Lecture 1 & 2

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Lecture 1
1.1 Why do we need local networks
1.2 LANs, WANs, and MANs
These 3 are all examples of communication networks. It’s a facility that interconnects a number of devices and provides a means for transmitting data from one attached device to another. LAN, WAN, and MAN are classified on the basis of geographical scope.

WAN: Wide-area networks
LAN: Local-area networks
MAN: Metropolitan-area networks

1.3 Backbone local networks
A single LAN in a premise is simple but not optimized. Here are the drawbacks:
Solution:
Divide the networks into several LAN groups where the devices have compatible data rate to each other, many slower LANs converge to a faster LAN and it becomes the backbone local networks for the facility. If greater distance is required, MAN can be used for the backbone.

LAN architecture
Typical scope: So it depends on the scope of each premise, the LAN architecture has to be proper design.


Lecture 2

In the previous section you learned how signals are transmitted over a network link. In this section we discuss the specific types of cable and wireless networking: Computers send electronic signals to each other using electric currents, radio waves, microwaves, or light-spectrum energy from the electromagnetic spectrum. These signals represent network data as binary impulses (0'S and 1's). The physical path through which computers send and receive these signals is called the transmission media.

Objective
Select the appropriate media for various situations.

Media choices include:
Situational elements include:
2.01 Circuit
In the world of computer networking, the term circuit is used in many different contexts. A circuit is basically a link between two devices. A voice telephone call is a dedicated circuit between two people. In a LAN, the physical wire may be shared by many different workstations, but when two stations are communicating with one another, the physical wire appears as a circuit between them.

Virtual circuits are common in WANs and internal networks that use ATM backbones. Basically, a virtual circuit appears to the end systems as a dedicated wire for transmitting information across a communication system. However, the underlying communication system may be cell, frame, or packet switched. The underlying network may be a mesh network on which a dedicated path through the network has been created. This path appears as a circuit.

Telephone companies offer circuit-switched services and packet-switched services. A circuit-switched service is one in which you make a call to another location to transmit voice or data. A packet-switched service is more like an any-to-any connection, in which packets from one site can be routed to many different destination sites over a packet-switched network. Put another way there are connection-oriented services that appear as circuits and connectionless services, which are often called datagram delivery services because they emulate a postal system in their deliver context.



2.01A Circuit-Switching Services
A circuit-switched service provides a temporary dedicated point-to-point circuit for data transmission through a carrier's switching systems (see "Circuit-Switching Services"). Customers can contract for various types of services, depending on their anticipated bandwidth needs. Each of the services discussed in the following paragraphs are covered in more detail under separate headings.

SWITCHED-56 SERVICES : Switched-56 is a digital switched service that operates at 56 Kbits /sec. A special Switched-56 data set device interfaces between the carrier's wire pairs and the customer's internal device (usually a router). Switched-56 services were originally intended to provide an alternate backup route for higher-speed leased lines such as T1 If a leased line failed, a Switched-56 line would quickly establish an alternate connection. Switched-56 can still be used in this way, but it is also used to handle peaks in traffic, fax transmissions, backup sessions, bulk e-mail transfers, and LAN-to-LAN connections. Rates are calculated by the minute in most cases.

DIGITAL 800 SERVICES: This is a carrier offering that expands on Switched-56. Basically, it provides toll-free (800 number) digital switched services. Multiplexing can be used to combine multiple circuits into a wide-bandwidth circuit as network traffic increases.

ISDN (INTEGRATED SERVICES DIGITAL NETWORK) : ISDN is a circuit-switched service that provides three channels for voice or data transmissions. Two of the channels provide 64-Kbit /sec data or voice, and a third provides signaling to control the channels. ISDN is offered in selected areas.



Circuit switching, as opposed to packet switching sets up a dedicated communication channel between two end systems. Voice calls on telephone networks are an example. For a home or office connection, a circuit starts out on a pair of twisted wires from the caller's location to a telephone switching center in the local area. If the connection is between two phones in the same area, the local switch completes the circuit. This is pictured as connection A1-A2 in Figure C-5. If the connection is between phones in two different areas, a circuit is set up through an intermediate exchange as shown by circuit C1-C2. Long-distance circuits are made through a remote switching office as shown by circuit B1-B2.

The difference between dedicated and switched circuits is that a dedicated circuit is always connected, and a switched circuit can be set up and disconnected at any time, reducing connect charges. The difference between circuit- and packet-switching services is pictured in figure below.


Switched circuits can supplement a dedicated line. For example, an appropriate bridge or router may use a dial-on-demand protocol to automatically dial a switched line if the traffic on the dedicated line exceeds its capabilities. Switched circuits are also used to perform occasional data transfers between remote offices. A switched circuit might connect every 15 minutes to transfer the latest batch of electronic mail.

In a packet-switching network, data is divided into packets and transmitted across a network that is shared by other customers. The packets are interleaved with packets from other sources. This uses the network more efficiently and reduces usage charges, but packet-switched networks are subject to delays if another customer overloads the network with too much traffic. The phone companies have developed high-speed switching networks that implement ATM (Asynchronous Transfer Mode) to solve this problem. ATM uses fixed-size cells and high-speed switching to improve service.

Note in the figure above that dedicated circuits are used to access a packet-switched network. These circuits are usually local leased lines or circuit-switched connections that funnel packets from a customer's site into the packet-switched network. They may be ISDN lines or high-capacity T-1 (1.544-Mbit / sec) lines.

Here are some things to note about circuit-switched services: As mentioned, ISDN (Integrated Services Digital Network) is an example of a circuit-switching service. Basic rate ISDN provides two 64-Kbit / sec circuit-switched channels that can be used for either voice calls or data communications. ISDN phones digitize analog voice into digital information for transmission across the circuit. The two 64-Kbit / sec channels can be combined into a single 128-Kbit / sec channel for data transfers. Users can "dial" any location to set up a circuit, thus the connection is switched. In contrast, broadband ISDN has a packet-switched orientation and can be scaled up to very high data rates. ISDN is a product of the phone company's desire to create a fully digital telephone system with circuit-switching capabilities. It was first proposed in the early 1980s and is still under construction.



2.01B Dedicated Digital Services
Digital circuits can provide data transmission speeds up to 45 Mbits /sec. Currently, digital lines are made possible by "conditioning" normal lines with special equipment to handle higher data rates. The lines are leased from the telephone company and installed between two points (point to point) to provide dedicated, full-time service. You'll need bridges or routers to connect LANs to digital lines. Voice and data multiplexors are also required if you plan to mix both voice and data channels.

The traditional digital line service is the T1 channel, which provides transmission rates of 1.544 Mbits /sec. T1 lines can carry both voice and data, so they are often used to provide voice telephone connections between an organization's remote sites. The lines are fractional, meaning that they can be leased as subchannels. T1 can be divided into 24 channels of 64-Kbit / sec bandwidth each, which is the bandwidth needed for a digitized voice call. Alternatively, a T3 line can provide the equivalent of 28 T1 lines for users who need a lot of bandwidth.



2.01C EMERGING SERVICES
DSL (Digital Subscriber Line) services are emerging that use the existing twisted-pair copper wire in the local loop to provide data rates of up to 60 Mbits / sec. Interestingly, the closer an end user is to the telephone company's switching office, the faster the data rate. In the past, these rates were not possible because the carrier's switching equipment was only designed to handle a narrow bandwidth for voice. But carriers see DSL as a way to provide bandwidth-hungry Internet users with all the speed they need. The equipment will typically require a PC with an Ethernet card and a DSL modem. The service is dedicated, not dial-up, so the typical configuration is to run a line from the customer's site to an ISP (Internet service provider). From there, customer data is packet-switched to appropriate destinations. Both voice and data can be transported using this scheme, so voice calls over the Internet products should become more popular as these schemes are put into place.



2.02 Packet-Switching Services

A packet-switched network transports data (or digitized voice) over a mesh of interconnected circuits. The term packet is used loosely here because the carriers deliver data in either frames (i.e., frame relay) or cells (i.e., ATM, Asynchronous Transfer Mode). Here, packet refers to a generic block of information that is transmitted through the mesh from one point to another. The important point about a switched service is that it provides any-to-any connections, as shown in figure above

The carriers prefer to preprogram VCs (virtual circuits) through their networks and lease them. You specify the locations where you want to send data (i.e., your remote branch offices) and the carrier programs the routers at each of the junctions to immediately switch the packets along an appropriate path.

Note that a circuit of appropriate bandwidth is required between the customer site and the carrier's access point into the switched network. This circuit might be a dial-up line or a dedicated circuit. However, because the distance between the customer and the access point is small, charges are minimal when compared to running a dedicated circuit end to end since such circuits carry distance charges. Organizations can use these services to create virtual data networks over wide areas that connect all of their remote sites.

2.02A - X.25: X.25 is a standard, well-tested, protocol that has been a workhorse packet-switching service since 1976. It is suitable for light loads and was commonly used to provide remote terminal connections to mainframe computer and because a large portion of the bandwidth is used for error checking. This error checking was important in the days of low-quality analog telephone lines, but today's high-quality fiber-optic circuits do not usually need these controls.

2.02B - FRAME RELAY: Frame relay provides services similar to X.25, but is faster and more efficient. Frame relay assumes that the telecommunication network is relatively error-free and does not require the extensive error-checking and packet acknowledgment features of X.25. Frame relay is an excellent choice for organizations that need any-to-any connections on an as-needed basis.

2.02C - CELL SWITCHING: Cell-switching networks, namely ATM, provide "fast-packet" switching services that can transmit data at megabit- and potentially gigabit-persecond rates. Carriers have already made a major switch to ATM switching and are now moving the services into local areas. The goal is to eventually use ATM all the way up to the customer premises.


Transmission Media

The electromagnetic spectrum provides a wide variety of ways in which signals may be passed through transmission media from one computer to another. The electromagnetic spectrum ranges from electric currents to infrared light and gamma rays. Figure 2.12 shows the electromagnetic spectrum divided into waveforms and their frequencies.



Transmission media are divided into two categories:


Networks that cover multiple sites frequently use combinations of cable and wireless media to link computers and devices. Each media type has certain characteristics that make it suitable for particular networks. To choose the best type of media for your network, you should know how each medium's characteristics relate to the following factors:



The following sections describe how these factors affect your network. The specifics of how each medium performs in these areas are covered later in this chapter.

Cost
The cost of each media type should be weighed against the performance it provides, and the available resources. For example, it is common practice among network integrators to attempt to run a network across unused, leftover telephone cabling. Although this could reduce costs, in many cases it is not a viable solution-for example, when cable drops of greater than 100 meters are required.

Installation
Each network installation is different, and you must look for the most affordable viable solution. You should take into consideration your real needs. For example, fiber-optic cable is fast, but you may not need that much speed. It's easy to spend too much if you're not writing the checks.

How difficult installation is depends on the individual situation, but some general comparisons between the media are possible. Some types of media can be installed with simple tools and little training; others require more training and knowledge and may be better left to professionals. For example, unshielded twisted-pair cable is easy to install, but fiber-optic cable requires professional training. To connect two lengths of fiber together, you may need to use electric fusion or a chemical epoxy process. These are jobs you probably don't want to undertake unless you know how to do them. Later sections of this chapter discuss installation in detail.

Bandwidth Capacity
The capacity of a medium is usually measured in bandwidth. In the world of networking, bandwidth is measured in terms of megabits per second (Mbps). Ethernet, for example, has a bandwidth of 1OMbps. A medium with a high capacity has a high bandwidth; a medium with a low capacity has a low bandwidth.

Note: In the field of communications, the term bandwidth refers to the range of frequencies a medium can accommodate. In networking, bandwidth is measured in terms of the number of bits that can be transmitted across a given medium per second

A high bandwidth normally increases throughput and performance, but the cable length and signaling techniques affect the bandwidth a cable can accommodate.

Node Capacity
Also of vital importance on a network is how many computers you can attach easily to the network cables. Each network cabling system has a natural number of computers that can be attached to the network before expensive devices such as bridges, routers, repeaters, and hubs must be used to expand the network.

Attennuation
Electromagnetic signals tend to weaken during transmission. This is referred to as attenuation. As the signals pass through the transmission medium, part of their strength is absorbed or misdirected. This phenomenon imposes limits on the distance a signal can travel through a medium without unacceptable degradation. The farther you are from a person, the harder it is to hear what that person is saying. Part of this is attenuation, and part is interference.
When the signal gets weak, it becomes difficult to tell a 1 from a 0, and errors can creep into the communication link. Because of attenuation and dispersion, you must be careful that network cables do not exceed the maximum length recommended for that type of cable. Exceeding the limits may lead to intermittent errors or network failure.

Electromagnetic Interference
Electromagnetic interference (EMI) affects the signal that is sent through the transmission media. EMI is caused by outside electromagnetic waves affecting the desired signal, making it more difficult for the receiving computer to decode the signal. Some media are more influenced by EMI than others. EMI is often referred to as noise. If you are in a quiet room, it is easier to hear a person than if you are at a rock concert.
A related concern is eavesdropping, especially if your network data requires a high level of security. The same characteristics of a cable that allow an external signal to interfere with the signal on the cable also make it easy for someone to detect the signal on the cable externally, without piercing the cable. Therefore, if you need a cable that is relatively invulnerable to eavesdropping, look for a cable that is relatively invulnerable to EMI.

2.1 CABLE MEDIA
Cables have a central conductor that consists of a wire or fiber surrounded by a plastic jacket. Three types of cable media are twisted-pair, coaxial, and fiber-optic cable. Two types of twisted-pair cable are used in networks: unshielded (UTP) and shielded (STP). Table 2.1 summarizes the characteristics of these types of cable media, which are discussed in the following sections.

Characteristics of cable media



2.1A Twisted-Pair Cable
Twisted-pair cable uses one or more pairs of two twisted copper wires to transmit signals. It is commonly used as telecommunications cable.
When copper wires that are close together conduct electric signals, there is a tendency for each wire to produce interference in the other. One wire interfering with another in this way is called crosstalk. To decrease the amount of crosstalk and outside interference, the wires are twisted. Twisting the wires allows the emitted signals from one wire to cancel out the emitted signals from the other and protects them from outside noise.
Twisted pairs are two color-coded, insulated copper wires that are twisted around each other. A twisted-pair cable consists of one or more twisted pairs in a common jacket.

Let's look at the two types of twisted-pair cable: unshielded and shielded.

2.1B Unshielded Twisted-Pair Cable
Unshielded twisted-pair (UTP) cable consists of a number of twisted pairs with a simple plastic casing. UTP is commonly used in telephone systems. Figure below shows a UTP cable.

The Electrical Industries Association (EIA) divides UTP into different categories by quality grade. The rating for each category refers to conductor size, electrical characteristics, and twists per foot. The following categories are defined:

Categories 1 and 2 were originally meant for voice communication and can support only low data rates, less than 4 megabits per second (Mbps). These cannot be used for high-speed data communications. Older telephone networks used Category 1 cable.

Category 3 is suitable for most computer networks. Some innovations can allow data rates much higher, but generally Category 3 offers data rates up to 16Mbps. This category of cable is the kind currently used in most telephone installations.

Category 4 offers data rates up to 20Mbps.

Category 5 offers enhancements over Category 3, such as support for Fast Ethernet, more insulation, more twists per foot, and data rates of 10OMbps and higher, but Category 5 requires compatible equipment and more stringent installation. In a Category 5 installation, all media, connectors, and connecting equipment must support Category 5, or performance will be affected.

Data-grade UTP cable (Categories 3, 4, and 5) consists of either four or eight wires. A UTP cable with four wires is called a two-pair. Network topologies that use UTP require at least two-pair wire. You may want to include an extra pair for future expansion. Figure below shows a four-pair cable.

Because UTP cable was originally used in telephone systems, UTP installations are often similar to telephone installations. For a four-pair cable, you need a modular RJ-45 telephone connector. For a two-pair cable, you need a modular RJ- 11 telephone connector. These connectors are attached to both ends of a patch cable. One end of the patch cable is then inserted into a computer or other device, and the other end is inserted into a wall jack. The wall jack connects the UTP drop cable (another length of cable) to a punch-down block. The other side of the punch-down block is wired to a patch panel. The patch panel provides connectivity through patch cables to other user devices and connectivity devices. Figure below shows how UTP might be installed.

UTP's popularity is partly because UTP was first used in telephone systems. In many cases a network can be run over the already existing wires installed for the phone system, at a great savings in installation cost.

Note: Before using existing telephone wiring for your data network make sure the wiring can handle data transmission. A telephone is much more forgiving than a data network and will operate when the wiring lacks proper twisting and other electrical characteristics that a computer network needs.

UTP cable has the following characteristics:
2.1C Shielded Twisted-Pair Cable
The only difference between shielded twisted-Pair (STP) and UTP is that STP cable has a shield-usually aluminum/polyester-between the outer jacket or casing and the wires. Figure below shows STP cable.

The shield makes STP less vulnerable to EMI because the shield is electrically grounded. If a shield is grounded correctly, it tends to prevent signals from getting into or out of the cable. It is a more reliable cable for LAN environments. STP was the first twisted-pair cable to be used in LANs. Although many LANs now use UTP, STP is still used.

Transmission media specifications from IBM and Apple Computer use STP cable. IBM's Token Ring network uses STP, and IBM has its own specifications for different qualities and configurations of STP. A completely different type of STP is the standard for Apple's AppleTalk networks. Networks that conform to each vendor's specifications have their own special requirements, including connector types and limits on cable length. STP has the following characteristics:

See Table of Chracteristics of Cable Media (shown earlier in this chapter) for a comparison of the characteristics of STP and UTP cable.

Note: Real world problem
Your company recently renovated its telephone system, including cabling, and had installed excess cables for future growth. Now you would like to use the excess cabling to network the computers in your company.

What sort of network performance (data capacity and cable length) can you reasonably expect from these new telephone cables?

You would like to use the old telephone cables in your building to network your computers.

What sort of network performance (data capacity and cable length) can you reasonably expect from these old telephone cables?

You will install a new physical plant (network cabling system) for your network. You will use Category-5 unshielded twisted-pair cabling throughout.

What sort of network performance (data capacity and cable length) can you reasonably expect from this new physical plant?

Your company has offices and manufacturing facilities in the same building, and you must network computers throughout the building.

In what circumstances might you use shielded twisted-pair cabling in your network?


2.1D Coaxial Cable
Coaxial cable, commonly called coax, has two conductors that share the same axis. A solid copper wire or stranded wire runs down the center of the cable, and this wire is surrounded by plastic foam insulation. The foam is surrounded by a second conductor, a wire mesh tube, metallic foil, or both. The wire mesh protects the wire from EMI. It is often called the shield. A tough plastic jacket forms the cover of the cable, providing protection and insulation. Figure below shows a coaxial cable.

Coaxial cable comes in different sizes. It is classified by size (RG) and by the cable's resistance to direct or alternating electric currents (measured in ohms, also called impedance).

The following are some coaxial cables commonly used in networking: Coaxial cable has the following characteristics:



2.1E PVC and Plenum Cable
Polyvinyl chloride (PVC) is commonly used in coaxial cabling because it is a flexible, inexpensive plastic well suited for use as insulation and cable jacketing. PVC is often used in the exposed areas of an office.

A plenum is the space between the false ceiling of an office and the floor above. The air in the plenum circulates with the air in the rest of the building, and there are strict fire codes about what can be placed in a plenum environment.

Because PVC gives off poisonous gases when burned, you cannot use it in a plenum environment. You must use plenum-grade cable instead. Plenum-grade cable is certified to be fire resistant and to produce a minimum amount of smoke. Plenum cable is also used in vertical runs (walls) without conduit (a tube to hold the cable). Plenum cable is more expensive and less flexible than PVC.

Note: Fire safety is a deadly serious issue. Consult your local fire and electrical regulations before running cable in your office.



2.2 Fiber-Optic Cable
Fiber-optic cable transmits light signals rather than electrical signals. It is enormously more efficient than the other network transmission media. As soon as it comes down in price (both in terms of the cable and installation costs), fiber-optic will be the choice for network cabling.

Note: Real world problems
The facilities manager for your industrial complex informs you that there is already plenty of coaxial cable in the buildings, installed for closed-circuit television. She suggests that you run your thicknet network over this already-installed cable instead of installing new cable

What is your reply to the facilities manager?

The ceilings in your office are a plenum environment.

How will this affect the installation of a network in your office?

You need to install a small network inexpensively.

Is coaxial cable a good choice, and if so, which type of coaxial cable would you use?


Each fiber has an inner core of glass or plastic that conducts light. The inner core is surrounded by cladding, a layer of glass that reflects the light back into the core. Each fiber is surrounded by a plastic sheath. The sheath can be either tight or loose. Figure below shows examples of these two types of fiber-optic cables.

Tight configurations completely surround the fibers with a plastic sheath and sometimes include wires to strengthen the cable (although these wires are not required). Loose configurations leave a space between the sheath and the outer jacket, which is filled with a gel or other material. The sheath provides the strength necessary to protect against breaking or extreme heat or cold. The gel, strength wires, and outer jacket provide extra protection.

A cable may contain a single fiber, but often fibers are bundled together in the center of the cable. Optical fibers are smaller and lighter than copper wire. One optical fiber is approximately the same diameter as a human hair.

Optical fibers may be multimode or single-mode. Single-mode fibers allow a single light path and are typically used with laser signaling. Single-mode fiber can allow greater bandwidth and cable runs than multimode but is more expensive. Multimode fibers use multiple light paths. The physical characteristics of the multimode fiber make all parts of the signal (those from the various paths) arrive at the same time, appearing to the receiver as though they were one pulse. If you want to save money, look into multimode, since it can be used with LEDs (light-emitting diodes), which are a more affordable light source than lasers. Figure below shows single-mode and multimode fibers.

Optical fibers are differentiated by core/cladding size and mode. The size and purity of the core determine the amount of light that can be transmitted. The following are the common types of fiber-optic cable: A typical LAN installation starts at a computer or network device that has a fiber-optic network interface card (NIC). This NIC has an incoming interface and an outgoing interface. The interfaces are directly connected to fiberoptic cables with special fiber-optic connectors. The opposite ends of the cables are attached to a connectivity device or splice center.

Splicing fiber-optic cable can involve electric fusion, chemical epoxy, or mechanical connectors. Fiber-optic cables, with cores as thin as 8.3 microns, can be very difficult to line up precisely.

Optical interface devices convert computer signals into light for transmission through the fiber. Conversely, when light pulses come through the fiber, the optical interface converts them into computer signals. For single-mode fiber, light pulses are created by injection laser diodes (ILDs), which create a higher quality of light. For multimode fiber, LEDs are used. When the light pulse is received, it is converted back into electric signals by P-intrinsic N diodes or photodiodes.

Fiber-optic cable has the following characteristics:

Real World Problems
You must connect a research institution's main offices with the testing site 30 miles away. You will connect them with fiber-optic cable.

Which type of fiber cable will you use? Why will you not use the other type?

You are replacing your thicknet backbone between your servers with a high-speed network cable. You are considering using Category 5 twisted-pair cable or multimode fiber cable.

Under what circumstances would you install the fiber?

Under what circumstances would you not install the fiber?



2.3 Wireless Media

WIRELESS MEDIA do not use an electrical or optical conduct( In most cases, the earth's atmosphere is the physical path for the data. Wireless media is therefore useful when distance or obstructions make bounded media difficult. There are three main types of wireless media: radio wave, microwave, and infrared.

2.3A Radio Wave Transmission Systems
Radio waves have frequencies between 10 kilohertz (KHz) and I gigahertz (GHz). The range of the electromagnetic spectrum between 10KHz and 1GHz is called radio frequency (RF). Radio waves include the following types:

Most radio frequencies are regulated. To use a regulated frequency, you must receive a license from the regulatory body over that area (the FCC in the United States). Getting a license can take a long time, costs more, and makes it more difficult to move equipment. However, licensing guarantees that, within a determined area, you will have clear radio transmission. The advantage of unregulated frequencies is that there are few restrictions placed on them. One regulation, however, does limit the usefulness of unregulated frequencies: unregulated frequency equipment must operate at less than 1 watt. The point of this regulation is to limit the range of influence a device can have, thereby limiting interference with other signals. In terms of networks, this makes unregulated radio communication bandwidths of limited use.

Because unregulated frequencies are available for use by others in your area, you cannot be guaranteed a clear communication channel.

In the United States, the following frequencies are available for unregu- lated use: Radio waves can be broadcast onmidirectionally or directionally. Various kinds of antennas can be used to broadcast radio signals. Typical antennas include the following: Figure below shows these common types of radio frequency antennas. The power of the RF signal is determined by the antenna and transceiver (a device that TRANSmits and reCEIVEs a signal over a medium such as copper, radio waves, or fiber-optic cables). Each range has characteristics that affect its use in computer networks. For computer network applications, radio waves fall into three categories:

Table below summarizes the characteristics of the three types of radio wave media, which are described in the following sections.

Factor Low power, single frequency High power, single frequency Spread Spectrum
Freq. Range All radio freq. ( typically low GHz range ) All radio freq. ( typically low GHz range ) All radio freq. ( typically 902-928 MHz in US, 2.4GHz also used )
Cost Moderate for wireless Higher than low power, single freq. Moderate
Installation Simple High Moderate
Bandwidth capacity 1-10 Mbps 1-10 Mbps 2-6 Mbps
Attenuation High ( 25 meter ) Low High
EMI Poor Poor Fair


A. Low-Power, Single- Frequency
As the name implies, single-frequency transceivers operate at only one frequency. Typical low-power devices are limited in range to around 20 to 30 meters. Although low-frequency radio waves can penetrate some materials, the low power limits them to the shorter, open environments.
Low-power, single-frequency transceivers have the following characteristics:



B. High-Power, Single-Frequency
High-power, single-frequency transmissions are similar to low-power, singlefrequency transmissions but can cover larger distances. They can be used in long-distance outdoor environments. Transmissions can be line-of-sight or can extend beyond the horizon as a result of being bounced off the earth's atmosphere. High-power, single-frequency can be ideal for mobile networking, providing transmission for land-based or marine-based vehicles as well as aircraft. Transmission rates are similar to low-power rates, but at much longer distances.

High-power, single-frequency transceivers have the following characteristics: C. Spread-Spectrum
Spread-spectrum transmissions use the same frequencies as other radio frequency transmissions, but instead of using one frequency, they use several simultaneously. You can use two modulation schemes to accomplish this: direct-sequence modulation and frequency hopping.

Direct frequency modulation is the most common modulation scheme. It works by breaking the original data into parts (called chips), which are then transmitted on separate frequencies. To confuse eavesdroppers, spurious signals can also be transmitted. The transmission is coordinated with the intended receiver, who is aware of which frequencies are valid. The receiver can then isolate the chips and reassemble the data while ignoring the decoy information. Figure below illustrates how direct frequency modulation works.


The signal can be intercepted, but it is difficult to watch the right frequencies, gather the chips, know which chips are valid data, and find the right message. This makes eavesdropping difficult.

Current 90OMHz direct-sequence systems support data rates of 2- to 6Mbps. Higher frequencies offer the possibility of higher data rates.

Frequency hopping rapidly switches among several predetermined frequencies. For this to work, the transmitter and receiver must be in nearly perfect synchronization. Bandwidth can be increased by simultaneously transmitting on several frequencies. Figure below shows how frequency hopping works.







Spread-spectrum transceivers have the following characteristics: