INTRODUCTION

 

Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The actual generation of the information is not part of data communications nor is the resulting action of the information at the receiver. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.

 

In Local Area Networks, we are interested in “connectivity”: connecting computers together to share resources. Even though the computers can have different disk operating systems, languages, cabling and locations, they still can communicate to one another and share resources.

The purpose of data communications is to provide the rules and regulations that allow computers with different disk operating systems, languages, cabling and locations to share resources. The rules and regulations are called protocols and standards in the communications.

 

BASIC COMPONENTS OF COMMUNICATION

 

 

 

 

 

Source: The transmitter of data. Examples are:

• Terminals
• Computers
• Mainframes

 

Medium: The communication stream through which the data is being transmitted. Examples are:

 

• Cabling
• Microwave
• fiber optics
• Radio Frequencies (RF)
• Infrared Wireless

Receiver: The receiver of the data transmitted. Examples are:

 

• Printers
• Terminals
• Mainframes
• Computers

 

 

 

DTE        DCE       Medium           DCE         DTE

 

 

	Source/ Sender		Host/ Receiver

 

 

 

 

 

 

 

 

 

         

DCE: The interface between the Source & the Medium, and the Medium & the Receiver is called the DCE (Data Communication Equipment) and is a physical piece of equipment.

 

DTE: Data Terminal Equipment is the telecommunications name given to the source and receiver’s equipment.

 

An example of this would be your PC dialing into a BBS (Bulletin Board System):

 

 

 

DTE          DCE          Medium        DCE DTE

 

 

 

 

 

 

PC+                             Modem          Phone              Modem           BBS’s

Communication                        Lines                                       Computer X

Software

 

 

DATA FLOW

Data flow is the flow of data between two points. The direction of the data flow can be described as:

Simplex: data flows in only one direction on the data communication line (medium). Examples are radio and television broadcasts. They go from the TV station to your home television.

 

 

Half-Duplex: data flows in both directions but only one direction at a time on the data communication line. For example, a conversation on walkie-talkies is a half-duplex data flow. Each person takes turns talking. If both talk at once-nothing occurs!

 

Full-Duplex: data flows in both directions simultaneously. Modems are configured to flow data in both directions.

 

 

MODEMS

 

A modem (Modulator / Demodulator) connects a terminal/computer (DTE) to the Voice Channel (dial-up line).

 

Basic Definition

 

The modem (DCE-Data Communication Equipment) is connected between the terminal/computer (DTE-Data Terminal Equipment) and the phone line (voice channel). A modem converts the DTE (Data Terminal Equipment) digital signal to an analog signal that the voice channel can use.

 

A modem is connected to the terminal/computer’s RS-232 serial port (25 pin male D connector) and the outgoing phone line with an RJ11 cable connector (the same as on a telephone extension cord). Male connectors have pins, female connectors have sockets.

 

DIGITAL CONNECTION

 

The connection between the modem and terminal/computer is a digital connection. A basic connection consists of a Transmit Data (TXD) line, a Receive Data (RXD) line and many hardware handshaking control lines.

 

 

 

 

 

                 TXD (2)                                    TXD (2)

                          RXD (3)                                    RXD (3)

 

 

Terminal/Computer                         Control Lines                                        Modem

            The control lines determine whose turn it is to talk (modem or terminal), if the terminal/computer is turned on, if the modem is turned on, if there is connection to another modem, etc.

 

 

ANALOG CONNECTION

 

The connection between the modem and the outside world (the phone line) is an analog connection. The voice channel has bandwidth of 0-4 KHz but only 300-3400 Hz is usable for data communications.

 

 

 

 

 

 

0 – 4 KHz BW

 

Analog

Modem                        Voice Band

 

 

 

The modem converts digital information into tones (frequencies) for transmitting through the phone lines. The tones are in the 300-3400 Hz Voice Band.

 

EXTERNAL/INTERNAL MODEMS

 

There are 2 basic physical types of modems: Internal & External modems. External modems sit next to the computer and connect to the serial port using a straight-through serial cable.

 

An Internal modems is a plug-in circuit board that sits inside the computer. It incorporates the serial port on-board. They are less expensive than external modems because they do not require a case, power supply and serial cable. They appear to the communication programs as if they were an external modem for all practical purposes.

 

 

MODEM TYPES

 

There are many types of modems, the most common of which are:

 

1. Optical Modem- Uses optical fiber cable instead of wire. The modem converts the digital signal to pulses of light to be transmitted over optical lines (more commonly called a media adapter or transceiver).

 

1. Short Haul Modem- A modem used to transmit data over 20 miles or less. Modems we use at home or to connect computers together among different offices in the same building are short haul modems.

 

1. Acoustic Modem- A modem that couples to the telephone handset with what looks like suction cups that contain a speaker and microphone. Used by traveling salespeople to connect to hotel phones.

 

1. Smart Modem- A modem with a CPU (microprocessor) on board that uses the Hayes AT command set. This allows auto-answer & dial capability rather than manually dialing & answering.

 

1. Digital Modem- Converts the RS-232 digital signals to digital signals more suitable for transmission. (also called a media adapter or transceiver)

 

1. V.32 Modem- A milestone modem that uses a 2400 baud modem with 4 bit encoding. This results in a 9600 bps (bits per second) transfer rate. It brought the price of high speed modems below $5,000.

 

 

Baud is the speed at which the analog data is changing on the voice channel and bps is the speed at which the decoded digital data is being transferred.

 

FEATURES OF MODEMS

 

1.                 Speed – The speed at which the modem can send data in bps (bits per second).          Typical modem speeds are: 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K,   28.8K bps.

 

2.                 Auto Dial/Redial – Smart modems can dial the phone number and auto re dial if a busy signal is received.

 

1. Auto Answer – Most modems have Ring Detect capability and can automatically answer the telephone when an incoming call comes in.

 

1. Self Testing – Newer modems have self-testing features. They can test the digital connection to the terminal/computer and the analog connection to a remote modem. They can also check the modem’s internal electronics.

 

1. Voice Over Data – Voice Over Data modems allow a voice conversation to take place while data is being transmitted. This requires both the source and destination modems to have this feature.

 

1. Synchronous or Asynchronous Transmission – Newer modems allow a choice of synchronous or asynchronous transmission of data. Normally, modem transmission is asynchronous (we send individual characters with just start and stop bits). Synchronous transmission or packet transmission is used in specific applications.

 

PHYSICAL CONNECTION

 

The physical connection determines how many bits (1’s or 0’s) can be transmitted in a single instance of time. If only 1 bit of information can be transmitted over the data transmission medium at a time it is considered a serial communication.

 

 

If more than 1 bit of information is transmitted over the data transmission medium at a time then it is considered a parallel communication.

 

Parallel Communication

 

Communications	Advantages	Disadvantages
Parallel	Fast Transfer Rates	Short distances only
Serial	Long Distances	Slow transfer rates

 

TRANSMISSION MEDIA – GUIDED

 

There are 2 basic categories of transmission media: guided and unguided.

 

Guided transmission media uses a cabling system that guides the data signals along a specific path. The data signals are bound by the cabling system. Guided media is also known as bound media. “Cabling” is meant in a generic sense, and is not meant to be interpreted as copper wire cabling only.

Unguided transmission media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and are therefore often called unbond media.

 

There 4 basic types of guided media:

 

1. Open Wire
2. Twisted Pair
3. Coaxial Cable
4. Optical Fiber

 

OPEN WIRE

 

            Open wire is traditionally used to describe the electrical wire strung along power poles. There is a  single wire strung between poles. No shielding or protection from noise interference is used. We are going to extend the traditional definition of open wire to include any data signal path without shielding or protection from noise interference. This can include multi conductor cables or single wires. This medium is susceptible to a large degree o noise and interference and consequently is not acceptable for data transmission except for short distances under 20 ft.

 

 

TWISTED PAIR

 

                      The wires in twisted pair cabling are twisted together in pairs. Each pair consists of a wire used for the +ve data signal and a wire used for the –ve data signal. Any noise that appears on 1 wire of the pair will also occur on the other wire. Because the wires are opposite polarities, they are 180 degrees out of phase (180 degrees – phasor definition of opposite polarity). When the noise appears on both wires, it cancels or nulls itself out at the receiving end, Twisted pair cables are most effectively used in systems that use a balanced line method of transmission: polar line coding (Manchester Encoding) as opposed to unipolar line coding (TTl logic).

 

 

 

UNSHIEDED TWISTED PAIR

 

The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference. To further improve noise rejection, a foil or wire braid “shield” is woven around the twisted pairs. This shield can be woven around individual pairs or around a multi-pair conductor (several pairs).

 

 

SHIELDED TWISTED PAIR

 

Cables with a shield are called shielded twisted pair and are commonly abbreviated STP. Cables without a shield are called unshielded twisted pair or UTP. Twisting the wires together results in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for Ethernet 10BaseT Cable.

 

UTP or unshielded twisted pair cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (Rj45, RJ11, etc….)

 

STP or shielded twisted pair is used with the traditional Token Ring cabling or ICS-IBM Cabling System. It requires a custom connector. IBM STP (shielded twisted pair) has a characteristic impedance of 150 ohms.

 

COAXIAL CABLE

 

Coaxial cable consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.

 

 

The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnest and Thick net. The excellent control of the impedance characteristics of the cable allow higher data rates to be transferred than with twisted pair cable.

 

OPTICAL FEBER

 

Optical fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding. A typical core diameter is 62.5 microns (1 micron = 10-6meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is protective coating consisting of plastic, it is called the jacket.

 

 

 

Jacket      Cladding        Core

 

Side View                                           End View

 

An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it “bends” as it passes from one medium to the other. An example of this is when we look into a pond of water.

 

 

If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted.

 

 

 

 

Incident Ray          Light

Rays                        Incident Ray

Light

Rays

Angle of Incidence                   Air      Angle of Incidence

q                                                                  q                       Air

Water                                                        Water

 

 

Refraction                                                  Reflection

 

 

Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it – it bounds the medium!

 

OPTICAL TRANSMISSION MODES

 

There are three primary types of transmission modes using optical fiber. They are

 

1. Step Index

 

1. Graded Index

 

c.         Single Mode   

 

Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This causes some rays to take a longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at different times. The signal becomes longer than the original signal. LED light sources are used. Typical core: 62.5 microns.

 

 

STEP INDEX MODE

 

Graded index has a gradual change in the core’s refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core: 62.5 microns.

 

 

GRADED INDEX MODE

 

Note: Both step index and graded index allow more than one light source to be used (different colors simultaneously), so multiple channels of data can be run be run at the same time!

 

Single mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small: 9 microns.

 

 

SINGLE MODE

 

Comparison of Optical Fibers

 

 

We don’t use frequency to talk about speed any more, we use wavelengths instead. The wavelength of light sources is measured in nanometers or 1 billionth of a meter.

 

Indoor cable specifications:

 

• LED (Light Emitting Diode) light source
• 3.5 db/km Attenuation (loses 3.5 dB of signal per kilometer)
• 850 nM – wavelength of light source
• Typically 62.5/125 (core dia/cladding dia)
• Multimode – can run many light sources.

 

Outdoor cable specification:

 

• Laser Light Source
• 1dB/Km Attenuation (loses 1dB of signal per kilometer)
• 1170 nM – wavelength of light source
• Monomode (single mode)

 

Advantages of Optical Fiber:

 

• Noise immunity: RFI and EMI immune (RFI – Radio Frequency Interference, EMI – Electro Magnetic Interference)
• Security: cannot tap into cable.
• Large Capacity due to BW (bandwidth)
• No corrosion
• Longer distances than copper wire
• Smaller and lighter than copper wire
• Faster transmission rate

 

Disadvantages of optical fiber:

 

• Physical vibration will show up as signal noise!
• Limited physical are of cable. Bend it too much and it will break!
• Difficult to splice

 

The cost of optical fiber is a trade – off between capacity and cost. At higher transmission capacity, it is cheaper than copper. at lower transmission capacity, it is more expensive.

 

MEDIA VERSUS BANDWIDTH

 

The following table compares the usable bandwidth of the different guided transmission media.

 

 

Cable Type	Bandwidth
Open Cable	0 -5 MHz
Twisted Pair	0 -100 MHz
Coaxial Cable	0 – 600 MHz
Optical fiber	0 -1 GHz

 

TRANSMISSION MEDIA – UNGUIDED

 

Unguided transmission media is data signals that flow through the air. They are not guided or bond to a channel to follow. They are classified by the type of wave propagation.

 

RF Propagation

 

There are three types of RF (radio frequency) propagation:

 

• Ground Wave
• Ionospheric
• Line of Sight (LOS)

 

Ground wave propagation follows the curvature of the Earth. Ground waves have carrier frequencies up to 2 MHz. AM radio is an example of ground wave propagation.

 

 

Ionospheric propagation bounces off of the Earth’s ionospheric layer in the upper atmosphere. It is sometimes called double hop propagation. It operates in the frequency range of 30-85 MHz. Because it depends on the Earth’s ionosphere, it changes with the weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.

 

 

Line of sight propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called space waves or tropospheric propagation. It is limited by the curvature of the Earth for ground – based stations (100 km, from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and satellite.

 

 

RADIO FREQUENCIES

 

The frequency spectrum operates from 0 Hz (DC) to gamma rays (1019 Hz)

 

Name	Frequency (Hertz)	Examples
Gamma Rays	1019 +
X-Rays	1017
Ultra -Violet Light	7.5 x 1015
Visible Light	4.3 x 1014
Infrared Light	3 x 1011
EHF – Extremely High Frequencies	30 GHz (Giga = 109)	Radar
SHF – Super High Frequencies	3 GHz	Satellite & Microwaves
UHF – Ultra High Frequencies	300 MHz (Mega = 106)	UHF TV (Ch. 14-83)
VHF – Very High Frequencies	30 MHz	FM & TV (Ch2 – 13)
HF – High Frequencies	3 MHz2	Short Wave Radio
MF – Medium Frequencies	300 kHz (kilo = 103)	AM Radio
LF – Low Frequencies	30 kHz	Navigation
VLF – Very Low Frequencies	3 k Hz	Submarine Communications
VF – Voice Frequencies	300 Hz	Audio
ELF – Extremely Low Frequencies	30 Hz	Power Transmission

 

 

Radio frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

 

MICROWAVE

 

Microwave transmission is line of sight transmission. The transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth’s curvature is only 50 Km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

 

 

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to their large bandwidth.

 

Advantages:

 

1. They require no right of way acquisition between towers.
2. They can carry high quantities of information due to their high operating frequencies.
3. Low cost land purchase: each tower occupies only a small area.
4. High frequency/short wavelength signals require small antennae.

 

Disadvantages:

 

1. Attenuation by solid objects: birds, rain, snow and fog.
2. Reflected from flat surfaces like water and metal.
3. Diffracted (split) around solid objects.

 

1. Refracted by atmosphere, thus causing beam to be projected away from receiver.

 

SATELLITE

 

Satellites are transponders (units that receive on one frequency and retransmit on another) that are set in geostationary orbits directly over the equator. These geostationary orbits are 36,000 Km from the Earth’s surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earth’s rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out into space.

 

 

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations because they are located on the Earth. The footprint is the “shadow” that the satellite can transmit to, the shadow being the area that can receive the satellite’s transmitted signal.

 

IRIDIUM TELECOM SYSTEM

 

The Iridium Telecom System is a new satellite system that will be the largest private aerospace project. It is a mobile telecom system intended to compete with cellular phones. It relies on satellites in lower Earth orbit (LEO). The satellites will orbit at an altitude of 900 – 10,000 Km in a polar, non-stationary orbit. Sixty-six satellites are planned. The user’s handset will require less power and will be cheaper than cellular phones. There will be 100% coverage of the Earth.

 

Unfortunately, although the Iridium project was planned for 1996-1998, with 1.5 million subscribers by end of the decade, at the time of this writing, it looked very financially unstable.