1 rick graziani [email protected] [email protected] [email protected]...

1

Rick GrazianiRick Graziani [email protected]@cabrillo.edu [email protected][email protected]

Long Distance Communications (Carriers, Modulation, and Modems)

Computer Networks and Internets by Douglas Comer

Rick Graziani, [email protected] Graziani, [email protected] 2

Other information

For most of this chapter we will rely on other sources. Comer does a good job in explaining “what happens” but

does not provide enough information to see “how it works.”

Sources used for this presentation:


Digital and Analog Bandwidth

Bandwidth = The width or carrying capacity of a communications circuit.

Digital bandwidth = the number of bits per second (bps) the circuit can carry

used in digital communications such as T-1 or DDS measure in bps T-1 -> 1.544 Mbps

Analog bandwidth = the range of frequencies the circuit can carry

used in analog communications such as voice (telephones) measured in Hertz (Hz), cycles per second voice-grade telephone lines have a 3,100 Hz bandwidth



Available at http://www.thinkgeek.com



GOLDMAN: DATACOMM FIG.02-14

DTE DCE

DTE DCE

Modulation

Demodulation

digital analog

digital analog

PSTN Dial-up network


Digital Signals digital signal = a signal whose state consists of discrete

elements such as high or low, on or off

Analog Signals analog signal = a signal which is “analogous” to sound waves telephone lines are designed to carry analog signals


Sound Waves


Analog Signals, Modulation and Modem Standards

A perfect or steady tone makes a wave with consistent height (amplitude) and pitch (frequency) which looks like a sine wave. (Figure 4-15)

A cycle or one complete cycle of the wave The frequency (the number of cycles) of the wave is

measured in Hertz Hertz (Hz) = the number of cycles per second


Transmission Terminology (whatis.com)

Broadband transmission = In general, broadband refers to telecommunication in which a wide band of

frequencies is available to transmit information. Because a wide band of frequencies is available, information can be multiplexed

and sent on many different frequencies or channels within the band concurrently, allowing more information to be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time).

Baseband transmission 1) Describing a telecommunication system in which information is carried in digital

(or analog) form on a single unmultiplexed signal channel on the transmission medium. This usage pertains to a baseband network such as Ethernet and token ring local area networks.

Narrowband transmission Generally, narrowband describes telecommunication that carries voice

information in a narrow band of frequencies. More specifically, the term has been used to describe a specific frequency range

set aside by the U.S. Fcc for mobile or radio services, including paging systems, from 50 cps to 64 Kbps.


Carrier Signal or Analog Wave = An electronic signal used to modulate data in broadband transmission, usually a sine wave.

Carrier Signal


Three parts of any analog wave are: 1. amplitude - the height of the wave2. frequency - the pitch of the wave3. phase - the shift or position of the wave

These are the three parts we can modulate or change the carrier signal or wave!

Modulate = ChangeMore in a moment.

Carrier Signal


Telephone Lines, Modems, and PSTN

Voice grade telephone lines and equipment are designed to transmit tones between 300 and 3,400 Hertz

bandwidth = 3,100 Hz or 3.1 KHz “most” of our human voice falls into this range Economics dictated the size of this bandwidth (Keyboard example) The “maximum” number of cycles (highest frequency) of an

analog signal over voice grade telephone lines is 3,400 Hz (cycles per second)


Telephone Lines, Modems, and PSTN

GOLDMAN: DATACOMM FIG.02-14

DTE DCE

DTE DCE

Modulation

Demodulation

digital analog

digital analog



Modem MOdulator/DEModulator converts analog signals to digital and digital signals to analog used for transmitting digital information between computers over

voice-grade telephone lines Computers use transmission interface standards such as RS-232-

C using positive and negative voltages which form square waves, whereas the PSTN is designed to carry analog signals (sine waves)


Modulation

modulation = 1. the process of varying the characteristic of an electrical carrier

wave (analog, sine wave) as the information on that wave varies Three types of modulation 1. amplitude modulation 2. frequency modulation 3. phase modulation

2. the process of converting digital signals to analog


Amplitude Modulation (AM) a modulation technique to vary the height the electrical signal

(the sine wave or carrier wave with modems) to transmit ones and zeroes, while the frequency of the wave remains constant

different amplitudes for 0’s and 1’s a.k.a. amplitude shift keying, ASK Figure 4-22 frequency for each bit remains constant volume = amplitude

Amplitude Modulation


Different amplitudes for 0’s and 1’s, while the frequency of the wave remains constant

Full duplex different amplitudes and frequencies are used for different

directionsDisadvantage Voice-grade telephone lines are susceptible to distortions which

affect amplitudes, as volume fades, the amplitude lowers Amplitude modulation only effective for low speed transmissions

Amplitude Modulation


Frequency Modulation a modulation technique to vary the frequency of the sine wave

(or carrier wave) to transmit ones and zeroes, while the amplitude remains constant

different frequencies for 0’s and 1’s a.k.a. frequency shift keying, FSK Figure 4-23 two separate frequencies for ones and zeroes

Frequency Modulation


Full Duplex requires a minimum of four frequencies, two frequencies for

each direction i.e. CCITT V.21 for 300 baud modems: Originating Sending Modem Modem 1270 Hz 1 2225 Hz 1070 Hz 0 2025 Hz loss of amplitude will not cause errors in transmission



Looking at waves

Wave Generator Tone Generator Adjust the amplitude and frequency of a sign wave Modulation



Conceptually:If voice-grade telephone lines can transmit a “maximum” of

3,400 Hz (cycles per second), between 300 Hz and 3,400 Hz,ANDIf one cycle = 1 bit,

Then a maximum of 3,400 bits per second can be transmitted over voice grade telephone lines? (Hold that thought!)


Phase Modulation (PM) a modulation technique to vary the phase of the sine wave (or

carrier wave) to transmit ones and zeroes, while the amplitude and the frequency remains constant

sine waves repeat themselves indefinitely shifting the wave breaks the wave abruptly and starts it again a

few degrees forward or backward A different phase shift, 0 to 360 degrees, is used to transmit

one or more bits

Phase Modulation


A different phase shift, 0 to 360 degrees, is used to transmit one or more bits

Full Duplex requires a minimum of two frequencies, one frequency

for each direction

Phase Modulation


Bits per second vs. Baud and High-speed modems

So far, discussed transmission of one bit at a time, via high or low amplitude, high or low frequency, phase shift or no phase shift

older modems sent only one bit per signal change, bps = baud

baud rate = the number of these signal changes per second

What if we could transmit more than one bit with each signal change (baud), amplitude, frequency of phase shift?

Remember, voice-grade phone lines limit transmission to 3,400 Hz or 3,400 bps with 1 cycle per bit


Dibit Modulation

Dibit Modulation 2 bits per baud, per cycle Two bits or dibit modulation: 00, 01, 10, 11Using Amplitude Modulation use four different amplitudes (wave heights)Using Frequency Modulation use four different frequenciesUsing Phase Modulation use four different phases

Dibit Amplitude modulation


•Amplitude Modulation (AM)

•Frequency Modulation (FM)

•Phase Shift Keying (PSK)

Summary of Modulations

25


Multiplexing


Multiplexing

Multiplexing and WAN (Wide Area Networks) The ability to establish, maintain and terminate multiple wide

area system-to-system connections over a single wide area link.

Data/Voice systems to Data/Voice systems LAN to LAN Terminal to Host


Multiplexer (mux) = A device which allows several devices to share the same communications circuit (cable, airwaves, etc.).

Common Types of Multiplexing Time Division Multiplexing (TDM) Statistical Time Division Multiplexing (STDM) Frequency Division Multiplexing (FDM)

Multiplexing

Adtran TSU (T1) Multiplexer


Adtran T3SU 300 (T3) Multiplexerhttp://www.adtran.com

Blackbox Multiplexer

http://www.blackbox.com

Multiplexing


Time Division Multiplexing = A multiplexer which allows devices to transmit information (data/voice) over the circuit by quickly interleaving information.

Train Example: Five Accordion Manufacturers with 20 box cars of

accordions needed to get to their destination ASAP SF to New York Three solutions1. Build 5 sets of tracks2. Build 1 set of tracks and have 5 separate trains3. Build 1 set of tracks and share a single train (multiplexing)

Time Division Multiplexing


3. Build 1 set of tracks and share a single train with the box cars lined up as:

Company Box Car A 1 B 2 C 3 D 4 E 5 A 6 B 7 etc.



Each source connected to the TDM mux has the entire bandwidth for a portion of time.

TDM constructs a “frame” consisting of one or more time slots for each input source.

TDM scans each input source for data during its designated time slot. If the source has no data to transmit, TDM mux inserts null data and the time slot is wasted.



The TDM channel or circuit must be able to handle the sum of the data rates of all its input sources plus overhead (later).

TDM can handle input sources with different data rates. A slower device may be assigned one time slot, where a

faster device may be assigned two or more time slots.



Multiplexing where input devices share the bandwidth of the circuit by dividing the link into many separate frequencies.

Involves modulating the signal from digital to analog and any other modulation techniques such as TCM.

Each user has the full bandwidth of the circuit at all times.

Frequency Division Multiplexing


More later…

T1 Multiplexing is covered in later in CCNA semester 4 and in CCNP Remote Access.

This would be another excellent project topic.

35


Chapter 6 – Long Distance Communications (Carriers, Modulation, and Modems)

Computer Networks and Internets by Douglas Comer

Instructor: Rick Graziani, August 1, 2002

CST 311 Introduction to Telecommunications

Includes material from other sources.

University transferable option of CIS 181 at Cabrillo College

1 rick graziani [email protected] [email protected] [email protected]...

Documents