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If you are confused about the different terms used in data communications this article written by Mark Barratt should help to clear things up.

Bandwidth is the difference between the highest and lowest frequencies which will propagate through an equipment or system. In many cases, the lower limit is DC, zero hertz, and so the bandwidth is the same as the upper frequency limit. The public telephone system constrains all signals to the range 300 Hz – 3 kHz. Its bandwidth is therefore 2.7kHz.

In the most obvious method of modulation (representing data electrically), two different voltages are used to represent a ‘1′ and a ‘0′. The receiver expects a data bit at a certain time, and samples the input voltage to determine the value of the bit. This is called “amplitude shift keying” (ASK). The maximum frequency of the signal will depend upon the slew rate (the time taken to change from 0 to 1, or vice versa). The maximum slew rate is the upper frequency limit, and the slew rate, in turn, limits the maximum data rate.

Plainly, the bandwidth of such a system directly limits the data rate, but in theory it need not. Consider a protocol which uses “frequency shift keying” instead. Here, two different frequencies (both of them within the legal bandwidth) are used to represent 1 and 0. The maximum data rate is now the maximum speed at which you can shift between the two frequencies. This is still limited by the bandwidth, but not so directly – the resulting maximum data rate is higher. And what happens if you use more than two frequencies? You can then transmit more than one bit of information per signal transition, upping the data rate again without increasing the maximum frequency of the signal.

It is techniques such as these which have allowed the development of 56k modems. Using a combination of multiple-level amplitude, frequency and phase modulation, they manage to extract up to 56,000 bits per second of performance from the aforementioned 2.7 kHz bandwidth. To achieve this using plain 2-level ASK would require a bandwidth of hundreds of kilohertz.

“Baud rate”, strictly, is a measure of “signal elements” per second, and is not a useful measure where the above signalling techniques are being used. Such systems are generally rated in “bits per second” bps. It is worth noting that manufacturers will claim the highest figure they can for this parameter, so that the figure will include bits which are part of the signalling protocol rather than the user’s data, and may even incorporate an assumption about the compressibility of the data. It is rarely (if ever) valid to divide bps by 8 to arrive at bytes of data transmitted/expected per second.

Balanced line operation is a transmission method which helps to eliminate the effects of noise on the cable. In the first diagram a coaxial cable is transmitting a 4V signal, this is unbalanced as all of the 4V signal is carried by the centre core of the coax with respect to the grounded screen. If 1V of noise is introduced, it adds to the signal being transmitted making 5V, this could interfere with our data.

With a balanced line transmission our 4V signal is split into +2V and -2V on one twisted pair, so we still have 4V between the two. Now when we introduce the 1V of noise, the +2V becomes +3V, and the -2V becomes -1V, but the potential difference between the two is still 4V. The devices we put on the ends of the cable to make the line balanced are called baluns, this name is derived from the function of the devices of converting between balanced and unbalanced transmission modes.

These days, more and more equipment is being designed to operate on balanced lines without the need for baluns, but there are still a lot of older systems out there that still use these converters.

So what causes the signal to attenuate?, and where does the crosstalk come from?
Below are of some of the terms used in high performance cable testing, and a description of what they mean.

Length
The length of a cable is one of the more obvious causes of attenuation because the longer it is, the more resistance it has, and therefore less of the signal will get through. To measure the length, a cable tester uses Time Domain Reflectometry (TDR). A pulse is sent down the cable and when it reaches the far end it reflects back, by measuring the time it takes to travel down the cable and back again, the tester can determine how long the cable is. To do this, the tester also needs to know how fast the pulsed signal is travelling, this is called the Nominal Velocity of Propagation (NVP) and is expressed as a percentage of the speed of light. The NVP is usually somewhere between 60% and 90% of the speed of light, with most Cat 5E cables being around 70%. Due to the twists in the cable, the measured length will be greater than the physical length, so if a run looks like it might be over 80m it would be wise to check it before it is tied up and terminated.

Wire Map
This test is to ensure that the two ends have been terminated pin for pin, i.e. that pin 1 at the patch panel goes to pin 1 at the outlet, pin 2 goes to pin 2 etc. etc. The wire map also checks for continuity, shorts, crossed pairs, reversed pairs and split pairs. A Split pair is probably the only thing that requires an explanation here, as they are undetectable with a simple continuity tester, this is because pin for pin they seem to be correct. As explained on the Cabling Basics page, balanced line operation requires that the signal is transmitted over a pair of wires that are twisted together, with a ’split pair’ the signal would be split between two different pairs.

Return Loss
When a cable is manufactured there are slight imperfections in the copper. These imperfections all contribute to the Structural Return Loss (SRL) measurement because each one causes an impedance mismatch which adds to the cables attenuation.DC loop resistance
This is simply the resistance between the two conductors of a twisted pair which is looped back at the far end. The primary purpose of this test is to make sure that there are no high resistance connections in the link.

Attenuation
This is the decrease in signal strength (expressed as negative dB) from one end of a cable to the other. The main causes of attenuation are impedance,
temperature, skin effect and dielectric loss. Impedance is the combination of resistance, inductance and capacitance in a cable, it is measured in Ohms and opposes the flow of current. Skin effect is phenomena which happens at high frequencies where the signal tries to escape from the confines of the copper and into the air. The signal travels along the outer ’skin’ of the copper which effectively reduces the cross sectional area of the cable and therefore increases its resistance.

NEXT
This stands for Near End cross Talk, and it occurs because alternating current flow produces an electromagnetic field around the cable, this field then induces a current flow in adjacent cables. The strength of this field increases with the frequency of the signal, and because the speed of data transmissions is ever increasing, NEXT is a big problem.The name ‘Cross Talk’ comes from the telecommunications industry, you may have heard a faint conversation in the background while on the phone yourself, this is caused by the electromagnetic effect between adjacent telephone wires. In the transmission of data, cross talk is at its highest level in the RJ45 connection as it enters the cable, or at the ‘Near End’. The term ‘Near End’ is slightly confusing because data can travel in both directions, and the NEXT test is carried out in both directions automatically by the tester, so the NEXT result is relative to the end of the cable that it was carried out on.
The twists in a cable help to cancel out the effects of NEXT and the more twists there are, the better the cancellation, however, the twists also increase attenuation, so there is a trade off between NEXT cancellation and attenuation. The twist rates in data cables are optimised for the best overall performance, the twist rates are also varied for each pair within the cable to help combat crosstalk.

PSNEXT
This stands for Power Sum Near End Cross Talk and is actually just a calculation. When a tester carries out the NEXT test it measures the cross talk on each pair as affected by each of the other three pairs individually, PSNEXT is simply the addition of the three NEXT results for each pair. So this is the combined effect that a pair would be subject to when used in a network that supports a four pair transmissions method, e.g.. Gigabit Ethernet.

FEXT, ELFEXT and PSELFEXT
Basically, Far End Cross Talk (FEXT) is like NEXT but it is measured at the far end (well that seems logical!). However, on its own FEXT doesn’t mean much because the length of the cable determines how much the signal is attenuated before it can affect the pairs at the far end. To compensate for this, and to provide a more meaningful result, the attenuation is subtracted from the FEXT test and the result is then called Equal Level Far End Cross Talk (ELFEXT).
And of course, no test parameter these days would be complete without adding the results together for each pair and calling it a Power Sum measurement, so now we have Power Sum Equal Level Far End Cross Talk or PSELFEXT for short.

Delay
This is the propagation delay or the time it takes for the signal to travel from one end of the cable to the other, it is not very important on it own because it value is directly proportional to the length of the cable. What is important is the relationship between the delays on each of the four pairs. This brings us nicely on to …………………….

Delay Skew
Now this is important, Delay Skew is the difference between the fastest and slowest pairs. Some networks use a four pair transmission method, this means that the signal is split into four, sent down the four pairs in the cable and re-combined at the far end. It is essential that the signals reach the far end at near enough the same time, otherwise the signal will not be re-combined correctly.

The first thing to understand about testing data cables is the ACR, this stands for Attenuation to Crosstalk Ratio. The pink area in the graph is the attenuation, this can be caused by several things as will be explained below, and the blue area is the crosstalk. Attenuation is the reduction in signal strength over the length of the cable and frequency range, the crosstalk is the external noise that is introduced into the cable. So, if the two areas meet, the data signal will be lost because the crosstalk noise will be at the same level as the attenuated signal.

ACR is the most important result when testing a link because it represents the overall performance of the cable.

Q. I just found your web site and i thought that it was very informative, but i could not find any thing on the difference between stranded and solid cabling, could you tell me what the difference between them is? (I think stranded is used as patch leads and solid is used as a connector between the patch panel and the wall outlet).
A. You are correct, stranded cable is used for patch leads because it is more flexible than solid copper. The solid cable is used in the fixed part of the installation, ie. the cable between the patch cabinet and the wall outlets. Solid cable has better performance characteristics than stranded and it is cheaper to make.

Q. Thank you for your help, it was very useful. One other thing you could help me with, is the way that the solid and stranded cable are wired up different? because i know that you can get RJ45 plugs for solid and stranded cable (i know how to wire up stranded cable to a RJ45 plug), if so then how is the solid cable wired up to the RJ45 plug.
A. The colour codes are the same for solid and stranded cables, the difference is in the IDC (insulation displacement connector) in the RJ45 plug. Because the cores are different the contacts have to be slightly different to ensure a good contact is made.

Q. Our offices are moving into an existing building which was wired by the previous owners, or a contractor for them, and their wiring is one I have never seen before. Each wall outlet has two data jack which are “sharing” a single UTP cable, 2 pairs to the left jack and the other two pairs to the right jack. What problems are we likely to encounter with this setup? Our normal wiring method is one jack one cable…..

A. It sounds like costs were an issue when this building was cabled, but if it is configured for Ethernet (using pins 1, 2, 3 & 6) it should be OK for 10BaseT. I wouldn’t like to speculate on whether it will run at higher speeds because that is dependent on the quality of the installation and the amount of network traffic.

If the installation was originally wired for Token Ring then it will use pins 3, 4, 5 & 6 which will not work with Ethernet.
Crosstalk could be an issue if the pairs have been split between outlets and/or between pins on the jack, (pins 1 & 2 should be a pair and 3 & 6 should be a pair).

I hope this helps, and although it is possible that you will have no problems, I would strongly advise a rewire. This is because it will be easier at this time to carry out the work and, as you move to higher speeds in the future this wiring configuration will undoubtedly start to cause problems.

Q. The company I work for is interested in installing network cabling for residential homes in the construction phase (as the home
is being built) well I am supposed to make some brochure about this and I have no idea where to even start. Some of the things I now I need (text information) are: why is it going to benefit the consumer and builder, what are the perks of having network cable, and why is it easier and a more efficient way to connect to the internet. I am not asking you for the answers but I would like to know if you can suggest any web sites that I may find information about the questions I asked above.
Thank you for your time

A. This is a big question!

First of all, we are in the ‘Information Age’ and more and more households are using the internet and have more than one computer. Because of this, home networking is becoming commonplace and it is a more efficient method of connecting two or three computers to the internet over one phone line or DSL (Digital Subscriber Line).

The main benefits of installing the cabling during construction are the cost and ease of implementation. Cable is fairly inexpensive and installing it during the building work is far easier than trying to do it once the property is finished and decorated. I get a lot of email from people all over the world who are cabling their houses to connect PC’s together, and hiding cables in wall cavities or chasing out and re-plastering is not an easy task for them, unlike most office environments where this is not such an issue.

Another point worth raising is that residential cabling is not just for connecting PC’s together. ‘Smart Houses’ also use intelligent devices such as alarm systems, refrigerators and heating/air conditioning systems all of which can be networked to a controlling PC or accessed via the Internet when you are away from home. OK, you may be asking why would we need to? but it is starting to happen. And, of course, the cabling system can also be used for telephones, which makes adding an extension phone as simple as plugging it in at the nearest cabling outlet and patching it through at the panel.

Q. We have a small Pier to Pier network setup with 13 systems and one printer connected to 2 eight port hubs. We are having all kinds of instability with the network. A station might see all the other stations on the network but they can’t see him or visa-versa. Sometimes it will appear and other times not.
When we made the cables for the stations, we used cat 5 cable (4prs) with the following wiring:
Pin 1 wht/org, Pin 2 org/wht, Pin 3 wht/grn, Pin 4 grn/wht, Pin 5 wht/blu, Pin 6 blu/wht, Pin 7 wht/brn, Pin 8 brn/wht.

We tested them using a TBase cable tester, and they passed just fine. Looking at your diagram for correct wiring, it looks like you have some colored wires in different locations, but it is still pin for pin. Is there a reason why ours would be causing our problems?

A. Regarding your question on RJ45 pinouts, I would say that you are probably experiencing crosstalk problems.

The reason is this, Ethernet uses pins 1 & 2 and 3 & 6 on 10/100BaseT networks (I know it’s a funny way to do it but thats how it is!). One pair is used for transmit and the other pair for receive, so pins 1 & 2 at one end will connect to pins 3 & 6 on the other end and vice versa. In the correct wiring configuration each signal path (transmit or receive) is on its own twisted pair of wires, the twists help to eliminate crosstalk. With your wiring, pins 3 & 6 are split between the green and blue pairs making them susceptible to crosstalk (or noise), this also gets worse when using higher speeds (100BaseT or Gigabit Ethernet).

The reason your tester didn’t pick it up is because it only tests for continuity, shorts, crossed pairs etc. (not split pairs) you would have to use a scanner (like the Fluke DSP4000 or MicroTest OmniScanner) to find a split pair.

Although most all network are a mesh it is good to know the basics.

Q. what are some advantages and disadvantages about star, ring, and bus topologies?

A. In answer to your question regarding Network Topologies I hope the following helps.

Bus Topology
Advantages – Simple to implement, all machines are ‘daisy chained’ which makes wiring as easy as stringing coax cable from one computer to the next and so on.
Disadvantages – Not very versatile when machines have to be moved, as rewiring part of the network is necessary. Not very fault tolerant, on some systems (10Base2) if one part of the bus is disconnected the whole segment of the network goes down. Not suitable for voice.

Ring Topology
Advantages – Fairly simple to wire. Quite fault tolerant, with a Token Ring network if the main ring is disconnected anywhere, the ring uses a loop back system to maintain ring integrity.
Disadvantages – It is only really used with Token Ring networks these days and it uses Type 1 cable, this is very bulky and is not really suitable for running voice. Again, moves and additions mean rewiring and re-routing cables.

Star Topology
Advantages – These days most buildings are cabled using a star topology with Cat 5e or Cat 6 cable. This gives the network a centralized wiring point which makes connecting and disconnecting machines as simple as plugging and unplugging an RJ45 connector in to a hub or switch (Ethernet, Token Ring and any other type of network or service that is required). It can also be used for voice, data, video and any low voltage application.

Disadvantages – Requires a centralized wiring point, which can be just a cabinet on the wall for small installations, or a dedicated air conditioned room with racks and cabinets in large office accommodation. On large installations the sheer bulk of cable coming back to the wiring closet can be difficult to control and keep tidy.

How can I tell if a cable is a crossover or straight through?
Q. I currently have 2 PCs, one running windows xp and one running windows 98 which I would like to network together using the XP machine as the host. I have 2 rj45 ethernet cards and a 10 metre cat 5e twisted pair cable.
Is there any way to tell if I have the right cable as my computers aren’t communicating and I have tried every other troubleshooting resource, and the only conclusion I can come to is that my cable is not a crossover type.
Any advice would be GREATLY appreciated!

A. If you hold both of the RJ45 connectors side by side and look at them from the bottom (ie. the clip is away from you) you can make out which colour is connected to which pin. The pins you are interested in are (from left to right) 1 & 2 and 3 & 6. If they are the same either end then it is a straight through cable, if pin 1 (usually orange/white, but not necessarily) is connected to pin 3 at the other end and 2 is connected to pin 6 (and vice versa) then you have a crossover cable. If you look at the testing page of the Network Cabling Help website, about half way down there are some pictures of the pin outs for RJ45 connectors. The ‘crossed pair’ image is actually the correct wiring for a crossover cable and above it is the correct wiring for a straight through cable. If you have a straight through cable then it won’t work without a hub, if pins 1 & 2 and 3 & 6 are crossed over then the problem lies with the way the network is setup.

Cat 5 crossover cable?
Q. I have been looking at your site and it was very helpful.
I’m trying to connect two computers with a cat 5 crossover cable. The cable that I received in the mail was crossed correctly, but it was also crossed at 4 &5. My question is will this cable work by just joining PC to PC without any hubs or anything else. I bought the cable from an individual.

A. If you are using it for 10BaseT or 100BaseT Ethernet then the wiring should be 1 to 3, & 2 to 6
Pins 4 & 5 aren’t used on 10/100BaseT Ethernet systems so it should be OK. If you are planning to use Gigabit Ethernet or Token Ring then it won’t work.

Cable length limits?
Q. I know that cables have restrictions on distance, but I really would like to know why those limits are for each cable. E.g. Why is Cat X cable limited to 100m, and thinnet 185M etc. I have just accepted these values but have been asked why and I do not know a technical enough answer as I have never been a cabler. I could not find this on your site and was hoping it might be added or if you could email me back with why. Thanks.

A. The length limits are not for the particular cables as such, they are for the type of data signal that they carry.

Let me try and explain!

Thinnet (RG58 coax) was used for 10Base2 Ethernet, at 10Mbps on RG58 coax Ethernet can reliably operate upto a distance of 185m. The native cabling environment of the AS400 is Twinax and the standard operating speed is only 1Mbps. At this speed it has a maximum distance of 1800m, however, if Cat 5 forms part or all of the link the distance can drop to between 36m and 364m.

So for a proprietary network such as Thinnet, the distance is set at the maximum length that the signal will work reliably at a given speed over a given type of cable. So far so good!

Now, when we talk about Cat 5, 5e, 6 etc. these are cabling ‘Standards’ which define a method of connecting all types of networking protocols, over a cabling system that uses a common media, common connectors and a common topology. So the length limit was arbitrarily set for the worst case scenario. 10BaseT may well work on Cat 5 for 150m but ATM, AS400, Token Ring etc. may not, and because a structured cabling system has to work for all networking methods, a limit had to be set.

I don’t think I have explained this very well but I hope you get idea. Incidentally, I have heard talk of the length limit being dropped from the standards as it is the overall Attenuation to Crosstalk Ratio which determines a cables ability to transmit a signal successfully, and not the length of the cable. If anyone can elaborate on this point please let me know.

Cat 5 and Cat 5E
The basic Cat 5 system used to be the only real choice, but developments in Ethernet technology led to the introduction of ‘Enhanced Category 5′ or Cat 5E. Both systems are capable of transmission rates up to 100MHz, but the test parameters for Cat 5 assumed that data signals would only use two of the four pairs (one pair for transmitting and one pair for receiving) and crosstalk measurements were only taken between each pair combination. With Gigabit Ethernet however, all four pairs can be used to transmit simultaneously, and so the cross talk on each pair has to be measured for the combined effects of the other three pairs.

Cat 6
At last! the standard for Cat 6 has been approved for publication by the EIA (TIA/EIA-568-B.2-1). Category 6 is capable of transmission frequencies up to 250Mhz and has a positive power sum attenuation to crosstalk ratio upto 200MHz using improved cables and RJ45 connectors. The problem that manufacturers have, is that to meet the Cat 6 specification, requires the use of cables and connectors which are designed to work together as a ‘tuned’ system. This means that if you install a Cat 6 system the manufacturer will only guarantee performance if all of the components including the patch leads are from their Cat 6 product range. In fact, by mixing Cat 6 components from different manufacturers you could end up with a system with worse performance characteristics than a conventional Cat 5e system. That said, it is worth noting that Cat 6 systems are backwards compatible with Cat 5/5e cabling and when mixed with these lower bandwidth systems the performance criteria of the lower specification will still be met.

Testing Cat 6 cables can be a frustrating process, apart from taking longer because the tester has to scan frequency steps up to 200MHz instead of 100MHz, the fine line between pass and fail is accentuated it seems by the slightest kink and twist. The most significant factor when testing a Cat 6 system can be return loss failures due to the test leads themselves. All connectors have a life cycle and with the average RJ45 connector this is around one or two thousand insertions, so test leads should be replaced after every 1000 tests or so. OK, not a problem but at around $200 per set this cost will have to be considered when pricing jobs.

Fluke seem to have a solution to this problem with their DSP-LIA101S Permanent Link Adapters. The connector at the end of the leads are interchangeable and replaceable with connectors from different manufacturers to ensure compatibility with the system under test. Although a good idea, the adapters are over $500 and a new pair of “Personality Modules” cost over $100. Surely the test plugs should now be considered as ‘consumables’ and the price lowered to reflect this.

Cat 7
This is proposed to be a 600MHz system using a shielded cable with individually screened pairs and a new type of connector. The cable and connectors are slightly bigger than Cat 5e and installation time can be increased because of the complexity of the termination. There are two main draw backs with installing this type of cabling, the first is the additional cost involved, and the second is that almost all networking hardware uses RJ45 jacks. To connect to the cabling system, you have to use Cat 7 to Cat 5e patch leads, and because any system is only as good as its weakest link, your speed is back down to 100MHz. Ratification of the Cat 7 standard could be two years away by which time fibre might be a cheaper alternative.

Shielded or Unshielded
This is a subject that has been debated and argued over for a long time, and as yet, there are still no definite answers. Most countries in Europe, and in particular Germany, argue that apart from protecting data signals against high frequency noise from outside sources, shielded cable also protects the humans against the possibility of having their brains fried due to the effects of high frequency emissions from the cable itself. Other countries, such as the UK, US and Canada, aren’t particularly bothered by this because nothing has been proved, and after all, millions of people wander around with mobile phones pressed against the side of their heads with no apparent side effects, er… yet. My advice would be to install unshielded cable unless the customer insists on a shielded system.

Shielded cables and components are more expensive and are more time consuming to terminate, you should also bear in mind that a shielded cable that isn’t properly grounded has worse performance characteristics than an unshielded cable. If a shielded cable isn’t grounded at all, the screen can act like an antenna and induce all manner of noise on to the data signal.

Low Smoke Zero Halogen
In public buildings, such as airports, shops and hospitals, then the cable should be Low Smoke Zero Halogen (LS0H or LSZH).

There are three main cabling standards:

• EIA/TIA 568A – This is the American standard and was the first to be published (1991).
• ISO/IEC 11801 – The International standard for structured cabling systems.
• CENELEC EN 50173 – The European cabling standard (the British version is BS EN 50173).

The reason for having a ‘Standard’ is to define a method of connecting all types of vendors voice and data equipment, over a cabling system that uses a common media, common connectors and a common topology. This means that a building can be cabled for all its communications needs without the planner or architect ever having to know what type of equipment will be used.

It is advisable to get a copy of one of the cabling standards documents, although once you have read through it once and understood some of what it describes, it will probably be filed away and never opened again. If you have ever tried to read a standards document you will know that it is hard work. Trying to separate the useful information from all the technical jargon can be very time consuming and even then you may not find the answer to your question. The bad news is, the Cabling Standards are no different, they are full of cross references, formulas and tables all of which can be a very daunting prospect and can make the installation engineer think twice about installing the stuff.

Now for the good news, the standards are mostly concerned with the performance criteria of the components of a cabling system, and, as that is guaranteed by the manufacturers of the different cabling components, you don’t have to worry about it. Great eh!

First you need some training, there are a few recognised courses on structured cabling systems which offer some hands-on experience, or you could take one of the many courses offered by the manufacturers of cabling components. Obviously the manufacturers try to sell their own products, but their courses are usually cheaper and they can still provide some of the basic cabling skills.

There are also lots of books on the subject of cabling and a selection of these can be found at Amazon.com, although my personal favourite is The Cabling Handbook 2nd Edition by John Vacca. It has over 1300 pages covering all aspects of network cabling and includes chapters on The Standards, Network Design, Wireless Communications, Fibre and Home Wiring, plus a whole lot more.

If you don’t want to invest any money on training until you are seeing some financial results, then you can gain valuable experience by actually doing some work for an existing cabling company.

The next thing to do is open accounts with the suppliers of data cabling materials in your area, or try some of the online suppliers. One such company is Network Cables Online.

Here at Network Cables Online (aka NCO) we are going to start blogging on various networking technologies, cable technologies, communications standards and anything related to what we do at Network Cables Online, LLC.