# Chinese (Mandarin)/Pinyin

!\"Double Happiness\" Ink and color on silk by the Chinese artist Cui
Bo, active during the reign of
Shenzong.{width="420"
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The reading materials of this book are written in bite size for learning
easier, can be used for learning Pinyin Chinese\*
as well as English. (\*For learning Hanzi Chinese,
please click ***here***.)

-   You can search related topics of this book from the following box,
    or browse the ***resources***.

# The formulation of Pinyin

## Historic background

**Pinyin** was
officially declared by the government of the
People\'s Republic of China in
1958.

# The explanation of Pinyin

## What is Pinyin?

**Pinyin** (also called Hanyu Pinyin,
Romanized Chinese or Pinyin Chinese) is a
type of
transliteration for Putonghua - the
Standard Chinese language (a tonal language)
where tone marks are used to show tones. It is the
official form of the Latin
alphabet transliteration used for the
People\'s Republic of China and most of the world.
And it is the standard form of Chinese
Romanization for the United
Nations.

## Pronunciation

## Orthography

We should divide Pinyin text by words and write
syllables connectedly, such as \"I am a
foreigner\" should be written as \"Wǒ shì
wàiguórén\" in Pinyin.

## Syllable-dividing mark

Syllable-dividing mark is the mark for dividing syllables, used before
the syllables starting with vowels \"a\", \"o\", or
\"e\", such as \"pí\'ǎo\".

# The application of Pinyin

## To spell Chinese language

### Phonetic notation of Hanzi

### For spelling Putonghua

Chinese is normally written by
ideographics. But for non-Chinese-speaking
people, it is hard to recognize them. Pinyin can
help Chinese learners recognize them more easily.
This is a useful way to learn Chinese. Pinyin can
also be used in place of Hanzi when Hanzi is not convenient.

## Application technology

### Indexing

#### Indexing problems

There is no particular order to
Hanzi as it does not use the Roman
alphabet (also called the Latin
alphabet, i.e. ABC), so ordering by
alphabetical order is inconvenient. There are
currently many indexing methods to Hanzi,
including character
stroke, character radical,
Four-Corner System,
Zhuyin, Pinyin and etc. The
structural problems of Hanzi cause indexing
difficulty.

##### Solutions to indexing problems

Related governments together stipulate a
unified Hanzi strokes and
radicals standard.

There have been suggestions to use Pinyin as
the indexing method.
Pinyin adopts internationally used Roman
alphabet, makes convenient
file order. Pinyin uses
phonetic values, avoiding the
problem created by the lack of
unity between traditional and simplified
character strokes.

### Technical terms translation

#### Technical terms translation problems

Majority of written language uses Roman alphabet (also called Latin
alphabet). Hanzi (also called Chinese character) is not an alphabetic
written language and is not convenient for translation, causing a lot of
confusion. Technological terms such as Internet can be translated as
互联网 (Hùliánwǎng), 国际互联网 (Guójì Hùliánwǎng), 因特网 (Yīntèwǎng);
laser translated as 雷射 (léishè), 镭射 (léishè), 莱塞 (láisài), 激光
(jīguāng). Brand names such as National, Panasonic, Technics are
translated as 乐声牌 (Lèshēng-pái), 松下 (Sōng-xià); Sharp is translated
as 声宝 (Shēngbǎo), 夏普 (Xiàpǔ); Sony is translated as 新力 (Xīnlì),
索尼 (Suǒní). Place names such as 北京 (Běijīng) is translated as
Peking, Beijing; 广州 (Guǎngzhōu) is translated as Canton, Kwangchow,
Guangzhou. People names such as the surname 罗 (Luó) is translated as
Luo, Lo, Law; 李 (Lǐ) is translated as Lee, Li; Nixon is translated as
尼克逊 (Níkèxùn), 尼克松 (Níkèsōng). The same person can be translated
into different names.

##### Technical terms translation problem solutions

When translating foreign languages, directly transliterating foreign
languages can solve problems. For example, Internet directly translates
to the Internet; laser directly translates to the laser; National,
Panasonic and Technics directly translate to National, Panasonic and
Technics, or as kanji of Japan: 松下 (Sōng-xià). Names of Chinese
people, places and technical terms all use Pinyin to transliterate to
foreign languages. For example, 北京 (Běijīng) 邓小平 (Dèng Xiǎopíng)
and 普通话 (Pǔtōnghuà) use Pinyin to transliterate to Beijing, Deng
Xiaoping and Putonghua.

### Standardization of person and place names

### Romanization of technical terms and code names

#### Romanization

## Learn Chinese

Pinyin is a tool for learning Mandarin, and is used to explain both the
grammar and spoken Mandarin. Books containing both Hanzi and Pinyin are
used by learners of Chinese; Pinyin\'s role in teaching pronunciation is
similar to Furigana-based books (with Hiragana letters written above or
next to Kanji, directly analogous to Zhuyin) in Japanese or fully
vocalised texts in Arabic (\"vocalized Arabic\").

### Pinyin reading materials

**Pinyin reading materials**
are commonly used for learning Chinese.

## Hanzi input

### Pinyin input method

Pinyin input method is a popularly used phonetic input method. To key in
Putonghua\'s pinyin which will automatically convert into Hanzi. For
example: \"BABA\" is for inputting \"爸爸\".

# Resources

-   :Category:Book:Pinyin
-   See also

# Appendix

## The Problems of Hanzi Application in IT

## Pinyin Orthography

-   Basic Rules of Pinyin Orthography
    (Summary)

## Dictionaries

-   Pinyin vocabulary
-   Wenlin Pinyin dictionary

## Pinyin reading matters

The reading materials of this book can be used for learning Chinese as
well as English.

-   **Please see  for Pinyin reading
    matters**

```{=html}
<!-- -->
```
-   **Pinyin reading matters from
    Wikimedia**

## Pinyin tone marking

ā á ǎ à a = a1 a2 a3 a4 a5 = a ar aa ah \'a \*

āi ái ǎi ài = ai air aai aih

ān án ǎn àn = an arn aan ahn

āng áng ǎng àng = ang arng aang ahng

ē é ě è = e er ee eh

\- ér ěr èr = - err eer erh

nǖ nǘ nǚ nǜ = nv nvr nvv nvh

lǖ lǘ lǚ lǜ = lv lvr lvv lvh

------------------------------------------------------------------------

**de** = **d**, **dy**, or **de**, can be written distinguishably as
follows:

**d** indicating subordination; suffix indicating an adjective

**dy** -ly, suffix indicating an adverb

**de** indicating a verb followed by an adverb or adverb clause; infix
indicating be able to

------------------------------------------------------------------------

**le** indicating a past tense; indicating a new situation

**bu** not, no; non-, un-; be unable to

**\'g** non-specific measure word

**\'r** non-syllabic diminutive suffix; retroflex final

-   \* Syllable-dividing mark can be replaced by grave mark (\`) when
    apostrophe (\') is used for indicating neutral tone.

```{=html}
<!-- -->
```
-   Alternative methods can be used when diacritics are not convenient.


# Chinese (Mandarin)/Planning

**Welcome to the main Planning page for the Chinese Wikibook.** Unless
your comment only pertains to a particular lesson, discuss your ideas
here so that the overall planning discussion is not spread across many
lesson pages. Initial planning and continued coordination of effort is
extremely important to help reduce the need for reworking later. New
issues are entered here, with the most recent at the bottom of the page.
Please review the *Table of Contents* to see if your issue has already
been raised; also check the archives (see below) in case it was
discussed some time ago.
![](Haldeman_and_Ehrlichman_discuss_policy,_1973.png "Haldeman_and_Ehrlichman_discuss_policy,_1973.png"){width="150"}
**Please observe the following guidelines:**

1.  Place your question **at the bottom** of the list;
2.  **Title** the question (by placing the title between equals signs
    like this: == *title* ==);
3.  **Sign** your name and date (by adding four tildes: \~\~\~\~).

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## Chinese Wikibook Purpose and Audience

This book is intended to be a complete learning resource center for
students of Mandarin Chinese. Dialects, such as Cantonese, will be
covered by their own Wikibooks and be linked to in the See Also section
of the Table of Contents (TOC). It should teach listening comprehension
and speaking as well as reading and writing using whatever technologies
are most appropriate (includes audio for text and animations for stroke
orders).

The audience is the serious studier of Chinese, either at the high
school or college level. Casual learners looking for a few choice
phrases or unwilling to spend the time learning characters can be served
adequately by the Chinese Phrasebook on Wikivoyage. Younger audiences
can have age-appropriate material created for them in WikiJunior.
Intermediate- or Advanced-Level modules may be added, but because of the
cumulative nature of a language text (explained below), it would
probably be best to focus efforts on the Introductory Level first and
make continuations of the series later.

## The Need for Planning

Language Wikibooks faces some unique challenges from a planning
perspective. It\'s harder to produce a quality, integrated work in
language instruction than in other subjects, like Biology or Physics.
Those you can break into discrete units and still read about it---topics
within the subject can be rearranged or meaningfully read even in
isolation from the rest of the text.

Not so with an elementary language text. All the grammar and vocabulary
that you learn is cumulative, so everything can only stack one way.
Flipping ahead (or falling behind) more than a few chapters and you\'re
lost. Order matters, so we use the a sequential naming scheme (Lesson 1,
Lesson 2,\...), not a topic-based one (Asking Questions, Giving
Directions,\...).

To avoid late-stage reorganizations that would necessarily be painful
with lots of work going to waste, we should agree on a \'Lesson
Roadmap\' beforehand and then flesh it out. I think a Wiki can really
work for this, but that the project still needs a common format and
approach. A standard outline for lessons would help a lot with that, so,
may I suggest that the lessons of this text each include the components
outlined below.

## *Lesson Roadmap*

1.  **Lesson 1: Hello!
    (你好！)** - Basic
    Sentences and Questions
    -   Simple Sentences
        -   SVO sentence structure
            -   The equational verb shi \[是\] and its negation with bu
                \[不\]
            -   Verb 叫
    -   Intro to Questions
        -   Ma \[吗\] and ne \[呢\] particles
        -   Question words (for now, only shei \[谁\] and na/nei \[哪\])
2.  **Lesson 2: Are you busy today? (今天你忙不忙?)**
     - Measure Words,
    Possession and Affirmative-Negative Questions
    -   Measure Words (the most commonly used, like 个、本、张)
    -   The possessive verb you \[有\] and its negation with mei \[没\]
    -   More Questions! Affirmative-Negative
        -   是不是 Questions
        -   有没有 Questions
3.  **Unit 3: 今天星期三** - Numbers, Days, Aux. Verbs, de \[的\]
    -   Numbers (to 100)
    -   year, month, day, week etc.
    -   Auxiliary Verbs, the de \[的\] particle
4.  **Lesson TBA: Unordered List of Grammar Explanations** - Even w/o
    example sentences grammar can be explained nonetheless
    -   Comparisons Using bǐ \[比\]
    -   The le \[了\] particle
        -   indicating a completed action
        -   indicating a past event
        -   expressing change of state or situation
        -   adding emphasis
    -   Time
        -   when - time of day, hour and minutes, morning/night
        -   how long - duration of time
    -   Directions/Placement Words (NESW, L/R, li3, li2, zai4, bian1)
    -   Pronunciation of yi1 (b/c of tone changes)
    -   Complement of degree de
    -   ordinal numbers di
    -   Complement of direction qu/lai
    -   Compliment of result
    -   ba
        -   the suggestion particle
        -   as preposition
    -   guo denoting experience of something
    -   (some) members of family
    -   reduplication
    -   emphasis using shi\...de
    -   How to use a Radical Index (for C-E Glossary)
    -   Basic rules for writing Chinese characters

## Subjects Areas to Cover

**Greetings**

**Getting around (a city)**

-   Where is the post office?
-   How far is it to the school?
-   Public transport
-   Conversation in a taxi

**Chapter Three: Buying Things**

-   How much is that shirt?
-   Haggling

**Eating**

-   Eating Out
-   At the supermarket
-   The grocer
-   A meal at home
-   Traditional Chinese cooking

**Sports**

-   What do you play?
-   Talking about a match
-   Skiing
-   Mountaineering
-   Yachting
-   Surfing

**Your House**

-   Description of house
-   Traditional Chinese furniture
-   Living in a community

**Immediate family and relatives**

**Education**

-   This is my school
-   Afterschool activities
-   School subjects

**Going to the Zoo / Wo men qu dong wu yuan**

## Decided Conventions

1.  **The Title Page leads to the main TOC**, not the Cover Page. The
    Cover Page can be linked from the TOC and used in Print versions.
2.  **Hanyu Pinyin is used** as the only Romanization format, though
    equivalency charts to other systems provided
    1. Tone marks are used
    instead of tone numbers.
3.  **Traditional or Simplified** characters appear only in the Lesson
    Text and in parentheses in the Vocabulary sections---not in titles,
    example sentences, or exercises.
4.  **Traditional pages are linked as parallel subpages** of every
    Simplified page. On every Traditional page, a parentdirectory link
    links back to the Simplified version, always providing a toggle
    button between versions.

## Unresolved Issues


# Chinese (Mandarin)/Development History

This history of the Chinese Wikibook highlights milestones along the
book\'s development.

## 2006

-   **March 17** A PDF
    Version_v0.2.pdf "wikilink") is created
    and the first audio
    samples
    are added to vocabulary
-   **March 13** Print
    version
    created

## 2005

-   **October** Chinese is elected October\'s Book of the
    Month,
    the first language text to do so
-   **June 12** First stroke order
    images
    linked from vocabulary
-   **March** First
    nomination
    for Book of the Month. One vote (or 37 minutes) shy of High school
    extensions\' 5 votes. Chinese
    would also be the runner-up in August and September.
-   Translated versions of the Chinese Wikibook appear in
    Spanish
    (March 13),
    Italian
    (July 19) and
    Polish
    (August 20).

## 2004

-   **December 29** First Table of
    Contents
-   **December 22**
    Planning
    page added
-   **December 19** Major
    rewrite
    of Lesson 1

## 2003

-   **December 13** First
    edit
    by Yacht


# Communication Systems/Communications Introduction

## Introduction

People are prone to take for granted the fact that modern technology
allows us to transmit data at nearly the speed of light to locations
that are very far away. 200 years ago, it would be deemed preposterous
to think that we could transmit webpages from China to Mexico in less
than a second. It would seem equally preposterous to think that people
with cellphones could be talking to each other, clear as day, from miles
away. Today, these things are so common, that we accept them without
even asking how these miracles are possible.

### What is Communications?

Communications is the field of study concerned with the transmission of
information through various means. It can also be defined as technology
employed in transmitting messages. It can also be defined as the
inter-transmitting the content of data (speech, signals, pulses etc.)
from one node to another.

## To Whom is This Book For?

This book is for people who have read the Signals and
Systems wikibook, or an equivalent
source of the information. Topics considered in this book will rely
heavily on knowledge of Fourier Domain representation and the Fourier
Transform. This book can be used to accompany a number of different
classes spanning the 3rd and fourth years in a study of electrical
engineering. Knowledge of integral and differential calculus is assumed.
The reader may benefit from knowledge of such topics as semiconductors,
electromagnetic wave propagation, etc., although these topics are not
necessary to read and understand the information in this book.

## What will this Book Cover

This book is going to take a look at nearly all facets of electrical
communications, from the shape of the electrical signals, to the issues
behind massive networks. It makes little sense to be discussing these
subjects outside the realm of current examples. We have the Internet, so
in discussing issues concerning digital networks, it makes good sense to
reference these issues to the Internet. Likewise, this book will attempt
to touch on, at least briefly, every major electrical communications
network that people deal with on a daily basis. From AM radio to the
Internet, from DSL to cable TV, this book will attempt to show how the
concepts discussed apply to the real world.

This book also acknowledges a simple point: It is easier to discuss the
signals *and* the networks simultaneously. For this kind of task to be
undertaken in a paper book would require hundreds, if not thousands of
printed pages, but through the miracle of Wikimedia, all this
information can be brought together in a single, convenient location.

This book would like to actively solicit help from anybody who has
experience with any of these concepts: Computer Engineers,
Communications Engineers, Computer Programmers, Network Administrators,
IT Professionals. Also, this book may cover all these topics, but the
reader doesn\'t need to have prior knowledge of all these disciplines to
advance. Information will be developed as completely as possible in the
text, and links to other information sources will be provided as needed.

## Where to Go From Here

Since this book is designed for a junior and senior year of study, there
aren\'t necessarily many topics that will logically follow this book.
After reading and understanding this material, the next logical step for
the interested engineer is either industry or graduate school. Once in
graduate school, there are a number of different areas to concentrate
study in. In industry, the number is even higher.

## Division of Material

Admittedly, this is a very large topic, one that can span not only
multiple printed books, but also multiple bookshelves. It could then be
asked \"Why don\'t we split this book into 2 or more smaller books?\"
This seems like a good idea on the surface, but you have to consider
exactly where the division would take place. Some would say that we
could easily divide the information between \"Analog and Digital\"
lines, or we could divide up into \"Signals and Systems\" books, or we
could even split up into \"Transmissions and Networks\" Books. But in
all these possible divisions, we are settling for having related
information in more than 1 place.

### Analog and Digital

It seems most logical that we divide this material along the lines of
analog information and digital information. After all, this is a
\"digital world\", and aspiring communications engineers should be able
to weed out the old information quickly and easily. However, what many
people don\'t realize is that digital methods are simply a subset of
analog methods with more stringent requirements. Digital transmissions
are done using techniques perfected in analog radio and TV broadcasts.
Digital computer modems are sending information over the old analog
phone networks. Digital transmissions are analyzed using analog
mathematical concepts such as modulation, SNR (signal to noise ratio),
Bandwidth, Frequency Domain, etc\... For these reasons, we can simplify
both discussions by keeping them in the same book.

### Signals and Systems

Perhaps we should divide the book in terms of the signals that are being
sent, and the systems that are physically doing the sending. This makes
some sense, except that it is impossible to design an appropriate signal
without understanding the restrictions of the underlying network that it
will be sent on. Also, once we develop a signal, we need to develop
transmitters and receivers to send them, and those are physical systems
as well.

### *Systems* Approach

It is a bit confusing to be writing a book about *Communication System*s
and also considering the **pedagogical** *Systems Approach*. Although
using the same word, they are not quite the same thing.

This approach is almost identical to the description above (Signals &
Systems) except that it is not limited to the consideration of signals
(common in many university texts), but can include other technological
drivers (codecs, lasers, and other components).

In this case we give a brief overview of different communication systems
(voice, data, cellular, satellite etc.) so that students will have a
context in which to place the more detailed (and often generic)
information. Then we can then zoom in on the mathematical and
technological details to see how these systems do their magic. This
lends itself quite well to technical subjects since the basic systems
(or mathematics) change relatively slowly, but the underlying technology
can often change rapidly and take unexpected terns.

I would like to suggest that the table of contents in this book be
rearranged to reflect this pedagogical approach: Systems examples first,
followed by the details.

### Why would anyone want to study (tele)communications?

Telecommunications is an alluring industry with a provocative history
filled with eccentric personalities: Bell, Heavyside, Kelvin, Brunel and
many others. It is fraught with adventure and danger: adventure spanning
space and time; danger ranging from the remote depths of the ocean floor
to deep space, from the boardrooms of AT&T to the Hong Kong stock
exchange.

Telecommunications has been heralded as a modern Messiah and cursed as a
pathetic sham. It has created and destroyed empires and institutions. It
has proclaimed the global village while sponsoring destructive
nationalism. It has come to ordinary people, but has remained largely in
the control of the 'media' and even \'big brother\'. Experts will soon
have us all traveling down a techno-information highway, destination ---
unknown.

Telecommunications has become the lifeblood of modern civilization.
Besides all that, there's big bucks in it

## About This Book

There are a few points about this book that are worth mentioning:

-   The programming parts of this book will not use any particular
    language, although we may consider particular languages in dedicated
    chapters.


# Communication Systems/Communications History

This page will attempt to show some of the basic history of electrical
communication systems.

## Chronology

1831 Samuel Morse invents the first repeater and the telegraph is born

1837 Charles Wheatstone patents \"electric telegraph\"

1849 England to France telegraph cable goes into service---and fails
after 8 days.

1850 Morse patents \"clicking\" telegraph.

1851 England-France commercial telegraph service begins. This one uses
gutta-percha, and survives.

1858 August 18 - First transatlantic telegraph messages sent by the
Atlantic Telegraph Co. The cable deteriorated quickly, and failed after
3 weeks.

1861 The first transcontinental telegraph line is completed

1865 The first trans-Atlantic cable goes in service

1868 First commercially successful transatlantic telegraph cable
completed between UK and Canada, with land extension to USA. The message
rate is 2 words per minute.

1870 The trans-Atlantic message rate is increased to 20 words per
minute.

1874 Baudot invents a practical Time Division Multiplexing scheme for
telegraph. Uses 5-bit codes & 6 time slots---90 bps max. rate. Both
Western Union and Murray would use this as the basis of multiplex
telegraph systems.

1875 Typewriter invented.

1876 Alexander Graham Bell and Elisa Grey independently invent the
telephone (although it may have been invented by Antonio Meucci as early
as 1857)

1877 Bell attempts to use telephone over the Atlantic telegraph cable.
The attempt fails.

1880 Oliver Heaviside\'s analysis shows that a uniform addition of
inductance into a cable would produce distortionless transmission.

1883 Test calls placed over five miles of under-water cable.

1884 - San Francisco-Oakland gutta-percha cable begins telephone
service.

1885 Alexander Graham Bell incorporated AT&T

1885 James Clerk Maxwell predicts the existence of radio waves

1887 Heinrich Hertz verifies the existence of radio waves

1889 Almon Brown Strowger invents the first automated telephone switch

1895 Gugliemo Marconi invents the first radio transmitter/receiver

1901 Gugliemo Marconi transmits the first radio signal across the
Atlantic 1901 Donald Murray links typewriter to high-speed multiplex
system, later used by Western Union

1905 The first audio broadcast is made

1910 Cheasapeake Bay cable is first to use loading coils underwater

1911 The first broadcast license is issued in the US

1912 Hundreds on the Titanic were saved due to wireless

1915 USA transcontinental telephone service begins (NY-San Francisco).

1924 The first video signal is broadcast

1927 First commercial transatlantic radiotelephone service begins

1929 The CRT display tube is invented

1935 Edwin Armstrong invents FM

1939 The Blitzkrieg and WW II are made possible by wireless

1946 The first mobile radio system goes into service in St. Louis

1948 The transistor is invented

1950 Repeatered submarine cable used on Key West-Havana route.

1956 The first trans-Atlantic telephone cable, TAT-1, goes into
operation. It uses 1608 vacuum tubes.

1957 The first artificial satellite, Sputnik goes into orbit

1968 The Carterphone decision allows private devices to be attached to
the telephone

1984 The MFJ (Modification of Final Judgement) takes effect and the Bell
system is broken up

1986 The first transAtlantic fiber optic cable goes into service

## Claude Shannon

## Harry Nyquist


# Communication Systems/Telephone System

From the western perspective, the telephone is ubiquitous. However, the
reality is that 80% of the world's population have limited access to
one.

To the average person, the telephone system is simply a 'black box'.
Relatively few people need to know how the PSTN works. The main
consideration is that it works, and is relatively inexpensive. The PSTN
has a designed reliability of 99.999% (called 5 nines reliability).

### A Telephone Call

To make a telephone call, one simply picks up the handset, enters a
number, and waits for the system to perform its magic:

::\*Lifting the handset from its cradle releases a hook switch and
causes a dc current to flow (20 - 120 ma). The central office monitors
this loop current and interprets it as a request for service.

::\*The office acknowledges the request for service by sending dial
tone. This normally occurs in less time than it takes to pickup the
handset and place it to the ear. Once dial tone has been received, the
subscriber starts to dial.

::\*In the past, when dialing, a rotary dial switch opened and closed
the loop in a predetermined manner. If one was very coordinated, it was
possible to perform the same task by flashing the hook switch. To assure
the customer that the system is responding, dial tone is removed once
dialing starts. In most systems today DTMF (**Dual Tone Multi
Frequency**) signaling is used How Telephones
Work

::\*Depending upon the office type and digits received, a number of
things might happen. In most cases, end-users are attached to what is
called a class 5 or end-office. These are the most common types of
telephone exchanges. Each class 5 office has one or more, three digit
exchange numbers. These are the first three digits in an ordinary
7-digit telephone number.

::\*If the central office includes the customer dialed exchange number,
it will know that the call is local and the other party is connected to
the same office. The office will therefor control the entire call setup
and takedown.

::\*If the first three digits do not correspond to an exchange handled
by the end-office, it will have to find a trunk line to an office that
can handle the call. This means that each office must know the exchange
numbers of all the offices within its calling area, and how to get to
them. The call setup and takedown will therefor be shared between the
two exchanges. They must monitor the call in progress and inform each
other of any change in call status.

::\*If the first digit dialed is a one, the office will recognize this
as a long distance call, and will start looking for a spare toll trunk.
A toll office has a greater knowledge base as to where distant exchanges
are located and how to get to them.

::\*The telephone system must be intelligent enough to recognize that in
a local call, only seven digits are usually required. Some very small
exchanges however, allow local calls by omitting the exchange number and
using only the last 4 digits or extension number. In large urban areas,
it may be necessary to prefix local calls with a 3-digit area code. An
international call may require up to 16 digits.

::\*Once the entire number has been received, the office at each end of
the connection must alert both parties as to what is happening. At the
originating end, a ringing tone is sent to the speaker in the handset.
At the terminating end, the office is generating a much larger ringing
voltage to activate a bell.

::\*The far-end-office monitors the line to determine if someone answers
the ringing phone. This is done by examining the DC current drawn when
the far-end customer lifts the handset, inducing loop current through
the hook switch. The far-end-office must then disconnect the ringing
before the handset reaches the ear, and signals back to the originating
office that someone has picked up the phone. The origination office must
then disconnect the ring back tone and complete the voice connection.

::\*Both end-offices monitor their respective loop currents during the
entire call to determine if one party hangs up. Once this happens, one
end-office signals the other, and dial tone is placed on the loop. This
alerts the remaining party that the connection has been terminated.

::\*If the line is in use, the central office will not set up the
connection and return a busy tone to the originator. By doing this
switching, call processing, and transmission resources are not being
tied up unnecessarily. However, there are a number of options such as
call forwarding and call waiting which modify this process.

::\*With call forwarding, a call to a busy number is routed to an
answering service. With call waiting, the calling party hears a ringing
tone, and the called party hears a beep, which they can either ignore or
signal back to the office that the new call should be given priority
over the existing call. If however, the call cannot be completed because
the system itself is too busy, it returns a fast busy tone to the
originator.

::\*In a touch-tone environment, the same procedure is followed, except
that tones are used to convey numbers to the local office instead of
interrupting loop current. Some calling features, generally known as
CLASS (Custom Local Area Signaling Services) , are available in areas
with touch-tone service.

In summary, the steps in completing a local telephone call are as
follows:

-   Initially calling party goes in off hook condition.
-   Once the DC current flows into the loop,the switching machine sends
    a dial tone to the calling party in order to make sure that it
    access to the switching machine.
-   Now the caller starts dialing the destination telephone number
    either by rotary dialer or touch tone keypad.
-   The switching machine will locate the corresponding local loop for
    the destination number.
-   After locating,the switching machine will check for on hook or off
    hook condition of the destination subscriber soas to make dc current
    to flow.Simultaneously the switching machine provide a signal path
    through it for the two local loops.
-   The switching machine sends a busy dial tone to the called
    subscriber when the destination party is off hook.
-   If the destination part is off hook when the destination party
    answers to the call and completes the loop.
-   When the destination party answers the called party,DC current flows
    through the loop.The flow of DC current through the loop indicates
    the switching machine that destination party answered the call.As a
    result,the switching machine remove the ringing and ring back
    signals.
-   If both stations are on hook,switching machine recognizes on open
    circuit on that loop and drops the connection through the switching
    machine.

#### But where do the telephone wires go?

The telephone line goes to a terminal block in a service area interface.
These are often located on a pole or small enclosure on the street. The
service area interface bundles the subscriber drop cables into a single
larger cable. These are in turn gathered together to form larger feeder
cables. The entire wiring system somewhat resembles a huge tree.

Cables coming out of a central office may have hundreds or even
thousands of pairs bundled together however by the time the cable gets
to the end user, it is generally down to about 50 pairs. An individual
subscriber consists of many cable sections spliced together. Bellcore
claims that the average U.S. subscriber line has twenty-two splices.

Feeder cables enter the central office in a large underground room
called a cable vault. Each feeder may contain hundreds of pairs of
wires, and be pressurized in order to prevent moisture or ground water
from entering and affecting the transmission characteristics of the
wire. A typical vault may contain tens of thousands of wire pairs.

The cables pass through the vault and are terminated on the vertical
side of the MDF (main distribution frame) . To protect the central
office equipment from high voltage transients, such as lightning
strikes, which may travel down the wire, the lines are surge protected
by carbon blocks or gas tubes.

The horizontal side of the MDF, connects the incoming telephone lines to
the peripheral equipment. All that is required to connect a line
appearance to a specific interface is to place a jumper between the
vertical and horizontal sides of the MDF.

Signals coming from an end-user are generally analog in nature.
Consequently, the peripheral equipment converts the signals to digital
form before passing them on to the rest of the network.

Incoming trunks from other central offices are comprised of specialized
carrier systems. They may be either analog or digital, but all new
systems are strictly digital.

Most end-user voice & data interfaces are multiplexed on to high-speed
paths, which pass through the internal switching, network before being
routed to outgoing lines or trunks. Incoming digital carrier systems may
be accepted directly into the switching network through a cross-connect
or may be demultiplexed prior to switching.

## PSTN Hierarchy

:   ![](Local_network.gif "fig:Local_network.gif")

Historically the telephone network was composed of a hierarchical
structure consisting of 5 different office types. The most common of
these is the class 5 end-office. An end-office connects directly to
subscriber telephone sets and performs switching functions over a
relatively small area. Telephone exchanges connect to subscribers by
means of local loops or lines, generally one per customer. Telephone
offices connect to each other by means of trunks.

A class 5 or end-office interconnects telephones throughout a small
service area. Each end-office may contain several three-digit exchange
numbers and is aware of other local exchange numbers held by other
offices.

Calls between offices are routed over interoffice or tandem trunks. Long
distance calls are routed to toll offices via toll trunks. The average
class 5 office serves approximately 41,000 subscribers, and covers 30
square km in an urban environment.

Some nodes may have no customers at all, and may be connected only to
other nodes. These inter-node or trunk connections are usually made by
FDM or TDM transmission links.

### Exchange Area Network

:   ![](Trunk_connection.gif "fig:Trunk_connection.gif")

An exchange network consists of local and tandem exchanges connected by
trunks. A tandem office interconnects class 5 offices by means of
twisted pair, coax, microwave, or fiber optic carriers. Alternate
routing paths between local exchanges are provided if the direct trunks
are occupied.

An exchange area includes all of the offices, which are aware of each
other, but do not involve long distance charges. In very large urban
areas, there is an overlap between exchange areas, which may also cross
over area code boundaries.

### Long Haul Network

:   ![](Long_haul_network.gif "fig:Long_haul_network.gif")

A long haul network consists of exchanges interconnected by toll
offices. Toll offices keep track of long distance charges and are
typically confined to national boundaries. These trunks consist of high
capacity coax, microwave, or glass fiber.

Messages used to control the call setup and takedown can be sent by two
basic methods. Traditionally, inter-office messages are sent over the
same channel that will carry the voice path, but in newer systems,
common channel signaling is being employed. In this method, the offices
have dedicated facilities, which are used to send inter-office messages.
There are some advantages to this, perhaps the notable being the added
degree of difficulty encountered if one wants to defraud the system.
When in-band signaling was used, it was possible for people to dial long
distance calls without being charged, if they created the tones used to
disable the toll circuit.

### Interoffice Signaling

Trunks are used to interconnect the various levels of telephone
exchanges. It is necessary for these links to exchange on a wide range
of information including:

:

    :   • Call related signaling messages
    :   • Billing information
    :   • Routing and flow control signals
    :   • Maintenance test signals

There are two ways for telephone offices to communicate with each other
and pass on routing information. Information can be conveyed in the same
channel that will be used to convey the voice signal, or it may be
completely disassociated with it.

#### CAS

The CAS (Channel Associated Signaling) approach uses the voice channel
to send information through a trunk. For example, a 2600 Hz tone is used
in interoffice trunks to signal on-hook. A major disadvantage of this
system is that subscribers can bypass toll centers by injecting the
appropriate tones. One way to avoid this problem is by using out-of-
band signals on toll trunks. Since the customer's signal must pass
through an audio anti-aliasing filter, it is not possible to inject the
out-of-band signaling tone.

A principle advantage of in-channel signaling is that the integrity of
the voice path is checked each time a connection is established.
Out-of-band signaling allows for continuous supervision of the
connection throughout the call.

#### CCIS

The CCIS (Common Channel Interoffice Signaling) approach has the
signaling information conveyed on a facility completely separated from
the customer's voice path. This allows for a faster, more efficient
control, however the reliability of the CCS network must be considerably
greater than that of the individual voice paths. The signaling channel
may follow the same route as the final connection path, or it may be
completely disassociated with it. STPs (Switch Transfer Point) are need
in the network if the signaling path is disassociated, thus effectively
creating two networks: a speech network and a signaling network overlay.

#### SS7

Virtually all calls requiring tandem or toll office routing are
established and controlled by the SS7 signaling network.

The SS7 signaling network is a packet switching facility comprised
primarily of STPs (Signaling Transfer Point) and SCPs (Service Control
Point) connected to the PSTN SSP (Signal Switching Point). STPs are
deployed in pairs and are the brains of the system. They determine which
trunks and offices should be used in establishing inter-office
connections.

The SCP is a database that keeps track of such things as: credit card
authorization, virtual network subscriber listings, 800 number
conversion tables, billing, and other special services.

## Class 5 Office

A telephone central office
is often referred to as a switch because it switches or routes calls.
Regardless of who makes them, all class 5 offices have the same
objectives, and therefor have similar structures. The three major
components found in any modern switching systems are; the central
control, network, and peripherals.

### A Generic Communication Switch

:   ![](Generic_switch.gif "fig:Generic_switch.gif")

The internal architecture of a telecommunications switch is somewhat
like the organization of the entire system. The internal structure is
often illustrated by the traditional pyramid or hierarchical
arrangement. The control or brains of the operation are shown at the
top, and the peripheral units that connect to the outside world are
placed at the bottom.

Physically, the switch is simply a series of boxes, full of electronics:

:   ![](Nortel_DMS100_public_voice_exchange.jpg "fig:Nortel_DMS100_public_voice_exchange.jpg")

The MTBF (mean time between failure) for any PSTN switch must be very
long, since business would soon grind to a halt if telephone traffic was
interrupted for a prolonged period, but more importantly, emergency
services would be severely curtailed. For these reasons, large public
switches have a great deal of redundancy built in. Redundancy is
provided in two basic ways; hot standby and load sharing.

In the hot standby arrangement, two or more processors are fed with the
identical information and are making decisions, however, only one of
these processors is in charge and is executing decisions. In the event
of a failure, the healthy unit assumes the full load. There is no
degradation in performance, and no calls in progress should be lost.

In a load-sharing configuration, all processors are actively working but
not to their full capacity. In the event of a failure, the defective
processor is isolated from the system, and the others pick up the slack.
There may be degradation in performance, and calls in progress on the
defective processor may be lost.

#### Peripheral Layer

The periphery is the outermost layer in any system. It contains the
interface to all outgoing lines and trunks.

#### Network Layer

The network switch does the actual routing of signals from one customer
or port to another. Switching can be done in either the space or time
domain. Initially, all switching was done by mechanical contacts in the
space domain. Today most switches are digital in nature and operate
mainly in the time domain. However, some degree of space switching is
always required since signals must ultimately be routed from one line to
another.

Networks always provide some form of concentration. That is to say, not
all customers can be handled simultaneously. Statistical analysis shows
that in the majority of cases, the switch needs only to handle about 20%
of all the subscribers at one time. This is similar to highway systems
where the roads are designed to handle not all vehicles simultaneously,
but only a certain peak load.

#### Central Control

This contains the system intelligence and customer database. It knows
who the customers are, what they want, and how to provide the service
they require.

In a step x step \[step by step\] switch, the intelligence is fully
distributed and there is no central control, whereas in a crossbar
facility, all of the intelligence is resident in a central controller or
computer. In all modern systems the intelligence is somewhat
distributed, with various functional blocks contributing to the decision
making process.

At onetime there was a sharp distinction between computers and telecom
switches, but today this division is less clear, and central controllers
may be regarded as a specialized computer.

## Specialized Components

There are some components associated with telephone systems that deserve
special consideration.

### Line Cards

:   ![](LineCard2.jpg "fig:LineCard2.jpg")

Line cards are the single most common component in a telephone office.
It is a very complex device that contains a wide range of technologies.

The (telephone) line interface is often referred to as a BORSCHT
circuit.

This acronym describes the functional requirements of a standard
telephone line interface. The tip and ring leads of the telephone set
are wired through some protection devices to the line interface located
in the peripheral module. This interface must perform the following
functions:

:

    :   B Battery feed
    :   O Over voltage protection
    :   R Ringing
    :   S Supervision & Signaling
    :   C Coding
    :   H Hybrid
    :   T Test

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:   ![](Telephone_line.gif "fig:Telephone_line.gif")

Many of these functions can be integrated into a single IC, often called
a SLIC chip (subscriber line interface chip). SLICs have been available
for the PBX market for over a decade. Recently however, they have also
become available for the central office environment as well.

:   **B - Battery Feed**

Most domestic appliances are powered from an electric utility grid. The
notable exception to this is the telephone. This is because the
telephone should still operate in the event of a power failure. Indeed,
the telephone is vital in case of disaster or emergency.

The telephone office provides a nominal -48 volt dc feed to power the
phone. This magnitude is considered the maximum safe dc operating
potential. It would not be in the telephone company's best interest to
provide a dc voltage, which could electrocute its customers, or it's own
employees. A negative potential was chosen to reduce corrosive action on
buried cables.

Multi-function telephones cannot always be powered from the telephone
exchange and often require an alternate power source. For this reason,
sophisticated line interfaces such as ISDN SAA interfaces have a 'fail
to POTS' mode. If the electric power fails, the complex phone cannot
function to full capacity. The telephone exchange can sense the local
power outage through the telephone loop and switches to POTS only
service.

The POTS loop requires a nominal -48 v at 20 -- 100 ma dc to maintain a
voice and signaling path. The earpiece in the handset does not require
biasing, but the carbon microphone does. Subscriber signaling is
performed by temporarily placing a short circuit on the loop thus
changing the loop current, which is then sensed at the central office.

:   ![](Voltage_feed.gif "fig:Voltage_feed.gif")

There are several ways to provide loop current, the simplest being a
resistor in series with a battery.

:   ![](Current_feed.gif "fig:Current_feed.gif")

Another way to provide loop current is by an electronic current source.

Although this method is quite complex, it has become quite popular with
the advent of high voltage bipolar technology. One of the more difficult
requirements to meet is the 60-dB longitudinal line balance requirement.
To achieve this, the impedance to ground on each side of the loop, must
match within 0.1%. This is easy to do with laser trimmed thick film
resistors, but a bit tricky with current sources.

:   ![](AWG_loop_length.gif "fig:AWG_loop_length.gif")

A standard telephone requires a minimum of about 20 ma. This means that
the maximum possible loop resistance is about 2000 $\Omega$. In actual
practice, the loop is generally limited to 1250 W. The maximum loop
length is determined by the wire gauge.

:   **O - Over-voltage Protection**

The two major types of over-voltage that can occur are lightning strikes
and power line contact. In both cases, the circuit must either recover
or fail-safe. Under no circumstances can a surge be allowed to propagate
further into the system, or create a fire.

Initial surge protection is provided at the MDF by gas tubes and/or
carbon blocks, which arc if the applied voltage exceeds a few hundred
volts. Since these devices take a finite time to respond, high-speed
diodes are also used at the line circuit inputs.

:   **R - Ringing**

Ringing is often provided by means of a dedicated ringing generator that
is connected onto the loop by means of a relay. It is possible to
generate ringing voltages at the line interface if the current
generators have a high enough voltage source available to them. Or
alternately, a switching converter with step up capability can be place
on the interface.

In Canada, the ringing voltage is a nominal 86 Vrms at 20 Hz, with a 2
second on and 4 second off cycle. On rural party lines, ringing codes of
long and short rings are sometimes used.

In the U.S. there are a number of fully selective and semi-selective
ringing methods used on party lines. One employs different frequencies
ranging from about 16 -- 66 Hz. In such cases, each telephone ringer is
tuned to its own frequency. Other methods use positive and negative
battery voltages or apply ringing on either the tip or ring side of the
line with respect to ground.

:   **S - Supervision & Signaling**

The central office must supervise the loop in order to identify customer
requests for service. A request for service is initiated by going
off-hook. This simply draws loop current from the CO.

Loop current at the far-end is monitored during ringing to enable the CO
to disconnect the ringing generator when the phone is answered. The
office continues to monitor the loop current at both ends of the
connection throughout the call, to determine when the call is terminated
by hanging up.

Signaling is a way to inform the CO what the customer wants. The two
basic signaling methods used in customer loops are dial pulse and
touch-tone. It is interesting to note that preferred customer loop
signaling method in analog exchanges is digital, while the preferred
method in digital exchanges is analog!

:

    :   **MF Signaling Tones**

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:   ![](DTMF_keypad.gif "fig:DTMF_keypad.gif")

Two tones are used to perform the signaling function to eliminate the
possibility that speech be interpreted as a signal. At one time DTMF
decoders were costly and bulky devices located in a common equipment
bay, but today with the advent of LSI technology, this function can be
performed on a chip. An example is the Mitel MT8865 DTMF filter, and
MT8860 DTMF decoder.

Positions 11 to 14 are not presently being used.

:   **C - Coding**
:   ![](Companding_curves.gif "fig:Companding_curves.gif")

Telecommunications signals are seldom linearly encoded, but rather are
companded (a combination of compression & expansion). This allows for a
more uniform S/N ratio over the entire range of signal sizes. Without
companding, a 12 bit linear encoding scheme would be needed to obtain
the same S/N ratio at low volume levels. It also reduces the noise and
crosstalk levels at the receiver.

There are two principal international standards used to implement coding
algorithms today: A-law, which is used in Europe, and µ-law, which is
used in North America. These both involve 8 kHz sampling, and 8 bit A/D
& D/A conversion, thus resulting in 64 kbps digital bit streams.

Although the actual compression algorithms are continuous functions, the
codec approximates them by linear segments. A-law has 13 linear
segments, and µ-law has 15 linear segments or cords.

:   ![](Codec-segments.gif "fig:Codec-segments.gif")

Another important difference between the European and North American
codecs, can be seen by the position of the decision threshold and its
digital value.

:   ![](Codec_steps.gif "fig:Codec_steps.gif")

When a telephone call is placed between Europe and North America, it is
essential that all of these differences be accounted for. It is possible
to regenerate the analog voice by passing it through the same type of
codec that originally processed it, and then re-code with the other. An
alternate approach is to use lookup tables that translate the binary
values of one system to the other.

:   ![](Simplified_codec.gif "fig:Simplified_codec.gif")

Since the highest frequency passed is about 3.4 kHz, a great deal of
ingenuity is required to pass data at 4.8, 9.6 kbps or even higher. Note
that these are well above the Nyquist rate but considerably below the
Shannon-Hartley limit.

All modern telephone systems today employ codecs in the BORSCHT
interface to digitize the incoming analog signals. It is ironic that
although the telephone system has been updated to digital technology,
the telephone set and loop has remained analog.

By international agreement, all voice codecs use an 8 kHz sampling rate.
Since each transmitted sample is 8 bits long, the analog voice signal is
encoded into a 64 kbps binary steam. This rate determines the basic
channel data rate of most other digital communications systems.

By bypassing the codec, it is possible to send 64 kbps customer data
through the telephone system. However, because of old style signaling
schemes still in use, digital data rates are often limited to 56 kbps.

:   **H - Hybrid**

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:   ![](2-4_wire_hybrid.gif "fig:2-4_wire_hybrid.gif")

A diplexer performs a bi-directional 2-wire to 4-wire conversion. It
allows two unidirectional electrical paths to be combined into a single
bi-directional one, and vice versa. It is advantageous to separate
transmit and receive portions of the signal since it is easier to make
unidirectional amplifiers, filters, and logic devices.

One of the simplest ways to create an audio band hybrid is to use a
transformer hybrid.

:

    :   **Single Core Transformer Hybrid**

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:   ![](Single_core_hybrid.gif "fig:Single_core_hybrid.gif")

There are several ways to split transmit and receive paths, the simplest
method uses a single core hybrid transformer.

The basic defining transformer equations are:

$$\frac{{V_1 }}
{{n_1 }} = \frac{{V_2 }}
{{n_2 }} = \frac{{V_3 }}
{{n_3 }} =  \cdots \quad \quad \quad and\quad \quad \quad I_1 n_1  + I_2 n_2  + I_3 n_3  +  \cdots  = 0$$

For a single core hybrid with a center-tapped secondary, the impedance
relationships for proper operation (conjugate matching) are:

$$for\quad \quad \quad n_2  = n_3 \quad \quad \quad Z_2  = Z_3  = 2Z_4 \quad \quad \quad and\quad \quad \quad Z_1  = \left( {\frac{{n_2 }}
{{n_1 }}} \right)^2 Z_4$$

:   ![](Hybrid_operation.gif "Hybrid_operation.gif") Note what happens
    if the transformer is driven from one of the secondary windings:

$$let\quad \quad \quad n_1  = n_2  = n_3 \quad \quad \quad \therefore I_3  =  - I_1  - I_2$$

:

    :   But *I~1~* and *I~2~* flow in the opposite directions, therefor:

$$I_3  =  - \left( { - I} \right)_1  - I_2 \quad \quad \quad and\;if\quad \quad \quad \left| {I_1 } \right| = \left| {I_2 } \right|\quad \quad \quad then\quad \quad \quad I_3  = 0$$

This last requirement can be satisfied by adjusting the impedances
Z~1~ - Z~4~ to make the currents equal. From this we observe that
signals injected into any port emerge only at adjacent ports but not at
the opposite one.

:   ![](THL.gif "fig:THL.gif")

In a properly balanced single core hybrid the typical throughput or
insertion loss is about 3.5 dB and the THL (trans hybrid loss) is about
25 dB.

:

    :   **Double Core Hybrid**

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:   ![](Double_core_hybrid.gif "fig:Double_core_hybrid.gif")

When properly balanced, a 2-core network can achieve a THL of 50 dB
while the insertion loss remains at about 3.5 dB. It has better
performance than the single core device, but is bulkier and more
expensive.

:

    :   **Balancing Networks**

All telecom equipment is tested and characterized against standard
impedance terminations. These impedances are based on line surveys and
are approximate equivalent circuit representations of the outside
cabling plant. For this reason, these networks vary from country to
country. In North America, IRL (input return loss) is measured against:

:   **T - Testing**

In order to maintain a high degree of service (99.999%), the equipment
must be capable of detecting and repairing faults before the customer is
even aware that there may be a problem. As a result, a separate test
buss and access relay is provided on a line interface. Tests may be
performed in a bridged mode or with the loop and line card disconnected
from each other.

Testing can be done in three basic directions:

:

    :   • From the line interface looking out towards the subscriber
        loop
    :   • From the loop connection looking into the line card
    :   • From the central office side of the line card

These tests are generally automated and are conducted late at night when
there is little chance that the customer will request service, thus
interrupting the test. Some of the scheduled tests may include:

:

    :   • Transmit and receive levels
    :   • Transmit and receive frequency response
    :   • Insertion loss
    :   • Trans-hybrid loss
    :   • Quantization distortion
    :   • Aliasing distortion

Some other tests that may be performed when commissioning a line or when
a complaint is lodged, include:

:

    :   • Impulse noise test
    :   • C-message noise
    :   • Longitudinal balance

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:   **Repeaters**

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:   ![](2_wire_repeater.gif "fig:2_wire_repeater.gif")

By placing two hybrids back to back, it is possible to create a
bidirectional amplifier or repeater. The total gain in the 4-wire path
within the repeater must not exceed the combined transhybrid loss of the
transformers. If this happens, the circuit will oscillate or sing.

The total gain in the 4-wire path within the repeater must not exceed
the combined transhybrid loss of the transformers. If this happens, the
circuit will oscillate or sing.

### Space Division Switching

The physical path between any two customers on a space switch is not
shared with anyone else. Crosspoints made from electromechanical relays
have been used to perform the interconnection, but newer systems use
semiconductors.

In a crossbar matrix, the number of inputs and outputs do not have to be
equal thus facilitating either concentration or expansion. In any case,
a total of NxM crosspoints are required. Although only one contact is
shown, many systems require two contacts if they keep the integrity of
the tip and ring leads throughout the system.

:   ![](Space_division_switch.gif "fig:Space_division_switch.gif")

The folded matrix treats inputs and outputs identically and requires
only *N(N-1)/2* crosspoints. The major disadvantage of this type of
switch is the rapid increase of crosspoints as the system size
increases. The number of crosspoints increases as *N^2^* and yet only a
maximum of *N* crosspoints can be active at any given time. Failure of a
single crosspoint prevents communication between the two devices sharing
that crosspoint.

**Multiple Stage Space Switch**

One way to avoid the cost penalties associated with full matrices, is to
organize the contacts into smaller groups. This impacts the call
processing since the switch controller must manage several contacts per
connection. Furthermore, the connection between any two subscribers may
take any one of a number of paths, thus further complicating the
decision making process.

The following sketch shows a simple three-stage space switch. It should
be remembered that since a full duplex connection is required, a second
switch supplying the return path must be provided.

:   ![](3_stage_space_switch.gif "fig:3_stage_space_switch.gif")

The total number of crosspoints in the above illustration is:

PLACE EQUATIONS HERE

The number of crosspoints required is dramatically reduced in this
multiple stage switch in comparison to a square matrix. The central
stage allows several ways to make a connection between two subscribers
therefor, single crosspoint failures can be bypassed. This results in a
more flexible and reliable system, but demands a more complex control
structure.

**Blocking**

A non-blocking network is capable of finding a path between any idle
input to any idle output. This does not mean that the system be able to
handle simultaneously all customer requests for service. In such a case,
the system may overload but the customer is not able to distinguish the
difference between blocking and overload.

The following illustration shows how blocking can occur. The solid lines
represent connections in service.

PLACE ILLUSTRATION HERE

The connections in service are not necessarily the optimum routing and
may have been forced by the prior connections. Note that in this case,
it is not possible for the last customer on switch 1:1 to contact the
last customer on switch 1:3 because there is not a free center switching
stage common to both. To overcome this, an additional center stage can
be added. To prevent blocking, 2n-1 center arrays are necessary.

The total number of crosspoints (NoC) in a non-blocking network is
therefore:

PLACE EQUATIONS HERE

As the total number of lines in increase, the total number of
crosspoints can be approximated by:

A non-blocking network of this type is known as a Clos switch, after its
inventor. Its basic characteristics include:

:

    :   • Expansion in the first switching stage
    :   • An odd number of center stages
    :   • Concentration in the last stage

### Time Division Switching

Time domain switching is simply an application of time domain
multiplexing and may be performed on analog or digital signals. Any
number of inputs may be sequentially routed to a single output.

:   ![](TDM1.gif "fig:TDM1.gif")

This technique increases the transmission link utilization and can be
modified to support circuit switching. If a multiplexer is placed at the
input, a demultiplexer is placed at the output. This system can be used
to multiplex either analog or digital signals.

:   ![](TDM2.gif "fig:TDM2.gif")

Each customer is assigned a unique switch, but all customers share the
same internal signal path. For N customers there are 2N switches. TDM
bus switching occurs when the input sequence is not the same as the
output sequence.

Each customer is given access to the common structure for a brief
moment. If higher data rates are needed, multiple inputs can be
assigned, thus giving the customer more time to transmit a signal.

**Time Slot Interchange \[TSI\]**

If information can be arranged into a sequence, it can also be
rearranged, much like shuffling a deck of cards. This is the task of the
time slot interchange unit. Full duplex operation is achieved by
interchanging time slots in both directions.

:   ![](TSI.gif "fig:TSI.gif")

The incoming TDM channels are mapped sequentially into RAM while the
outgoing channels are read out non-sequentially. This output address
generator is simply a memory-mapped pointer governed by a central
controller.

The required memory access time can be approximated as the inverse of
the channel rate. The RAM width is determined by the number of bits in a
channel, and the length by the number of channels in a frame.

**Time Multiplexed Switching**

Combining TDM and TSI allows a channel from one digital bit stream to be
switched to any channel on another digital bit stream. A multistage time
switch can consist of cascaded switching modules. To prevent blocking, 3
or more stages are required.

:   ![](TMS_switch.gif "fig:TMS_switch.gif")

Although it appears that only time domain switching is used in this
example, it is also known as a Time-Space-Time switch or simply *TST*.
This is because the center stage is actually switching different input
lines in space to a common output line. Since there is an ambiguity in
the terminology, some manufacturers of telecommunications equipment may
refer to this as a *TTT* switch if all of the signals are digital.

For small-scale switches, space switching is most efficient. However, as
switch size increases, time domain switching tends to gain the
advantage.

It is sometimes difficult to make direct comparisons between various
telecommunications switches because the internal architectures may be
quite different. However, it is possible to compare BHCA (busy hour call
attempts) capacity or performance such as traffic intensity, under a
specified set of circumstances.

Traffic Intensity is the product of average holding time and the calling
rate and is expressed in CCS (hundred call seconds) or Erlangs.

CCS is defined as: (number of calls per hour)x(call holding time in
seconds)/100seconds Therefore, 36 CCS = 1 Erlang

An erlang is a measure of the traffic intensity and is equal to the
average number of simultaneous calls at any given moment. It represents
the total circuit usage during any time interval, divided by that
interval. It also corresponds to the minimum number of channels
necessary to carry the traffic, if it could be scheduled.

**Example**

:

    :   Imagine for a moment that there are 8 customers in a small
        telephone system making random calls:

```{=html}
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```

:   ![](Erlang_example_a.gif "fig:Erlang_example_a.gif")

```{=html}
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```

:

    :   These calls could conceivably be arranged as:
        ![](Erlang_example_b.gif "Erlang_example_b.gif")

Three, 1 hour channels could carry this traffic and still have 25
minutes left over to spare. The traffic intensity is therefor less than
3 Erlangs:

## Questions for Research

1\. What is the purpose of dial tone?

2\. What are the advantages of CAS and CCIS interoffice signaling?

3\. What is the difference between a line and a trunk? 4. How does the
line circuit monitor the state of the telephone?

4\. Why does the standard telephone exchange need to provide BORSCHT?

5\. List the differences between North American and European codecs.

6\. Under what circumstances can poor THL be tolerated?

7\. What is the difference between an echo suppresser and an echo
canceler?

8\. Where do the telephony terms tip and ring originate? Tip and Ring
come from switchboard days. The plug on operators cords had a center
\'tip\' for one conductor, and the outer \'ring\' for the other
conductor. 9. Define blocking.

10\. Why are multiple stage space (or time) switches more practical than
a single stage switch?


# Communication Systems/Cellular Systems

## Cellular Voice Networks

Mobile radio service was introduced in St. Louis in 1946. This radio
dispatching system had an operator who patched the caller to the PSTN.
Later, IMTS allowed customers to dial their calls without an operator.
From this humble beginning came the present cellular phone system.

The cellular network is viewed by the PSTN as just another end-office
where calls originate and terminate.

:

    :   ![](PSTN_MSC_cloud.gif "fig:PSTN_MSC_cloud.gif")

The STP (signal transfer point) handles the network routing by
establishing the route to the HLR (home location register) for a
specific mobile user. This simplifies network management, because only
the routing tables in the STP need to be updated as the system grows.
Each MSC (mobile switching center) does not have to maintain full
routing tables to all other MSCs.

**Cellular Voice Reference Model**

:   ![](Cellular_voice_reference_model.gif "fig:Cellular_voice_reference_model.gif")

```{=html}
<!-- -->
```

:

    :   AC --- Authentication Center. This manages the authentication of
        the end user or equipment on behalf of the MS. It may serve many
        HLRs or in fact be an HLR itself.
    :   BS --- Base Station. The base station manages one or more cell
        sites and consists of a controller and one or more radio
        transceivers.
    :   EIR --- Equipment Identity Register. This is not presently well
        defined, but is used to identify end user equipment and reduce
        the incidents of fraud.
    :   HLR --- Home Location Register. This identifies the particular
        user and their service profile. It also records their current
        location and authorization period. The HLR may be distributed
        over more than one entity.
    :   MS --- Mobile Station. This is the actual radio based terminal
        that provides customer access to the network.
    :   MSC --- Mobile Switching Center. The telecommunications switch
        which routes calls between mobile users and the PSTN.
    :   VLR --- Visitor Location Register. This allows visitors to roam
        on other systems.

This reference model distinguishes between various tasks and does not
necessarily reflect the actual physical equipment.

Unfortunately, there is a wide range of incompatible technologies being
used at the Um air interface. Consequently, although it is possible to
communicate with people all over the world, it is not possible to take
your phone with you everywhere. This makes it difficult to adapt the
present system to support global PCS.

The cellular infrastructure market is dominated by four major
corporations: Lucent Technologies, Nortel, Ericsson, and Motorola.

The current cellular system is experiencing unprecedented growth in
subscribers, services, and technological innovation. Some of the offered
services include:

:

    :   • Paging, vehicle location
    :   • Text, data, facsimile transmission
    :   • Traffic, weather information
    :   • Emergency aid dialing
    :   • Electronic funds transfer for fare payment

There are however, some areas of concern:

:

    :   • Computerized call hand-offs between cell sites
    :   • Caller identification
    :   • Remote diagnostics
    :   • Reliability
    :   • Technology mix \[digital, analog, UHF, audio, computer\]
    :   • Billing
    :   • Long distance paging

Each subscriber is assigned transmit and receive frequencies for the
duration of a call. The frequency pair is sometimes referred to as the
forward and reverse channels, or the up and down link. Under control
from the cell site, the cellular phone must be able to tune to any of
the hundreds of frequency channels in the system.

Some of the adjacent cells will monitor the signal strength from the
portable unit in order to help the central exchange to determine which
cell site should handle the call. If at some time, this signal becomes
weaker because the customer has moved into another cell service area, a
call hand--off will be necessary. This requires that the cellular phone
switch to another set of frequencies, and the calling path be rerouted.
All of this must appear transparent to the user.

### AMPS

AMPS is an analog system based on FDMA and used extensively in North
America. It has been slightly modified since its inception and some of
its characteristics are:

:

    :   • Channel width: 30 KHz
    :   • Duplex spacing: 45 MHz
    :   • Data transmission: BFSK ±8 KHz deviation
    :   • Output power: 0.6 to 4 watts

The present system divides the 832 channels into two blocks of 416
channels. Within each block, 21 channels are reserved for signaling.

Some of the weaknesses associated with this system include:

:

    :   • Call blocking during busy hours in urban areas
    :   • Misconnects and disconnects due to rapidly fading signals
    :   • Lack of privacy and security
    :   • Limited data transmission \[1200 bps\]

One technique, which has been put forward to resolve some of the
congestion problem now found in the major urban areas, is to narrow the
channel broadcast bandwidth. NAMPS(narrow AMPS) increases system
capacity by splitting each 30 kHz AMPS channel into three 10 kHz
channels. This is seen as an interim solution until a better scheme is
agreed upon.

**Cell Channel Allocation**

:

    :   ![](Amps_cell_group.svg "fig:Amps_cell_group.svg")

It is not possible to assign all channels to each cell since adjacent
cells using the same frequencies would interfere with each other. The
channel distribution in the 832-channel system is as follows:

:

    :   • A cell group of 7 adjacent cells share 416 full duplex
        channels
    :   • No cell contains any adjacent frequency channels
    :   • 4 cells are assigned 56 channels
    :   • 3 cells are assigned 57 channels
    :   • 21 channels are reserved for control

Frequency utilization can be improved by cell splitting and sectoring.
These approaches effectively reduce the size of the customer service
area and allow frequencies reuse. This increases the number of hand-offs
and other demands on the MTX.

:

    :   ![](Cell_spliting.gif "fig:Cell_spliting.gif")

Cell splitting involves the creation of a new smaller cell from two
larger ones, while sectoring is the breakup of a single cell into
smaller ones. Typically, cell sites are split 3 or 4 to 1.

To minimize spill over into nearby cells, the cell antennas are given a
slight downward tilt, and the output power is limited to 100 Werp.

Another way to increase utilization is by channel borrowing. A few
channels are allowed to violate the normal frequency assignments and
move between cells. This allows the system to dynamically vary the
number of customers that can be served in a given cell. Careful
consideration must be given to potential co-channel interference

This principle can be further extended to provide dynamic channel
assignment, where the assigned cell frequencies are continually changing
to meet the shifting demand patterns.

### GSM

GSM is a third generation wireless telephone technology that allows
roaming in 17 European countries. It started off as a pan-European
standard but quickly gained world wide acceptance for its comprehensive
and thoughtful implementation.

GSM uses a TDMA access format and has a call hand-off capability, thus
increasing customer mobility and allowing inbound calls. Base stations
can handle 124 frequency bands. Channel 0 is performs a dual role of
providing a signaling channel and monitoring signal strength. All other
channels can be assigned to subscribers.

:

    :   • Up Link Frequency 890 - 915 MHz
    :   • Down Link Frequency 935 - 960 MHz
    :   • Carrier Spacing 200 KHz
    :   • Can support 1000 speech or data channels
    :   • Uses GMSK† modulation

UMTS

## Cellular Data Networks

## Cellular Satellite Networks


# Communication Systems/Packet Data Systems

## Carrier Grade Packet Networks

In most data applications, user data is decomposed into a string of
packets. This requires that additional information be added to the
user\'s data to ensure its safe arrival at the correct destination.

If the packets are rather long or of variable length, the system may be
referred to as frame relay. If the packets are small and of fixed
length, the system may be referred to as cell relay.

The user data may be organized into fixed or variable length packets
depending on the type of system. In any case, additional information is
appended to the customers data in the form of a header.

Some of the header contents may include:

:   • Source address
:   • Destination address
:   • Time stamp or packet sequence number
:   • Maximum allowable delay (priority)
:   • ACK/NACK
:   • Network control/billing
:   • Error checking etc.

However, simplex connections with transmission delays are not suitable
for voice connections, but are not a concern in data transmission. Each
input data packet is stored in an input buffer or queue and sent to the
appropriate output queue when resources become available. If the packets
and queues are sufficiently short, a virtual full duplex link may be
established, and interactive transactions may be possible.

In order to understand the operation of these systems, it is often
necessary to resort to a standard reference model. The OSI model
outlines the various functions and attributes, which are inherent in any
communications system.

A further necessity is the standardization of the implementation of the
OSI model. This work has largely been done by the IEEE and ITU (formerly
CCITT).

The OSI model was started in 1978 and has evolved to provide a framework
for interoperability. Standardization is required in order that various
types of equipment or systems can be joined in some useful way. Some of
the activities, which are regulated, include: • Interprocess
communications • Data representation • Data storage • Resource and
process management • Security • Program support

+---------------------------------------------------+
| OSI Model                                         |
+===================================================+
|                                                   |
+---------------------------------------------------+
| Host\                                             |
| layers                                            |
+---------------------------------------------------+
| 6\. Presentation |
+---------------------------------------------------+
| 5\. Session           |
+---------------------------------------------------+
| Segment                                           |
+---------------------------------------------------+
| Media\                                            |
| layers                                            |
+---------------------------------------------------+
| Frame                                             |
+---------------------------------------------------+
| Bit                                               |
+---------------------------------------------------+

The majority of the OSI model is implemented in microprocessor based
hardware. This means that new applications, network architectures, and
protocols can generally be accommodated by making software changes. It
also means that the traditional gap between the telecommunications and
computer industry is being reduced.

Significant advances have been made in the physical layer interfaces, as
networks migrate from twisted pair cabling to fiber optics. These
developments have introduced new applications, architectures and
protocols.

Some applications for public data packet networks include:

:   • Electronic funds transfer: Banks, Automated tellers,
    Clearinghouses, Stock exchanges
:   • Point-of-sale terminals
:   • Credit card verification
:   • Electronic mail
:   • Electronic purchasing
:   • Inventory management
:   • Database interworking

## The Internet

## LANs

LANs are used to connect a group of terminals or workstations together.
These may be found in a relatively confined space such as within a
building or campus setting. The attachments may be dumb or intelligent
terminals, file servers, routers, and repeaters.

The majority of LANs are privately owned. They can be connected to the
PSTN or some other carrier to form MANs or WANs.

LANs can be categorized in a number of ways. One method examines how
information is placed on the interconnecting transmission medium.
Baseband LANs directly inject logic levels of on the medium and share
access by some form of TDM. Broadband LANs however, use high frequency
carriers and share access by FDM.

Some authors prefer to segregate a special category, the PBX:

:   LAN - local area network (baseband)
:   HSLN - high-speed local network (may be baseband or broadband)
:   PBX - private branch exchange (baseband)

The most widely used LANs today are ethernet, token ring, token bus, and
Appletalk. They can be interconnected by gateways, routers, bridges, or
repeaters to form MANs, which can in turn be used to create WANs.
**Topology**

![](LAN_topology.gif "LAN_topology.gif")

LANs can be configured in three basic ways, namely: as a ring, star, or
bus. Another arrangement known as a tree topology is really a variation
of the bus. Cable TV is an example of a tree network employing frequency
division multiplexing.

:   **Star**

The end office of the PSTN is configured as a star where all lines
radiate from a single location. This type of wiring is also used in all
modern office buildings. This center of the star or hub may contain a
network manager of some other piece of hardware that exercises control
over the LAN. This type of arrangement is used in a PBX, and the most
common connection media is a twisted pair of wires.

Some networks may be configured as a physical star, but may have a
different logical topology, such as a ring.

:   **Ring**

A ring consists of a closed loop, where each station is connected by an
active or passive tap. An active tap is one has electronic components
inserted into the loop to both extract and inject signals, thus all
stations are effectively connected in series. A passive tap simply comes
in contact with the loop., thus all stations are effectively connected
in parallel.

A station wanting to transmit waits its turn to inject a packet onto the
ring. This implies some sort of distributed protocol. This is most often
implemented by means of a token.

:   **Bus**

A distributed protocol arrangement or token is needed to resolve bus
contention problems. Each terminal monitors the common bus for any data
with their address appended, at which point it simply makes a copy. This
arrangement is often found in computer systems where a number of
peripheral devices may be connected on a high-speed bus.

Two important issues in this type of arrangement are access and signal
level. In a baseband or digital environment, there is often a polling
mechanism imposed to create order. Also, the signals degenerate as they
propagate down the line, this means that some sort of compensation must
be made.

RS-422 is a simple protocol that is implemented on a twisted wire bus.
The system allows a couple of dozen devices to be networked as far as
1 km, with a data rate of up to 1 Mbit/s.

Ethernet is an example of a baseband system implemented on a bus.

## Further reading

-   Serial Programming/Forming Data
    Packets


# Communication Systems/Satellite Systems

## Satellite Applications

Satellites support a number of applications including:

:

    :   • Communications systems
    :   • Broadcasting systems
    :   • Remote sensing
    :   • Global positioning and navigation
    :   • Search and rescue

:   • Weather and pollution monitoring

    :   • Surveillance

**Some suggested topics** .TV Broadcasting

## Satellite Frequency Bands

## Satellite Link Budgets

## DBS (Direct Broadcast Satellite)

## VSAT (Very Small Aperture Terminals)

## GPS (Global Positioning System)

It refers to a system of satellites that constantly transmit a signal,
and a GPS terminal that picks up those signals and calculates its
position on Earth by measuring the distance between itself and two or
more GPS satellites (by measuring the time it takes to receive the
signals).

## Satellite Orbits

### Geostationary Orbit

A satellite which orbits around the Earth at the same rate that the
Earth turns is known as a synchronous orbit. Synchronous orbits can be
of any inclination. If they are polar orbiting, the satellite will
appear to be over the same spot at the same time every day.

Remote sensing satellites can be place in orbits that are synchronous
with the Earth's rotation over a longer period than a day, and thus will
be able to view the entire ground surface over a number of orbits.

If a geosynchronous orbit is placed over the equator, something very
interesting happens. The satellite will appear to stop moving in the
sky. This is referred to as a geostationary orbit.

The gravitational acceleration of an object as a function of altitude
is:

$$a_{c}=\left( \frac{r_{e}}{r_{s}} \right)^{2}g$$

:

    :   where:

$$r_{e}=$$ equatorial radius of the Earth (6378.388 km)

$$r_{s}=$$ radius at the satellite position

$$g=$$ Earth\'s gravitational constant (9.80665 m/sec^2^)

The centrifugal acceleration on a satellite is given by:

$$a_{c}=r_{s}\omega ^{2}=r_{s}\left( \frac{2\pi }{T_{s}} \right)^{2}$$

:

    :   where:

$$\omega =$$ angular velocity in radians/sec

$$T_{s}=$$ time for 1 orbit (86400 seconds \[24 hrs\] for geostationary
orbit)

For a stable orbit, the two forces associated with these accelerations
must be equal

$$\left( \frac{r_{e}}{r_{s}} \right)^{2}g=r_{s}\left( \frac{2\pi }{T_{s}} \right)^{2}$$

![](Geostationary_plot.gif "Geostationary_plot.gif")

Solving for *r~s~* we obtain:

$$r_{s}=\sqrt[3]{\left( \frac{r_{e}T_{s}}{2\pi } \right)^{2}g}=\sqrt[{}]{\left( \frac{6378\ km\times 86400\ \sec }{2\pi } \right)^{2}9.8065\ m/\sec ^{2}}=42,254.22\ km$$

Therefor the height above the Earth is 42,254 km - 6378 km = 35,876 km
or 22,292 miles.

The velocity of a satellite in a circular orbit is given by:

This works out to 3.073 km/Sec for a satellite in a geostationary orbit.

#### Path Length

The transmission path length can be found by applying Pythagorean
Theorem:

In actual practice, the angle of elevation must be at least 5 degrees
above the horizon. Therefore the actual geometry is closer to:

By applying the law of sines, a better approximation of the maximum path
length can be determined, as well as the minimum distance from the pole
required to see the satellite.

Using the polar radius to calculate the total arc length of the Earth,
with the polar curvature, we obtain an effective polar circumference of:

Which means that the minimum distance d from the pole, at which the
satellite can be seen is:

It is evident from the sketch, that the further north one goes, the
greater the signal path length through the atmosphere.

### Molniya Orbit

The former USSR is not able to make great use of geostationary
satellites, because of the northern latitude of the country.
Consequently, communications satellites have required a slightly
different approach. The term molniya means "flash of lightning".

### Sun Synchronous Orbit

Many satellites which require sunlight for imaging use sun-synchronous
orbits. The sun-synchronous orbit provides a constant node-to-sun angle,
and the satellite passage over a certain area occurs at the same time of
the day.