Communications Fundamentals

CircuitsA circuit is the physical path that runs between two or more points. It terminates on a port (i.e., a point of electrical or optical interface), and that port can be in a host computer, on a multiplexer, on a switch, or in another device.There are two types of circuitsTwo-Wire CircuitsA two-wire circuit has two insulated electrical conductors: One wire is used for transmission of the information, and the other wire acts as the return path to complete the electrical circuit.

Continue………..Four-Wire CircuitsA four-wire circuit has two pairs of conductors. That is, it has two sets of one-way transmission paths: one path for each direction and a complementary path to complete the electrical circuit.A four-wire circuit has two pairs of conductors. That is, it has two sets of one-way transmission paths: one path for each direction and a complementary path to complete the electrical circuit . Four-wire circuits are used where the distance between the termination points requires that the signal be strengthened (amplified) periodically. So, for example, four-wire circuits connect the various switches that make up the public switched telephone network (PSTN).

Continue…………ChannelsA channel defines a logical conversation path. It is the frequency band, time slot, or wavelength (also referred to as lambda, λ) allocated to a single conversation. channels, greatly increasing the carrying capacity of an individual circuit. Because we are becoming more digitalized all the time, people often refer to the number of channels rather than the number of circuits.

Lines and TrunksLines and trunks are basically the same thing, but they're used in different situations. A line is a connection configured to support a normal calling load generated by one individual. A trunk is a circuit configured to support the calling loads generated by a group of users; it is the transmission facility that ties together switching systems. A switching system is a device that connects two transmission lines.

Continue…………..Virtual CircuitToday, because of the great interest in and increased use of packet switching, most networks use virtual circuits. Unlike a physical circuit, which terminates on specific physical ports, a virtual circuit is a series of logical connections between sending and receiving devices. A virtual circuit is a connection between two devices that acts as though it's a direct connection, but it may, in fact, be composed of a variety of different routes. The routes might change at any time, and the incoming return route doesn't have to mirror the outgoing route. The term virtual circuit is largely used to describe connections between two hosts in a packet-switching network, where the two hosts can communicate as though they have a dedicated connection, although the packets may be taking very different routes to arrive at their destination

Types of Virtual CircuitsPermanent virtual circuitA PVC is a virtual circuit that is permanently available; that is, the connection always exists between the two locations or two devices in question. A PVC is manually configured by a network management system, and it remains in place until the user reconfigures the network. Its use is analogous to the use of a dedicated private line because it provides an always-on condition between two locations or two devices.

Switched virtual circuitIn contrast to PVCs, SVCs are set up on demand. They are provisioned(provided) dynamically by using signaling techniques. An SVC must be reestablished each time data is to be sent; after the data has been sent, the SVC disappears. An SVC is therefore analogous to a dialup connection in the PSTN. The main benefit of an SVC is that you can use it to access the network from anyplace. The predominant application for SVCs is to accommodate people who are working at home, in a hotel, at an airport, or otherwise outside the physical location of the enterprise network.

Types of Network ConnectionsSwitched network connections A switched connection is referred to as a dialup connection. This implies that it uses a series of network switches to establish the connection between the parties.

Leased-line network connections A leased line is also referred to as a private line. With a leased line, the same locations or the same devices are always connected, and transmission between those locations or devices always occurs on the same path.

Dedicated network connections In essence, a dedicated line works exactly like a leased line. It is always connected, and it always uses the same path for transmission. However, the end user may own the transmission facility (rather than lease it) such that it is exclusive to that user

Multiplexing

Multiplexers are extremely important to telecommunications. Their main reason for being is to reduce network costs by minimizing the number of communications links needed between two points. The capability to manage transmission resources on a dynamic basis, with such things as priority levels. If you have only one 64Kbps channel left, who gets it? Or what happens when the link between San Francisco and Hong Kong goes down? How else can you reroute traffic to get the high-priority information where it needs to go? Multiplexers help solve such problems

Multiplexing Types

Frequency Division Multiplexing (FDM)FDM modulates each message to a different carrier frequency. The modulated messages are transmitted through the same channel and a bank of filters separates the messages at the destination. The frequency band of the system is divided into several narrowband channels, one for each user. Each narrowband channel is reserved for one user all the time. FDM has been used in analog carrier systems in the telephone network. The same principle is also used in analog cellular systems in which each user occupies one FDM channel for the duration of the call. In such a case, we call the process frequency-division multiple access (FDMA) because the frequency-division method is now used to allow multiple users to access the network at the same time.

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Time Division Multiplexing (TDM)A more modern method of multiplexing is TDM, which puts different messages from different users, in non overlapping time slots. Each user channel uses a wider frequency band but only a small fraction of time, one time slot in each frame. In addition to the user channels, framing information is needed for the switching circuit at the receiver that separates the user channels (time slots) in the demultiplexer. When the demultiplexer detects the frame synchronization word, it knows that this is the start of a new frame and the next time slot contains the information of user channel.

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Note:FDM and TDM can be combined. For example, you could use FDM to carve out individual channels and then within each of those channels apply TDM to carry multiple conversations on each channel. In fact, this is the way that some digital cellular systems work (for example, Global Systems for Mobile Communications [GSM]).

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Statistical time division Multiplexing (STDM)In a synchronous time division multiplexer, it is generally the case that many of the time slots in a frame are wasted. A typical application of a synchronous TDM involves linking a number of terminals to a shared computer port. Even if all terminals are actively in use, most of the time there is no data transfer at any particular terminal.An alternative to synchronous TDM is statistical TDM or intelligent TDM. The statistical multiplexer exploits this common property of data transmission by dynamically allocating time slots on demand.NOTE: Diagram will explain on board

Transmission Media

Transmission is the physical path on which data/information is transferred from one location to another.

Types of Media• Guided Media : Cables (UDP,STP, Coaxial Cable) Cant support Advance applications of the future.• Unguided Media: Wireless ( Radio frequency, Infrared, Microwave and Satellite)

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Shielded Twisted Pair (STP)Used in local loop.

Cant support advance applications due to low bandwidth. Requires short distances between repeaters

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High support of EMI, RFI, moisture and corrosion as compare to UTP

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Fiber optic• A fiber-optic cable is made of glass or plastic and transmits

signals in the form of light.• 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 of1600Gbps.

• 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 5km for coaxial or twisted-pair cable.

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

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

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

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Disadvantages• Installation and maintenance. Fiber-optic cable is a

relatively new technology. Its Installation and maintenance require expertise that is not yet available everywhere.• Unidirectional light propagation. Propagation of light is

unidirectional. If we need bidirectional communication, two fibers are needed.

• Cost. The cable and the interfaces are relatively more expensive than those of other guided media.

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Unguided MediaUnguided 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.

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Figure

Continue………….Unguided signals can travel from the source to destination in several ways:• ground propagation.• sky propagation.• line-of-sight propagation. 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.In sky propagation, higher-frequency radio waves radiate upward into the ionosphere (the layer of atmosphere where particles exist as ions) where they are reflected back to earth. This type of transmission allows for greater distances with lower output power.

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In line-or-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other,

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Figure

Note:The electromagnetic spectrum defined as radio waves and microwaves is divided into eight ranges, called bands, each regulated by government authorities. These bands are rated from very low frequency (VLF) to extremely high frequency(EHF).

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Radio WavesElectromagnetic waves ranging in frequencies between 3kHz and 1GHz are normally 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. 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.

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Figure

Omnidirectionalantenna

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Radio wavesRadio 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 abuilding. It is a disadvantage because we cannot isolate a communication to just inside or outside a building.

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Radio wavesThe omnidirectional characteristics of radio waves make them useful for multicasting, in which there is one sender but many receivers. AM, FM radio and television.

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Microwaves Electromagnetic waves having frequencies between 1 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.

Continue……..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 do not allow two short towers to communicate by using microwaves. Repeaters are often needed for long distance communication.Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside buildings.Microwaves, due to their unidirectional properties, are very useful when unicast (one-to-one) communication is needed between the sender and the receiver. They are used in cellular phones, satellite networks and wireless LANs.

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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 highfrequencies, cannot penetrate walls. This advantageous characteristic prevents interference between one system and another; a short-range communication system in one room can not 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.

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Infrared Applications Communication between PC and Keyboard Communication between PC and Printer Communication between TV and Remote

Control Data transferring between Mobile sets