PLANT LAYOUT

PLANT LAYOUT

DEFINITION

Plant Layout is defined as, “A technique of locating machines, processes and plant services within the factory in order to secure greater possible output of high quality at the lowest possible total cost of production.”

Plant layout provides a broad framework within which production and all other activities have to take place. All facilities like equipment, raw-materials, machinery, tools, fixtures, finished goods, in-process inventories, workers and even scrap and waste etc., are given a proper place in the layout. The design of plant layout is a strategic decision and the analysis and planning of a sound plant layout is very important.

OBJECTIVES OF GOOD PLANT LAYOUT

Some of the important objectives of a good plant layout are:
1) Overall simplification of production process in terms of equipment utilization, minimization of delays, reducing manufacturing time and better provisions for maintenance.
2) Overall integration of man, materials, machinery supporting activities and other considerations in a way that results in the best compromise.
3) Minimization of material handling time and cost by suitably placing the facilities in the best flow sequence.
4) Effective space utilization.
5) Reduced inventory-in-process and easy availability of materials for assembly.
6) Better supervision and control.
7) Worker satisfaction and reduction fatigue.
8) Better working environment and present look to create the same.
9) Minimization of waste and higher productivity.
10) Avoid unnecessary capital investment.
11) Higher flexibility and adaptability to changing conditions.

DETERMINANTS OF LAYOUT

The following factors should be taken into consideration while determining the layout for a factory.

1) Type of Product
The type of product to be produced affects the layout strongly. The layout depends on whether the products are goods or services. If they are goods, then whether they are small or light, heavy or bulky or fragile. Layout designs depend on product designs and quality standards to be maintained as well. Product layout is preferred for one or a few standardized products whereas process layouts are useful for producing a large variety of non standardized products.

2) Type of Production Process
This relates chiefly to the production technology used and the type of materials handled. The type of production system i.e., continuous production, job production, process production and on largely governs the type of plant layout.

3) Volume of Production
The plant layout in a large scale organization will be different from the same in the small scale manufacturing industry especially with respect to material handling equipment, space utilization, communication, etc.

4) Management Policy
A layout can often reflect the policy of the management. It is the Management, within its cost constraints, which has to decide on many matters like nature and quality of products, size of the plant, plans for expansion, storage facilities, employee facilities etc.

5) Service Facilities
The layout of Factory must include proper service facilities required for the comfort and welfare of workers. These include canteen, lockers, gardens, parking area, drinking water, first aid etc.

6) Possibility of Future Expansion
The type of layout depends upon the possibility of future expansion and installation of additional facilities.

TYPES OF LAYOUTS

Plant Layouts can be classified as four basic types. But most of the practical layouts are a suitable combination of these basic types to match the requirements of activities and flow for a particular organization.
The basic types are:
1) Product or Line Layout.
2) Process Layout or Functional Layout or Job Shop Layout.
3) Cellular or Group Layout.
4) Fixed Position Layout.

Product or Line Layout

In this type of layout, only one product, or one type of product is produced in a given area. A product layout is one where work centers and equipment are arranged in a sequence such that the raw material enters at one end of the line and goes from one work centre to the next rapidly in the smooth flow and the finished product is delivered at the other end of the line. In this, each unit of output requires the same sequence of operations from beginning to end.
Eg: Automobile assembly lines, Beverage bottling, Cafeteria, Automatic car washing etc.
Product layouts are suitable for continuous production and are adopted by those organizations which produce a few products in large volume.

A line layout or product layout can be adopted on conditions that

i) Product is standardized.
ii) There is a reasonably stable product demand.
iii) There is a continuous supply of raw material.
iv) There is no breakdown of machinery or absenteeism of key personnel.

Disadvantages:

1) Lack of Flexibility - any change in product requires the modification of layout.
2) Sequence of operation is disturbed if there is any problem at any of the work centers.
3) Capital investment is high.
4) Absence of labor at any of the work centers stops production.

Process Layout or Job Shop Layout

In this type of layout, similar equipments and operations are grouped together to perform similar work in each area. Process layouts are widely used both in manufacturing and other service facilities especially in job and batch production, and non-repetitive type of work. It is employed when designs are not stable and volume of production is small. The path of flow of raw materials through the various sections varies from one product to another. Usually the paths are long and there will be possibility of backtracking.

Eg: Job shops, Hospital, Universities, Large Officers, Paper mills, Cement industries, Chemical industries etc.

Advantages of process layout:

1) It allows variety of products can be made on the same equipment.
2) The equipment is general purpose and less expensive than equipment used in product layouts.
3) The operations can continue, if some equipment is unavailable because of breakdown or planned maintenance.
4) It is suitable for low volume variable demand.
5) Products can be made for specific orders.

Disadvantages of process layout:

1) Scheduling work on equipment is complicated and must be done continuously.
2) High levels of operator skills are needed.
3) Large amount of work-in-progress, more waiting for next operation.
4) Higher total production time.
5) Multiple handling of materials leads to higher materials handling cost.
6) Effective and quick supervision is difficult.
7) Large floor space is required.
8) Rate of production is low. Not feasible to incorporate automation.

Cellular or Group Layout

This is a combination of product and process type of layouts. In this type the area is divided into several cells. Each cell has a few different equipment or facilities so that a 'family' of parts which require similar processing can be produced in each cell. Each member of this 'family' of parts is made complete in this small specialized area with all the necessary machining sequences. Families of parts come together later in another cell for assembly.

This layout is called Group layout since Group Technology is used. Group Technology (GT) is the analysis and comparison of components so as to group them into families with similar characteristics. GT is used to develop a hybrid between pure process layout and pure product layout. This technique is very useful for companies that produce variety of parts in small batches. Each batch can be processed in each cell taking the advantage of a flow line. The application of Group Technology involves two basic steps. The first step is to determine component families or groups. The second step is to arrange equipment used to process a particular family of components. This is similar to having small plants within the plant.

Advantages of Cellular Layout:

1) Reduced material handling cost.
2) Less work-in-process Inventory.
3) Simplified Production Planning and Control.
4) Better Utilization and Specialization of labor.
5) Rate of production is high. Feasible to incorporate automation in each cell.
6) Product variety can be higher than line layout but lesser than process layout.
7) Suitable for incentive Pay Scheme.
8) Delivery times can be estimated more precisely.

Disadvantages:

1) Increased machine down time since machines dedicated to a particular cell may not be used all the time.
2) Cells having a particular combination of facilities may become out-of-date as products and processes change.

Fixed Position Layout

In this type of layout, the product stays in one location while tools, equipments and workers are brought near it and fabrication is carried out. A fixed position layout is appropriate when it is not feasible to move the product because of its size. This type of layout is suitable,
1) when one or few pieces of identical heavy products are to be manufactured
2) when the assembly consists of large number of heavy parts
3) when the cost of transportation of the products being processed are higher than the cost of movement of tools and equipments
Eg: Ship building, building of bridges, agricultural operations, satellite erection, etc.,

Advantages of Fixed Position Layout:

1) Capital investment is lower in the layout.
2) Flexibility to changes in product design.
3) Responsibility for quality can be pin-pointed.
4) Helps in job enlargement and upgrades the skills of the Operator.
5) The workers identify themselves with the product and take extra interest and pride in doing the job.

Disadvantages:

1) Equipment needed for fabrication may not be mobile
2) Work may suffer due to climatic conditions.

LINE BALANCING IN ASSEMBLY LINE

It is an important method of minimizing costs in product or line layout. As we already know, a line layout is one where work centers are arranged in a sequence such that raw material enters at one end of the line and goes from one work centre to the next and the finished product is delivered at the other end of the line.

Although the product layout produces a large volume of goods in a relatively short time, once the line is established there are numerous problems that arise in connection with this type of layout that do not become important in the process layout. One of these complex problems is the problem of line balancing, which might be considered the problem of balancing operations or stations in terms of equal times and times required to meet the desired rate of production. In practical cases, perfect balance is achieved in straight line layouts.

The problem in line balancing is minimizing the idle time on the line for all combinations of workstations subject to certain restrictions. An important restriction is the production volume that is to be produced. If the demands for the product change, then there should be a change in line balancing. Usually an assembly line is used for a variety of products; it becomes necessary to consider a fixed number of workstations.

In general, there are two types of line-balancing situations, each of which involves different considerations. It is sometimes difficult in practical cases to distinguish between the two categories, but it is useful to consider the line balancing problem as: (1) assembly-line balancing and (2) fabrication-line balancing. The distinction refers to the type of operations taking place on the line to be balanced. The term "assembly line" has gained a certain popular interpretation as it is used with reference to the automotive industry. The term "fabrication line", on the other hand, implies a production line made up of operations that form or change the physical or chemical characteristics of the product involved. Machining operations would fit into this classification, as would heat-treat operations.

DESIGN OF MATERIAL HANDLING SYSTEM

Materials handling systems are closely integrated with a plant layout. Many different combinations of equipment could be used to achieve this purpose. In order to move materials at minimum cost the alternative equipment and materials handling system must be carefully evaluated before installation. In addition, the systems that are installed must be reviewed periodically to ensure that it continues to be as effective as possible.

The rapid changes in materials handling technology can make an existing approach obsolete and non-competitive. An increase in production delays by lower machine utilization and greater idle labor time or rise in material breakage or spoilage rates are danger signals.' Such inefficient practices and transferring materials from one container to another can be a major handling problem.

A check list of factors to consider during an audit or in the initial design of a material handling system has been developed. It is the result of considerable experimentation and experience and may serve, as a rough guide in analyzing a materials handling problem; however

• Eliminate all handling as for as possible,
• Maintain a simple line of flow,
• Maintain a steady rate of material flow,
• Mechanized handling wherever economically feasible,
• Accommodate the largest workload possible,
• Minimize travel distance,
• Use flexible equipment wherever possible.

The initial step in the design of a materials handling system is to determine what material must be transported. This may be accomplished by preparing a list of all end products, sub-assemblies, components and raw materials, involved in the production process. When will be the move take place and how much will be moved? To determine this production forecast for end product must be extended until it is possible to estimate the total amount of sub-assemblies, components and raw materials that must be moved. Then the average daily movement required to meet production forecast is determined.

KANBAN SYSTEM

KANBAN SYSTEM

“Kanban” is a pull-based material replenishment system that uses visual signals, such as color-coded cards, to signal to upstream workstations when inputs are required at a downstream workstation. In effect, Kanban is a communication tool for pull-based production. A Kanban could be an empty bin, a card, an electronic display or any suitable visual prompt.
Typically there are two main kinds of Kanban:

1. Production Kanban – A signal from the internal customer to the internal supplier that something is required from the internal supplier.

2. Withdrawal Kanban – A signal from the internal supplier to the internal customer that the supplier has produced something which is available to be withdrawn by the internal customer. In such case the internal supplier doesn’t produce more until the withdrawal is made by the internal customer.

There are many variations on the Kanban system and in fact there are many books dedicated to the topic of how to best apply Kanban.

Many people think the Toyota production system a Kanban system: this is incorrect. The Toyota production system is a way to make products, whereas the Kanban system is the way to manage the Just-in-time production method. In short, the kanban system is an information system to harmoniously control the production quantities in every process. It is a tool to achieve just-in-time production. In this system what kind of units and how many units needed are written on a tag-like card called Kanban. The Kanban is sent to the people of the preceding process from the subsequent process. As a result, many processes in a plant are connected with each other. This connecting of processes in a factory allows for better control of necessary quantities for various products. The Kanban system is supported by the following:

1. Smoothing of production
2. Reduction of set-up time design of machine layout
3. Standardization of jobs
4. Improvement activities
5. Autonamation
   

Kanban is usually a card put in a rectangular vinyl envelope. Two kinds are mainly used: Withdrawal Kanban and Production-ordering Kanban. A Withdrawal Kanban details the kind and quantity of product which the subsequent process should withdraw from the preceding process, while a Production-ordering Kanban specifies the kind and quantity of the product which the preceding process must produce. The Withdrawal kanban shows that the preceding process which makes this part is forging, and the carrier of the subsequent part must go to position B-2 of the forging department to withdraw drive pinions. The subsequent process is machining. The Kanban that shows the machining process SB-8 must produce the crank shaft for the car type. The crank shaft produced should be placed at store F26-18. These cards circulate within Toyota factories, between Toyota and its many co-operative companies, and within the factories of co-operative companies. In this manner, the Kanban can contribute information on withdrawal and production quantities in order to achieve Just-in-time production. Suppose we are making products A, B, and C in an assembly line. The parts necessary to produce these products are a and b which are produced by the preceding machining line. Parts a and b produced by the machining line are stored behind this line, and the production-ordering Kanbans of the line are attached to these parts.

The carrier from the assembly line making product A will go to the machining line to withdraw the necessary part a with a withdrawal kanban. Then, at store, he picks up as many boxes of this part as his withdrawal kanbans and he detaches the production-ordering kanban attached to these boxes. He then brings these boxes back to his assembly line, again with withdrawal kanbans. At this time, the production-ordering Kanbans are left at store a of the machining line showing the number of units withdrawn. These Kanbans will be the dispatching information to the machining line. Part a is then produced in the quantity directed by that number of Kanbans. In this machining line, actually, parts a and b are both withdrawn, but these parts are produced according to the detached order of the production-ordering Kanban.

JUST-IN-TIME PRODUCTION

JUST-IN-TIME PRODUCTION

The idea of producing the necessary units in the necessary quantities at the necessary time is described by the short term Just-in-time. Just-in-time means, for example, that in the process of assembling the parts to build a car, the necessary kind of sub-assemblies of the preceding processes should arrive at the product line at the time needed in the necessary quantities. If Just-in-time is realized in the entire firm, then unnecessary inventories in the factory will be completely eliminated, making stores or warehouses unnecessary. The inventory carrying costs will be diminished, and the ratio of capital turnover will be increased. However, to rely solely on the central planning approach which instructs the production schedules to all processes simultaneously, it is very difficult to realize Just-in-time in all the processes for a product like an automobile, which consists of thousands of parts.

Therefore, in Toyota system, it is necessary to look at the production flow conversely; in other words, the people of a certain process go to the preceding process to withdraw the necessary units in the necessary quantities at the necessary time. Then what the preceding process has to do is produce only enough quantities of units to replace those that have been withdrawn.

Radio Frequency Identification (RFID)

Radio Frequency Identification (RFID) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders. An RFID tag is a small object that can be attached to or incorporated into a product, animal, or person. RFID tags contain silicon chips and antennas to enable them to receive and respond to radio-frequency queries from an RFID transceiver. Passive tags require no internal power source, whereas active tags require a power source.

RFID tags can be either passive, semi-passive and active.

Active

Unlike passive and semi-passive RFID tags, active RFID tags (also known as beacons) have their own internal power source which is used to power any ICs and generate the outgoing signal. They are often called beacons because they broadcast their own signal. They may have longer range and larger memories than passive tags, as well as the ability to store additional information sent by the transceiver. To economize power consumption, many beacon concepts operate at fixed intervals. At present, the smallest active tags are about the size of a coin. Many active tags have practical ranges of tens of meters, and a battery life of up to 10 years.


Semi-passive

Semi-passive RFID tags are very similar to passive tags except for the addition of a small battery. This battery allows the tag IC to be constantly powered. This removes the need for the aerial to be designed to collect power from the incoming signal. Aerials can therefore be optimized for the backscattering signal. Semi-passive RFID tags are faster in response and therefore stronger in reading ratio compared to passive tags.

Passive

Passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio frequency signal provides just enough power for the CMOS integrated circuit (IC) in the tag to power up and transmit a response. Most passive tags signal by backscattering the carrier signal from the reader. This means that the aerial (antenna) has to be designed to both collect power from the incoming signal and also to transmit the outbound backscatter signal. The response of a passive RFID tag is not just an ID number (GUID): tag chip can contain nonvolatile EEPROM(Electrically Erasable Programmable Read-Only Memory) for storing data. Lack of an onboard power supply means that the device can be quite small: commercially available products exist that can be embedded under the skin. As of 2006, the smallest such devices measured 0.15 mm × 0.15 mm, and are thinner than a sheet of paper (7.5 micrometers).[4] The addition of the antenna creates a tag that varies from the size of postage stamp to the size of a post card. Passive tags have practical read distances ranging from about 2 mm (ISO 14443) up to a few meters (EPC and ISO 18000-6) depending on the chosen radio frequency and antenna design/size. Due to their simplicity in design they are also suitable for manufacture with a printing process for the antennae. Passive RFID tags do not require batteries, and can be much smaller and have an unlimited life span. Non-silicon tags made from polymer semiconductors are currently being developed by several companies globally. Simple laboratory printed polymer tags operating at 13.56 MHz were demonstrated in 2005 by both PolyIC (Germany) and Philips (The Netherlands). If successfully commercialized, polymer tags will be roll printable, like a magazine, and much less expensive than silicon-based tags.

Because passive tags are cheaper to manufacture and have no battery, the majority of RFID tags in existence are of the passive variety. As of 2005, these tags cost an average of Euro 0.20 ($0.24 USD) at high volumes.

Conficker

Conficker

Conficker, also known as "Downup", "Downadup" and "Kido", is a computer worm that surfaced in October 2008 and targets the Microsoft Windows operating system. The worm exploits a previously patched vulnerability in the Windows Server
service used by Windows 2000, Windows XP, Windows Vista, Windows Server 2003, Windows Server 2008, Windows 7 Beta, and Windows Server 2008 R2 Beta. The worm has been unusually difficult for network operators and law enforcement to counter because of its combined use of advanced malware techniques.

Symptoms

* Account lockout policies being reset automatically.
* Certain Microsoft Windows services such as Automatic Updates, Background Intelligent Transfer Service (BITS), Windows Defender and Error Reporting Services disabled.
* Domain controllers responding slowly to client requests.
* Congestion on local area networks.
* Web sites related to antivirus software becoming inaccessible.

points for choosing a right Processor

points for choosing a right Processor

Which processor is actually the fastest and how important are Hertz when you select new computer? I will clarify concepts and answer some of the most common questions about processors.

1. What is a processor?

Processor, often called the CPU (Central Processing Unit), is the brain of the computer where it accounts for most of the calculations made. There are processors in many types of gadgets, everything from microwave ovens to cars, here we focus however on the ones we have in our computers.
Modern processors can perform billions of calculations every second and they are made up of millions of small transistors mounted on a surface in a few square centimeters.

2. What is the difference between a CPU and a GPU?

CPU is not the only circuit in a computer that handles calculations, the graphics card in the computer is also usually a self-processor that only handles graphics calculations, this is sometimes called the GPU (Graphics Processing Unit).
Even the sound card may have an advanced dedicated processor.

3. What should I choose - AMD or Intel?

Since immemorial time we have AMD and Intel's been on the performance throne and there has always been arguments for both sides. What is the fastest, most power-efficient and most affordable? In the current situation Intel leads in terms of pure performance, while AMD instead focusing on being the affordable option for the budget with a little narrower. And so it has, with few interruptions, been quite a while. Important to know is that when you select the processor, select the platform, thus moderate style and in some cases a common style, so it will be easy to switch to another processor if you would change your mind.

4. How much power, the processor?

This has been the development properly forward the last two to three years and even if you select a fast dual core processor it need not absorb very much power. In this context used TDP (Thermal Design Power) as a unit, in practice the maximum effect that the cooling device must support to the CPU so that it does not overheat. The latest and fastest Core 2 Quad models from Intel is about 65-95 watts TDP, while a simpler dual core processor in a new laptop has TDP-values of around 25-35 watts.

Ultra Low Voltage flavors draws no more than 5-6 watts TDP and the latest Atom processor from Intel, a model that will appear in the mini-computers in the year, draws as little as 2.5 watts. Now that defines Intel and AMD TDP-value in different ways so the value, unfortunately, is not comparable across different platforms.

5. How many cores do I need?

The difference between a single core and a dual core processor is quite large but it is not as obvious to go up to Quad Core. If you're running heavy applications for audio or video editing, the four cores will be preferred (if your program supports this) but if you're looking for the best gaming performance, it is usually better to put money on a fast dual core processor and a fast graphics card. Few games can use more than two cores in the current situation and usually the graphics card is the weak link in terms of performance.

6. How many gigahertz do I need?

A few years ago you could compare hertz rates between different processors and thus get a decent idea of what was the fastest but it is as simple as it no longer. AMD and Intel use different processor architectures and the factors that bus, cache and number of grains to make it properly when you choose. Here it is more important than ever to read the tests and that you select a new and fresh processor so you do not stand with an old platform that you can not upgrade.

7. How much the cache?

The cache is built into the processor itself, which means that the data does not need to detour through working memory (RAM) before being processed. This means that when the cache is used calculations are much faster than otherwise. The more cache you have in the processor the faster the work is, but it is also an expensive component that draws up the overall price. This memory is also divided into different levels, L1, L2, etc. The lower the number the closer the actual processor (and faster).

8. Will I earn something on overclocking?

Overclocking can be a cheap way to get out a little more performance from your computer while you are charged to the processor more than its actual performance. Make sure you have a big cooler and check that your CPU is overclocking friendly (read in properly before). The rule on overclocking is that drawing up the speed of the system bus, thus increasing the CPU clock frequency, but at the same time, insert the motherboard to the test. Many new motherboards have overclocking features built-in smartly that you can control directly from Windows software.

9. Can I change the processor itself?

Before you buy a new processor to your computer, it is important that you have stone eye on the new processor fits in your motherboard. Not only physically but also to the motherboard software (bios) to identify the processor. The actual change is quite simple but you have my thumb in the hand it may be worthwhile to ask someone with experience for help. It can be an expensive business if you slip with the screwdriver when you mount the cooler.

And you are thinking of changing the processor in your laptop then usually just forget. If you are lucky, processor is not soldered but only to disassemble the chassis can be a nightmare and it's not safe to bios support your new CPU and cooler whether it can handle the heat development.

10. How important are the bits?

The major advantage of 64-bit processor and 64-bit operating system is that it can use significantly more working memory, is still software support, however, very limited. If you do not need to work with a workstation that will render 3D animations or similar applications, we recommend 32 bits. To take advantage of a 64-bit processor, software must have support for it.

PC Hardware Troubleshooting Tips

PC Hardware Troubleshooting Tips

First, of course, you need to check the stupid stuff. You'd probably be amazed how many "problems" are solved by connecting the cables, or turning on the power switch that you swear you just did. Beyond that, double-check the snugness of your connections—jiggling in a new add-in card or screwing in a cable connection can (and often does) make a difference. You may even want to check the integrity of your cables and connectors. I once solved a baffling SCSI problem by noticing that one of the pins in a miniature 50-pin SCSI-II connector was bent. I had mistakenly presumed, prior to that, that a bent pin would have prevented me from making a connection at all, but it didn't. Unfortunately, as a result, I wasted several hours on something that could've taken two minutes.

Finally, whenever you install something new, whether it's more memory, a new drive, a plug-in card or what-have-you, and something doesn't work, it's more than likely because you made a mistake somewhere in the installation process. Step back through the process again, double- and triple-check your connections, and then try one more time. In the case of RAM that doesn't work or isn't recognized, it could be an incompatibility with the specific manufacturer of the RAM and your motherboard, so see if you can try a different brand before you give up hope. Finally, in some instances installing something new causes a conflict with something else--which is what the rest of this article is all about.

Driver Dilemmas

Once you're past the basics, it’s on to step two. As long as your computer boots, then there's a good chance the problem is related to missing, damaged, incompatible or improperly installed driver software, otherwise known simply as drivers. (If your PC doesn't start up, you may want to create a boot floppy disk—see "How to Create a 'Real' Windows 95 (or 98 or ME) Boot Disk" for more. In addition, you should check out the "PC Startup Troubleshooting Tips" article for more.)

Virtually every piece of hardware located inside or connected externally to your PC requires a driver to communicate and function with the operating system, applications, and other hardware components in your machine. Drivers essentially translate messages back and forth between the hardware in question and the operating system, thereby allowing your computer system to work as a unified whole (at least, in theory). The truth is, though appearances may suggest otherwise, any computer system is actually made up of a bunch of specialized pieces that don't speak the "native language" of other components and, therefore, require a great deal of translation to communicate and work effectively with them. When any of these various levels of translation break down, well, that's when you get problems.

The Device Manager is Your Friend

If you're running Windows 95, 98, or ME, your first stop after the computer finishes the startup process should be the Device Manager, a piece of operating system software that helps you manage the various pieces of your PC. You can get to the Device Manager in several different ways: the two easiest are right-clicking on the My Computer icon on your desktop and selecting Properties from the context menu that pops up when you do this, or by going to the Start menu, going up to Settings, selecting Control Panel from the list of choices and then double-clicking on the System control panel. Either way, you’ll be presented with a tabbed dialog box; click on the tab that says Device Manager, and you’re there.

The first thing to look for is a yellow exclamation point or red international no sign (you know, the circle with the slash through it) next to one of your devices. You may need to click on all the little plus signs next to each category of devices to see the full outline-like list of everything in your computer. If you see the yellow or red symbol, you know something is amiss. My first suggestion is to highlight the offending device and click on the Remove button. What this does is essentially erase the driver software associated with the device as well as references to it within the Windows 95/98/ME Registry. (The Registry tracks all the hardware and software you install, the preferences you set for each, as well as lots of other stuff). The fancy term for this is "logically" removing the device because, even though it may still be physically attached, the computer no longer has any record of its presence.

Next, you should restart your computer, let Windows' Plug-and-Play feature "find" the device again, and redo the driver installation process by either using the driver suggestions that Windows finds and makes or clicking on the Have Disk button and using the installer disks that came with the device. (If both options are possible, I’d go with the disks that came with the hardware—unless you’re sure they are an older version.) If the computer doesn’t find the device, then I would suspect the connections or the device itself. Buy another cable, unplug and re-plug the connectors, remove and then physically reinstall any new plug-in cards or do whatever you have to to ensure that the connection is solid. Once you’ve done that, you’ll find that many times, simply reinstalling the drivers solves the problem.

Other times the problem is due to an older version of a driver conflicting with something else on your system. It’s always a good idea to check for and get the latest versions of drivers that your PC needs. Check the manufacturer’s web site first and if you can’t find the driver, or drivers, there (or if the company is no longer in business), try searching for it at one of the web’s driver repositories, such as The Driver Zone, WinDrivers.Com, WinFiles, or Frank Condron's World o' Windows. If you're running Windows 98 or Windows ME, you may also be able to get a new driver for your hardware via the Windows Update feature off the Start menu.

If you find and download a new driver version, you can often update the driver by clicking on the device listing in the Device Manager, clicking on Properties, selecting the Driver Tab and then clicking on the Update Driver button. From there, depending on what version of Windows you’re using (the original Win95A, the revised Win95B, Win98, or Windows ME), you’ll either need to manually find where the file is located on your hard disk, and then select it, or the computer will attempt to find it for you. Either way, once the correct file is found, you initiate the update process by simply clicking a button.

Some driver updates force you to create new floppy disks, which you then use to update the driver. Frankly, though it’s more work initially, this is a good solution in the long run, I think, because if you ever have to completely reinstall Windows (such as, when you buy yourself a big new hard drive), you’ll generally want to have copies of your drivers on floppies anyway.

Mind your Ps and IRQs

Finally, if that still doesn’t work, you may need to futz with the dreaded IRQs, or Interrupt Requests. Most hardware devices on your computer need attention from the processor on a fairly regular basis (to check their status) and the mechanism for doing that is called Interrupt Requests (because the device politely asks the processor to interrupt what it’s doing at the time and give it some attention—well, sort of). Because of the need to maintain compatibility with older hardware, today’s PCs are still limited to 16 IRQs (numbered 0-15), which is turning out to be a fairly big problem on many newer, well-equipped computers (read one of my InfoWorld Electric "Plugged In" columns to learn more on the subject).

The general principle with IRQ troubleshooting is that two devices cannot typically share an IRQ (an important exception is with some PCI-based add-in cards), and if they try to, one or sometimes neither of the devices will work properly. If you find that you have an IRQ conflict, where two ISA cards or other non-PCI devices are trying to use the same IRQ, you’ll need to change the settings on one of the devices to an open IRQ. The problem is, not every device is able to use every IRQ, so even though you have other IRQs available, the problem device may be incapable of using one of the open IRQs. If that’s the case, you may need to move another device using one of the IRQs that the problem hardware does work with first, and then free up an IRQ for the problem hardware. So, for example, if IRQ 8 is open but you have a SCSI card that only works with IRQs 9 or 11, you may first need to move whatever’s on 11 to 8, and then set the SCSI card to IRQ 11. Unfortunately, this can sometimes lead to an infuriating puzzle game where you try to match devices with your IRQs.

To find out what IRQs are in use by your computer, double click the computer icon at the top of the Device Manager. You’ll see a new window pop up that shows which devices are using which IRQs. Unfortunately it doesn’t leave a blank for any IRQs that are not in use, so if you need to find an available IRQ you’ll have to look close, count the numbers and see if any are missing. If a number between 0 and 15 is missing, that means that particular IRQ is available.

Some older plug-in cards require you to change the IRQs by setting tiny DIP switches on the card itself, or via a dedicated configuration utility. Most newer Plug-and-Play cards can be changed via Windows 95 or Windows 98. To do so, go to the Device Manager, highlight the product in question, click on the Properties button and then go to Resources Tab. Generally, you’ll have to deselect the Use Automatic Settings button to make any changes. Most devices offer several Basic Configuration choices, which is what you should try first. These are different combinations of IRQs, memory ranges, and I/O ranges. All you typically need to worry about is the IRQ. Some devices also let you adjust these parameters individually by clicking the Change Settings button.

Thankfully, you rarely have to worry about IRQ problems if you’re using Windows 95, 98 or ME because they all do a pretty good job of automatically fixing them before they arise. In fact, this is one of their most important, yet little discussed improvements over Windows 3.1. Unfortunately, Windows 3.0 and 3.1 users still have to worry about this kind of stuff on a semi-regular basis. The problem is not unheard of under Windows 95, 98 or ME, however (I ran into it myself), which is why I’ve included it here.

The End

These tips won’t solve all the hardware problems you may run into, but they should solve a good number of them. The important thing to remember when doing any troubleshooting is that computers really are logical devices and there’s always a logical reason for why something isn’t working. Discovering what that reason is and then applying the right solution isn’t always easy (or intuitive), but if you think about the problem logically and work through it step-by-step, there’s a good chance you’ll be able to solve it on your own. And, if worse comes to worse, you can always just nuke everything and start over (see "Starting Over: Repartitioning, Reformatting and Reinstalling" for more on exactly how to do that). Good luck!

File Systems in Detail

File Systems in Detail

What is a File System?

A file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. File systems may use a data storage device such as a hard disk or CD-ROM and involve maintaining the physical location of the files, they might provide access to data on a file server by acting as clients for a network protocol, or they may be virtual and exist only as an access method for virtual data. It is distinguished from a directory service and registry. More formally, a file system is a special-purpose database for the storage, organization, manipulation, and retrieval of data.

Most file systems make use of an underlying data storage device that offers access to an array of fixed-size blocks, sometimes called sectors, generally a power of 2 in size. The file system software is responsible for organizing these sectors into files and directories, and keeping track of which sectors belong to which file and which are not being used. Most file systems address data in fixed-sized units called "clusters" or "blocks" which contain a certain number of disk sectors. This is the smallest amount of disk space that can be allocated to hold a file.

However, file systems need not make use of a storage device at all. A file system can be used to organize and represent access to any data, whether it be stored or dynamically generated.

File Names Conventions

Whether the file system has an underlying storage device or not, file systems typically have directories which associate file names with files, usually by connecting the file name to an index in a file allocation table of some sort, such as the FAT in a DOS file system, or an inode in a Unix-like file system. Directory structures may be flat, or allow hierarchies where directories may contain subdirectories. In some file systems, file names are structured, with special syntax for filename extensions and version numbers. In others, file names are simple strings, and per-file metadata is stored elsewhere.

Metadata

Other bookkeeping information is typically associated with each file within a file system. The length of the data contained in a file may be stored as the number of blocks allocated for the file or as an exact byte count. The time that the file was last modified may be stored as the file's timestamp. Some file systems also store the file creation time, the time it was last accessed, and the time that the file's meta-data was changed. Other information can include the file's device type, its owner user-ID and group-ID, and its access permission settings.

Arbitrary attributes can be associated on advanced file systems, such as XFS, ext2/ext3, some versions of UFS, and HFS+, using extended file attributes. This feature is implemented in the kernels of Linux, FreeBSD and Mac OS X operating systems, and allows metadata to be associated with the file at the file system level. This, for example, could be the author of a document, the character encoding of a plain-text document, or a checksum.

Hierarchical file systems

The hierarchical file system was an early research interest of Dennis Ritchie of Unix fame; previous implementations were restricted to only a few levels, notably the IBM implementations, even of their early databases like IMS. After the success of Unix, Ritchie extended the file system concept to every object in his later operating system developments, such as Plan 9 and Inferno.

Facilities

Traditional file systems offer facilities to create, move and delete both files and directories. They lack facilities to create additional links to a directory (hard links in Unix), rename parent links, and create bidirectional links to files.

Traditional file systems also offer facilities to truncate, append to, create, move, delete and in-place modify files. They do not offer facilities to prepend to or truncate from the beginning of a file, let alone arbitrary insertion into or deletion from a file. The operations provided are highly asymmetric and lack the generality to be useful in unexpected contexts. For example, interprocess pipes in Unix have to be implemented outside of the file system because the pipes concept does not offer truncation from the beginning of files.
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Types of file systems

File system types can be classified into disk file systems, network file systems and special purpose file systems.

Disk file systems

A disk file system is a file system designed for the storage of files on a data storage device, most commonly a disk drive, which might be directly or indirectly connected to the computer. Examples of disk file systems include FAT [FAT12, FAT16, FAT32, exFAT], NTFS, HFS and HFS+, HPFS, ext2, ext3, ext4, ISO 9660, ODS-5, ZFS and UDF. Some disk file systems are journaling file systems or versioning file systems.

Flash file systems

A flash file system is a file system designed for storing files on flash memory devices. These are becoming more prevalent as the number of mobile devices are increasing, and the capacity of flash memories increase.

While a disk file system can be used on a flash device, this is suboptimal for several reasons:

Erasing blocks: Flash memory blocks have to be explicitly erased before they can be rewritten. The time taken to erase blocks can be significant, thus it is beneficial to erase unused blocks while the device is idle.
Random access: Disk file systems are optimized to avoid disk seeks whenever possible, due to the high cost of seeking. Flash memory devices impose no seek latency.
Wear levelling: Flash memory devices tend to wear out when a single block is repeatedly overwritten; flash file systems are designed to spread out writes evenly.

Database file systems

A new concept for file management is the concept of a database-based file system. Instead of, or in addition to, hierarchical structured management, files are identified by their characteristics, like type of file, topic, author, or similar metadata.

Transactional file systems

Each disk operation may involve changes to a number of different files and disk structures. In many cases, these changes are related, meaning that it is important that they all be executed at the same time. Take for example a bank sending another bank some money electronically. The bank's computer will "send" the transfer instruction to the other bank and also update its own records to indicate the transfer has occurred. If for some reason the computer crashes before it has had a chance to update its own records, then on reset, there will be no record of the transfer but the bank will be missing some money.

Transaction processing introduces the guarantee that at any point while it is running, a transaction can either be finished completely or reverted completely (though not necessarily both at any given point). This means that if there is a crash or power failure, after recovery, the stored state will be consistent.

This type of file system is designed to be fault tolerant, but may incur additional overhead to do so.

Journaling file systems are one technique used to introduce transaction-level consistency to filesystem structures.

Network file systems

A network file system is a file system that acts as a client for a remote file access protocol, providing access to files on a server. Examples of network file systems include clients for the NFS, AFS, SMB protocols, and file-system-like clients for FTP and WebDAV.

Special purpose file systems

A special purpose file system is basically any file system that is not a disk file system or network file system. This includes systems where the files are arranged dynamically by software, intended for such purposes as communication between computer processes or temporary file space.

Special purpose file systems are most commonly used by file-centric operating systems such as Unix. Examples include the procfs (/proc) file system used by some Unix variants, which grants access to information about processes and other operating system features.

Deep space science exploration craft, like Voyager I & II used digital tape-based special file systems. Most modern space exploration craft like Cassini-Huygens used Real-time operating system file systems or RTOS influenced file systems. The Mars Rovers are one such example of an RTOS file system, important in this case because they are implemented in flash memory.

Crash counting is a feature of a file system designed as an alternative to journaling. It is claimed that it maintains consistency across crashes without the code complexity of implementing journaling.

File systems and operating systems

Most operating systems provide a file system, as a file system is an integral part of any modern operating system. Early microcomputer operating systems' only real task was file management — a fact reflected in their names. Some early operating systems had a separate component for handling file systems which was called a disk operating system. On some microcomputers, the disk operating system was loaded separately from the rest of the operating system. On early operating systems, there was usually support for only one, native, unnamed file system; for example, CP/M supports only its own file system, which might be called "CP/M file system" if needed, but which didn't bear any official name at all.

Because of this, there needs to be an interface provided by the operating system software between the user and the file system. This interface can be textual or graphical. If graphical, the metaphor of the folder, containing documents, other files, and nested folders is often used.

Flat file systems

In a flat file system, there are no subdirectories—everything is stored at the same (root) level on the media, be it a hard disk, floppy disk, etc. While simple, this system rapidly becomes inefficient as the number of files grows, and makes it difficult for users to organize data into related groups.

Like many small systems before it, the original Apple Macintosh featured a flat file system, called Macintosh File System. Its version of Mac OS was unusual in that the file management software (Macintosh Finder) created the illusion of a partially hierarchical filing system on top of MFS. This structure meant that every file on a disk had to have a unique name, even if it appeared to be in a separate folder. MFS was quickly replaced with Hierarchical File System, which supported real directories.

A recent addition to the flat file system family is Amazon's S3, a remote storage service, which is intentionally simplistic to allow users the ability to customize how their data is stored. The only constructs are buckets and objects. Advance file management is allowed by being able to use nearly any character including '/' in the objects name, and the ability to select subsets of the bucket's content based on identical prefixes.

File systems under Unix-like operating systems

Unix-like operating systems create a virtual file system, which makes all the files on all the devices appear to exist in a single hierarchy. This means, in those systems, there is one root directory, and every file existing on the system is located under it somewhere. Unix-like systems can use a RAM disk or network shared resource as its root directory.

Unix-like systems assign a device name to each device, but this is not how the files on that device are accessed. Instead, to gain access to files on another device, the operating system must first be informed where in the directory tree those files should appear. This process is called mounting a file system. For example, to access the files on a CD-ROM, one must tell the operating system "Take the file system from this CD-ROM and make it appear under such-and-such directory". The directory given to the operating system is called the mount point - it might, for example, be /media. The /media directory exists on many Unix systems and is intended specifically for use as a mount point for removable media such as CDs, DVDs and like floppy disks. It may be empty, or it may contain subdirectories for mounting individual devices. Generally, only the administrator or root may authorize the mounting of file systems.

File systems under Linux

Linux supports many different file systems, but common choices for the system disk include the ext family such as ext2 and ext3, XFS, JFS and ReiserFS.

File systems under Solaris

The Sun Microsystems Solaris operating system in earlier releases defaulted to UFS for bootable and supplementary file systems. Solaris defaulted to, supported, and extended UFS.
Support for other file systems and significant enhancements were added over time, including Veritas Software Corp. VxFS, Sun Microsystems QFS, Sun Microsystems UFS, and Sun Microsystems ZFS.

Kernel extensions were added to Solaris to allow for bootable Veritas VxFS operation. Logging or Journaling was added to UFS in Sun's Solaris 7. Releases of Solaris 10, Solaris Express, OpenSolaris, and other open source variants of the Solaris operating system later supported bootable ZFS.

Logical Volume Management allows for spanning a file system across multiple devices for the purpose of adding redundancy, capacity, and/or throughput. Legacy environments in Solaris may use Solaris Volume Manager Multiple operating systems may use Veritas Volume Manager. Modern Solaris based operating systems eclipse the need for Volume Management through leveraging virtual storage pools in ZFS.

File systems under Mac OS X

Mac OS X uses a file system that it inherited from classic Mac OS called HFS Plus. HFS Plus is a metadata-rich and case preserving file system. Due to the Unix roots of Mac OS X, Unix permissions were added to HFS Plus. Later versions of HFS Plus added journaling to prevent corruption of the file system structure and introduced a number of optimizations to the allocation algorithms in an attempt to defragment files automatically without requiring an external defragmenter.

Filenames can be up to 255 characters. HFS Plus uses Unicode to store filenames. On Mac OS X, the filetype can come from the type code, stored in file's metadata, or the filename.

HFS Plus has three kinds of links: Unix-style hard links, Unix-style symbolic links and aliases. Aliases are designed to maintain a link to their original file even if they are moved or renamed; they are not interpreted by the file system itself, but by the File Manager code in userland.

Mac OS X also supports the UFS file system, derived from the BSD Unix Fast File System via NeXTSTEP. However, as of Mac OS X 10.5, Mac OS X can no longer be installed on a UFS volume, nor can a pre-Leopard system installed on a UFS volume be upgraded to Leopard.

File systems under Microsoft Windows

Windows makes use of the FAT and NTFS file systems.

The File Allocation Table (FAT) filing system, supported by all versions of Microsoft Windows, was an evolution of that used in Microsoft's earlier operating system. FAT ultimately traces its roots back to the short-lived M-DOS project and Standalone disk BASIC before it. Over the years various features have been added to it, inspired by similar features found on file systems used by operating systems such as Unix.
Older versions of the FAT file system (FAT12 and FAT16) had file name length limits, a limit on the number of entries in the root directory of the file system and had restrictions on the maximum size of FAT-formatted disks or partitions. Specifically, FAT12 and FAT16 had a limit of 8 characters for the file name, and 3 characters for the extension (such as .exe). This is commonly referred to as the 8.3 filename limit. VFAT, which was an extension to FAT12 and FAT16 introduced in Windows NT 3.5 and subsequently included in Windows 95, allowed long file names (LFN). FAT32 also addressed many of the limits in FAT12 and FAT16, but remains limited compared to NTFS.

NTFS, introduced with the Windows NT operating system, allowed ACL-based permission control. Hard links, multiple file streams, attribute indexing, quota tracking, compression and mount-points for other file systems are also supported, though not all these features are well-documented.

Unlike many other operating systems, Windows uses a drive letter abstraction at the user level to distinguish one disk or partition from another. For example, the path C:\WINDOWS represents a directory WINDOWS on the partition represented by the letter C. The C drive is most commonly used for the primary hard disk partition, on which Windows is usually installed and from which it boots. This "tradition" has become so firmly ingrained that bugs came about in older versions of Windows which made assumptions that the drive that the operating system was installed on was C. The tradition of using "C" for the drive letter can be traced to MS-DOS, where the letters A and B were reserved for up to two floppy disk drives. Network drives may also be mapped to drive letters.
Data retrieval process

The operating system calls on the IFS manager. The IFS calls on the correct FSD in order to open the selected file from a choice of four FSDs that work with different storage systems—NTFS, VFAT, CDFS, and Network. The FSD gets the location on the disk for the first cluster of the file from the FAT, FAT32, VFAT, or, in the case of Windows NT based, the MFT. In short, the whole point of the FAT, FAT32, VFAT, or MFT is to map out all the files on the disk and record where they are located.

Digital Video Broadcasting (DVB)

Digital Video Broadcasting (DVB)

Digital Video Broadcasting (DVB) is a suite of internationally accepted open standards for digital television. DVB standards are maintained by the DVB Project, an international industry consortium with more than 270 members, and they are published by a Joint Technical Committee (JTC) of European Telecommunications Standards Institute (ETSI), European Committee for Electrotechnical Standardization (CENELEC) and European Broadcasting Union (EBU). The interaction of the DVB sub-standards is described in the DVB Cookbook. Many aspects of DVB are patented, including elements of the MPEG video coding and audio coding.

Transmission

DVB systems distribute data using a variety of approaches, including by satellite (DVB-S, DVB-S2 and DVB-SH; also DVB-SMATV for distribution via SMATV); cable (DVB-C); terrestrial television (DVB-T, DVB-T2) and digital terrestrial television for handhelds (DVB-H,DVB-SH); and via microwave using DTT (DVB-MT), the MMDS (DVB-MC), and/or MVDS standards (DVB-MS).

These standards define the physical layer and data link layer of the distribution system. Devices interact with the physical layer via a synchronous parallel interface (SPI), synchronous serial interface (SSI), or asynchronous serial interface (ASI). All data is transmitted in MPEG-2 transport streams with some additional constraints (DVB-MPEG). A standard for temporally-compressed distribution to mobile devices (DVB-H) was published in November 2004.

These distribution systems differ mainly in the modulation schemes used and error correcting codes used, due to the different technical constraints. DVB-S (SHF) uses QPSK, 8PSK or 16-QAM. DVB-S2 uses QPSK, 8PSK, 16APSK or 32APSK, at the broadcasters decision. QPSK and 8PSK are the only versions regularly used. DVB-C (VHF/UHF) uses QAM: 16-QAM, 32-QAM, 64-QAM, 128-QAM or 256-QAM. Lastly, DVB-T (VHF/UHF) uses 16-QAM or 64-QAM (or QPSK) in combination with COFDM and can support hierarchical modulation.

The DVB-T2 standard is expected to be published at the end of June 2008 and is expected to be approved and submitted to ETSI during 2008.

The DVB-T2 standard will give more-robust TV reception and increase the possible bit-rate by over 30% for single transmitters (as in the UK) and is expected to increase the max bit-rate by over 50% in large single-frequency networks (as in Germany, Sweden).

Encryption and metadata

The conditional access system (DVB-CA) defines a Common Scrambling Algorithm (DVB-CSA) and a physical Common Interface (DVB-CI) for accessing scrambled content. DVB-CA providers develop their wholly proprietary conditional access systems with reference to these specifications. Multiple simultaneous CA systems can be assigned to a scrambled DVB program stream providing operational and commercial flexibility for the service provider.

DVB is also developing a Content Protection and Copy Management system for protecting content after it has been received (DVB-CPCM), which is intended to allow flexible use of recorded content on a home network or beyond, while preventing unconstrained sharing on the Internet. DVB-CPCM has been the source of much controversy in the popular press and it's said that CPCM is the DVB's answer to the failed American Broadcast Flag.

DVB transports include metadata called Service Information (DVB-SI, ETSI EN 300 468, ETSI TR 101 211) that links the various elementary streams into coherent programs and provides human-readable descriptions for electronic program guides as well as for automatic searching and filtering.

Recently, DVB has adopted a profile of the metadata defined by the TV-Anytime Forum (DVB-TVA, ETSI TS 102323). This is an XML Schema based technology and the DVB profile is tailored for enhanced Personal Digital Recorders. DVB lately also started an activity to develop a service for IPTV (DVB-IPI, ETSI TR 102033, ETSI TS 102034, ETSI TS 102814) which also includes metadata definitions for a broadband content guide (DVB-BCG, ETSI TS 102 539).

COMMUNICATION NETWORK

COMMUNICATION NETWORK

A communication network, consists of a set of equipment and facilities that provide a service, the transfer of information between users located at various geographical points. For example telephone networks provides telephone service, computer networks, television broadcast networks, cellular networks and Internet.

Communication network and transportation networks have become essential infrastructure in every society.

The capacity to transfer high volume of data for a long distance almost immediately are the features of network-based services. For example Internet provides e-mail, information search and retrieval, various forms of electronic service.

EVOLUTION OF NETWORK ARCHITECTURE AND SERVICES

A communication service which is used of transfer of information. Different services differ in the details of how and in what form information is transferred.

Let us consider three example networks
i) Telegraph networks
ii) Telephone networks
iii) Computer networks

TELEGRAPH NETWORKS AND MESSAGE SWITCHING

In the year 1987 Samuel B. Morse demonstrated a practical telegraph that provided the basis for telegram service, the transmission of text message over long distance. Here the text was encoded into sequence of dots and dashes. Each dot and dash which is used to be represented by short and long pulses of electrical current over a copper wire. By relying on two signals, telegraphy made use of digital transmission system. In Morse code the pause between letter was 3 dots and for word is 5 dots.

In store-and-forward process, the message is completely received and stored at an intermediate point and then forward to the next node depending on the availability of link. The process of selecting an appropriate link is known as routing. If the information is message, then it is known as message switching. In simpler terms, message switching is a switching method that sends data from point-to-point with each intermediate node storing the data , waiting for a free transmission channel and forwarding the data to the next point until the destination is reached.

In the beginning communication channel used to carry information sent by one person. Naturally transmission rate (in bits/second) was less. To increase the transmission rate multiplexing was developed. Multiplexing is a technique used to place multiple signals on a single communication channel. Multiplexing partitions a channel into many separate channels, each capable of transmitting its own independent signal, thereby enabling many different transmission over a single medium.

One approach of multiplexing involves modulation. Binary symbols can be transmitted by sending a sinusoidal signal of a given frequency for a given period of time. Multiple sequence of binary symbols could be transmitted simultaneously by using multiple pairs of frequencies for the different telegraphy signals. This concept is used in Modems.

We know characters can be represented by ASCII ( American Standard Code for Information Interchange) , is a 7 bit code. The equipments can communicate with ASCII code.

The telegraph service involves the transmission of text messages between geographically far places. To provide the service , the architecture of the telegraph network contains the following key elements or functions.

i) Digital transmission takes in the network that is information is represented either 0 or 1. It can make use of ASCII code also. The transmission medium can be copper wire and radio.
ii) To identify the messages exactly, messages are covered by beginning and ending by sequence of characters. This is known as framing.
iii) There should be destination address that is to whom it want to deliver. Routing procedure determines the path that a message follows across a network of telegraph stations interconnected by digital lines

TELEPHONE NETWORKS AND CIRCUIT SWITCHING

In 1876 Alexander Graham Bell developed a device that can transmit voice signals. The device that is known as Telephone. The telephone network provides a two-way, real-time transmission of voice signals across a network.

Telephone service became popular due to its voice service and expert operator with knowledge of Morse code is not required like in the case of telegraph. Here voice signals are converted into equivalent electrical signal and passed through cable.