Network Cable Testing – An Ultimate OC Guide
Computers can be connected to each other to form a network that allows them to share resources.
Because the resources shared are principally various forms of digital data (organized as binary bits), and this data is transferred from one computer, or computing device, to another over a distance, then a data link is needed to connect the network, and this makes the data link the backbone of this network.
The computer network can, therefore, be described as a functional digital telecommunication network.
This is because digital data is converted into binary signals that are transmitted in the network through data links, and this is analogous to the way a communication channel called a link allows for signal transmission between two terminals in a telecommunication network.
A binary signal is also a logical signal as it has two distinguishable states, 0 and 1, which can be converted into 2 voltage bands – the reference (zero or ground for 0 state) voltage value and the supply voltage value (for the 1 state) – that can be transmitted through electrical conductors connecting two computers.
This explains why data transfer cables, or data link cables, are made from electrical conductors. Because the link is a cable, this cabled network is made up of computers that communicate with each other only, and such a network is called a local area network (LAN) or simply, a wired assembly.
Local Area Network (LAN) explained
In a LAN or wired assembly, the data cable is called the signal cable, and this is usually an ethernet cable. The three main types of ethernet cables are the twisted pair cable (most commonly used), fiber optic cable, and the co-axial cable.
Understanding that signal cables uses electrical charges to transfer data is important because it allows one to comprehend the basics of computer networks and cable integrity testing.
Likewise, it explains why there are different signal cables for transferring different types of data, as well as why specific signal cables can transmit data at higher speeds as compared to others.
These are discussed later in this review. The medium through which the signal travels through, such as a wired link/cable, is called the transmission medium.
Even so, data transmission can fail, and in most cases, the signal cable is at fault. As expected, a notification about transmission failure is convenient for the user, as it allows him/her to go check for the problem.
This type of notification is normally provided when transmission integrity tests are performed prior to commencing data transfer.
Normally, the integrity of data transmission is secured by a connection-oriented protocol such as the ubiquitous transmission control protocol (TCP) which ensures that the communication channel is functional and reliable so that all data packets sent from the sender (computer) is received in the right order in the receiver (computer).
TCP initially establishes a session using a three-way handshake before allowing for data transfer, and this session must be acknowledged by both computers before data transfer commences.
The three-way handshake involves the sender (computer) sending a SYN message to the receiver, with the receiver acknowledging that it has received the SYN message by sending a SYN-ACK message to the sender.
The sender then acknowledges receipt of the SYN-ACK message by sending an ACK RECEIVED message to the receiver, and afterwards data transfer can commence.
As expected, if the data link is broken, the SYN message will not be received, and if the data link is unreliable, either of the three messages will not reach its destination, for example, SYN can be sent but SYN ACK cannot reach its destination.
This way TCP notifies the user that data transmission cannot occur because a reliable data link cannot be established.
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On the other hand, if integrity of data transmission is supported by a connectionless-oriented protocol called the user datagram protocol (UDP), which does not establish nor acknowledge a session before sending data, then one cannot know if data has been received at the other end, unless s(he) confirms it by physically checking for it in the receiver computer.
Use of UDP necessitates the user to ascertain that the data link cables transmit all the binary signals that are sent by a computer, and this is done by checking the continuity of the cables using a cable tester.
This is critical because even though UDP provides faster data transmission speeds as compared to TCP, it is more prone to data corruption and data non-delivery.
UDP vs TCP
Failure of data transmission
As explained, failure of data transmission occurs because the links fail to transmit their signals, which in a LAN means that data transfer between communicating computers is interrupted or compromised.
Therefore, the cable needs to be tested for its continuity to ensure that there is no break in the electrical path that connects two computers together.
The best way to test for this, and get a reliable confirmation is to use a specialist tool called a network cable tester to check if signals are being transmitted through a signal cable, and if the strength and fidelity of the signals are maintained throughout their electrical paths.
To understand how failure of data transmission through signal cables develop, it is important to understand how computer networks are created and how they operate.
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Computer Network Basics – Establishment, Operation, and Cabling
As mentioned earlier, a computer network is made up of interconnected computers and computing devices that share resources, which is data that is exchanged through data links.
Each computer or computing device (such as a printer) that participates in data exchange is called a node; and the node from which data originates from, and the one the data is routed through, as well as the node which receives the data ( or where the data terminates into) are called network nodes.
Network nodes from which data originates from, and terminates into are called hosts, while those nodes that route data are collectively described as the network hardware.
Two hosts are described as networked if they can exchange information, either through direct connection via a single data link, or indirectly through networking hardware.
The information to be exchanged is first formatted into user data (called the payload) that is then sandwiched between control information (made up of a header and a trailer), and together they form a unit of data called a packet, which can then be transmitted through a digital network.
As expected, a packet is a series of bits, but these bits are constituted into two different formats – the header (alongside a trailer) and payload.
The header is control information that directs the packet to the right destination node, where the original data, packaged as the payload can be extracted and used.
Therefore, packet headers direct how data flows through a networked system, and this type of network system is called a packet-switched network; and further elaboration of its operation is beyond the scope of this review.
The computer network has two layers, the physical link (or hardware) layer and the data (or logical software) layer. There needs to be rules that govern how each layer operates.
In a networked system, there are sets of standard rules that govern how data is exchanged among the nodes; and the most commonly used set of standards that apply to the data layer is known as Ethernet.
Use of these standards allow an ethernet-compliant device to be added to an existing ethernet-based networked system.
As expected, the networking hardware and data link must also comply with Ethernet standards, and this impacts their design and construction.
The data link cable that allows for data exchange between two Ethernet-compliant devices is called an ethernet cable.
Simple Single-Link Network
So how is a network established? This can be answered by considering a very simple question – how does one get two computers to communicate with each other when connected together using a cable?
To begin, for two computers to initiate communication, they need to be connected together using an ethernet cable to form the most rudimentary form of a computer network – a single link direct network of 2 computers.
At either end of the cable must be connectors that support data transfer, and each connector must fit into a compatible port that is part of a component called the network interface card (NIC).
The NIC is plugged into the motherboard of the computer so as to allow its protocol suite to convert signals from the ethernet cables into bits that can be processed by the central processing unit (CPU of the computer).
Therefore, data flows from computer A through its NIC and then via a connector into the Ethernet cable which transfers it to the connector plugged into the NIC of computer B. The NIC is not part of a networking hardware, rather it is a piece of computer hardware.
Electromagnetic Interference and Cross-Talk
As noted earlier, data packets are transmitted in bits which are converted into two different levels of voltages, which serve as signals that can be conducted in the copper wires of the ethernet cable.
The voltage drop between supply and reference voltages creates an electromotive force (EMF, a force that moves electrons) that causes electrons to move through the conductor, and this creates an electric current that in turn creates a magnetic field.
Likewise, the wire has an inherent resistance against current flow through it when EMF is applied, and this resistance is called impedance.
The magnetic field created in the ethernet wire creates a magnetic field in a nearby electric wire, which results in voltage being inducted into this adjacent wire that subsequently creates (induced) current flow in a direction opposite to (original) current flow in the ethernet wire.
If EMF is applied to this adjacent wire so that current flows in the same direction as that of the ethernet wire, then the induced current would resist this current flow, and this phenomenon is called inductance.
Inductance is measured in ohms (Ω). Therefore, placing two ethernet wires side-by-side and applying EMF to both causes them to induce resistance against current flow in each other due to inductance, and this phenomenon is called electromagnetic interference (EMI).
For ethernet cables, EMI slows down transmission of digital signals and can even stop transmission. So, how does one deal with this problem when the ethernet wires need to be arranged in pairs?
This is dealt with by twisting the wires so that EMI caused by one wire is out-of-phase with the EMI created by the other wire, and this causes the EMI to cancel each other out.
As expected, the number of twists affects the efficiency of EMI cancellation, with numerous twists offering better performance as compared to fewer twists.
The straight distance from one end of the twist to the other end is called the twist pitch, and as expected short twist pitch offer better protection from EMI as compared to long pitch.
The standard ethernet cable used in computer networking has 8 insulated 24-gauge or 22-gauge (according to the American Wire Gauge [AWG]) copper wires arranged into 4 pairs, with each pair being a twisted pair. These 4 pairs are sheathed inside an insulation sheath that serves as their cable jacket.
This type of Ethernet cable is called an unshielded twisted pair (UTP) cable.
Each of the 8 wires features a colored insulation, with the insulation of each wire having its own unique colors so as to allow for color-coding of the wires.
Because each pair has 2 wires of matched impedance that are of the same length, then a common-mode EMI is created by either wire, and if they persist to the terminal node, then the EMI can be removed in the NIC through common-mode rejection.
This allows each pair of signal wires to create a balanced signal pair that operates as a balanced line. Therefore, there are 4 balanced lines in the standard ethernet cable.
To ensure that each balanced line does not create an EMI in the 3 adjacent balanced lines, the spacing of twists in each line is different so that each line has its own unique twist pitch.
This not only protects each balanced line from signal interference from other balanced lines, a phenomenon called cable-pair crosstalk, but it also serves to protect the 4 balanced lines from signal interference caused by external electrical equipment such as radios, machines, and fluorescent bulbs.
The UTP cable can be converted into a high-performance Ethernet cable by sheathing the twisted pairs inside a foil sheath so as to create a shielded twisted pair (STP) whose foil shield provides protection against leakage of EMI into, or out of the twisted pairs.
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Categories of Ethernet Cables
Ethernet was initially standardized as IEEE 802.3 in 1983, and it started by using coaxial cables as the transmission medium.
Later, the twisted pair cable was developed, and it was far more superior to the coaxial table in terms of speed and efficiency of signal transmission, as well as lower attenuation (loss of signal strength).
Even so, UTP ethernet cables can work best when transmitting data for short distances, usually up-to 100 meters.
If one needs network cables that can transmit data over long distances, then fiber optic cables should be used. Fiber-optic cables use unique connectors, mainly the subscriber connector (also called standard or square connector, SC) and small-form/small-factor pluggable (SFP) connectors.
In terms of construction, the fiber optic cable is made up of a glass core that is surrounded by a layer of glass cladding, which is in turn sheathed by a plastic buffer coat.
A braid is made around this buffer coat using Kevlar in order to make a Kevlar braiding, that is then sheathed by a plastic jacket.
For UTPs, their twist pitch and insulation can only prevent crosstalk as long data speed is kept below a certain level, and this means that the functionality of UTPs are limited by the maximum data speeds that they can handle.
Also, a UTP cable uses its entire bandwidth. A cable bandwidth defines a range of frequencies that can be used for data transfer, and as expected high bandwidth corresponds to a high potential to transmit large amounts of data of varying frequencies.
- Category 1 UTP is a telephone cable used for voice-only communication, and achieves a data transmission speed of 20 kilobits-per-second.
- Category 2 UTP has a bandwidth of 1 megahertz (1Mhz) and supports data transfer rate (DTR) of up-to 4 megabits-per-second (Mbs), which makes it suitable for local-talk applications.
- Category 3 UTP has a bandwidth of 16Mhz and supports a DTR of 10Mbs which makes it the first category that can be described as a true ethernet cable.
- Category 4 ethernet UTP has a DTR of 20Mbs which makes it suitable for token ring LAN. Its bandwidth is 20Mhz.
- Category 5 UTP has a DTR of 100Mbs which makes it suitable for fast ethernet LAN. Its bandwidth is 100Mhz.
- Category 5 enhanced (Cat 5e) UTP has a DTR of 1000Mbs (1 gigabit-per-second [1 Gbs]) which makes it a Gigabit ethernet cable. Its bandwidth is 200Mhz.
- Category 6 UTP has a DTR of 2500Mbs which makes it an ultra-fast Gigabit ethernet cable. Its bandwidth is 250Mhz
- Category 6 augmented (Cat 6a) UTP has a DTR of 10Gbs which makes it an ultra-fast Gigabit ethernet cable.
- Category 7 UTP is simply a Cat 6a UTP whose wires benefit from additional shielding against cross talk. Its bandwidth is 600Mhz. As expected, its DTR is 10Gbs.
- Category 8 is not a UTP, rather it is a copper STP with a DTR of 40Gbs, but its maximum cable length per data link is limited to 30 meters.
Most modern LAN uses Cat 5e UTP and higher categories, which means that Cat 5 and Cat 3 UTP are being rendered obsolete, though one can find them in old LANs.
Cat 6 UTP cables and lower categories of cable are limited to 100 meters of cable length per single link in a network, while Cat 6a and higher categories can achieve longer network distances because they benefit from additional shielding against cross-talk.
Relatedly, solid core Cat 5e UTP cables are ideal for use in structural installation, while patch cables can use those with stranded cores.
Another unique type of UTP cable is the screened UTP cable which features a foil screen, hence its designation as an F-UTP.
Its cable length is limited to 98 meters, but its screen size makes it unsuitable for crimping into standard modular connectors, though it offers better crosstalk protection as compared to the standard UTP.
In Cat 5 Ethernet cables, there are 8 wires that need to be connected to the network through an ethernet port, and this is achieved by connecting these wires into a special device called a connector, which provides a hardware interface between the cable wires and the ethernet port of a computer.
This connector is designated as an 8P8C connector because it has 8 positions that maintain continuity with 8 contacts.
Each position is connected to a wire, and this position extends into a contact that is coupled to a corresponding contact in the ethernet port, therefore allowing each wire to communicate to its corresponding contact in the ethernet port.
Understanding this is important when using network cable testers because one can find that the tester shows that continuity is intact, but when cable is connected to a computer or router, no data is exchanged; and this indicates that there are faulty or dirty contacts in the ethernet port.
Likewise, the fault can lie in the 8P8C connector and this can also be detected by this tester.
8P8C is a crimp-type modular connector that is popularly known as RJ-45 because of the wire arrangements that it supports.
Other forms of modular connector plugs are the 6P6C, 4P4C, and 6P4C connectors. Inside the 8P8C connectors are 8 pins arranged horizontally in a row, with each pin numbered from 1 to 8.
Each of the 8 wires in the ethernet cable needs to be connected to a specific pin in the 8P8C, and this ensures that cable and connector are complementarily-structured to work with each other when transferring data.
Each connector is designed to latch to its corresponding ethernet port, with a plastic (latching) tab on the connector allowing for latching, which can be made more secure by eliminating snagging.
This is done by sheathing the latching tab with a boot to protect the tab from breaking, and this creates a snagless connector.
Even so, one may need to pull back the sheath in this snagless connector when inserting it into the RJ45 port in the cable tester.
Also, one must also note that there are modular connectors that are dimensionally indexed, that is, its dimensions are non-standard which prevents one from fitting such a connector to a standard ethernet port.
This is important when one is using a tester that comes with a standard ethernet port, as one will be forced to uncrimp the wires from the connector and then crimp them to a standard connector or test each wire individually.
What are the 8 wires for?
There are 8 wires in the ethernet cable, but according to 10BaseT and 100BaseT protocols of the Ethernet standards, only 2 pairs are used.
This means that only 4 wires are used and these wires are connected to the connector pins 1,2, 3, and 6. Pins 1 and 2 in the 8P8C connector send signals to the ethernet port, while Pins 3 and 6 receive signals from the ethernet port, and send them to the ethernet cable.
Therefore, pins 1 and 2 are designated as transmitting plug pins, while pins 3 and 6 are designated as receiving socket pins.
The term 10BaseT means the following – the cable can handle a clock rate of 10Mhz which allows it to achieve a data transfer rate of 10Mbps if 1 bit is transferred per time, while Base stands for baseband signaling, while T stands for twisted pair.
Therefore, a 100BaseT and 1000BaseT cables can achieve data transfer rates of 100Mbps and 1000Mbps respectively.
As mentioned earlier, the ethernet cable is color coded, and the four twist pairs have an orange (O), green (G), brown (Br), and blue (Bl) cables, and each of these cables is paired with a white cable with a color strip that matches it.
This means that the orange cable is paired with a white cable with an orange strip (WO) to make a twisted pair, while the green, blue, and brown cables are paired with a WG (white with green), WBl (white with blue), and WBr (white with brown) cables respectively.
When crimping the cable to the 8P8C connector using 100BaseT protocols, then Pin 1 goes with the WG wire, while pin 2 goes with the G wire.
As mentioned earlier, crosstalk must be prevented in a balanced line, and for the line that carries wire 1 and 2, this is achieved by mirroring the signal so that two signals are obtained, the original signal and the mirrored signal with an inverse polarity (designated as – or negative signal).
Mirroring ensures that the mirrored signal creates an EMI that is opposite to the EMI created by the original (+) signal, and when created together, the EMIs cancel each other hence eliminating electromagnetic noise that can cause data corruption.
Wire 1 transmits the + signal (transmitting data +, TD+), while wire 2 transmits the – signal (the TD- wire), and because these are mirrored signals, the same data is transmitted twice with the NIC comparing the data in each signal to ensure that they are the same.
Pin 3 goes with the WO wire, and this is the + wire for receiving data (RD+). Meanwhile, pin 6 goes with the O wire which receives the – signal, hence it is the RD- wire.
In a technical sense, mirroring of signals means mirroring of the bits so that Bit 1 is mirrored into Bit 0 and vice versa. Each bit has its own amplitude.
Likewise, when comparing the mirrored signals, any difference in signal amplitude is due to introduction of external EMI noise, and the receiver (NIC) can just discard the noise to remain with intact mirrored signals of equal but inverted amplitudes.
EMI reduction is best achieved by having short twist pitch, and in Cat 5 ethernet cables, the pitch is 3 twists per inch.
Use of two pairs of wires allows for data transfer in full duplex mode, that is data is transmitted and received at the same time.
On the other hand, in the half duplex mode, data can either be transmitted or received at any particular time.
In Gigabit ethernet, all 8 wires are used, and mirroring is done in each balanced line.
It also uses a unique modulation technique called 4D-PAM5 (which stands for 4 dimensions pulse-amplitude modulation with 5 voltage levels) that allows 2 bits to use four different voltage levels to generate 4 signals that are transmitted simultaneously so as to achieve a transfer rate of 1000Mbs at a clock rate of 125MHz based on this formula:
DTR = Clock Rate X Number of Bits
This is possible because each of the four balanced lines can be used for both data transmission and reception, hence each pair provides for bidirectional data transfer.
- Data link pair 1 – Pins 1 and 2.
- Data link pair 2 – Pins 3 and 6.
- Data link pair 3 – Pins 4 and 5 (which makes it a Registered Jack 45 [RJ45] connector).
- Data link pair 4 – Pins 7 and 8.
Modern computer motherboards come with onboard gigabit ethernet ports, which eliminates the latency associated with using NIC expansion cards that are plugged into their PCI buses.
As mentioned earlier, wires in the UTP ethernet cable are color-coded, and this allows for ordering of the wires when fitting them into connectors.
This wire ordering is done according to a wiring standard accepted by the Telecommunications Industry Association (TIA), which also means that the standard is accredited by the American National Standards Institute (ANSI).
For structured cabling using UTP ethernet cables, the wiring standard used is the ANSI/TIA-568 which covers wired data networks.
There are two modes in this standard which allows for two forms of wiring orders: 568A and 568B.
In the ANSI/TIA-568A standard, the wiring order from pin 1 to pin 8 is as follows: WG-G-WO-Bl-WBl-O-WBr-Br; while the wiring order of ANSI/TIA-568B standard is as follows; WO-O-WG-Bl-WBl-G-WBr-Br.
Evidently, the order of Pin 1,2,3 and 6 determine the version of standard used, with Pin 1 and 2 order used in 568A being used for Pin 3 and 6 in 568B, while Pin 3 and 6 order used in 568A is used for Pin 1 and 2 in 568B.In the US, ANSI/TIA-568B is the favored standard.
Both ends of the ethernet cable are crimped to connectors, and when the same wiring standard is used in both ends of the cable, then this is described as a straight or patch cable.
However, if one standard is used in one end, and another standard used on the other end, for example 568A is used in one connector and 568B in the other connector, then this type of connection creates a crossover cable.
A straight cable allows for end-to-end signal transmission in the same wires, also called straight-through data transfer, and this makes it useful when connecting a computer to a networking hardware such as a router, hub, or switch; or put simply, connecting dissimilar devices in a LAN network.
On the other hand, a crossover cable is used to connect two similar devices in a LAN, for example, 2 computers, 2 hubs, or 2 routers together.