Encoding and Modulatind

Data stored in a computer in the form of 0s and 1s.  To be carried from one place to another (inside or outside the computer), data are usually converted to digital signals.  This is called digital signal conversion or encoding digital data into a digital signal.

Digital to Digital Conversion-   Digital to Digital encoding or conversion is representation of digital information by the digital signal.  For Example, when you transmit data from your computer to your printer, both the original data and he transmitted data are  digital.  In this type of encoding, the binary ‘1s’ and ‘0s’ generated by a computer are translated in to a sequence are voltage pulses that can be propagated over a wire.

            Of the many mechanism for Digital to Digital encoding, we will discuss only those most useful for data communication.  These fall in to three broad categories Unipolar, Polar and By Polar.

Unipolar-  Digital transmission system work by sending voltage pulses along a medium link usually a wire or cable in most type of encoding one voltage level stands for binary ‘0’ and another level stands for binary ‘1’.  The polarity of a pulses refers to whether it is positive of negative.  Unipolar encoding is so named because it uses only one polarity.  This polarity is assigned to one of the two binary states, usually the 1.  The other state, usually the ‘0’ is represented by ‘0’ voltage.

DC Component-  The average amplitude of a Unipolar encoded signal is nonzero.  This creates what is called a direct current (DC) component (A component with zero frequency) when signal contains a DC component.  It cannot travel through media that cannot handle DC components.

Polar-   Polar encoding uses who voltage levels, one positive and one negative by using both levels, in most popular encoding methods the average voltage level on the line is reduced and the DC component problem of unipolar encoding is alleviated.

Polar encoding uses two levels (Positive and negative) of amplitude.

Non Return to Zero-  The NRZ encoding the level of the signal is always either of positive or negative.  The two most popular methods of NRZ transmission are discussed blow.

NRZ-L-  An NRZ-L encoding the level of the signal depend on than type of bit it represents.  A positive voltage usually mean s the bit is a “0”.  And a negative voltage means the bi is a 1 (or vice versa); thus the level of the signal is dependent upon the state of the bit.

In NRZ-L the level of the signal is dependent upon the state of the bit.

NRZ-I-  In NRZ-I an inversion of the voltage level represents a 1 bits,  It is the transition between a positive and negative voltage not the voltage themselves that represent a 1 bit is encountered.  The existence of 1s in the data stream allows the receivers to resynchronize its timer to actual arrival of the transmission.  A string of ‘0s’ can still cause problems.  But because ‘0s’ are not as likely, they are less of a problem.

In NRZ-I the signal is inverted if a 1 is encountered.

Bipolar-   The ‘1s’ are represented by alternating positive and negative voltages.  If  the first ‘1’ bit is represented by the negative amplitude, the third by the positive amplitude and so on.

In bipolar encoding we use three levels positive zero and negative.          

Bit Rate and Baud Rate-  Two terms use frequently to data communication are bit rate and baud rate. “Bits rate is number of bits transmitted during one second.

Baud rate is refer to the number of signal units per second Baud rate is less than or equal to the bit rate.

Example-  An analog signal carries four bits in each signal element.  If 1000 signal elements are sent per second find the baud rate and the bi rate.

  Baud Rate= Number of signal element x 1000 bits per seconds

  Bit Rate=      Baud Rate X Number of bits per signal element

                         1000          X 4 =4bps

Example-  The bit rate of a signal is 3000.  If each signal element carries 6 bits what is the baud rate.

Baud Rate=Bit Rate X Number of bits per signal element

                     3000      X  6 =500 Baud per secound       

Frame Relay

Frame Relay is a virtual – circuit technology that provide low level (Physical and data link layer) service in response to the following demands.

Higher Data Rate at lower Cost-  In the post many organizations used a Wan Technology such as leased line or x.25 to connect single computer.  That data rate was relatively low.  Today, most organization use high speed LAN and want to use WAN to connect this LANs.  One solution to use “T-Line”, but these line provide only point to point connections, no many too many.  Creating a Mash Network out of T-Lines is a very expensive.  For Example – To connect 6 LANS, we need 15 T-Lines.  On the other hand, we need only six-T-Lines to connect the same six LANs to a frame Relay Network.  Frame Relay provides the same type of service at lower cost.

               

            Although the Frame Relay originally was designed o provide a 1.544-Mbps data rate (equivalent to a T-1 Line) today most implementations can handle up to 44.376 Mbps(equivalent to a T-3 Line).

Virtual Circuits-  Frame Relay operates using virtual circuits as opposed to the actual circuits that leased Lines used.  These virtual circuits are what link together the thousand of devices connected o he providers “Cloud” There are two type of virtual circuits permanent and switched.

            Permanent virtual circuits (PVCs) are by far the most common type in use today.  What “permanent” means here is the Telco creates the mapping inside their gear and as long as you pay the bill, they will remain in place.

            Switched virtual circuits (SVCs) are more like a phone call.  The virtual circuit is established when data needs o be tansmitted, then  it’s taken down when the data transfer is complete.

Unguided Media

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. Unguided signals can travel from the source to destination in several ways: ground propagation, sky propagation, and 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. Distance depends on the amount of power in the signal: The greater the distance.

In sky propagation higher- frequency radio waves radiate upward into the ionosphere (the layer of atmosphere where particle exist as ions) where they are reflected back to earth.  This type of transmission allows for greater distances with lower output power.

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, 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.

SIGNAL

Analog data-   Analog data is human voice.  When somebody speaks, a continuous wave is created   in the air.  This can be captured by a microphone and converted to the analog signal.

Digital data-   Digital data is data stored in memory of the computer in the form of 0s to 1s.  It is usually converted to a digital signal.  When it is transferred from one position to another position.

Analog and digital Signal-   Signal can be analog and digital.  Analog signal can have any value in range.  Digital signals can have only a limited number of value.

                              

Periodic Signals-   A periodic signal consists of a continuously repued pattern.  The period of signal (T) is expressed in seconds.

                                      

                                                  Example of Periodic and aperiodic signal

Aperiodic Signals-   An aperodic or nonperiodic signal has no repetitive pattern.

Amplitude-   Amplitude refer to height of signal.  The unit for amplitude depend on the type of the signal.  For electrical signal, the unit is normally volts ampere or watts.

Sound level	Type of Sound
40db	normal speech
90db	lawn mowers
110db	shotgun blast
120db	jet engine taking off
120db+	rock concerts

Frequency-  The frequency of a periodic function is the number of complete cycles that can o cccur per second,  Frequency is denoted with a lower-case f.  It is defined in terms of the period, as follows:

Period of frequency-   Period refers to the amount of time in seconds, a signal needs to complete one cycle.  Frequency refer to the number of period in  one second.

Unit of Period-   Period is expressed in seconds the communication industry uses five unit to measure period second (s) millisecond, microsecond, nanosecond and picoseconds.

Unit of Frequency-   Frequency is hertz, after the German Physicist Heinrich Rudolf Hertz.  The communications industry uses five units to measure frequency. Hertz, Kilohertz, Megaherth, Gigahertz and Terahertz.

Unit of Frequency-  

Frequency	1 mHz (10−3)	1 Hz (100)	1 kHz (103)	1 MHz (106)	1 GHz (109)	1 THz (1012)
Period (time)	1 ks (103)	1 s (100)	1 ms (10−3)	1 µs (10−6)	1 ns (10−9)	1 ps (10−12)

Digital Signal-   In addition to being represented by an analog signal, data can also be represented by a digital signal.

             For example, A “1” can be encoded as a positive voltage and a “0” as a zero voltage.

Bit Interval and Bit Rate-   Bit interval and bit rate are used to describe digital signals.

        The bit interval is the time required to send one single bit.

        The bit rate is the number of bit interval per second.  This means that the bit rae is the number of bits sent in one second usually expressed in bits per second (bps).  

           

Example of bit rate and bit interval

Problems of using Voice Channels for Digital Transmission
A digital signal is comprised of a number of signals. Specifically, the signal is represented as follows,

signal = f + f3 + f5 +f7 +f9 +f11 +f13 ....f(infinity)

This means a digital signal has a base frequency, plus another at three times the base frequency, plus another at five times the base frequency etc. f3 is called the third harmonic, f5 the fifth harmonic and so on. The third harmonic is one third of the amplitude of the base frequency (called the fundamental frequency), the fifth harmonic is one fifth the amplitude of the fundamental and so on.

In order to send a digital signal across a voice channel, the bandwidth of the channel must allow the fundamental plus third and fifth harmonic to pass without affecting them too much. As can be seen, this is what such a signal looks like, and is the minimum required to be correctly detected as a digital signal by the receiver.

Lets consider sending a 2400bps binary digital signal down a voice channel rated with a bandwidth of 3.1KHz. The base frequency of the digital signal is 1200Hz (it is always half the bit rate), so the fundamental frequency will pass through the channel relatively unaltered. The third harmonic is 3600Hz, which will suffer attenuation and arrive severely altered (if at all). The fifth harmonic has no chance of passing the channel.

In this case it can be seen that only the base frequency will arrive at the end of the channel. This means the receiver will not be able to reconstruct the digital signal properly, as it will require f3 and f5 for proper reconstruction. This results in errors in the detection process by the receiver. Baud Rate-The baud rate indicates the number of bits per second That are transmitted. For example: 300 baud means hat 300 bits are trans mitted each second (abbreviated 300bps).  Assuming asynchronous communication, which requires 10 bits per character, this translates to 30 characters per second (cps).  For slow rates (below 1200 baus), you can divide the baudby 10 o see how many characters per second are sent.

Type of Cable

10BASE2

The 10Base2 Ethernet standard uses thin coaxial (RG-58) cable.  10Base2 has a maximum length of 185 Meters (607feet) and because of its composition offers a fair amount of resistance to interference.  One of the more important is that it is limited to transfer speed of 10 Mbps.  This speed limitation reduces its effectiveness in many network environments.

10BASE5

The 10BASE5 standard uses the thick coaxial (RG-62) cable.  It is a transfer speed of 10Mbps and runs distance of 500 meter.  The 10BASE5 standard specifies a true bus topology, meaning that computer attached to the network cable itself using special cable and connectors it does no use additional networking equipment.

            10BASE5 network must be connected with a minimum distance of 2.5 meters between each other to prevent noise on the cable.

10BASE T

That a normal Ethernet network is one that operates at 10 Mbps and has  the name 10Base T.  Now-a-days more and more origination s are looking for even faster ways to connect there 3, 4 and 5 UTP cable and is limited to distance of hundred meters.

Fast Ethernet 100BASE T

100baseT is a 100 mbps networking standard based on  the principle of standard Ethernet.  A number of standard fall under the fast Ethernet banner; the most commonly implemented is 100BaseTX which use category 5(or higher)  UTP cabling.  The following sections describe he fast Ethernet standards.

10BASE FX

This is the standard of the running fast Ethernet over fiber optic cable.  Due to the cast of implementations this is the rarely used.  One of the largest advantages is its distance capabilities.  100BaseFX can reach an impressive10, 000 meters.

100BASE T4

100BASET4 can use category 3 4and 5 cable to reach speed of 100mbps with regards to distance limitation and other characteristics 100BASE T4  is comparable 100BASE TX.

100BASE TX

This is the fast Ethernet standard of choice on day network. 100BASETX is most often implemented with UTP cable, but it has been known to be used with STP.  100BASE TX operated at speed of 100 mbps and offers a maximum transmission

distance of 100 meters.  The difference between TX and T4 uses all four pairs of wire within the UTP cable TX only use two paired.

  

CSMA/CD

When a device on a network want  to transmit to another device.  The first device  to check to see if the line is clear and then transmit.  The only problem is that two devices perform this action at the same time.  They will both line is clear and so will both attempt to transmit.

Ethernet can accept only one signal at a time.  So the two transmission with collide with each other.  These collisions damage the data packet on the network.  To avoid another collision each system will wait a period of time, known as the back off, before retransmitting the data.  If  data collide again the delay before retransmitting in increase and data is sent again.

The more device that are connected o an Ethernet network the more likely it is there will be collisions on the network this decrease in  performance has driven  improvements in the structures of Ethernet network, improvement include substation of older hubs with new high performance, Ethernet switches.

Ethernet Addressing

Ethernet Addressing-  Ethernet are the physical addressing of the device.  Every LAN card is identify as the Unicast  Ethernet  Address.  The LAN address are as per the IEEE standard.   The IEEE  802.3 standard the assigning the LAN Card.

IEEE assigning  OUI (Organization Uniquely Identification).

Ethernet Frame-   Ethernet frame are used by the data link layer to transfer the packet of the frame the data frame to network layer.  These are mode at the data link layer frame perform data security check and ensure the data is no corrupt.