, TX+; pin 2, TX-; pin 3, RX+; pin 6, RX-. When the four-pair cable is used for 1000Mbps ethernet (1000BASE-T), all four pairs are used. All cores can carry bidirectional signals. Pin 1, lane A +; pin 2, lane A -; pin 3, lane B +; pin 4, lane C -; pin 5, lane C +; pin 6, lane B -; pin 7, lane D +; pin 8, lane D -. Interpretation of RouterOS 'Cable Pair' result in the 'cable-test' report - - - - Pin order RJ45 RouterOS 'Cable Pair' listing notes 1 light orange pair 2 reported in first listed pair used in 100b-T 2 orange pair 2 reported in first listed pair used in 100b-T 3 light green pair 3 reported in second listed pair used in 100b-T 4 blue pair 1 reported in second listed pair 5 light blue pair 1 reported in third listed pair 6 green pair 3 reported in third listed pair used in 100b-T 7 light brown pair 4 reported in fourth listed pair 8 brown pair 4 reported in fourth listed par ------------------------------------ Interpretation of 'cable-test' report 1. Ethernet port, with a cable connected and an active IP connection: Status link ok Cable Pairs [no values given] With this result, there is no ambiguity. If the link is active, this is always seen in my experience. 2. Ethernet port, with no cable connected to that port: Pattern A Status no link Cable Pairs open/0 open/0 open/0 open/1 or Pattern B Status no link Cable Pairs normal normal normal normal or Pattern C Status no link Cable Pairs open/0 shorted/0 normal normal 'Open' means open-circuit, i.e. no connection. The digit after the slash is an estimate of the distance in meters from the switch-chip. Because the precision of this estimate approaches a metre, 0 and 1 are equivalent. Here the distance from the switch chip is close to 0 m, i.e. no cable. In webfig the values may fluctuate between zero and one while the test continues. Pattern B shows 'normal' for all 4 pairs. It was seen at empty ethernet sockets on a RB493G and is not known why this sometimes occurs. Pattern C was also seen on a RB493G, at several empty ethernet sockets. 3. Ethernet port, with a cable connected to that port but its other end free: Status no link Cable Pairs open/3 open/3 open/3 open/3 The number after the slash will vary with different cable lengths. 'Open' once again is expected. The number after the slash, in this case 3, indicates the approximate length in metres from the switch chip of the continuous pairs. Again the length is only an estimate and there may be some variation in length reported between the pairs. Taking into account this variation, the lengths normally match the length of the attached cable. If the four pairs' lengths are similar but shorter than the cable length, there may be a break in all the pairs at that shorter distance, i.e. 'distance-to-fault' (DTF). See 4. below. If one or more pairs are open at a shorter distance than others, there could be isolated breaks in the pair(s) at that shorter DTF. 4. Ethernet port with a defective cable connected to it: Status no-link Cable-pairs open/4 shorted/0 normal/? normal/? The result is variable: this is one example. Each pair of cores shows 'open', 'shorted' or 'normal'. 'Open' (open-circuit) means pair ends in a discontinuity, which could be the end of the cable (see 2. and 3. above), or a break. 'Shorted' means the cores of that pair are in contact with each other. The presence of 'shorted' on any pair normally confirms that the cable is faulty. The digits after the slashes indicate the distance to the break or short. 'Normal' is not clearly documented but may indicate that the pair is connected to the corresponding pins at a socket in a machine at the far end, without an link being present. However it has been seen where the cable was not connected to another machine. 5. SPF port with no cable connected to that port: Status no link Cable Pairs [blank] ------------------------------------ Experiments • a 15 cm length of twisted-pair ethernet cable plugged in port 10 of a RB3011 (the cable was not connected at the far end and all results stated 'no link') gave the following: Cable Pairs open/1 open/1 open/1 open/1 (numeral fluctuating between 0 and 1 while test in progress) The free ends were shorted in various combinations. Test 1. Connecting together cores 5 and 6 from different pairs produced no change in the result displayed. Test 2. Connecting together cores 7 and 8 i.e. pair 4 gave the result Cable Pairs open/1 open/1 open/1 shorted/1 Interpretation. 'short' is tested between cores of the same pair, not between cores of different pairs, i.e. the result short indicates that a pair is internally shorted. Test 3 (this test also carried out on RB193G with the same result). Connecting together cores 1 and 2 i.e. pair 2 gives Cable Pairs shorted/1 open/1 open/1 open/1 Test 4. Connecting together cores 4 and 5 i.e. pair 1 gives Cable Pairs open/1 open/1 shorted/2 open/1 Interpretation (of tests 3 and 4). The testing is not carried out in the sequence pair 1 through pair 4, but across the pins from pin 1 to pin 8. The result of test 4 implies that the short is reported on the second pin of the shorted pair. However the previous Test 1 does not support this. Test 5. Cores 7 and 8 (pair 4) were connected while maintaining the previous connection of the cores of pair 1: Cable Pairs open/1 open/1 shorted/2 shorted/1 Interpretation. Shorts in multiple pairs can be detected and are listed separately. --------- Cable with active link. A cable working at 1Gbps has be seen to fall back to 100Mbps when the connection is interrupted (eg by punched-down cores in a patch panel being dislodged). It may be inferred that pairs 2 and 3, necessary for 100b-t ethernet, are intact (the oranges and the greens, according to the 'RJ45' pairing scheme). In this case the cable-test continues to report 'link up', rather than showing cable pair information. If a 100b-t connection is active, no results are supplied for the un-needed pairs, i.e. those necessary for 1000b-t transmission (ie pairs 1 and 4 in RJ45 nomenclature). Remains to be tested: disconnecting an active 100b-t connection at the far end when the cable has a fault preventing 1000b-t operation, and performing the cable-test. ------------------------------------ Refs 1. Mikrotik: https://wiki.mikrotik.com/wiki/Manual:Interface/Ethernet#Detect_Cable_Problems