Adaptive Hardware Infrastructures for SAP
Chapter 11: Local Area Network Solutions
In this section, learn why high bandwidth does not mean high frequency in an SAP infrastructure. Discover why copper cables are easier to install and more cost-effective than optical fibers, and how deformation or mechanical stress during installation can cause cable failures. Also, find out why sporadic disturbances and breakdowns take place in data networks without a clear reason.
Adaptive hardware infrastructures for SAP, Ch. 11
Table of contents:
How to attain high availability for SAP and local networks
Configuring wires and fibers in adaptive SAP hardware infrastructures
WLAN standards and integrating WLAN into SAP hardware infrastructures
Chapter 11: Local Area Network Solutions
11.2 Wires and Fibers
Today, basically two cable types are used for local networks: lines with twisted pairs of copper wires and fiber optics cables. Both have types have advantages and disadvantages, because of their physical characteristics.
11.2.1 Copper Cables
The area of end device connections is generally based on twisted pair copper wires. This type of cable has existed since the first telephone signals were transferred. Throughout the years, on the one hand, the transfer frequencies have become much higher; on the other hand, there are essentially more sources of disturbance. Fortunately, twisted pair cables were also developed to the same extent. Therefore, we can say with assurance that twisted copper cables actually do meet the requirements of high-speed data transfers. Compared with optical fibers, copper cables are much easier to install and, consequently, are more cost-effective.
Twisted pair cable consists of two copper wires. Each wire is encased in its own color-coded insulation, twisted around one another. Multiple pairs are packaged in an outer sheath, or jacket, to form a twisted-pair cable. The twist of the cable is essential for electrical noise immunity and must go as near as possible to the connectors of the wall receptacles and patch-panels. By varying the length of the twists in nearby pairs, the crosstalk between pairs in the same cable sheath can be minimized. The typical nominal impedance is 100 ohms.
For data networks in companies, structured cabling in accordance with EN 50173-12, ISO/IEC, or EIA/TIA-568 category 5 onwards has become standard.
The decisive quality attribute for top quality data cables is the symmetry of the cable. Different twist lengths of pairs that are placed next to each other avoid crosstalk. In this context, it is important that the cables are not only symmetrically stranded but also precisely finished.
High Bandwidth is not Equal to High Frequency
|There only appears to be a connection between the transfer frequency and the achievable bandwidth. By using highly developed signal encoding processes, all high-speed technologies such as Fast Ethernet and Giga Ethernet, as well as ATM, don't exceed 310 MHz as the transfer frequency. Technologies with higher bandwidth are based on fibre optic cables.
Contemporary data networks operating with frequencies that are located in the middle area of the VHF radio band (Very High Frequency). The metallic conductors act like antennas for these frequencies, for receiving as well as for transmitting. STP cables contain a metal shield to reduce the potential for electromagnetic interference (EMI). EMI is caused by alternating electromagnetic fields from other sources such as electric motors, power lines, high power radio and radar signals but also by flickering fluorescent tubes in the vicinity that may cause disruptions or interference, called noise.
There are, however, a variety of shielding solutions for data cables. From the most simple aluminum polyester compound film, through combinations of tin-plated twisted meshwork and compound film, to expensive metal shields, you can find all possible constructions.
The names of the various shielded (Shielded Twisted Pairs, STP) and unshielded (Unshielded Twisted Pair, UTP) cable types are quite confusing. STP also encompasses Screened Shielded Twisted Pair (ScTP) and Foil Twisted Pair (FTP) cables. Within UTP, there are paradoxically also Shielded Unshielded cables (S-UTP) with a complete external shielding, but without individual shielding of the pairs.
At first glance, STP cables appear to be immune to any interferences, because of their shielding. But, unfortunately, this is not the case. As the grounded shield also acts as an antenna and transforms the incoming interferences into a current, which induces a current in the signal wires in the opposite direction. As long as both currents are symmetrical, they eliminate each other. Any discontinuity in shielding or asymmetry in the currents between the shield and signal wires acts as a source for electronic noise. Therefore, STP cables are effective only if the entire link from one end to the other is continuously shielded and properly grounded. However, this can in turn cause severe problems by amperage flow over the shield in cases, where the electrical supply grid has no separated grounding.
For UTP cables, the physical shield is replaced by improved variations of the twisting as well as sophisticated filtering techniques in the network devices. Disturbances are equally induced in both conductors and therefore eliminate each other. Throughout the years, the UTP cables have constantly been improved so that they now fully meet the requirements of category 5.
Despite a heated debate over the years about the advantages and disadvantages of shielded versus unshielded twisted pair cables, a final conclusion has still not been reached. In Europe, STP is the main preference, not only to protect data signals against outside emissions, but because corresponding regulations require the protection of the environment against the emissions of data signals. UTP cables are used generally in the rest of the world. In any case, reliability is always determined by the quality of the cable manufacturing and proper installation.
Electromagnetic Compatibility (EMC)
| Another factor to consider when choosing a cabling system relates to electromagnetic compatibility (EMC). In the U.S. and Germany, EMC regulations have existed for years. However, the implementation of the European EMC Directive 89/336/EEC in 1989 has refocused attention on EMC. With the increased amount of electronic equipment in the average workspace, EMC becomes increasingly more important. Excess radiation from one piece of equipment can adversely affect performance of another piece of equipment. EMC refers to the ability of an electronic system to function properly in an environment where several pieces of equipment radiate electromagnetic emissions. This means that every electronic system, which includes all copper based cabling systems, must meet this directive.
11.2.2 Fiber Optics Cables
On the other hand, fiber optics cables also have some disadvantages:
A fiber optics cable consists of a bundle of optical threads (fibers), in which messages modulated onto light waves propagate along the direction of the fiber because of internal reflection. The reflection occurs due to the different refraction indices between the core and the coating. Fiber-optic network cabling is made up of at least two strands of optical fiber running parallel to each other in a plastic "zip-cord" jacket, or multiple fibers in a single jacket.
There are two different types of optical fibers: Multi-Mode Fibers (MMF) and Single-Mode Fibers (SMF). In this context, "mode" refers to the effect of spreading a light signal in an optical fiber, resulting in light-rays following different paths (or modes) down the fiber (modal dispersion). Multimode fiber (MMF), with a core diameter of 62.5μm allows a light signal to take various zigzag paths. This modal dispersion causes some light rays to arrive later at the end of the fiber. Due to the undesired runtime differences caused by these different modes, the range of MMF cables is limited to 1.5 miles (2 km).
Single-mode fiber (SMF) with a core diameter of only 9 microns allows only one path for the light to take due to the fiber's very small diameter. Single-mode fibers and components are more expensive than Multimode fiber, but allow connectivity up to 12 miles (20 km). Cheap Plastic SMF fibers can be used only for very short connections, they absorb the light rays earlier because plastic is not as clear as glass.
Multi-mode fiber is designed for coupling light from low cost LED3-based transmitters. Single-mode fiber is only suitable for laser-based transmission.
There are varieties of connectors (FDDI-MIC, ST, SC-Duplex etc.) for MMF as well as for SMF. Be sure to use the same diameter and connectors throughout your infrastructure. Project deadlines are easily missed when plugs at fibers do not fit the active components, and adapter cables are not on hand.
Fiber optics cables can be used for all current network technologies. In comparison to network connections made of metal, fiber optic cables have numerous advantages:
- Optical fibers enable a larger bandwidth than copper cables.
- Optical fibers are not sensitive to electromagnetic radiation and don't emit any by themselves. This means that all regulations are meet by default.
- Optical fibers are immune to lightning strikes and power line transients.
- Metal free Optical fibers cannot generate any ground loops.
- Optical fibers are much thinner and lighter in weight than metal wires.
- It is very difficult to tap eavesdrop on optical fibers without being noticed, making this very secure from electronic eavesdropping.
- Fiber optics cable connectors are high precision parts. An exact alignment of the fiber inside the connector housing and proper polish of the fiber end is crucial for connectivity quality.
- Installing connectors on site is time consuming and requires high precision work. The alternative of fusion welding strands with prefabricated connectors (called pigtails) on site needs expensive equipment.
- Together with the higher costs for the cable itself, the deployment of fiber-optic cable costs more than twice that of a category 5 copper connection. The "per port" price of active fiber optic components (hubs, switches, router modules) is typically twice the price of their copper counterparts. The prices for long range single-mode cables and components are even higher than for multi-mode
- Glass fibers are more fragile than wire and sensitive and age under the impact of hydrogen ions. They must therefore be protected against moisture through special coatings. However, this protective layer is also subject to aging.
Case Example: Mice in Cable Conduit
| In a company, a complete administration building was suddenly without network based IT services. The reason behind this was a fiber optics cable that had been gnawed through in a cable conduit. Rodents like to build their houses in cable conduits, and their offspring like to test their teeth on the cables. Since fiber optics cables can only be spliced by using special tools, this led to a downtime lasting several days. Even rodent-safe cables and mousetraps are therefore investments that increase availability of enterprise service architectures and IT services in general.
11.2.3 Installation Guidelines for Cable Networks
Good craftsmanship, together with using high quality components, has a direct relationship to how long your cabling infrastructure will last. As mentioned before, high-speed data links have higher demands than plain-old telephone lines. To make matters worse, the effects of poor installation work may not be immediately evident!
Deformation or mechanical stress during installation causes most cable failures. Deformation changes the physical properties responsible for high frequency transmission. Even when the cable looks flawless from the outside, irreversible degradation of transmission properties is suspected when too much stress is applied to the cable. Mechanical stresses as well as temperature levels are part of the ISO/IEC 11801 standard "Generic Cabling for Customer Premises." The installation of network infrastructures should be dedicated to certified contractors, familiar to the special demands of data cabling.
However, even with equipment, which has been checked and conforms to the standards, problems still exist, as with increased demands on the networks, the reserves in the transfer parameters decrease.
Visual inspections An example is the standard of fitting the data cables in the connection components. The standard for this does provide for a visual inspection, but this is rarely carried out. Generally, people content themselves with the test logs created by cable scanners. For instance, in distributor panels where the cable sections that have been stripped of the isolation are narrowly guided along the blank wire ends of the neighboring cable, this can lead to a short circuit between the wire and the foil shield if the latter changes, for example, because the temperature of the cabinet interior increases. During an acceptance test on cabling, it is therefore critical that you perform visual inspections.
With fiber-optic cables, you can run into problems later on as well, even if they have been installed according to the standards. Here the problems can mainly be found in the preconfigured plugs. These plugs are very sensitive to scratches, dust, and inept treatment when being mounted. We also recommend that you use a microscope when conducting checks. You can find additional information and further considerations when executing acceptance tests in Mißbach/Hoffmann,4 which is also worthwhile reading for small changes or enhancements carried out by your internal electrician.
11.3 Potential Equalization, Grounding, and Lightning Protection
Frequently, sporadic disturbances and breakdowns occur in data networks without a clear reason. Connections become extremely slow, monitor screens flicker, assemblies burn through or after a thunderstorm, and entire facilities break down at once. Furthermore, individual employees may be marginalized, because strangely it is only always their PCs that go "mad."
In many cases, however, the real reason for these breakdowns can be found in the potential equalization and grounding. Apart from the data network cabling in every building, there is also cabling for the power supply. People often overlook the fact that these two cable networks are linked via the grounding, and massive disturbances of the data networks can occur if the power supply network is not designed as IT-compatible.
In order to ensure an electricity flow, a wire (L) is required from the electricity source to the consumer, as well as a retracting wire (N). A third wire is stipulated as a ground wire or protective earth conductor (PE). As the 230V alternating current is tapped from a 380V three-phase network, this results in a 5-wire network or TN-S system with three conducting phases and a common neutral and ground wire for each phase.
The security function of the protective earth conductor is ensured, even if the protective earth conductor is connected to the neutral cable that is also grounded, and thereby forms a "combined" PEN conductor (that is, a retracting circuit (N) plus a protective earth conductor (PE)). Such couplings are permitted and are commonly used in building installations, because this means that a cable can be saved. This form of network is called a 4-cable network or TN-C system. This variant has no negative effects on the lamps connected, whereas, when connecting other electronic appliances, this can lead to considerable problems.
Since an increasing number of electronic pre-connection units are being used for fluorescent tubes and switching power supplies for computers, the current flow is not sinusoidal. Instead, it contains considerable high frequency components. These can cause parasitic currents of several amperes, which results in a magnetic field that acts as if an irregularly functioning high-frequency transmitter is an integral part of the computer system.
To ensure a stable operation of the data network, an integrated 5-wire network with a clear grounding concept should be guaranteed. If necessary, a separate 230V power supply grid must be installed from the transformer of the sub-distributor. In this separate supply circuit, the neutral cable must not come into contact with the ground wire at any point (separate conduits for PE and N, so that no parasitic currents are possible through the data cables). The sockets for this network should be marked "only for IT devices." There should also be corresponding signs in the rooms of its installation to ensure that an inexperienced electrician does not create a connection between N and PE again.
In conjunction with a nearby lightning stroke, cable screens grounded on only one side, act as antennae into which high voltages are induced. The voltage will be discharged at the network board or the network socket, depending on where the disruption to the shield occurs. Even with a double-sided grounding due to the induction caused by a lightning stroke, a compensatory current flows in the data shield. This can lead to extensive destructions of the connected interfaces as a result of strong electric currents. Therefore, power sockets with integrated over voltage protectors are definitely advisable. The same is true for your house, where lightning bolts can induce voltage surges in the main telephone line, which destroy telephone systems and DSL routers, as one of the authors can attest from his own first-hand experience.
Ground loops represent another cause of parasitic currents, and one often overlooked. In the network cabling system, problems with ground loops occur mainly when shielded twisted-pair cables (STP) are used between parts of a building complex. If these parts have different earth potential, parasitic currents of several amperes can flow over the shield if grounded on both ends. These ground loops can lead to a degradation of network performance, and even to a damage of network components.
When installing STP cables, you must ensure that grounding occurs only at one end of the grounded link. For UTP cables, ground loops are not an issue, because of the design of these cables. Fiber-optic cables are the most secure way of eliminating damage caused by parasitic currents of any type.
| The insulation of data cables generally consists of flammable synthetic materials, mainly polyethylene and PVC. To reduce the threat to people and material, Building Codes generally require cables that does not generate toxic fumes when burning, such as FEP (fluoro-ethylene polymer) in air ducts, air plenums, and other environmental air spaces.
In the event of a fire, these cables also don't generate corrosive gases and the smoke gas density is considerably lower. Therefore, PVC cables are banned in building installations in an increasing number of countries.
It is for good reason that building insurers in recent years placed major emphasis on the issue of fire protection. According to the insurers, all cable conduits in ceilings that run at a right angle to an emergency route must have a full fire protection. This also applies to all later changes. You should therefore always ensure that the cable installations are in accordance with the regulations of the property insurers.
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