High-bandwidth and high-speed demands are pushing the limits of data transmission in cable assemblies. The demands of advanced applications such as Fast Ethernet, CDDI, ATM and Gigabit Ethernet require better methods of determining whether cabling systems can reliably transfer voice, video and data information. Cable assembly suppliers say they are stepping up to the plate with solutions. As networking demands escalate, new and forthcoming network applications have higher and higher data transmission rates

Back in the days of 10 Base T, you could run data on web string or barbed wire. Five years ago you could hook up a relatively generic cable and use it for testing and be pretty certain it worked. Now you're getting a lot more emphasis on the interconnect and cable because you're pushing the limits of what copper is capable of doing. Today we need to run data over some really fast speeds, all the connectors and cable assemblies have to be absolutely dead perfect because you can't get away with any variances. We're finding out that most of the assemblies used in our business just won't make it in the very high-speed data transmissions. The industry has to be reinvented.

Importance of structured cabling
Network cabling infrastructure is the most important part of a network, and an essential building block without which, a system may fail to function properly or at optimum efficiency. All too often, organisations fail to identify the importance of structured cabling systems and do not allow for future expansion as more and more bandwidth is required to cope with ever more demanding business applications. Although the sluggish movement of data is usually blamed on hardware or software, 70% of all network problems are related to cabling systems.

It is very unlikely that businesses that wish to continuously upgrade to the latest network technologies will ever see a cable plant with a lifespan of more than five years. Considering the trend in the recent past, with technology of 1985 the thick and thin coax supporting 10-Mbps Ethernet is still being used. Later in 1987 we started using the UTP to support 10Base-T, and in the early 1990s was replaced by Cat3 and soon thereafter by Cat4. By 1995 we were looking at 100Base-TX Fast Ethernet over Cat5. Cat4 was available for only a very brief time because Cat5 followed so quickly on its heels. In just 10 years we progressed through five cabling standards!

Five years later, in 2000, Gigabit Ethernet (1000Base-T) revealed deficiencies in some Cat5 installations. So, in early 2001, Cat5 had already been superceded by Cat5E. Cat4 and Cat5 are both defunct, and the development of Cat6 standards is well under way. Cat6 promises to replace Cat5E within a year or so and there's also talks of developing a Cat7 standard!

The evolving Cat 7
Copper cabling standards remain uncertain. Few people believe that Cat 7 will ever be viable. If and when a version does arrive, it will not be UTP; existing proposals call for a shielded or screened twisted pair (STP or ScTP).

Everything about Cat 7 promises to be outrageously expensive and extremely difficult to install. The cable will be heavier and bulkier, costing more to ship and install, and if the shielding is not grounded properly it will create more problems than UTP ever had. There are significant physical differences between category 6 and category 7 components. The "fully shielded" construction of category 7 cable results in a larger outside diameter and less flexibility than UTP or ScTP. These attributes require greater care in the design of pathways and termination spaces to allow for more space and larger bend radii. Another potential problem is backward compatibility among cabling standards.

Now consider the 1000Base-T Gigabit Ethernet, which always energizes all four pairs at once! The bi-directional dual duplex transmission scheme employed by 1000Base-T actually requires each end of a channel to transmit on one conductor of each of the four pairs simultaneously. The specifications for ANSI/TIA/EIAC at 5 come nowhere close to supporting such an application. This is one of the primary reasons for developing newer cabling Categories such as Cat6 and Cat7. There are other problems with Cat7, most notably crosstalk and return loss, which are two of the biggest problems faced when attempting to support greater bandwidths.

The pending category 7 and class F standards will enable shielded cable installations to perform to their full potential in terms of bandwidth over twisted-pair cabling, versatility and ease-of-use. Category 7 cables will be "fully shielded", with individually screened twisted-pairs and an overall shield. This type of cable is dominant in several European countries. However, its global acceptance has been impaired by connecting components that are limited in terms of performance, ease of use, adaptability, and size. A category 7 interface that is specifically designed for fully shielded cables will overcome these connectivity issues.

Category 6/class E delivers the highest level of transmission performance available without individually screened pairs. For the vast majority of business and institutional applications, 250 MHz bandwidth is more than adequate for the life of the cabling system, making category 6/class E the perfect choice for generic premises cabling.

The goal for category 7/class F is to be as good or better than any other type of balanced media for each transmission parameter. For example, the channel will deliver positive power sum ACR up to at least 600 MHz.

Another difference is with the connecting hardware. The pending category 7 specification requires connectors to provide at least 60 dB of crosstalk isolation between all pairs at 600 MHz. This requirement is 32 dB more severe than category 5 at 100 MHz, and 20 dB more severe than category 6 at 250 MHz

Category 7 cabling advantages

• Higher Bandwidth of up to 600 MHz as compared with Cat 5e (100 MHz) & Cat 6 (250 MHz)
• Suitable for installing in strong RFI & EMI environment
• Individual pair shielding enable better NEXT isolation allowing different application to run in the same cable
• Lower cost than using a fiber LAN
• Moving from existing copper based LAN without having to change the existing electronics.
• Secure transmission shielding keeps signal within cable.

Category 7 cabling disadvantages

• Individual pair and overall shielding increases the overall weight and size of cable. Hence needs larger & stronger pathway and more stringent bend radius (100 mm or 4 inch)
• Individual pair and overall shielding means higher labor cost and longer time to terminate cable.
• · Bandwidth of 600 MHz is it truly useful· May have earth loop problems, if both ends of the cable are connected to ground.

FIBER OPTIC SOLUTION
Choosing between Copper and Fiber-optic solutions is sometimes difficult, as distance, cost, required bandwidth and specialized expertise need to be considered.

WHICH IS MORE PRACTICAL?
Remember the telcos are still using single mode fiber over ten years old and have upgraded from 145 MB/s to over 2.5 GB/s with no problems - and more is coming.

After proposing a solution for GigaBit Ethernet that ran on Cat 5, the proposal for Cat 5 required bi-directional signals on each pair, with multilevel coding to compress the data. With the bi-directional use of each pair, things like FEXT (far end crosstalk) and return loss (reflections at the interfaces) became critical issues. The transceiver needed digital signal processing at very high speeds to make it work.

Now the proposal seems to be changing to "lets run on a higher bandwidth cable plant and use less expensive electronics." The option is Cat 6. And if this proposal wins out, users will not be running Gigabit Ethernet on installed cable plants, unless, of course they are fiber! And if they have to install new cabling, will fiber be the medium of choice? At some point, it will be cheaper to use fiber than to "S T R E T C H" the UTP copper any farther.

The Moore's Law of Fiber Pricing?
Remember Moore's Law? It's credited to Gordon Moore of Intel and it states that the cost of computing power is halved every 18 months. Well, the very same price decline appears to be happening in fiber!

An excess supply of fiber coupled to the end of several key patents relating to the manufacture of the fiber appears to be causing severe erosion in the price of fiber. Imports of cheaper fiber made overseas are likely with the expiration of some of the patents.

Optical fiber cabling development

• 2500 BC Roman Times Glass is drawn into fibers· 1790 Claude Chappe invents "Optical Telegraph" in France
• 1958 Alec Reeves begins investigating optical communications at Standard Telecommunication Laboratories
• The rest is history - Wide use of Optical Fiber Cable

Advantages of Fibre Optic cables

• Long distance transmission
• Immune to RFI & EMI
• Intrinsic security of transmission
• Reduced system costs
• Reduced maintenance costs
• Dielectric nature
• Upgrading ease
• Light weight
• Lowest life cycle costs
• High capacity
• Smaller size
• Synergistic planning lowers overall system cost factors

3 Potential disadvantages

• Potentially higher cost than copper
• Requires higher skilled labor to terminate than copper
• Fiber connectors are less forgiving of abuse than copper connectors

Common questions on optic fibre cabling

What are the different types of fibre available? What are the figures used to describe them ?
There are three types of fibre available in data networking - 50/125mm multimode fibre, 62.5/125mm multimode fibre and 9/125mm single mode fibre.

The figures 50 mm, 62.5mm and 9mm refer to the diameter of the inner glass core in which the light travels. The figure 125mm refers to the diameter of the glass outer cladding. As each fibre shares the same outer diameter, the mechanical properties of the fibres are identical. However, the optical properties vary significantly. Multi mode fibres have a large core, which allows less critical alignment and can be used with low cost LED technology. However, because of the core diameter the bandwidth is limited.

Single mode fibre, on the other hand, has almost unlimited bandwidth due to the small core supporting only one light mode. But, this requires very high precision alignment in both joints and connectors and the need to use expensive laser technology to drive the fibre.

These factors combine to make a single mode installation approximately four times more expensive than a multi mode installation.

How is the distance over which a fibre can be used related to attenuation and bandwidth?
When specifying a fibre, the two main attributes are attenuation of a fibre (or link) is the light loss through the fibre (of system) measured in decibels. The task of measuring the loss of a link is relatively simple process using a light source and power meter at the required wavelength. This attenuation component of the fibre or link, along with the bandwidth of the fibre, characterizes the fibre link and data rate/distance capability of that link.

In the past, bandwidth over fibre was never much of an issue. There was always far more bandwidth than existing electronics could interface with. Now, however, bandwidth has become a significant factor due to the emergence of new applications such as the Gigabit Ethernet standard. In the past, at low data rates, the usable distance over fibre links was dictated by the attenuation, i.e. by the link loss budger. Now, due to the much greater bandwidth requirements of Gigabit Ethernet, we find that some multi mode fibre types are limited in usable distance by bandwidth rather than loss. The date rate versus length of a fibre link is related to the bandwidth of the fibre but is also a factor of the transceiver technology. The simplest way to demonstrate the length limitations of the various protocols of fibre types.

Sreelal can be contacted at ind-sn@panduit.com