2026-09-23 · class-notes, a-plus, comptia, messer, networking, network-tools, cabling, troubleshooting
Full class notes for Professor Messer A+ 220-1201 objective 2.8: crimpers, Wi-Fi and spectrum analysers, tone generator and probe, punch-down tools, cable testers, loopback plugs, taps and port mirrors.
Class notes · Professor Messer, CompTIA A+ 220-1201 Core 1 · Section 2, lesson 2.8 (Network Tools)
Quick reference: the short version of this lesson is the Network Tools cheat sheet in A+ Core 1 section 2. It follows the section's earlier lessons on ports, wireless protocols, network services, DNS, DHCP, VLANs and VPNs, network devices, IP addressing and internet connection types — the tools here are how you build and fix the physical layer those lessons depend on.
By the end of these notes you should be able to:
Sooner or later you are handed a bare cable with no connector on it — coax, twisted pair or fibre — and asked to put the right connector on the end. For copper, the tool is a cable crimper: it squeezes a connector permanently onto the cable so the two become one piece. The classic example is fitting an RJ45 connector to a twisted-pair cable.
Inside an un-crimped RJ45 connector, the eight metal contacts sit slightly proud of the body and each one has a sharp, pointed end. You slide the eight untwisted wires into the connector in order, and when the crimper closes, those points are forced down through the plastic insulation on each wire until they bite into the copper underneath. This is an insulation displacement connection: you never strip the individual wires.
The same squeeze also pushes a small plastic cable stay down onto the outer jacket, so a tug on the cable pulls against the jacket rather than against the eight delicate wires.
+------------------------------------------------------------------------------+
| Before the crimp After the crimp |
| |
| contacts stand proud contacts driven down |
| v v v v | | | | |
| +---------+ +---------+ |
| | RJ45 | | RJ45 | |
| ===| wires | ===| wires | |
| +---------+ +---------+ |
| ^ ^ |
| stay not yet pressed stay clamps the jacket |
+------------------------------------------------------------------------------+
A crimper alone is not enough. A realistic kit is:
| Tool | Job |
|---|---|
| Cable crimper | Presses the connector onto the cable |
| Electrician's scissors or cable snips | Cutting the cable and trimming the wires square |
| Wire stripper | Removing the outer jacket without nicking the pairs inside |
| RJ45 connectors | One per end, plus spares for the ones you get wrong |
| Cable tester | Proving the result before you install it (section 5) |
The first few attempts are slow, mostly spent getting eight wires into the right order and fully seated. After a few goes it becomes routine, and the payoff is being able to make cables to exactly the length you need instead of living with 3 m patch leads behind a rack.
Note (beyond this lesson): the wire order is a standard, not a preference. T568B is W-Or, Or, W-Gn, Bl, W-Bl, Gn, W-Br, Br. Use the same standard at both ends for a normal patch cable, and pick one standard for the whole site.
Caution: untwist as little as possible — no more than about 13 mm (half an inch) at the connector. The twists are what cancel interference; undo too many and a cable that passes a continuity test can still perform badly.
In the real world: many installers now use pass-through RJ45 connectors, where the wires poke out of the front and the crimper trims them flush. They are far easier to get right, but need a crimper designed for them.
Going wireless removes the cables but adds a different set of problems, and you cannot see radio. A Wi-Fi analyser makes it visible. It shows:
A Wi-Fi analyser only understands Wi-Fi. If you need to know what else is using the same frequencies, you need a spectrum analyser, which looks at the whole radio spectrum and shows every signal in the area, 802.11 or not.
| Wi-Fi analyser | Spectrum analyser | |
|---|---|---|
| Sees | 802.11 networks, channels, clients | All radio energy in the band |
| Finds | Channel overlap, weak signal, busy channels | Non-Wi-Fi interference from any source |
| Typically | An app on a laptop or phone | A dedicated device or USB probe |
| Costs | Little or nothing | Considerably more |
Exam tip: "another network is on my channel" is a Wi-Fi analyser problem. "Something that isn't Wi-Fi at all is jamming the band" is a spectrum analyser problem.
Note (beyond this lesson): common non-Wi-Fi sources on 2.4 GHz are microwave ovens, older cordless phones, Bluetooth devices, baby monitors and wireless cameras. A Wi-Fi analyser cannot see any of them; it just reports poor performance on a channel that looks empty.
A Wi-Fi analyser plots signal against noise. Far from the access point, the noise floor sits close to the level of your own signal, so there is little headroom and throughput suffers. Walk towards the access point and the signal rises clearly above the noise, which you can watch happening on the display.
+------------------------------------------------------------------------------+
| Far from the AP Close to the AP |
| |
| strength strength |
| | | ##### signal |
| | #### signal | ##### |
| | ~~~~ noise | ##### |
| | ~~~~ | ~~~~~ noise |
| +--------------- +--------------- |
| little headroom -> poor throughput clear headroom -> good |
+------------------------------------------------------------------------------+
Note (beyond this lesson): the gap between the two is the signal-to-noise ratio (SNR). Roughly 20 dB or more of SNR supports the fastest rates; below about 10 dB the link drops to slow rates or fails. That is why a survey records signal and noise, not signal alone.
In a building with hundreds or thousands of cables, the hard part is not making a connection — it is working out which of the many identical cables in the comms room is the one that ends at the desk you just left.
The kit is a pair:
The important point is that the probe never has to touch the copper. You wave it along the outside of the cable jackets and listen for the one that sings.
+------------------------------------------------------------------------------+
| |
| [Tone generator] --- clipped to the cable, injects an audio tone |
| | |
| | =============================================== |
| | the cable runs through walls, trays and ducts |
| | =============================================== |
| ))) field around the wire |
| [Inductive probe] -> speaker |
| move it across the bundle |
+------------------------------------------------------------------------------+
The generator can attach in whatever way the cable presents itself: a modular RJ45 connection, a coax connector, or clips straight onto a punch-down block. Then you take the probe to the far end and move from cable to cable until one makes a noise.
In the real world: work methodically. Split a large bundle into halves, probe each half, and keep halving the noisy one. Sweeping randomly across two hundred cables wastes far more time than a few deliberate passes.
Caution: do not connect a tone generator to a cable that is still plugged into live equipment, especially anything carrying PoE or a telephone line. Unplug the far end first.
Where a lot of cabling arrives in one place — the back wall of a comms room or data centre — the cables are terminated onto a punch-down block rather than fitted with plugs. Cross-connecting one cable to another then becomes a short jumper between block positions instead of a re-run of cable.
The punch-down tool does two things in one push: it seats each wire into the block's slot, where the metal contacts cut through the insulation to reach the copper and hold the wire so it cannot easily be pulled out, and it trims off the excess wire in the same movement, leaving a tidy termination.
A well-punched block shows the twists maintained right up to the point of termination. Those twists are what cancel interference, so untwisting a long tail before punching it down weakens the signal even though the connection is electrically fine. The lesson makes exactly this point about a good installation.
Blocks are numbered so that each terminated cable has an address. Record which number a cable is punched into and where its other end goes. When you later need to connect that run to a switch, the documentation, not a probe, should be the first thing you reach for.
+------------------------------------------------------------------------------+
| Punch-down block, numbered positions |
| |
| 01 |=| ==== to desk 12, port A (twists kept to the block) |
| 02 |=| ==== to desk 12, port B |
| 03 |=| ==== to meeting room camera |
| 04 |=| ==== spare |
| |
| Jumper 01 --> switch port 1/0/7 (record it, don't re-trace it) |
+------------------------------------------------------------------------------+
Note (beyond this lesson): the two families you will meet are the older 66 block (mostly telephone) and the 110 block (data). Punch-down tools take interchangeable blades for each, and one side of the blade cuts while the other does not — put it in the wrong way round and you cut off the wire you meant to keep.
After crimping a plug or punching down a block, a cable tester checks that the cable is wired the way you intended: pin 1 at one end reaches pin 1 at the other, pin 2 reaches pin 2, and so on through all eight. It finds:
A cable tester is a continuity test. It tells you the copper goes where it should; it says nothing about the quality of the link — bandwidth, crosstalk, attenuation or noise. A cable can pass a continuity test and still be far too long, badly untwisted, or run alongside a fluorescent light.
Exam tip: continuity is the key word. If a scenario says the cable "tests fine" but throughput is poor or errors are high, the answer is not another continuity test — think length, interference, untwisted pairs, or a certifier.
Note (beyond this lesson): the tool that does measure quality is a cable certifier, which tests against the TIA category standards (Cat 5e, Cat 6a and so on) and produces a pass or fail report. It costs orders of magnitude more than a continuity tester.
Many tone generators double as a continuity tester. In testing mode with nothing connected, the unit flashes to show it sees no wires. Plug one end of the cable into the main unit and the other into the remote end, and it steps through pins 1 to 8 in turn: a light for each pin that is connected, no light for one that is not, and lights out of order where wires are crossed. It is a simple test, but it answers the question "did I get the eight wires in the right order?" in a couple of seconds.
+------------------------------------------------------------------------------+
| Cable tester walking the pins |
| |
| Main unit Remote unit |
| [1][2][3][4][5][6][7][8] ==== cable ==== [1][2][3][4][5][6][7][8] |
| |
| all eight light in order = wired straight through |
| one pin never lights = open (missed or unseated wire) |
| lights out of sequence = crossed pair, wires in wrong order |
+------------------------------------------------------------------------------+
A device reports a lot of errors on a link. Is the cable at fault, or the interface on the device? A loopback plug answers that by removing everything else from the picture.
A loopback plug is a short connector that wires the interface's transmit path straight back into its own receive path. There are different plugs for serial, RJ45 and fibre connections. They are not crossover cables — a crossover joins two different devices; a loopback folds one interface back on itself.
+------------------------------------------------------------------------------+
| Normal link Loopback test |
| |
| [Device A] TX ------> RX [Device B] [Device] TX ---+ |
| RX <------ TX RX <--+ |
| |
| A fault could be at either end Only this one interface and |
| or anywhere in the cable between its own signalling are tested |
+------------------------------------------------------------------------------+
Exam tip: loopback plug = test one interface in isolation. Cable tester = test the cable. Between them you can say which half of the problem you have.
Note (beyond this lesson): an RJ45 Ethernet loopback plug joins pin 1 to pin 3 and pin 2 to pin 6 (the transmit pair to the receive pair); a 9-pin serial loopback joins pin 2 to pin 3. There is also a software loopback —
127.0.0.1and::1— which tests the TCP/IP stack rather than the hardware. The two ideas share a name and nothing else.
Eventually you will need to collect the actual packets crossing a link. A physical tap is inserted into the middle of the link: you break the connection and put the tap in line, so traffic passes through it and a copy goes out to your monitoring port.
Breaking a link means an outage, so this is rarely something you do to a production network during working hours. If a link is going to be captured often, the usual answer is to install the tap permanently while you have a maintenance window.
Fibre taps are commonly passive — they split off a small fraction of the light and need no power at all, so they cannot fail the link by losing power. Copper taps generally do need a power source.
When you cannot interrupt the link, use a feature built into enterprise switches: a port mirror, also called port redirection or SPAN (Switched Port ANalyzer). You plug your protocol analyser into one switch port and tell the switch to copy every frame from another port to it. You get the same copy of the traffic without touching the cabling — the switch itself acts as your tap.
+------------------------------------------------------------------------------+
| Physical tap (link broken, tap inserted) |
| |
| [Host] ==== [ TAP ] ==== [Switch] |
| || |
| \/ copy of both directions |
| [Analyser] |
| |
| Port mirror / SPAN (no cabling changed) |
| |
| [Host] ==== [Switch port 1] --- traffic flows as normal |
| || switch copies frames internally |
| [Switch port 9] ==== [Analyser] |
+------------------------------------------------------------------------------+
| Physical tap | Port mirror (SPAN) | |
|---|---|---|
| Installing it | Breaks the link; needs a maintenance window | Configuration change only |
| Needs | The tap hardware; power for copper taps | A managed switch that supports it |
| Fidelity | Sees everything on the wire, including errors | Can drop frames when the switch is busy |
| Affects the switch | No | Uses switch resources |
| Good for | Permanent monitoring points, forensics | Ad-hoc capture, anywhere you already have a managed switch |
In the real world: a mirror port can be oversubscribed. Mirroring two busy gigabit ports into one gigabit analyser port means frames get dropped, and the gaps are silent. If a capture must be complete, use a tap.
Scenario: a user's desktop is connected but the switch reports thousands of errors on that port. The cable was made on site.
Scenario: a meeting room has a live wall socket, and nobody knows which patch panel position it lands on. The comms room has around 200 identical cables.
These are physical-layer tools, and the physical layer is where a lot of defensive assumptions quietly break.
| Term | Meaning |
|---|---|
| Crimper | Tool that presses a connector permanently onto a copper cable |
| RJ45 | The eight-pin modular connector used on twisted-pair Ethernet |
| Insulation displacement | Contacts piercing a wire's insulation to reach the copper, with no stripping |
| Cable stay | The part of a connector that clamps the outer jacket so the wires take no strain |
| Wire stripper | Tool for removing the outer jacket without damaging the pairs |
| Wi-Fi analyser | Tool showing 802.11 channels, signal strength, interference and clients |
| Spectrum analyser | Tool showing all radio energy in a band, including non-Wi-Fi sources |
| Tone generator | Device that injects an analogue tone onto a cable for tracing |
| Inductive probe | Handheld receiver that hears the tone through a cable's insulation |
| Punch-down block | Terminating block where many cables land for cross-connection |
| Punch-down tool | Tool that seats a wire into a block and trims the excess in one push |
| Cable tester | Continuity tester that checks each pin reaches the matching pin |
| Continuity | Whether an unbroken electrical path exists; not a measure of quality |
| Loopback plug | Connector that feeds an interface's transmit back into its own receive |
| Diagnostic mode | Interface mode that sends a test pattern and checks what returns |
| Physical tap | Device inserted into a link to copy traffic to a monitoring port |
| Passive tap | A tap, usually fibre, that needs no power to work |
| Port mirror | Switch feature that copies one port's frames to another port |
| SPAN | Switched Port ANalyzer; another name for port mirroring |
| Protocol analyser | Software or hardware that captures and decodes network traffic |