Chapter 2.5 –2.8Jocelyn Etheredge Nikoya Shaw Crystal SandersO'Keevia Howard

USB and Scrambler The popular USB

(Universal Serial Bus) standard for connecting computer peripherals uses NRZI.

A scrambler works by XORing the data with a pseudorandom sequence before it is transmitted. Scrambling is attractive because it adds no bandwidth or time overhead.

Balanced signals Signals that have as

much positive voltage as negative voltage even over short periods of time are called balanced signals. They average to zero, which means that they have no DC electrical component.

Passband Transmission The absolute frequency values do not matter for

capacity. This means we can take a baseband signal that occupies 0 to B Hz and shift it up to occupy a passband of S to S + B Hz without changing the amount of information that it can carry, even though the signal will look different.

ASK, FSK, PSK, BPSK, and QPSK

In ASK (Amplitude Shift Keying), two different amplitudes are used to represent 0 and 1.

With FSK (Frequency Shift Keying), two or more different tones are used.

In the simplest form of PSK(Phase Shift Keying), the carrier wave is systematically shifted 0 or 180 degrees at each symbol period.

Because there are two phases, it is called BPSK(Binary Phase Shift Keying).

A better scheme that uses the channel bandwidth more efficiently is to use four shifts, called QPSK (Quadrature Phase Shift Keying).

Frequency Division Multiplexing

FDM (Frequency Division Multiplexing) takes advantage of passband transmission to share a channel.

It divides the spectrum into frequency bands, with each user having exclusive possession of some band in which to send their signal.

AM radio broadcasting illustrates FDM. When many channels are multiplexed together, 4000 Hz is allocated per channel. The excess is called a guard band, which keeps the channels well separated.

Orthogonal Frequency Division Multiplexing

In OFDM, the channel bandwidth is divided into many subcarriers that independently send data. The subcarriers are packed tightly together in the frequency domain.

TDM & STDM An alternative to FDM is

TDM(Time Division Multiplexing).

Here the users take turns(in a round-robin fashion), each one periodically getting the entire bandwidth for a little burst of time.

STDM(Statistical Time Division Multiplexing) indicates that the individual streams contribute to the multiplexed stream not on a fixed schedule, but according to the statistics of their demand.

Code division Multiplexing CDM is a form of spread spectrum communication

in which a narrowband signal is spread out over a wider frequency band. This can make it more tolerant of interference,

There are a lot of media which is used for telecommunicating

History of the Telephone Before 1984 The Bell system supplied

both local and long distance services for most of the US.

Because the government felt this system was being monopolized it was split up into “pieces” or smaller entities known as BOC’s or (Bell Operating companies)

The Local Loop contains:

Telephone Modems

MODEM: A device which converts between a

stream of digital bits and an analog signals that represents the bits.

ASDLThe customer subscribes to it an

entering or incoming line is connected using a different switch and making the local loop accessible along with the full capacity it has.

Stands for Asymmetric Digital Subscriber Line

Fibers Copper local loops tend to limit the

performance of ADSL and telephone modems.

So that they can provide faster and better service, telephone services continually upgrade loops by installing optical fibers to locations where needed i.e. Homes and Offices. This upgrade is called FttH which stands for Fiber to the Home.

Time Division Multiplexing Based on PCM which is used to carry

multiple calls over trunks. The T1 method is used in North America

and Japan T1 carrier consist of 24 voice channels ,

each channel inserts 8 bits into the output stream

Time Division Multiplexing

The T1 carrier is illustrated above (1.544 Mbps).

SONET/SDH Sonet: Synchronous Optical

Network *has four major goals *make it possible for different

carriers to interwork *unity of the U.S, Europe, and

Japan’s digital systems *multiplex multiple digital

channels *provide support for

operations, administration, and maintenance.

SDH: Synchronous Digital Hierarchy

Differs from sonnet only in minor ways.

These are the ITU recommendations.

SONET/SDH Illustrated below are two back-to-back

SONET frames

Wavelength Division Multiplexing

Four fibers come together at an optical combiner, each into a single shared fiber for transmissions to a distant destination.

WDM is popular because the energy on a single channel is typically only a few gigahertz wide.

Wavelength division multiplexing is demonstrated below.

Circuit Switching/ Packet Switching

Think about when your computer places a telephone call, the switching equipment within the telephone seeks out a physical path all the way from your telephone to the receivers' telephone this is circuit switching.

Packet switching is the alternative to circuit switching.

Packet switching network place a tight upper limit on the size of packets.

Telephone System First-Generation 1G Mobile Phones

Analog Voice Second-Generation 2G Mobile

Phones Digital Voice Third-Generation 3G Mobile Phones

Digital Voice + Data A shows frequencies are not reused

in adjacent cells. and b adds more users, smaller cells can be used

Second Generation (2G) Mobile Phones: Digital Voice The first generation was analog the second

generation is digital. Advantages of switching to digital:- It provides capacity gains by allowing voice

signals to be digitized and compressed- It improves security by allowing voice and

control signal to be encrypted.- It enables new services such as text

messaging.

Third Generation (3G) Mobile Phones: Digital Voice and Data The third generation of mobile phones

(3G) is all about digital voice and data.- Ex. Apple’s iPhoneThe only problem is iPhone uses a 2.5G

network (an enhanced 2G network not an actual 3G network) and there is not enough data capacity to keep users happy.

Cable Television Conceived in the late 1940s As a way to provide better reception to

people living in rural or mountainous areas Nowadays many people get their telephone

and internet service over cable. - Community antenna television- Internet over cable- Spectrum allocation- Cable modems- ADSL versus cable

Community Antenna Television

• An Early Cable Television System

Internet Over Cable

Cable Television

The Fixed Telephone System

Spectrum Allocation

Allocation in a typical cable TV system used for internet access

Cable Modems

Details of the upstream and downstream channels in North America.

ADSL Versus Cable Which is Better ADSL or Cable? - Depending on who you ask because

ADSL and cable are much more alike than they are different. They offer comparable service and, as competition between them heats up probably comparable prices.

Public Switched Network