Network Topology

Network Topology:

Topology is the layout of connected devices on a network. Think of it as the logical "shape" of the network wiring. This logical shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle, but it would be highly unlikely to find an actual ring topology there.

'Logical' means how it looks as a pure design concept, rather than how it actually looks physically. e.g. the topology pictures you will see have nice straight lines between bits of the network, they don't try to show all the corners that need to be turned and holes that have to be drilled in a real cable installation.

Each topology has its advantages and disadvantages: usually related to cost, complexity, reliability and traffic.

There are five basic types of topologies are normally used to implement network:

•          Bus
•          Ring
•          Star
•          Tree
•          Mesh

Bus Topology :

The physical Bus Network Topology is the simplest and most widely used of the network designs. It consists of one continuous length of cable (trunk) that is shared by all the nodes in the network and a terminating resistor (terminator) at each end that absorbs the signal when it reaches the end of line. Without a terminator the electrical signal would reach the end of copper wire and bounce back, causing errors on the network.

Bus networks usually use coaxial cables that connect to each of the computers through T-shaped connectors. A terminator specific to the type of cable used placed on each end node of the network. Since the bus network is nothing more than a set of cables, connectors, and terminators, the signal is not amplified when traveling through the wiring.

Bus networks are easy to assemble and expand. They only require a small amount of cable, compared to other network topologies. However, bus networks can suffer cable breakage, loss of information in the connectors and deficiencies in the necessary wiring length, often difficult to resolve. Any physical problem in the network, such as a loose connector, can crush the entire bus network.

Advantages of Bus Topology:

·         It uses established standards and it is relatively easy to install and the use for small networks.

·         It requires fewer medium than other topologies.

·         Failure of one node does not affect the network functioning.

·         Cost is less as only one main cable is required and least amount of cable is required to connect computers.

·         Expansion is easier. New node can be easily added by using a connector.

Disadvantages of Bus Topology:

·         If the main central line fails the entire network collapses.

·         The bus networks are difficult to reconfigure, especially when the acceptable number of connections or maximum distances have been reached.

·         They are also difficult to troubleshoot because everything happens on a single media segment.

·         Sharing a single communication channel results in slower access time.

Ring Topology:

The physical ring Topology is a circular loop of point-to-point links. Each device connects directly to the ring or indirectly through and interface device or drop cable. Message travel around the ring from node to node in a very organized manner. Each workstation checks the message for a matching destination address. If the address doesn't match the node simply regenerates the message and sends it on its way. If the address matches, the node accepts the message and sends a reply to the originating sender.

• In ring topology, the various nodes are connected in form of a ring or circle (physical ring), in which data flows in a circle, from one station to another station.

• It has no beginning or end that needs to be terminated.

• In this topology, each device or node has a dedicated point to point line configuration with only two devices on either side of it.

• Signal is passed along the ring in one direction from one station to another until it reaches destination.

• Each device in ring incorporates a repeater.

• When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

• There are two kinds of ring topologies:

1. Single Ring

2. Dual Ring

•. In this topology, each device or node has a dedicated point to point line configuration with only two devices on either side of it.

• Signal is passed along the ring in one direction from one station to another until it reaches destination.

• Each device in ring incorporates a repeater.

• When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

• There are two kinds of ring topologies:

1. Single Ring

2. Dual Ring

1. Single ring - In single ring network, a single cable is shared by all the devices and data travel only in one direction.

Each device waits for its turn and then transmits. When the data reaches its destination, another device can transmit.

2. Dual ring: This topology uses two rings to send the data, each in different direction. Thus allowing more packets to be sent over the network.

Advantages of Ring Topology

The advantages of Ring Topology are:

1.They are very easy to troubleshoot because each device incorporates a repeater.

2.A special internal feature called beaconing allows troubled workstations to identify themselves quickly.

There is no master computer on controller. Every computer has equal chance to place the data and access the token.

There are no collisions.

Data packets travel at greater speeds.

It is easier to locate the problems with device and cable i.e. fault isolation is simplified. If one device does not receive a signal within a specified time, it can issue an alarm. This alarm alerts the network operator to the problem and its location.

Disadvantages of Ring Topology

The disadvantages of ring topologies are:

A ring network requires more cable than a bus network.

A break in cable ring brings down entire network (in case of single ring).

Adding or removing the node disturbs the network activity.

In ring network, communication delay is directly proportional to the number of nodes in the network. Hence addition of new nodes in the network also increases communication delay.

It is considerably difficult to install and reconfigure ring Topology

Media failure on unidirectional or single loop causes complete network failure.

Star Topology :

The physical star Topology uses a central controlling or hub with dedicated legs pointing in all directions – like points of a star. Each network device has a dedicated point-to-point link to the central hub. There is no direct link between these computers and the computers can communicate via central controller only. This strategy prevents troublesome collisions and keeps the lines of communications open and free of traffic. Since in the star topology each computer on the network uses a different cable connection, this type of topology is expandable, only limited by the number of ports available in the hub (although it is possible to join several hubs to increase the number of ports ). The expansion of a star topology network does not present any difficulty, since adding another computer to the network means nothing more than placing a cable between the computer and the hub. In fact, the rest of the network users will not even notice the extension.

The routing function is performed by the central controller which centrally controls communication between any two computers by establishing a logical path between them. It means that if one computer A wants to send data to another computer B, Computer A sends the data to the controller & this controller then sends the data to computer B.

This Topology, obviously, require a great deal of cabling. This design provides an excellent platform for reconfiguration and trouble-shooting. Changes to the network are as simple as plugging another segment into the hub and a break in the LAN is easy to isolate and doesn't affect the rest of the network.

Advantages of Star Topology

The benefits of star topology are:

1. It is easier to add new node or modify any existing node without disturbing network i.e. expansion is easier.

2. Addition of new node does not increase communication delay.

3. If any local computer or link fails, the entire system does not collapse. Only that link or computer is affected.

4. It is easy to find device and cable problems i.e. fault identification and isolation is easier.

5. Media faults are automatically isolated to the failed segment.

Disadvantages of Star Topology

The disadvantages are considered as follows:

1. If the central controller or hub fails, entire system collapses.

2. Cabling cost is more as each node is connected individually to the hub.

3. Requires more cable than most topologies

4. Moderately difficult to install.

Mesh Topology:

The mesh network topology uses redundant connections between the node on the network, applying a fault tolerance strategy. Each node included in the network connected to the rest of the node, which explains why this type of topology requires extensive wiring. This type of topology can cope with the failure of one or two segments of the network without interrupting traffic since it has redundant lines.

Mesh networks are more expensive and difficult to install than other types of network topologies due to the large number of connections they require. In most cases, networks that use this redundant connection strategy included within the broader hybrid networks. In a hybrid network, only the most essential and crucial servers and computers configured with redundant connections. In this way, the fundamental segments of the corporate network protected without using multiple lines for each of the computers connected to the network.

Advantages of Mesh Topology

1. It is robust as the failure of one node does not collapse the entire system. If one link fails, the entire system continues to work.

2. There is no traffic congestion problem as dedicated links are being used.

3. Dedicated links ensure faster transmission without any delay.

4. Dedicated links also ensure data privacy and security.

5. Point to point links makes fault identification and isolation easier.

Disadvantages of Mesh Topology

1. Connecting each device to every other device in the network makes installation and reconfiguration difficult.

2. It has high cabling cost as n (n-l)/2 links are required to connect n nodes.

Tree Topology:

Tree or Hierarchical Topology: The type of  Topology in which a central 'root' node, the top level of the hierarchy, is connected to one or more other nodes that are one level lower in the hierarchy i.e., the second level, with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy, i.e., the third level, connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy – the hierarchy of the tree is symmetrical, each node in the network having a specific fixed number, f, of nodes connected to it at the next lower level in the hierarchy, the number, f, being referred to as the 'branching factor' of the hierarchical tree

Advantages:

Supported by several hardware and software venders.

It allows more devices to be attached to a single central hub and can therefore increases the distance a signal can travel between devices.

It allows the network to isolate and prioritize communication from different computers i.e. the computers attached to one secondary hub can be given priority over the computers attached to another secondary hub.

Disadvantages:

• Overall length of each segment is limited by the type of cabling used.

• If the backbone line breaks, the entire segment goes down.

• More difficult to configure and wire than other topologies.

• It has higher cabling cost in setting up a tree structure.

Hybrid Topology

The hybrid Topology is a type of Topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies in a single network that is composed of one or more interconnections of two or more networks that are based upon the same physical topology.

When two hubs of different topologies are joined so that the devices attached to them can communicate as in figure, it is called a Star-Bus network.

Network Layer Design Issues:

Store-and-Forward Packet Switching:

The major components of the system are the carrier's equipment (routers connected by transmission lines), shown inside the shaded oval, and the customers' equipment, shown outside the oval.

Host H1 is directly connected to one of the carrier's routers, A, by a leased line. In contrast, H2 is on a LAN with a router, F, owned and operated by the customer. This router also has a leased line to the carrier's equipment.

We have shown F as being outside the oval because it does not belong to the carrier, but in terms of construction, software, and protocols, it is probably no different from the carrier's routers.

Figure. The environment of the network layer protocols.

This equipment is used as follows.

A host with a packet to send transmits it to the nearest router, either on its own LAN or over a point-to-point link to the carrier. The packet is stored there until it has fully arrived so the checksum can be verified.

Then it is forwarded to the next router along the path until it reaches the destination host, where it is delivered. This mechanism is store-and-forward packet switching.

Services Provided to the Transport Layer:

The network layer provides services to the transport layer at the network layer/transport layer interface. An important question is what kind of services the network layer provides to the transport layer.

The network layer services have been designed with the following goals in mind.

1.      The services should be independent of the router technology.

2.      The transport layer should be shielded from the number, type, and topology of the routers present.

3.      The network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs.

Given these goals, the designers of the network layer have a lot of freedom in writing detailed specifications of the services to be offered to the transport layer. This freedom often degenerates into a raging battle between two warring factions.

The other camp argues that the subnet should provide a reliable, connection-oriented service. They claim that 100 years of successful experience with the worldwide telephone system is an excellent guide. In this view, quality of service is the dominant factor, and without connections in the subnet, quality of service is very difficult to achieve, especially for real-time traffic such as voice and video.

These two camps are best exemplified by the Internet and ATM. The Internet offers connectionless network-layer service; ATM networks offer connection-oriented network-layer service. However, it is interesting to note that as quality-of-service guarantees are becoming more and more important, the Internet is evolving.

Connection Oriented Communication Services:

There is a sequence of operation to be followed by the users of connection oriented service. These are:

1.      Connection is established.

2.      Information is sent.

3.      Connection is released.

In connection oriented service we have to establish a connection before starting the communication. When connection is established, we send the message or the information and then we release the connection.

Connection oriented service is more reliable than connectionless service. We can send the message in connection oriented service if there is an error at the receivers end. Example of connection oriented is TCP (Transmission Control Protocol) protocol.

Connection less Services:

It is similar to the postal services, as it carries the full address where the message (letter) is to be carried. Each message is routed independently from source to destination. The order of message sent can be different from the order received.

In connectionless the data is transferred in one direction from source to destination without checking that destination is still there or not or if it prepared to accept the message. Authentication is not needed in this. Example of Connectionless service is UDP (User Datagram Protocol) protocol.

Difference between Connection oriented and Connectionless service:

In connection oriented service authentication is needed, while connectionless service does not need any authentication.

Connection oriented protocol makes a connection and checks whether message is received or not and sends again if an error occurs, while connectionless service protocol does not guarantees a message delivery.

Connection oriented service is more reliable than connectionless service.

Connection oriented service interface is stream based and connectionless is message based.

TCP/IP Reference Model

The TCP/IP Reference Model

The ARPANET was a research network sponsored by the DoD (U.S. Department of Defence). It eventually connected hundreds of universities and government installations,using leased telephone lines. When satellite and radio networks were added later, the existing protocols had trouble inter-working with them, so a new reference architecture was needed. Thus, from nearly the beginning, the ability to connect multiple networks in a seamless way was one of the major design goals.This architecture later became known as the TCP/IP Reference Model, after itstwo primary protocols. 

The Link Layer

All the requirements of DoD led to the choice of a packet-switching network based on a connection less layer that runs across different networks.

The lowest layer in the model, the link layer describes what links such as serial lines and classic Ethernet must do to meet the needs of this connection less internet layer. It is not really a layer at all, in the normal sense of the term, but rather an interface between hosts and transmission links. The Internet Layer

The internet layer is the linchpin (hub) that holds the whole architecture together.It is shown in Fig. as corresponding roughly to the OSI network layer. Its job is to permit hosts to inject packets into any network and have them travel independently to the destination (potentially on a different network). They may even arrive in a completely different order than they were sent, in which case it is the job of higher layers to rearrange them, if in-order delivery is desired.

The internet layer defines an official packet format and protocol called IP(Internet Protocol), plus additionally coordinating protocol called ICMP (Internet Control Message Protocol) that helps it function. The job of the internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly a major issue here, as is congestion (though IP has not proven effective at avoiding congestion).

The Transport Layer

The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been defined here.

The first one, TCP(Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. It segments the incoming byte stream into discrete messages and passes each one on to the internet layer. At the destination,the receiving TCP process reassembles the received messages into the output stream. TCP also handles flow control to make sure a fast sender cannot swamp as low receiver with more messages than it can handle.

The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connection less protocol for applications that do not want TCP’s sequencing or flow control and wish to provide their own. It is also widely used for one-shot, client-server-type request-reply queries and applications in which prompt delivery is more important than accurate delivery, such as transmitting speech or video.

The Application Layer

The TCP/IP model does not have session or presentation layers. No need for them was perceived. Instead, applications simply include any session and presentation functions that they require.

On top of the transport layer is the application layer. It contains all the higher-level protocols. The early ones included virtual terminal (TELNET), file transfer(FTP), and electronic mail (SMTP).

Transmission Media

TRANSMISSION MEDIA

In data communication terminology, a transmission medium is a physical path between the transmitter and the receiver i.e. it is the channel through which data is sent from one place to another. Transmission Media is broadly classified into the following types:

Types of Transmission Media

A transmission medium can be broadly defined as anything that can carry information from a source to a destination. For example, the transmission medium for two people having a dinner conversation is the air. The air can also be used to convey the message in a smoke signal or semaphore. For a written message, the transmission medium might be a mail carrier, a truck, or an airplane.

In data communications the definition of the information and the transmission medium is more specific. The transmission medium is usually free space, metallic cable or optical cable. The information is usually a signal that is the result of conversion of data from another form.

Magnetic Media

One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or removable media (e.g., recordable CDs, DVDs), physically transport the tape or disks to the destination machine, and read them back in again.

Although this method is not as sophisticated as using a geosynchronous communication satellite, it is often more cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor.

A simple calculation will make this point clear. An industry-standard Ultrium tape can hold 800 gigabytes. A box 60 × 60 × 60 cm can hold about 1000 of these tapes, for a total capacity of 800 terabytes, or 6400 terabits (6.4 petabits). A box of tapes can be delivered anywhere in the United States in 24 hours by Federal Express and other companies. The effective bandwidth of this transmission is 6400 terabits/86,400 sec, or a bit over 70 Gbps. If the destination is only an hour away by road, the bandwidth is increased to over 1700 Gbps. No computer network can even approach this. Of course, networks are getting faster, but tape densities are increasing, too.

I. Guided Media

Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light.

1. Twisted-Pair Cable

A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together, as shown below figure.

Twisted Pair Cable

One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. The receiver uses the difference between the two.

In addition to the signal sent by the sender on one of the wires, interference (noise) and crosstalk may affect both wires and create unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because they are at different locations relative to the noise or crosstalk sources (e,g., one is closer and the other is farther). This results in a difference at the receiver. By twisting the pairs, a balance is maintained. For example, suppose in one twist, one wire is closer to the noise source and the other is farther; in the next twist, the reverse is true. Twisting makes it probable that both wires are equally affected by external influences (noise or crosstalk). This means that the receiver, which calculates the difference between the two, receives no unwanted signals. The unwanted signals are mostly canceled out. From the above discussion, it is clear that the number of twists per unit of length (e.g., inch) has some effect on the quality of the cable.

Applications

Twisted-pair cables are used in telephone lines to provide voice and data channels. The local loop-the line that connects subscribers to the central telephone office-commonly consists of

Unshielded twisted pair cables. The DSL line that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-pair cables. Local-area networks, such as lOBase-T and lOOBase-T, also use twisted-pair cables.

2. Coaxial Cable

Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover (below figure).

Coaxial Cable

Applications

Coaxial cable was widely used in analog telephone networks where a single coaxial network could carry 10,000 voice signals. Later it was used in digital telephone networks where a single coaxial cable could carry digital data up to 600 Mbps. However, coaxial cable in telephone networks has largely been replaced today with fiber-optic cable. Cable TV networks also use coaxial cables.

In the traditional cable TV network, the entire network used coaxial cable. Later, however, cable TV providers replaced most of the media with fiber-optic cable; hybrid networks use coaxial cable only at the network boundaries, near the consumer premises. Cable TV uses RG-59 coaxial cable. Another common application of coaxial cable is in traditional Ethernet LANs. Because of its high bandwidth, and consequently high data rate, coaxial cable was chosen for digital transmission in early Ethernet LANs.

3. Fiber Optic Cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light. Light travels in a straight line as long as it is moving through a single uniform If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction. Figure 7.10 shows how a ray of light changes direction when going from a more dense to a less dense substance.

Light Changing Direction

As the figure shows, if the angle of incidence I (the angle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser substance. Note that the critical angle is a property of the substance, and its value differs from one substance to another.

Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. See Figure below.

Fiber Optics Cable

Applications

Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost-effective. Today, with wavelength-division multiplexing (WDM), we can transfer data at a rate of 1600 Gbps. The SONET network provides such a backbone. Some cable TV companies use a combination of optical fiber and

coaxial cable, thus creating a hybrid network. Optical fiber provides the backbone structure while coaxial cable provides the connection to the user premises. This is a cost-effective configuration since the narrow bandwidth requirement at the user end does not justify the use of optical fiber. Local-area networks such as 100Base-FX network (Fast Ethernet) and 1000Base-X also use fiber-optic cable.

Advantages and Disadvantages of Optical Fiber Advantages

Fiber-optic cable has several advantages over metallic cable (twisted pair or coaxial).

1. Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Currently, data rates and bandwidth utilization over fiber-optic cable are limited not by the medium but by the signal generation and reception technology available.

2. Less signal attenuation. Fiber-optic transmission distance is significantly greater than that of other guided media. A signal can run for 50 km without requiring regeneration. We need repeaters every 5 km for coaxial or twisted-pair cable.

3. Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic cables.

4. Resistance to corrosive materials. Glass is more resistant to corrosive materials than copper.

5. Light weight. Fiber-optic cables are much lighter than copper cables.

6. Greater immunity to tapping. Fiber-optic cables are more immune to tapping than copper cables. Copper cables create antenna effects that can easily be tapped.

Disadvantages

There are some disadvantages in the use of optical fiber.

1. Installation and maintenance. Fiber-optic cable is a relatively new technology. Its installation and maintenance require expertise that is not yet available everywhere.

2. Unidirectional light propagation. Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed.

3. Cost. The cable and the interfaces are relatively more expensive than those of other guided media. If the demand for bandwidth is not high, often the use of optical fiber cannot be justified.

                        II. UNGUIDED MEDIA: WIRELESS

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them.

Ionosphere with Signal Propagation

Unguided signals can travel from the source to destination in several ways: ground propagation, sky propagation, and line-of-sight propagation, as shown in Figure In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth.

These low-frequency signals emanate in all directions from the transmitting antenna and follow the curvature of the planet. Distance depends on the amount of power in the signal: The greater the power, the greater the distance. In sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. This type of transmission allows for greater distances with lower output power.

In line of sight propagation, very high frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other, and either tall enough or close enough together not to be affected by the curvature of the earth. Line-of-sight propagation is tricky because radio transmissions cannot be completely focused.

1. Radio Waves

Waves ranging in frequencies between 3 kHz and 1 GHz are called radio waves. Radio waves, for the most part, are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned. A sending antenna sends waves that can be received by any receiving antenna. The omnidirectional property has a disadvantage, too.

The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band. Radio waves, particularly those waves that propagate in the sky mode, can travel long distances. This makes radio waves a good candidate for long-distance broadcasting

such as AM radio. Radio waves, particularly those of low and medium frequencies, can penetrate walls.

This characteristic can be both an advantage and a disadvantage. It is an advantage because, for example, an AM radio can receive signals inside a building. It is a disadvantage because we cannot isolate a communication to just inside or outside a building. The radio wave band is relatively narrow, just under 1 GHz, compared to the microwave band. When this band is divided into sub bands, the sub bands are also narrow, leading to a low data rate for digital communications.

Omnidirectional Antenna

Radio waves use omnidirectional antennas that send out signals in all directions. Based on the wavelength, strength, and the purpose of transmission, we can have several types of antennas. Below figure 7.20 shows an omnidirectional antenna.

Omnidirectional Antenna

 Applications

The omnidirectional characteristics of radio waves make them useful for multicasting, in which there is one sender but many receivers. AM and FM radio, television, maritime radio, cordless phones, and paging are examples of multicasting.

2. Microwaves

Electromagnetic waves having frequencies between I and 300 GHz are called microwaves. Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. The unidirectional property has an obvious advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas. The following describes some characteristics of microwave propagation:

1. Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall. The curvature of the earth as well as other blocking obstacles does not allow two short towers to communicate by using microwaves. Repeaters are often needed for long distance communication.

2. Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside buildings.

3. The microwave band is relatively wide, almost 299 GHz. Therefore wider sub bands can be assigned, and a high data rate is possible.

4. Use of certain portions of the band requires permission from authorities.

Unidirectional Antenna

Microwaves need unidirectional antennas that send out signals in one direction. Two types of antennas are used for microwave communications: the parabolic dish and the horn (see below figure). A parabolic dish antenna is based on the geometry of a parabola: Every line parallel to the line of symmetry (line of sight) reflects off the curve at angles such that all the lines intersect in a common point called the focus.

The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver. Outgoing transmissions are broadcast through a horn aimed at the dish.

The microwaves hit the dish and are deflected outward in a reversal of the receipt path. A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem (resembling a handle) and deflected outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the stem.

Unidirectional Antennas

3. Infrared

Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for short-range communication. Infrared waves, having high frequencies, cannot penetrate walls. This advantageous characteristic prevents interference between one system and another; a short-range communication system in one room cannot be affected by another system in the next room. When we use our infrared remote control, we do not interfere with the use of the remote by our neighbors. However, this same characteristic makes infrared signals useless for long-range communication. In addition, we cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication.

Applications

The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide bandwidth can be used to transmit digital data with a very high data rate. The Infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established standards for using these signals for communication between devices such as keyboards, mice, PCs, and printers. For example, some manufacturers provide a special port called the IrDA port that allows a wireless keyboard to communicate with a PC. The standard originally defined a data rate of 75 kbps for a distance up to 8 m. The recent standard defines a data rate of 4 Mbps.

Infrared signals defined by IrDA transmit through line of sight; the IrDA port on the keyboard needs to point to the PC for transmission to occur.