NETWORKING CONCEPTS

The Way of a WAN

To at least some extent, WANs are defined by their methods of transmitting data packets. True, the means of communication must be in place. True, too, the networks making up the WAN must be up and running. And the administrators of the network must be able to monitor traffic, plan for growth, and alleviate bottlenecks. But in the end, part of what makes a WAN a WAN is its ability to ship packets of data from one place to another, over whatever infrastructure is in place. It is up to the WAN to move those packets quickly and without error, delivering them and the data they contain in exactly the same condition they left the sender, even if they must pass through numerous intervening networks to reach their destination.
Picture, for a moment, a large network with many subnetworks, each of which has many individual users. To the users, this large network is (or should be) transparent—so smoothly functioning that it is invisible. After all, they neither know nor care whether the information they need is on server A or server B, whether the person with whom they want to communicate is in city X or city Y, or whether the underlying network runs this protocol or that one. They know only that they want the network to work, and that they want their information needs satisfied accurately, efficiently, and as quickly as possible.
Now picture the same situation from the network's point of view. It "sees" hundreds, thousands, and possibly even tens of thousands of network computers or terminals and myriad servers of all kinds—print, file, mail, and even servers offering Internet access—not to mention different types of computers, gateways, routers, and communications devices. In theory, any one of these devices could communicate with, or transmit information through, any other device. Any PC, for instance, could decide to access any of the servers on the network, no matter whether that server is in the same building or in an office in another country. To complicate matters even more, two PCs might try to access the same server, and even the same resource, at the same time. And of course, the chance that only one node anywhere on the network is active at any given time is minuscule, even in the coldest, darkest hours of the night.
So, in both theory and practice, this widespread network ends up interconnecting thousands or hundreds of thousands of individual network "dots," connecting them temporarily but on demand. How can it go about the business of shuffling data ranging from quick e-mails to large (in terms of bytes) documents and even larger graphic images, sound files, and so on, when the possible interconnections between and among nodes would make a bowl of spaghetti look well organized by comparison? The solution is in the routing, which involves several different switching technologies.
Switching of any type involves moving something through a series of intermediate steps, or segments, rather than moving it directly from start point to end point. Trains, for example, can be switched from track to track, rather than run on a single, uninterrupted piece of track, and still reach their intended destination. Switching in networks works in somewhat the same way: Instead of relying on a permanent connection between source and destination, network switching relies on series of temporary connections that relay messages from station to station. Switching serves the same purpose as the direct connection, but it uses transmission resources more efficiently.
WANs (and LANs, including Ethernet and Token Ring) rely primarily on packet switching, but they also make use of circuit switching, message switching, and the relatively recent, high-speed packet-switching technology known as cell relay.

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Circuit Switching

Circuit switching involves creating a direct physical connection between sender and receiver, a connection that lasts as long as the two parties need to communicate. In order for this to happen, of course, the connection must be set up before any communication can occur. Once the connection is made, however, the sender and receiver can count on "owning" the bandwidth allotted to them for as long as they remain connected.
Although both the sender and receiver must abide by the same data transfer speed, circuit switching does allow for a fixed (and rapid) rate of transmission. The primary drawback to circuit switching is the fact that any unused bandwidth remains exactly that: unused. Because the connection is reserved only for the two communicating parties, that unused bandwidth cannot be "borrowed" for any other transmission.
The most common form of circuit switching happens in that most familiar of networks, the telephone system, but circuit switching is also used in some networks. Currently available ISDN lines, also known as narrowband ISDN, and the form of T1 known as switched T1 are both examples of circuit-switched communications technologies.

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Message Switching

Unlike circuit switching, message switching does not involve a direct physical connection between sender and receiver. When a network relies on message switching, the sender can fire off a transmission—after addressing it appropriately—whenever it wants. That message is then routed through intermediate stations or, possibly, to a central network computer. Along the way, each intermediary accepts the entire message, scrutinizes the address, and then forwards the message to the next party, which can be another intermediary or the destination node.
What's especially notable about message-switching networks, and indeed happens to be one of their defining features, is that the intermediaries aren't required to forward messages immediately. Instead, they can hold messages before sending them on to their next destination. This is one of the advantages of message switching. Because the intermediate stations can wait for an opportunity to transmit, the network can avoid, or at least reduce, heavy traffic periods, and it has some control over the efficient use of communication lines.

Packet Switching

Packet switching, although it is also involved in routing data within and between LANs such as Ethernet and Token Ring, is also the backbone of WAN routing. It's not the highway on which the data packets travel, but it is the dispatching system and to some extent the cargo containers that carry the data from place to place. In a sense, packet switching is the Federal Express or United Parcel Service of a WAN.
In packet switching, all transmissions are broken into units called packets, each of which contains addressing information that identifies both the source and destination nodes. These packets are then routed through various intermediaries, known as Packet Switching Exchanges (PSEs), until they reach their destination. At each stop along the way, the intermediary inspects the packet's destination address, consults a routing table, and forwards the packet at the highest possible speed to the next link in the chain leading to the recipient.
As they travel from link to link, packets are often carried on what are known as virtual circuits—temporary allocations of bandwidth over which the sending and receiving stations communicate after agreeing on certain "ground rules," including packet size, flow control, and error control. Thus, unlike circuit switching, packet switching typically does not tie up a line indefinitely for the benefit of sender and receiver. Transmissions require only the bandwidth needed for forwarding any given packet, and because packet switching is also based on multiplexing messages, many transmissions can be interleaved on the same networking medium at the same time.

Connectionless and Connection-Oriented Services

So packet-switched networks transfer data over variable routes in little bundles called packets. But how do these networks actually make the connection between the sender and the recipient? The sender can't just assume that a transmitted packet will eventually find its way to the correct destination. There has to be some kind of connection—some kind of link between the sender and the recipient. That link can be based on either connectionless or connection-oriented services, depending on the type of packet-switching network involved.

·         In a (so to speak) connectionless "connection," an actual communications link isn't established between sender and recipient before packets can be transmitted. Each transmitted packet is considered an independent unit, unrelated to any other. As a result, the packets making up a complete message can be routed over different paths to reach their destination.

In a connection-oriented service, the communications link is made before any packets are transmitted. Because the link is established before transmission begins, the packets comprising a message all follow the same route to their destination. In establishing the link between sender and recipient, a connection-oriented service can make use of either switched virtual circuits (SVCs) or permanent virtual circuits (PVCs):

o        Using a switched virtual circuit is comparable to calling someone on the telephone. The caller connects to the called computer, they exchange information, and then they terminate the connection.

INTRODUCTION TO SYSTEM ENGINEERING

SYSTEM ENGINEERING CONCEPT

Computers are fast becoming our way of life and one cannot imagine life without computers in today’s world. You go to a railway station for reservation, you want to web site a ticket for a cinema, you go to a library, or you go to a bank, you will find computers at all places. Since computers are used in every possible field today, it becomes an important issue to understand and build these computerized systems in an effective way.

Building such systems is not an easy process but requires certain skills and capabilities to understand and follow a systematic procedure towards making of any information system. For this, experts in the field have devised various methodologies. Waterfall model is one of the oldest methodologies. Later Prototype Model, Object Oriented Model, Dynamic Systems Development Model, and many other models became very popular for system development. For anyone who is a part of this vast and growing Information Technology industry, having basic understanding of the development process is essential. For the students aspiring to become professionals in the field a thorough knowledge of these basic system development methodologies is very important.
In this web site we have explored the concepts of system development. The web site starts with the system concepts, making the reader understand what does system mean in general and what are information systems in specific. The web site then talks about the complete development process discussing the various stages in the system development process. The different types of system development methodologies, mentioned above, are also explained.
This tutorial is for beginners to System Analysis and Design (SAD) Process. If you are new to computers and want to acquire knowledge about the process of system development, then you will find useful information in this tutorial. This tutorial is designed to explain various aspects of software development and different techniques used for building the system. This tutorial is a good introductory guide to the need and overall features of software engineering.
This tutorial is designed to introduce Software Engineering concepts to the upcoming software professionals. It assumes that its reader does not know anything about the system development process. However it is assumed that the reader knows the basics of computers.

What is Software Engineering?

Software Engineering is the systematic approach to the development, operation and maintenance of software. Software Engineering is concerned with development and maintenance of software products.
The primary goal of software engineering is to provide the quality of software with low cost. Software Engineering involves project planning, project management, systematic analysis, design, validations and maintenance activities.

Analysis, design, and development systems, products, or services requires answering several fundamental questions:

·         WHAT is a system?

·         What is included within a system’s boundaries?

·         WHAT role does a system perform within the User’s organization?

·         What mission applications does the system perform?

·         WHAT results-oriented outcomes does the system produce?

These fundamental questions are often difficult to answer. If you are unable to clearly and concisely delineate WHAT the system is, you have a major challenge. Now add the element of complexity in bringing groups of people working on same problem to convergence and consensus on the answers. This is a common problem shared by Users, Acquirers, and System Developers, even within their own organizations.
At the end of this lesson you would be able to know about system's concepts, characteristics and various types of Information Systems. You would also be able to understand the system development process.

People often confuse the concepts of systems, products, and tools. To facilitate our discussion, let’s examine each of these terms in detail.

System Context

We defined the term system earlier in this section. A system may consist of two or more integrated elements whose combined—synergistic—purpose is to achieve mission objectives that may not be effectively or efficiently accomplished by each element on an individual basis. These systems typically include humans, products, and tools to varying degrees. In general, human-made systems require some level of human resources for planning, operation, intervention, or support.

Product Context

Some systems are created as a work product by other systems. Let’s define the context of product: a product, as an ENABLING element of a larger system, is typically a physical device or entity that has a specific capability—form, fit, and function—with a specified level of performance.
Products generally lack the ability—meaning intelligence—to self-apply themselves without human assistance. Nor can products achieve the higher level system mission objectives without human intervention in some form. In simple terms, we often relate to equipment-based products as items you can procure from a vendor via a catalog order number. Contextually, however, a product may actually be a vendor’s “system” that is integrated into a User’s higher-level system. Effectively, you create a system of systems (SoS).
Example
1. A hammer, as a procurable product has form, fit, and function but lacks the ability to apply its self to hammering or removing nails.
2. Ajet aircraft, as a system and procurable vendor product, is integrated into an airline’s system and may possess the capability, when programmed and activated by the pilot under certain conditions, to fly.

Tool Context

Some systems or products are employed as tools by higher level systems. Let’s define what we mean by a tool. A tool is a supporting product that enables a user or system to leverage its own capabilities and performance to more effectively or efficiently achieve mission objectives that exceed the individual capabilities of the User or system.
Example
1. A simple fulcrum and pivot, as tools, enable a human to leverage their own physical strength to displace a rock that otherwise could not be moved easily by one human.
2. A statistical software application, as a support tool, enables a statistician to efficiently analyze large amounts of data and variances in a short period of time.

HEATING ,COOLING AND LOAD CALCULATION

Heating, Cooling Loads and Energy The design of any commercial building HVAC system requires a licensed professional engineer and must be done according to all other aspects of the building as a system. Evaluating commercial building loads is complex and usually time consuming. A number of software programs are available to help designers proceed with this evaluation. However, preliminary design for simple buildings can still be evaluated using hand calculations or rudimentary spreadsheet programs Heating and Cooling Load Calculation The first step in the sizing process usually involves calculating each zone’s peak heating and cooling load as well as the whole-building peak loads. The following factors typically need to be considered when performing these calculationsSolar gains through windows: Standard double-glazed windows can let up to 75 percent of this energy penetrate the building, where it becomes a cooling load. Additional window treatments such as tinted and reflective glazing, shading and draperies can further reduce solar gains. Internal gains from occupants (including latent heat for cooling purposes): Each adult will typically generate about 75 W of sensible energy and 55 W of latent energy. Internal gains from lighting and equipment: Lighting power is often about 20 W/m² in office buildings but can be as high as 40 to 50 W/m². The equipment load (also called plug-load) is often in the 2- to 5-W/m² range but can be as high as 15 to 20 W/m². Outside air loads (sensible and latent) from ventilation and infiltration: All buildings should meet at least the minimum outside air requirements imposed in their local jurisdiction. The amount of outside air is often taken from ASHRAE Standard 62. A typical value for outside airflow rate is 15 L/s/occupant. Heat gains or losses through windows, walls, floors and roofs: These gains are mostly important for heating load calculation but may still have some impact for the cooling load, especially the windows, and heat gain. The amount of heat transfer through these components can be estimated using the following formula: Heat gain/loss = Area X (surface temperature outside – surface temperature inside)/RSI value. Using the very simplified values and formulas presented here can help in getting a rough estimate of a zone heating and cooling load. Some other important points to know about load calculations are as follows: The heating load calculation must be done without credit for occupants and internal gains, since this load usually occurs at night. Zone loads are calculated with consideration only to the zone’s peak gains (i.e., solar) or losses (for heating). Each zone’s peak loads may occur at different moments. However, for hand calculations, cooling loads are usually calculated during the hottest day of the summer for three different times for each zone. The greatest of the calculated loads are selected as the zone peak loads. Heating loads need only to be evaluated at the heating design temperature, since no credit for solar gain or internal gain is considered. However, since some areas in the core of a building may require cooling at all times, these zones may need to consider internal gain even under winter design conditions. Whole-building loads are calculated considering all zones’ loads. The whole-building peak loads may not occur at the same moment as that of any of its zones. Precise determination of the time of occurrence of the whole-building load requires either extensive hand calculations or, more realistically, an hourly computer simulation. Approximate cooling block load can be estimated using the greatest of the sum of all zone loads for the three time periods previously considered. Design temperatures must be obtained from a reliable source, such as ASHRAE Handbook Fundamentals, 2001. Typical values for building heating load range from 20 to 120 W/m². Cooling loads generally vary from 50 W/m² for buildings in cool climates with little internal gains to 200 W/m² or more for commercial buildings in hot climates with high internal gains. For a thorough calculation of the zones and whole-building loads, one of the following three methods presented in ASHRAE should be employed: Transfer Function Method (TFM): This is the most complex of the methods proposed by ASHRAE and requires the use of a computer program or advanced spreadsheet. Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF): This method is derived from the TFM method and uses tabulated data to simplify the calculation process. The method can be fairly easily transferred into simple spreadsheet programs but has some limitations due to the use of tabulated data. Total Equivalent Temperature Differential/Time-Averaging (TETD/TA): This was the preferred method for hand or simple spreadsheet calculation before the introduction of the CLTD/CLF method.

                             Networking Cable

The networking cable is also known as the network media. The network cable is a channel through which data flows within a network. It carries the electrical pulses (digital signals) from one computer to another, or to any other peripheral attached to the network.

There are two broad categories of networking cables namely:

·         The bounded/ guided media.

·         The unbounded/ unguided media. (boundless)

1. THE BOUNDED MEDIA

In the bounded media the signal is contained inside a physical cable. There are three common types of bounded media namely:-

• Coaxial cable
• Twisted pair cable
• Fiber optic cable

A. THE COAXIAL CABLE

The coaxial cable consists of a solid or stranded copper core (a central conducting core), surrounded by a dielectric (special insulator), a braided or woven copper mesh shielding layer (which is connected to signal ground and it absorbs Electromagnetic Interference-EMI) and a protective outer covering (insulating jacket). All these layers are concentric around a common axis thus the name coaxial.
Surrounding the core is a dielectric insulating layer that separates it from the wire mesh. The braided wire mesh acts as a ground and protects the core from electrical noise and crosstalk. (Crosstalk is signal overflow from an adjacent wire). Coaxial cable is largely immune to electrical interference and can carry data at higher rates over long distances than twisted pair cable.
The conducting core and the wire mesh must always be kept separate from each other. If they touch, the cable will experience a short, and noise or stray signals on the mesh will flow onto the copper wire. An electrical short occurs when any two conducting wires or a conducting wire and a ground come into contact with each other. This contact causes a direct flow of current (or data) in an unintended path. In the case of household electrical wiring, a short will cause sparking and the blowing of a fuse or circuit breaker. With electronic devices that use low voltages, the result is not as dramatic and is often undetectable. These low-voltage shorts generally cause the failure of a device; and the short, in turn, destroys the data.
A non-conducting outer shield—usually made of rubber, Teflon, or plastic—surrounds the entire cable this is for normal cable protection.

• The coaxial cable is more resistant to interference and attenuation than twisted pair cabling.
• It transmits voice, video, and data.

Types of Coaxial Cable
There are two types of coaxial cable namely:

1. Thinnet cable
2. Thicknet cable

Features of Thinnet Cable/10Base2 Ethernet:

• 10 refers to the rate of data transfer. It transfers data at the rate of 10Mbps (Megabits per second)
• 2 refers to distance allowed between computers it should be no more than 2 meters.
• Total segment length is 185 Metres (Distance between the farthest computers).
• Total number of nodes (devices) connected - 30 nodes per trunk.
• Thinnet cable is a flexible coaxial cable about 0.64 centimeters (0.25 inches) thick.

Features of Thicknet Cable / 10Base5 Ethernet

• 10 refer to the rate of data transfer. It transfers data at the rate of 10Mbps (Megabits per second)
• 5 refer to distance between computers it should be no more than 5 meters.
• A maximum of 100 workstations is allowed per trunk and the distance between should be a maximum of 5 meters.
• Segment length is 500 metres.
• Thicknet cable is a relatively rigid coaxial cable about 1.27 centimeters (0.5 inches) in diameter.

B. THE TWISTED-PAIR CABLE

• This netwok cable type consists of a number of insulated strands of copper wire twisted around each other. The twisting cancels out electrical noise from adjacent pairs (cross talk) and from other sources such as motors, relays, and transformers.
• The twisted-pair wires are often grouped together and enclosed in a protective sheath to form a cable. The total number of pairs in a cable varies.
• They make use of RJ-45 telephone-type connectors (larger than telephone and consists of eight wires vs. Telephone’s 4 wires).
• They are generally inexpensive.
• They are easy to install.

There are two types of Twisted Pair Cables:-

1. Unshielded twisted pair
2. Shielded twisted pair

Unshielded Twisted Pair (UTP)

The Unshielded Twisted Pair (UTP) /10 base T
This cable type uses the10 base T specifications, it is the most popular type of twisted-pair cable and is fast becoming the most popular for LAN cabling. These pairs are typically colour-coded to distinguish them. The maximum cable length segment is 100 meter, about 328 feet. If you exceed this segment length limitation, attenuation occurs.
The 568A Commercial Building Wiring Standard of the Electronic Industries Association and the Telecommunications Industries Association (EIA/TIA) specifies the type of UTP cable that is to be used in a variety of building and wiring situations. The objective is to ensure consistency of products for customers. These standards include five categories of UTP: The higher the grades number the more immune to the interference and the faster it can accurately transmit data, the categories are as follows:-
Categories

• Cat 1 Voice grade telephone cable.
• Cat 2 Data grade up to 4 Mbps, four twisted pairs.

Category 3 and above is needed for Ethernet networks. Cat 3, 4, and 5 use RJ-45 connectors

• Cat 3 Data grade up to 16 Mbps, four pairs.
• Cat 4 Data grade up to 20 Mbps, four twisted pairs.
• Cat 5 Data grade up to 100 Mbps, four twisted pairs.
• Cat 5e Data grade up to 100 Mbps, four twisted pairs.
• Cat 6 Data grade up to 1000 Mbps, four twisted pairs.

Advantages of using UTP cable

• Less vulnerable to network failures.
• UTP cable is the least costly of any cable type.

Drawbacks

1. A network using UTP cables requires distribution of hubs.
2. It requires more cabling.
3. UTP is particularly susceptible to crosstalk, which is when signals from one line get mixed up with signals from another.
4. Easily tapped (because there is no shielding).
5. 100 meters is maximum distance between the furthest devices, so attenuation is the biggest problem while using UTP cables.

Shielded Twisted Pair (STP)
Features of the STP

1. Uses a woven copper braid jacket and a higher quality protective jacket. Also uses foil wrap between and around the wire pairs.
2. Much less susceptible to interference and supports higher transmission rates than UTP.
3. Shielding makes it somewhat harder to install.
4. It has got the same 100 meter limit as UTP.
5. It is harder to tap.
6. Used in AppleTalk and Token Ring networks.

C. OPTICAL MEDIA
FIBER OPTIC CABLE
In many parts of Africa for example Kenya, the use of fiber cable to route internet communication to the rest of the world is being implemented gradually. This is is a very positive development because we expect internet cost to come down and to be accessible by everyone regardless of whether you are in the town or at the village level.
So what is this fiber optic cable?
An optic fiber cable consists of an extremely thin cylinder of glass called the core that is surrounded by a concentric layer of glass called a cladding. Optical fiber carries digital signals in the form of modulated pulses of light along a flexible glass tube. It does not use electricity, except to power the transmitting and receiving circuitry at either end.
The outer jacket is for protection while the cladding is used to reflect light signals back into the waveguide.
The center conductor of a fiber-optic cable is a fiber that consists of highly refined glass or plastic designed to transmit light signals with little loss. A glass core supports a longer cabling distance, but a plastic core is typically easier to work with. The fiber is coated with a cladding or a gel that reflects signals back into the fiber to reduce signal loss. A plastic sheath protects the fiber.
Unlike the other two types of cables, fiber optic cables do not leak signals and are immune to electromagnetic interference. They support greater bandwidth and can transmit data up to a maximum of 2km without the need of repeaters to regenerate the signals. However, they are expensive to buy and install.
The fiber optic strands transfer light in a single direction at a time. Hence 2 strands are placed in each cable to allow simultaneous transmission and reception at the same time.
A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines.
At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses.
A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are funneled into the fiber-optic medium where they transmit themselves down the line.
Light pulses move easily down the fiber-optic line because of a principle known as total internal reflection.“This principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in”. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses.

VIEW OF FIBER OPTIC CABLE ADVANTAGES OVER COPPER

VIEW OF FIBER OPTIC CABLE ADVANTAGES OVER COPPER

In recent years it has become apparent that fiber-optics are steadily replacing copper wire as an appropriate means of communication signal transmission. They span the long distances between local phone systems as well as providing the backbone for many network systems. Other system users include cable television services, university campuses, office buildings, industrial plants, and electric utility companies.
A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. Looking at the components in a fiber-optic chain will give a better understanding of how the system works in conjunction with wire based systems.
At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are funneled into the fiber-optic medium where they travel down the cable. The light (near infrared) is most often 850nm for shorter distances and 1,300nm for longer distances on Multi-mode fiber and 1300nm for single-mode fiber and 1,500nm is used for for longer distances.
Think of a fiber cable in terms of very long cardboard roll (from the inside roll of paper towel) that is coated with a mirror on the inside.
If you shine a flashlight in one end you can see light come out at the far end - even if it's been bent around a corner.
Light pulses move easily down the fiber-optic line because of a principle known as total internal reflection. "This principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses. The core must a very clear and pure material for the light or in most cases near infrared light (850nm, 1300nm and 1500nm). The core can be Plastic (used for very short distances) but most are made from glass. Glass optical fibers are almost always made from pure silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for longer-wavelength infrared applications.



There are three types of fiber optic cable commonly used: single mode, multimode and plastic optical fiber (POF).

Transparent glass or plastic fibers which allow light to be guided from one end to the other with minimal loss.
Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable through to the other end. The light source can either be a light-emitting diode (LED)) or a laser.
The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable converts the pulses back into the digital ones and zeros of the original signal.
Even laser light shining through a fiber optic cable is subject to loss of strength, primarily through dispersion and scattering of the light, within the cable itself. The faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners, called repeaters, may be necessary to refresh the signal in certain applications.
While fiber optic cable itself has become cheaper over time - a equivalent length of copper cable cost less per foot but not in capacity. Fiber optic cable connectors and the equipment needed to install them are still more expensive than their copper counterparts.
Single Mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission.  Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. Synonyms mono-mode optical fiber, single-mode fiber, single-mode optical waveguide, uni-mode fiber.
Single Modem fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fiber)
Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.  

Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.
Multi-Mode cable has a little bit bigger diameter, with a common diameters in the 50-to-100 micron range for the light carry component (in the US the most common size is 62.5um). Most applications in which Multi-mode fiber is used, 2 fibers are used (WDM is not normally used on multi-mode fiber).  POF is a newer plastic-based cable which promises performance similar to glass cable on very short runs, but at a lower cost.
Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.  
The use of fiber-optics was generally not available until 1970 when Corning Glass Works was able to produce a fiber with a loss of 20 dB/km. It was recognized that optical fiber would be feasible for telecommunication transmission only if glass could be developed so pure that attenuation would be 20dB/km or less. That is, 1% of the light would remain after traveling 1 km. Today's optical fiber attenuation ranges from 0.5dB/km to 1000dB/km depending on the optical fiber used. Attenuation limits are based on intended application.
The applications of optical fiber communications have increased at a rapid rate, since the first commercial installation of a fiber-optic system in 1977. Telephone companies began early on, replacing their old copper wire systems with optical fiber lines. Today's telephone companies use optical fiber throughout their system as the backbone architecture and as the long-distance connection between city phone systems.
Cable television companies have also began integrating fiber-optics into their cable systems. The trunk lines that connect central offices have generally been replaced with optical fiber. Some providers have begun experimenting with fiber to the curb using a fiber/coaxial hybrid. Such a hybrid allows for the integration of fiber and coaxial at a neighborhood location. This location, called a node, would provide the optical receiver that converts the light impulses back to electronic signals. The signals could then be fed to individual homes via coaxial cable.
Local Area Networks (LAN) is a collective group of computers, or computer systems, connected to each other allowing for shared program software or data bases. Colleges, universities, office buildings, and industrial plants, just to name a few, all make use of optical fiber within their LAN systems.
Power companies are an emerging group that have begun to utilize fiber-optics in their communication systems. Most power utilities already have fiber-optic communication systems in use for monitoring their power grid systems.

INTRODUCTION TO FIBRE OPTIC CABLE

INTRODUCTION TO FIBRE OPTIC CABLE
Fibre optics have found many uses in a variety of industries, but nowhere has it had such a profound effect as it has in telecommunications. Originally considered by many to be a prohibitively expensive technology in search of practical applications, it has now transformed the very infrastructure of PTOs. It has achieved this because of two very simple advantages it has over copper: (1) the ability to transmit data at higher transmission rates and with lower losses and, (2) the ability to do this at lower error rates. It should not be forgotten that it is only because of the widespread uptake of fibre optic transmission that many of the new high data-rate protocols such as, frame relay, SMDS, SDH and ATM have, or will, be made possible. Although fibre optics are at the bottom of the telecommunication value-added chain it is one, if not the, of the major technology drivers of our industry. This edition of Technology Watch looks at the basics of fibre optic technology and the changes that are currently impacting our business.

Copper Cables

Analogue and digital data has been transmitted from point-to-point using copper cable in a variety of forms for decades. The two major types used for the computer and telecommunication industries are twisted pair and coaxial cable. A twisted pair consists of a pair of individually insulated conductors twisted together. Performance of this type of cable is considerably improved by wrapping the pair in a copper sheaf which also provides screening against outside interference. Coaxial cable on the other hand, consists of a central copper conductor surrounded by a single or multiple braided/solid copper outer covering. This construction provides much better transmission characteristics than twisted pair. Some of the major limitations of copper based cables are:
High signal attenuation. Although coaxial cables are an order better that twisted pair in terms of signal attenuation, this is achieved by an increase in bulk. Low-loss coaxial cables have diameters between 1 and 3 cm and become progressively more costly and unwieldy as the diameter increases. Further, loss is frequency dependent in copper conductors: the higher the data rate used, the higher the losses. In practice, transmitting data at rates beyond a few hundred Mbit/s is just not cost competitive. Whereas with fibre optic cables attenuation is virtually flat up to rates of several Gbit/s providing sufficient bandwidth for even the highest data rates as needed in the near future such as the 2.4Gbit/s SDH rate.
Interference sensitivity. Copper cables are prone to electromagnetic interference such as that created when switching on a piece of electrical equipment. This can create high error rates in data links (explaining why X.25 error correction protocol was developed). The reverse is also true, signals within the cable radiate from its entire length making it very easy to eavesdrop on what is being transmitted. In contrast, fibre optic cable can be laid next to power distribution cables and suffer no interference at all.
Ground loops. Copper cables by default provide an electrical link between source and destination and as such can cause problems. Ground potentials can vary by several volts between sites at different locations, say, two office blocks. If not handled correctly, this can cause high DC currents to flow which will cause random data errors (it can cause 50Hz hum on HI-FI as well). Electrical storms can also induce large currents in a copper cable which cannot only cause data errors but can cause severe physical damage as well. Being an insulator, fibre optic cables do not suffer from this problem.

Fibre Optic Basics

A fibre optic cable consists of a glass silica core through which light is guided. This is covered with a material with a refractive index of slightly less than the core. This is called the cladding. The refractive index of the cladding need only be around 1% less than the core to achieve the total internal reflection necessary to confine the light to the core .
 Although it was stated earlier that the real benefit of fibre optic cable over copper was virtually infinite bandwidth, there are effects that limit the achievable bandwidth and the length of the cable that can be used before the need of a repeater.
Attenuation
Transmission of light by fibre optics is not 100% efficient. There are several reasons for this including absorption by the core and cladding (caused by the presence of impurities) and the leaking of light from of the cladding. When light reflects off the cladding /core interface it actually travels for a short distance within the cladding before being reflected back. This leads to attenuation (signal reduction) by up to 2db/Km for a multi-mode fibre. For example, with this level of attenuation, if light travelled over 10kM of cable only 10% of the signal would arrive at the following end.
The amount of attenuation for a given cable is also wavelength dependent. Figure 3 shows the attenuation profile for the two main types of fibre; multi-mode and single-mode cable. The absorption peak at 1000nm is caused by the peculiarities of single mode fibre while the peak at 1400nm is caused by traces of water remaining in the fibre as an impurity. Due to this water absorption peak there are two standard single-mode wavelengths in use, 1310nm and 1550nm. 1310nm has been a standard for many years, only now is there a trend towards using 1550nm brought about by the need to extend the distances between repeaters.
Modal Dispersion
An optical fibre is a form of waveguide (as encountered with microwave radio transmission) and light can travel through it in a stable number of ways or modes. This needs a little explanation as it is an important concept. A light ray entering a fibre at a specific angle will travel through the fibre with a specific number of hops or reflections following a particular path. If the angle of the light ray entering the fibre changes slightly it is possible that the light will follow a different path with fewer, or maybe more, hops or reflections. The greater the diameter of the core the more modes exist and thus there are more ways the light can travel through the fibre. This type of fibre is called multi-mode. As light rays travelling in each mode will travel a different distance they will arrive at the output at different times. This effect is called modal dispersion and is measured in picoseconds per nanometre per kilometre, or ps/nm/km. In practice it means that a step-edge light pulse will be severely degraded by the time it reaches the output. Put simply, dispersion limits the bandwidth of a fibre because as the data rate increases there is insufficient time for a logical '0' to recover to a logical '1' and vice-versa so the receiver is unable to differentiate between them. To reduce dispersion distortion, the number of modes the fibre supports must be reduced. This is achieved by reducing the diameter of the core as shown in Figure 5.
As the core of the fibre is reduced in diameter the situation is eventually reached where a light ray can only travel through the fibre via one path i.e. all light entering the fibre will traverse exactly the same path through the fibre before exiting at the same time - this is called a single-mode fibre. Because of this, the bandwidth of a single-mode fibre is much higher than that of a multi-mode fibre.

Fibre Types

The two main types of fibre in use today are step-index multi mode and step-index single mode fibre. The step-index part of the name can be understood by referring to Figure 5 which shows the cross-sections of these two types of fibre. Step-index refers to the abrupt change in refractive index between the core and cladding materials in contrast to graded-index fibres where refractive index changes gradually over the diameter of the fibre. Graded-index fibre will not be discussed here. Multi-mode fibres have cores of around 50µm and outside diameters of about 125µm. Single-mode fibre has a core reduced to below 10µm to allow only one mode of propagation to be supported.
Multi-Mode Fibre
Multi-mode fibre has the following characteristics:
The fibre can capture light from the light source and pass light to the receiver with high efficiency, so can be used with low-cost light emitting diodes (LEDs).
High precision connectors are not required because the large core diameter allows wide-tolerance on mechanics.
Low-cost comes at a cost! Multi-mode modal dispersion severely limits the usable bandwidth.
Multi-mode fibres suffer from higher losses than single mode fibres. For example, Mercury's Multimode fibre is specified at 0.8dB/km at a wavelength of 1310nm.
Multi-mode fibre has found some application in cost-sensitive areas such LAN (but even here it is too costly compared to copper solutions) and local-loop applications. But its poor bandwidth and high-loss characteristics has meant that its application in high-data rate PTO backbone links has been very limited. Indeed, with Mercury, only the original city fibre-optic network was deployed using multi-mode fibre. Since 1986, only single-mode fibre has been used.



Single-Mode Fibre
Single-mode fibre exhibits lower attenuation. Attenuation of Mercury's current single-mode fibre is specified at 0.37dB/km at 1310nm, in effect allowing a non-repeatered run to be increased by a factor of two over multi-mode fibre. The use of single-mode fibre completely eliminates modal-dispersion - the key cause of bandwidth limitation in multi-mode fibre optic fibre, but this does not mean that it has infinite bandwidth. What dispersion is left is called chromatic dispersion (so called as it is wavelength dependent). Chromatic dispersion is caused by the core material itself and is actually negative at short wavelengths and moves positive at longer wavelengths. This creates a 'magic' wavelength at which dispersion is actually zero.
This is, interestingly enough, at about 1310nm which explains the wide use of this particular wavelength . If 1310nm is used on a single-mode fibre it is easy to achieve a bandwidth of several Gbit/s with losses of around .37dB/km (Mercury's specification). Thus, in a single-mode fibre, attenuation is the limiting factor for long-distance transmission.
The characteristics of single-mode fibre are:
Bandwidth can be in the order of many Gbit/s with very low attenuation. This allows long-distance unrepeatered transmission up to around 50km.
The small diameter (10µm) of the core necessitates the use of expensive laser diodes to enable efficient light coupling and pass sufficient light into the fibre.
The small core diameter needs extremely precise connectors e.g. if two fibres are misaligned by only 1µm the overlap area is reduced by about 15% or attenuation equivalent to several km of fibre. Single-mode connectors are thus more expensive.
The performance of single-mode fibre is so good that it is the only type of fibre used for long distance links.

The Move to 1550 nm

Although the losses within silica based core materials are approaching theoretical limits, absorption by impurities keep them above those limits. Some impurities are inevitable as dopents need to be added to keep the refractive index of the core material above that of the cladding. A move was made to increase wavelength of the light source to 1550nm rather than use the more common wavelength of 1310nm. Indeed, this is what has happened, at a price. Attenuation drops from typically 0.35dB/km to 0.23dB/km, reducing attenuation by almost 35%. Looked at from another perspective, this equates to an increase in distance between repeaters in a long link from 50km to above 90km. In long links, the attenuation caused by splices becomes of more concern. Splices have typical attenuation values of 0.1dB which equates to around 0.4km of single-mode fibre!
Unfortunately, the price to be paid for lowered attenuation is an increase in chromatic dispersion. At 1310nm this is very near to zero, but at 1550nm it increases by a factor of six. Fortunately, even with that increase, it is still possible through careful system design to support the STM-16 SDH rates of 2.4Gbit/s over distances in excess of 100km.
A further way of reducing dispersion at 1500nm is to improve the quality of the laser-diode light sources used. Standard laser-diodes not only emit light at the primary or dominant wavelength but they also emit other wavelengths at the same time. These are close to the dominant wavelength but are at lower power levels. These sidelobes cause dispersion of digital signals being transmitted through a fibre optic fibre. Much work has gone into developing single-frequency lasers to minimise this effect.

Conclusions

Over the last few years fibre optic technology has advanced at a tremendous rate in a rather quiet and reserved manner driven by the need for higher bandwidths on long distance backbone links.
This performance enhancement has gone hand-in-hand with the development of suitable transmission and access methodologies such as SDH. The higher rates defined by SDH would not be possible without the improvements that have taken place in fibre optics.
Fibre optic technology is far from being plateaued. The next step will be coherent transmission systems that will improve the quality of fibre optic data transmission by at least an order of magnitude. In radio terms, it equates to the move from regenerative to superhet receivers and at the performance level it could be considered to equate to the move from PDH to SDH!

TYPES OF HVAC SYSYEM

Types of HVAC:

·                                 Central Heating

·                                 Forced Air Heating

·                                 Hydronic Heating

·                                 Hot Water Heating

·                                 Radiant Heating

·                                 Central Air Conditioning

·                                 Home Heating

HVAC Design Services include:

·                          Load Calculations (Cooling and Heating)

Using the ASHRAE standard to determine loads and air-flows. We ensure that both latent and sensible load components are accounted for in load calculations.

Refining Cost Estimation.

·                          HVAC Duct Design & its Layout

Determining the register locations and types, acoustic calculations, pressure drop calculations (internal and external), duct lengths and connections required to produce layout given construction constraints.

Specifying the material for ducts and their size according to SMACNA standards.

Design and layout of accessories such as diffuser, dampers, grills etc.

Layout of duct system on floor plan, accounting for the direction of joists, roof hips, fire-walls and other potential obstructions.

·                          Pipe sizing & its Layout

Calculating required water flow, size the pipes, decides fittings locations and dielectric isolators where needed to design the piping system.

Specifying piping material and criteria for pipe sizing.

Layout of piping system.

·                          HVAC Equipment Selection & Layout

Sizing HVAC equipments to sensible load using ASHRAE standards.

Selecting equipments like AHU, FCU, ACCU, Water Cooled Condenser, Air Cooled Condenser, Cooling Tower, Chiller Packages, DX Chiller, Flooded Chiller, Refrigerant Coil, Water Coil, Package Units, Pumps, Fans, Valves and other System Accessories and its layout.

·                          Schedule for Equipments

Chillers / DX Units and Air Handling Equipments Schedule.

Ducting Accessories (diffusers, dampers, grills etc) and Pump Schedule.

·                          Air Conditioning Design & Layouts

·                          Ductwork Design, Detailing & Drawings

·                          HVAC Consultation

INPUT

We accept wide variety of inputs. Inputs can be in the form of:

·                                 Design Guidelines

·                                 List of Symbols

·                                 Titled Blocks

·                                 Draftin/Drawing Standards

·                                 Schematic Design Report

·                                 Applicable Code List

·                                 Location, Orientation and Usage of Building

·                                 Elevation of Building

·                                 Architectural Layouts of Fixtures, Furniture, Cladding, etc

·                                 Reflected Ceiling Plan, Diffuser Location & Lighting Fittings

·                                 Cooling And Heating Equipment Types

HVAC System Design and Installation

The goal for a Heating, Ventilation and Air Conditioning (HVAC) system is to provide proper air flow, heating, and cooling to each room. This page sets out key criteria that describe a quality system, and key design and installation considerations that should be met to achieve this goal. The pages following contain more detailed information on design, fabrication, installation, and performance testing.

Criteria for a Quality HVAC System

An HVAC system should:

1. Be properly sized to provide correct air flow, and meet room-by-room calculated heating and cooling loads.
2. Be installed so that the static air pressure drop across the handler is within manufacturer and design specifications to have the capacity to meet the calculated loads.
3. Have sealed supply ductwork that will provide proper air flow.
4. Be installed with a return system sized to provide correct return air flow.
5. Have sealed return ductwork that will provide proper air flow to the fan, and avoid air entering the HVAC system from polluted zones (e.g., fumes from autos and stored chemicals, and attic particulates).
6. Have balanced air flows between supply and return systems to maintain neutral pressure in the home.
7. Minimize duct air temperature gain or loss between the air handler and room registers, and between return registers and the air handler.
8. Be properly charged with refrigerant.
9. Have proper burner operation and proper draft.

Procedures to Design and Install an Air Distribution System

The following steps should be followed in the design and installation of the HVAC system to ensure efficiency and comfort (for details, see Appendix 1):

1. Determine room-by-room loads and air-flows using ACCA Manual J calculation procedures (or substantially equivalent).
2. Layout duct system on floor plan, accounting for the direction of joists, roof hips, fire-walls, and other potential obstructions. Determine register locations and types, duct lengths, and connections required to produce layout given construction constraints.
3. Size duct system according to ACCA Manual D calculation procedures (or substantially equivalent).
4. Size HVAC equipment to sensible load using ACCA Manual S procedures (or substantially equivalent).
5. Install equipment and ducts according to design specifications, using installation requirements and procedures from the Uniform Mechanical Code, the Air Diffusion Council, SMACNA, California Residential Energy Efficiency Standards, and manufacturers' specifications (Title 24); Using these procedures and those in Appendix A, the duct system should be substantially air tight.
6. Charge the system appropriately, and verify charge with the evaporator superheat method or subcooling method (or substantially equivalent).
7. Check for proper furnace burner operation and fire-box drafting.
8. Test the system to ensure that it performs properly by determining (1) that the system is properly sized, (2) it does not leak substantially, and has either (3a) proper air handler fan flow, and proper plenum static pressures, or (3b) proper room and return air flows, and proper plenum static pressures. (Procedures are detailed in Appendix A.)

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MINIMUM MATERIALS SPECIFICATIONS

The following are minimum materials specifications recommended to achieve a substantially tight installation that will last:

All Materials

• Shall have a minimum performance temperature ratings per UL181 (ducts), UL181A (closure systems for rigid ducts), UL181B (closure systems for flexible ducts) and/or UL 181BM (mastic);
• Shall have a flame spread rating of no more than 25 and a maximum smoke developed rating of 50 (ASTM E 84)

Factory-Fabricated Duct Systems

• All factory-fabricated duct systems shall include UL 181 listed ducts with approved closure systems including collars, connections and splices;
• All pressure-sensitive and heat-activated tapes used in the manufacture of rigid fiberglass ducts shall be UL 181A listed;
• All pressure-sensitive tapes and mastics used in the manufacture of flexible ducts shall be UL 181B (tape) or UL 181BM (mastic) listed.

Field-Fabricated Duct Systems

• Ducts:
• Factory-made ducts for field-fabricated duct systems shall be UL 181 listed.
• Mastic sealants and mesh:
• Sealants shall be UL 181BM listed, non-toxic, and water resistant;
• Sealants for interior applications shall pass ASTM tests C 731 (extrudability after aging) and D 2202 (slump test on vertical surfaces);
• Sealants and meshes shall be rated for exterior use;
• Sealants for exterior applications shall pass ASTM tests C 731, C 732 (artificial weathering test), and D 2202.
• Pressure-sensitive tapes:
• Pressure sensitive tape shall be that recommended by and meet the requirements of the flexduct manufacturer;
• Tape used for duct board shall be UL 181A listed and so indicated with a UL 181A mark or aluminum-backed butyl adhesive tape (15 mil. minimum).
• Drawbands:
• Shall be either stainless-steel worm-drive hose clamps or uv-resistant nylon duct ties;
• Shall have a minimum performance temperature rating of 165 degrees Fahrenheit (continuous, per UL 181A-type test) and a minimum tensile strength rating of 50 pounds;
• Shall be tightened as recommended by the manufacturer with an adjustable tensioning tool.

DESIGN, FABRICATION AND INSTALLATION

The following are design, fabrication and installation guidelines, that, if carefully followed, will provide a duct installation that is substantially airtight:

General Issues

• Ducts, plenums, and fittings should be constructed of galvanized metal, duct board, or flexible duct. Building cavities may not be used as a duct or plenum without a sealed duct board or metal liner.
• The air handler box should be air-tight;
• Air filters should be easily accessible for replacement, and evaporator coils should be easily accessible for cleaning;
• Ducts should be configured and supported so as to prevent use of excess material, prevent dislocation or damage, and prevent constriction of ducts below their rated diameter;
• Flexible duct bends should not be made across sharp corners or have incidental contact with metal fixtures, pipes, or conduits that can compress or damage the ductwork;
• Sheet metal collars and sleeves should be beaded to hold drawbands.

DESIGN HVAC SYSTEM

Loads and CFM Calculation

• ACCA Manual J Load Calculation or equivalent required;
• Calculate heat loss and heat gain for each room;
• Total room loads to determine system requirements.

Lay Out Air Distribution System

• Lay out duct system on floor plan and determine register positions and duct paths to optimize room air circulation and minimize duct length;
• Duct paths must account for locations and directions of joists, roof hips, fire walls, and other potential obstructions;
• Duct paths must be planned to avoid sharp turns of flexduct that will kink the duct.

Size Air Distribution System

• ACCA Manual D Duct Design or equivalent required;
• Calculate correct cfm for each room and total for building for both supply and return;
• Size ducts according to Manual J loads, Manual D air flows, and final layout on plans;
• Choose registers to optimize air distribution and duct static pressure;
• Size and locate returns to optimize air flow per Manual D;
• For return-filter grills, calculate minimum return filter area per Manual D.

Select System

• ACCA Manual S Residential Equipment Selection or equivalent required. ACCA, 1515 16th St., NW, Washington, DC 20036, (202)483-9370;
• From Manual J loads and Manual D cfm, determine appropriate equipment
• Equipment should be sized to sensible loads;
• Equipment sensible capacity should not be more than 15% larger than the total sensible design load (as specified in Manual S).

FABRICATE AND INSTALL AN AIRTIGHT DUCT SYSTEM

All Duct Types

• All joints and seams of duct systems and their components should be sealed with mastic, mastic and embedded mesh, or pressure-sensitive tape approved for use by the duct manufacturer and meeting UL181 specifications ("approved tape"); this includes around junctions of collars to distribution boxes and plenums;
• All sealants should be used in strict accordance with manufacturer's installation instructions and within sealants moisture and temperature limitations;
• All tapes used as part of duct system installation should be applied to clean, dry surfaces and sealed with manufacturer's recommended amount of pressure or heat. If oil is present, taped surfaces should be prepared with a cleaner / degreaser prior to application;
• It is recommended that all register boxes should be sealed to the drywall or floor with caulking or mastic.

Flexible Ducts

• Flexible ducts should be joined by a metal sleeve, collar, coupling, or coupling system. At least 2 inches of the beaded sleeve, collar, or coupling must extend into the inner core while allowing a 1 inch attachment area on the sleeve, collar, or coupling for the application of tape;
• The inner core should be mechanically fastened to all fittings, preferably using drawbands installed directly over the inner core and beaded fitting. If beaded sleeves and collars are not used, then the inner core should be fastened to the fitting using #8 screws equally spaced around the diameter of the duct, and installed to capture the wire coil of the inner liner (3 screws for ducts up to 12" diameter, and 5 screws for ducts over 12" diameter);
• The inner core should be sealed to the fitting with mastic or approved tape;
• Tape used for sealing the inner core should be applied with at least 1 inch of tape on the duct lining, 1 inch of tape on the fitting of flange, and wrapped at least three times;
• The outer sleeve (vapor barrier) should be sealed at connections with a drawband and/or three wraps of approved tape;
• The vapor barrier should be complete. All holes, rips, and seams must be sealed with mastic or approved tape.

Metal Ducts and Plenums

• Metal-to-metal connections should be cleaned and sealed in accordance with manufacturer's specifications;
• Openings greater than 1/16 inch should be sealed with mastic and mesh, or butyl adhesive tape;
• Openings less than 1/16 inch should be sealed with mastic or UL-181A listed tape;
• Special attention should be paid to collar connections to duct-board and/or sheet metal; seal around the connection with mastic;
• Connections between collars and distribution boxes should be sealed with mastic or approved tape;
• At least three equally-spaced #8 screws should be used to mechanically fasten round ducts (3 screws for ducts up to 12" diameter, and 5 screws for ducts over 12" diameter);
• Crimp joints should have a contact lap of at least 1-1/2 inches;
• Square or rectangular ducts should be mechanically fastened with at least one screw per side.

Duct Board

• Duct board connections should be sealed with adhesive, mastic, or UL 181A listed pressure-sensitive or heat-activated tape in accordance with manufacturer's specifications.

Duct Support

• Supports should be installed per manufacturer's specifications or per UMC requirements;
• Supports for flexible ducts should be spaced at no more than 4 foot intervals;
• Flexible ducts should be supported by strapping having a minimum width of 1-1/2 inches at all contact points with the duct;
• Supports should not constrict the inner liner of the duct;
• Flexible ducts should have maximum of 1/2 inch sag per foot between supports;
• Flexible ducts may rest on ceiling joists or truss supports as long as they lie flat and are supported at no more that 4 foot intervals.

Boots

• After mechanically attaching the register boot to floor, wall, or ceiling, all openings between the boot and floor, wall, or ceiling should be sealed with caulk or mastic.

Seal Air Handler

• Openings greater than 1/16 inch should be sealed with mastic and mesh, or butyl adhesive tape;
• Openings less than 1/16 inch should be sealed with mastic or UL 181A listed tape;
• Unsealed access doors should be sealed with UL 181A listed tape.

CHECK REFRIGERANT CHARGE

• For systems with fixed metering devices use evaporator superheat method:
• Indoor coil airflow must be greater than 350 cfm/ton;
• Refrigerant system evacuation must be complete (all non-condensables must be removed from the system;
• In hot, dry climates be cautious to be within range of superheat charging chart or use a different method.
• For systems with thermostatic expansion valves use the subcooling method.

CHECK COMBUSTION PERFORMANCE

• Check each chamber for correct flame;
• Check for proper drafting.

REFERENCES

1991 Uniform Mechanical Code (UMC) Sections 1002 - 1005 and Appendix A, Standard No. 10-5.
Air Diffusion Council, Flexible Duct Performance & Installation Standards.
ACCA Manual J, Seventh Edition, 1986
ACCA Manual D, New Edition, 1995
ACCA, 1515 16th St., NW, Washington, DC 20036, 202-483-9370 ASTM E 84 Test for Surface Burning Characteristics of Building Materials
ASTM C 731 Extrudability After Aging
ASTM C 732 Artificial Weathering Test
ASTM D 2202 Slump Test on Vertical Surfaces
SMACNA Manual Installation Standards for Residential Heating and Air Conditioning Systems
UL Standard 181 Standard for Factory-Made Air Ducts and Air Connectors
UL Standard 181A Standard for Closure Systems for Use With Rigid Air Ducts and Air Connectors
UL Standard 181B Standard for Closure Systems for Use With Flexible Air Ducts (in progress)
UL Standard 181BM Standard for Mastic Materials
ACCA substantial equivalents:
ASHRAE, 1791 Tullie Circle, N.E., Atlanta, GA 30329, 404-636-8400