Interview

1.My long-range goal is become best telecommunication engineer of Mongolia. So I must hardworking. Thus my second objective is perfect life. For example great family, nice work and wonderful home.
2 My short-range goal is do my task on time. Because I should raise my score. It is very important issue. May be my life will depend on it. So I must endeavor for now.
3 I hope I can make a contribution to your organization. Because I studied of telecommunication career for 4 years. And in this way I have many skills. So I believe necessary.
4 I like participation in activities which outside from class. This has been become more and more abilities for me. And I skilled in teamwork and able to rise easily.
5 I like fair everything. So I want to determine for justices. Head person must say openly to all success. I usually define my success for estimation of other people.
6 I think every people must put high goals. Because people live as yourself opinion. So my organization is become best telecommunication engineer of Mongolia. And I try to reach it.
7 My work is a very interesting and when I was student I don’t know it. After I study this school I understand it
8 Firstly I don’t know about this career. My sister decide to enter this school me. So I become must study by this direction. After it become interesting.
9 In my work important issue is become a very hardworking. I can it.
10 My career is telecommunication engineer. Almost we fix the broken cable. But women often do operator. In my opinion I want be header engineer. So I need I can any thing.
11 it is most important issue. I demand to interactive with all user. Because I should know for request of subscribers.
12 Yes. I always try to view everything on practice. So I went to ATS many times. The engineer’s are very friendly and kind people. Whenever they ready for help us. I saw optic fiber, mufti, and equipment at here.

essay

I’m Batsuure Burmaa graduated school of Information Telecommunication and technology in 2012 by professional telecommunication engineer. I’m keep time and team works.
I’m problem solving skills and ability to work under pressure also I’m hardworking, expertise and positive attitude I have got two weaknesses. I’m stubborn. Also I like to be pressed many for works.
I will be working your company. I think that my qualification is very well. My qualification will be most in demand in branch of telecommunications. Especially your company will be need this professional engineer.
I will have been studying in China. I raise one’s qualification. I will have been study many new skills. I will be studying optic cable and china language.
I spend my free time play basketball, volleyball and chess. I have studying basketball during 3 years, Volleyball 5 years, chess 8 years. Li have got many medals.
I studied many skills. I studied to lead a team because I was leader of team. Also I have become keep time and hardworking .also I studied to work by team and to communicate.
I thought about it. I think f\perfect company is with pleasant around , right poling of working management always . new service in costumers. So that I think perfect company is your company and I decided to work in your company.
Today, technology is developing quickly. I’m researching to made on new technology next if for me to convenient, I’m bring at company this is engineering responsibility.
Main of telecommunication professional is telecommunicate with others. To lay your selt method is nicely communicate with others.People between communicate everyday.
I’m imagining nicely a future. I will have been working your company, many practice experience and knowledge.
I will be can a good engineer and employee.

Application for employment

Personal information date of application_2010.__
Name: batsuure burmaa
last first middle
Address: BZD Ulaanbaatar, Mongolia
Street (apt) city, state zip
Altemate address: BZD Ulaanbaatar, Mongolia
Street city, state zip
Contact information____( 96560556 )_____(burmaa_tc@yahoo.com )
Home telephone mobile e-mail
How did you learn about our company?__________________________
Postion sought:_______foreman__ available start date today
Desired pay range: 600000 are you currently employed no by our or salary

education _________________________________________________
name and location graduate?-degree? Major/subjects of study
High school BKH aimag Ulziit sum “sain oyut” Ulziit sum Math, Mongolian language
Collage or university UB IICT Bachelor, IICT Optic,English
Specialized training ,trade school , etc… Telecommunication
engineer Bachelor, IICT Optic,English
other education ___ ___ __

Please list your areas of highest proficiency ,special skills or other items that may contribute to your abilities in performing the above mentioned potion.

Abstract chapter19

The telecommunications industry experienced some spectacular failures around the end of the twentieth century and the start of the twenty-first, but few could compare to satellite companies Iridium and Teledesic.
Its original plans called for a network of 840 satellites. This dropped to 288 satellites, then in February 2002 Teledesic announced that it was planning 20 medium Earth orbit (MEO) satellites.

Satellite communications had its beginning in 1962 with AT&T’s launch of its Telstar 1. Orbiting the Earth in about 2 h, Telstar was visible from the Earth station for less than half an hour, as the antennas followed its track across the sky.
From 1970, it is started satellite station in Mongolia.

Another application, direct broadcast television, was a long time in coming, but now high-quality TV can be received, even in remote locations. Very small aperture terminal (VSAT) enables users to mount small antennas on rooftops to run a multitude of applications such as point-of-sale, which need low bandwidth facilities distributed over a wide range.

A transponder is a radio relay station on board the satellite. Transponders are technically
complex, but their functions are identical with those of terrestrial microwave radio relay stations. Satellites are powered by solar batteries. Power is conserved by turning off
unused equipment with signals from the Earth.

Satellites are connected via intersatellite links to the four nearest neighbors
and to an Earth station.
The hubs control demand assignment to the satellite and monitor and diagnose
network performance.
The remote station has an antenna and a receiving unit, which is about the
size of a personal computer base unit. The primary advantage of VSAT is its ability to support multiple locations.

Chapter 25

TDM Customer-Premise
Switching Systems

PBXs have evolved in a path parallel to central offices. The earliest versions
were manual switchboards, followed by electromechanical dial, and then stored
program systems that matured into the current TDM architecture. As with central
offices, PBXs are on the cusp of a transition from TDM to VoIP and for much the
same reasons. The legacy TDM technology is still adequate for most offices and in
the absence of compelling reasons for replacing existing systems, the transition is
likely to be gradual. Most manufacturers have modified their TDM systems to
support VoIP, which further postpones the transition to an all-IP system because
upgrades to most existing systems can provide equivalent services. Moreover,
many of the justifications for replacing existing systems are difficult to quantify in
tangible terms. We will discuss these considerations in the “Applications” section
at the end of the next chapter.
All CPE equipment manufacturers are touting their IP systems to the point
that the TDM alternatives are obscured in the hoopla, but TDM switches serve the
majority of CPE lines and will continue to do so for the next several years. Feature
development has reached its zenith with TDM systems, however, as manufacturers
concentrate their development efforts on their IP products. The primary interest
most companies have in TDM systems is preserving the existing investment, the
life span of which has historically been in the 7-to-10-year range.
This chapter discusses the architectures and configuration of the three TDM
product lines on the market: key systems, hybrids, and PBXs. Although the distinction
between the product lines is somewhat blurred, if an organization requires more
than about 100 central office line and station ports, a PBX is usually most effective
because of its greater line, trunk, and intercom capacity. Hybrids can grow to about
250 ports, and are satisfactory for midsized offices at a lower cost than PBXs. Key systems
are used in smaller line sizes. When the system supports more than a dozen
central office lines it is set up with pooled trunk capabilities and defined as a hybrid.
437
TDM KEY AND HYBRID ARCHITECTURES
Key and hybrid systems are typically wall mounted in cabinets similar to the one
in Figure 25-1. Many small systems have a fixed size and cannot be expanded.
These are designated by their line and station size such as 3×8 or 6×16. When the
system reaches capacity, it must be replaced if further growth is needed, often preserving
only the telephone instruments. Larger hybrid systems may also be designated
by line and station size, but the basic unit is expandable. These systems
are also housed in wall-mounted cabinets, but they have expansion slots to accept
additional line and station cards.
All key and hybrid systems support loop-start central office lines. Some
high-end systems support ground start, T1/E1, BRI, and PRI trunks. Some also
support tie lines between other switches, and may support networking either
between themselves or with the manufacturer’s PBX product line. All hybrids and
key systems are processor controlled with either a proprietary or a commercial
microprocessor. The generic program is usually stored in a chip, which is replaced
to upgrade to a later software release. This is in contrast to PBXs, in which the program
is upgraded in the field from tape or CD-ROM. Key and hybrid line and
station translations are typically stored in flash memory, which enables them to
restart quickly in case of power failure.
438 PART 4 Customer Premise Systems
F I G U R E 25-1
Nortel MICS Key System (Photo by Author)
KTSs usually include one or more intercom lines. These are used for stationto-
station communication—in smaller systems the intercom is primarily for conversations
between the attendant and the called party. In larger systems multiple
intercom paths are provided so several internal conversations can be held simultaneously.
Most systems provide a built-in speaker so the intercom line can be
answered without using the telephone handset. Optionally, the user can lift the
handset for privacy. The number of intercom lines provided is a feature that distinguishes
a PBX from a hybrid. Many hybrid systems support a limited number
of intercom paths, which may preclude their use in offices that require a large
amount of internal calling. Most PBXs have enough time slots that virtually all
stations can be talking simultaneously.
While the attendant can answer and transfer calls from an ordinary telephone,
a special telephone is often provided. The attendant has all the features of
regular stations and may also have a busy lamp field (BLF) to show which stations
are occupied and a DSS field, which allows the attendant to transfer calls to stations
by pushing a button instead of dialing the station number. To support the
attendant, many systems include paging either to an overhead system or to telephone
speakers. The paging system is accessed by pushing a button or dialing a
code and can be divided into zones if the building is large enough to warrant it.
Many systems provide for parking a call so a paged user can go to any telephone,
dial a park number, and pick up an incoming call.
Several manufacturers produce multiline systems that do not require a KSU.
Most KSU-less systems require one pair of wires per line, which limits the size of
the system to four or fewer lines. The primary advantages of KSU-less systems are
low cost and ease of installation. Anyone who knows how to install a single-line
telephone can install KSU-less telephones because they do not require setup,
which makes them ideal for small offices and residences. The primary drawbacks
of KSU-less systems are limited expandability and lack of features. Since the systems
have no KSU, the only features available are those contained in the telephone
set itself. Some KSU-less systems also lack an intercom path, which means calls
cannot be announced over an intercom as they are with most key systems.
Voice mail is provided almost as a matter of course in practically all key and
hybrid installations. Other add-on peripherals such as ACD and call accounting
are available for many product lines.
PBX ARCHITECTURES
PBX technology has progressed through three generations and is now starting the
fourth generation of IP PBXs. First-generation systems were manual switchboards,
which were common in the first half of the twentieth century. Secondgeneration
systems used wired logic and analog step by step or crossbar switching
fabric. Second-generation telephones were nonproprietary rotary dial or DTMF
analog sets. If key features were needed with a second-generation PBX, a separate
CHAPTER 25 TDM Customer-Premise Switching Systems 439
key telephone system was required. The third generation introduced stored program
control processors. Some used analog switching fabric and others were
TDM. The processor-controlled logic enabled PBXs to support proprietary telephones,
which controlled a limited number of key telephone features. All systems
supported POTS phones, which required users to dial feature access codes.
Proprietary telephones either used an ISDN-like interface with a separate signaling
channel, or they signaled over wires separate from the talking path.
All of today’s third-generation switches employ TDM switching technology
and support both analog and proprietary digital telephones. TDM PBXs are the de
facto standard of the industry, having been operation for nearly three decades.
Products have continually improved with feature and hardware enhancements
and have reached the stage where the risk is slight in buying a PBX from any
reputable manufacture and distributor. Proponents of IP PBXs contend that
TDM PBXs are obsolete, and eventually they will be, but TDM systems meet the
communications needs of most users and will continue to do so for several years.
All TDM PBXs are processor controlled. Some use proprietary processors
and some use industry-standard microprocessors. PBXs are rated by busy hour
call attempts (BHCA) and busy hour call completions (BHCC). A call attempt is
any event that accesses the processor. This includes functions such as switch hook
flashes and feature accesses to hold, transfer, conference, park, or any such event
that occurs during the normal course of a call. The number of call attempts
an office requires is difficult to estimate, but the BHCA and BHCC factors are
convenient metrics for comparing products.
The switching matrix is evaluated by busy hour hundreds of calls seconds
(BHCCS). Traffic engineers use CCS in evaluating trunking requirements. A discussion
of traffic engineering is beyond the scope of this book, but it is covered in
the companion volume, The Irwin Handbook of Telecommunications Management.
A circuit occupied for an hour is busy for 3600 s or 36 CCS. If a switching matrix
is nonblocking, it is capable of supporting all stations in an off-hook condition
simultaneously on station-to-station calls. Trunk groups are never engineered to
permit all stations to connect to trunk calls simultaneously, so the switching network
would never be overloaded by trunk calls. Since each call involves two
stations, a nonblocking switch must support half the quantity of stations times
36 CCS per station. To illustrate, assume a PBX has 100 stations. If it is nonblocking,
the switching network should support 50×36 or 1800 BHCCS.
The architecture of a TDM PBX is similar to that of a TDM local switching
system. As Figure 25-2 shows, PBXs have a line side and a trunk side. The interfaces
are contained in hardware modules that fit into slots in a card cage. The line
and trunk ports connect to the TDM switching matrix, which makes the connections
under processor control. The features and configuration information are
contained in memory. Smaller systems use flash memory for the program and
configuration information. Larger systems usually employ volatile memory for the
program and for station and trunk translations. Generic programs are upgraded
440 PART 4 Customer Premise Systems
from tape or CD. Larger systems usually provide tape drives for backing up the
program and configuration, which must reload if the power fails.
The expansion cards plug into the PBX’s backplane, which ties the lines,
trunks, and central control circuits to the switching fabric and busses over which
the circuit elements communicate. Although the structure of PBXs is similar, there
are significant differences in products on the market and the way features are
packaged and sold. PBXs grow incrementally up to the total capacity of the system.
The main module contains common equipment cards such as processor, memory,
TDM switch, power supply, and tape and CD-ROM drives. These fit in dedicated
slots that cannot be used for any other purpose. System recovery following power
failure is much slower than key and hybrid systems, typically requiring 3 to
10 min. Figure 25-3 shows a cabinet stack from a Nortel SL-100 PBX.
Added to the main module are modules that support line and trunk cards.
These plug into a backplane that connects to the TDM switch modules. Figure 25-2
shows separate line and trunk modules for clarity, but most systems have universal
card slots—that is, either line or trunk cards can plug into any slot. Up to a given
line size, any port can connect through the TDM switch to any other port. At some
point, depending on the manufacturer’s design, direct port-to-port connectivity
becomes impractical and a center-stage switch is required to connect modules.
Station Connections
Stations connect to the line ports through a 64 Kbps circuit that runs on twisted-pair
wire. The typical digital station range is in the order of 1500 to 2500 ft (460 to 760 m).
CHAPTER 25 TDM Customer-Premise Switching Systems 441
Voice Mail
ACD Positions
Line Modules Trunk Modules
Remote Switch Unit
Local Trunks
IXC Trunks
Tie Trunks
Local Trunks
Fax
Maintenance
Terminal
Switch Modules
Processor
Bus
CTI Server
Database
F I G U R E 25-2
TDM PBX Architecture
Analog telephones connect to analog ports, or in some systems to digital ports
through an analog adapter. PBXs have at least two different types of line interface
cards, analog and digital. Most systems also support BRI cards, which are typically
used for videoconferencing. Digital line cards support proprietary telephones that
work only with that manufacturer’s system. Analog and ISDN cards support
telephone sets that are independent of the PBX manufacturer. Although ISDN
telephones should work with any manufacturer’s PBX, ISDN standards do not
define all of the features that the system may be capable of. Therefore, ISDN sets
from the PBX manufacturer will usually provide features that other manufacturers’
telephones cannot support.
442 PART 4 Customer Premise Systems
F I G U R E 25-3
Nortel SL-100 PBX (Photo by Author)
Line card density, which ranges from eight to 32 ports, is a distinguishing
feature among products. High-density cards allow for smaller cabinet size, which
is usually a plus. In smaller PBXs, however, high-density cards may result in more
spare ports than the owner would normally purchase. For example, if the system
has 32-port cards and 33 ports are required, 64 ports must be purchased, leaving
nearly half of them unused.
Trunk Connections
PBXs, like central offices, interface the outside world through trunk circuits that
exchange signals with other switching systems through a variety of signaling
interfaces. TDM systems support both analog and digital trunks, but as the network
evolves toward all digital, analog trunk interfaces for the PBX are gradually
disappearing. Transmission quality is better on digital trunks and they take fewer
card slots. Analog trunks are usually provided over three separate trunk groups:
incoming to the main listed number, outgoing, and DID. A separate type of analog
trunk card is required for DID trunks in most PBXs, although some manufacturers
offer universal trunk cards that will support either DID or two-way CO.
Incoming and outgoing analog trunks can be combined into a two-way trunk
group. Calls to the main listed number route to the console, and any trunk can be
seized for outgoing calls. Analog trunk cards contain from 4 to 16 trunks per card.
Analog trunk cards support two-way central office trunks and foreign exchange
lines. Trunks to the IXC are normally digital.
T1/E1 trunk cards support 24 or 32 circuits. Some PBXs use a single type of
card for T1/E1 or PRI; others have separate card types. It is important to understand
the difference between a line-side and trunk-side T1/E1. Line-side connections,
which are always the case with analog trunks, allow the user to flash the
central office and get second dial tone to activate features such as conferencing
and transfer. Trunk-side connections do not offer this capability. Most key systems
and hybrids allow the user to flash the central office, but PBXs do not. Therefore,
even though the central office may provide line-side features toward a PBX, they
cannot be accessed. For example, users may want incoming calls transferred to
cell phones. With a line-side connection, an attendant can flash the line, receive
second dial tone, and transfer the call. The connection is made in the central office
and the line is released. A PBX with T1/E1 trunk-side connections can also transfer
calls, but only if the switch has been configured for trunk-to-trunk transfer.
Many PBXs have this feature deactivated for reasons that will be explained later,
but if it is permitted, two trunks are tied up for the duration of the call. If the PBX
has PRI trunks, and if both the PBX and central office are equipped for antitromboning,
one of the two trunks will be released. Line-side T1 cards are used to connect
the PBX to peripherals such as interactive voice response and voice mail.
Most LECs offer both PRI and digital trunks. The latter may be connected to
the line side or the trunk side of the central office, but PRI is always a trunk-side
CHAPTER 25 TDM Customer-Premise Switching Systems 443
connection. Depending on the LEC, non-PRI T1 trunks may be set up to provide
two-way DID, offering a service similar to PRI, but with in-band signaling. The
main impediment to the all-digital PBX trunking network is the premium prices
that many LECs charge for digital or ISDN trunks, but even this is disappearing
as competition lowers trunk prices. Most LECs now offer digital trunks, either as
separate two-way and DID trunks or combination trunks that can be used for
either DID or outgoing service. Digital trunks have two drawbacks. One is their
lack of scalability. When additional trunks are required, they are added a full
T1/E1 at a time. The other is their greater vulnerability to failure since a single
failure can kill all the trunks to a PBX.
Signaling compatibility is an important issue in connecting a PBX to a central
office. If analog trunks are used, PBXs use ground start trunks to prevent glare
as discussed in Chapter 12. DID trunks are used for incoming service only, so they
may be loop start from the central office, with the DID digits passed with DTMF
signaling. Two-way DID trunks are normally connected to the central office as tie
lines using E&M signaling. PRI trunks use the D channel for signaling.
As discussed in Chapter 15, PRI in North America provides 23 64-Kbps
B channels plus one D channel. The rest of the world uses E-1, with 32 channels,
of which two 64 Kbps channels are reserved for signaling and controlling. PRI is
preferable to T1 because, among other advantages, it supports caller ID and callby-
call service selection, which permits the PBX and the central office to determine
for each call what type of service is needed. For example, if a PBX supports video
conferencing, multiple channels on the PRI will be needed to provide the desired
degree of picture quality. The PBX and the central office set up the appropriate
bandwidth by exchanging messages on the D channel. Many PBXs use BRI on the
line side to support video conferencing.
PBXs require an access digit, usually 9, to connect station lines to central
office trunks. When the user dials 9, the PBX seizes an idle central office trunk and
connects the talking path through to the station if the station is permitted off-net
dialing. The station hears central office dial tone as a signal to proceed with
dialing.
Trunk-to-trunk transfer enables a user to link an incoming trunk to an outgoing
trunk and hang up. The feature is available in most PBXs, but it is often disabled
to prevent misuse. For example, unauthorized callers may request
confederates on the inside to connect them to long distance trunks. The feature has
many authorized uses, however, such as transferring a call to a cell phone, so
many companies activate trunk-to-trunk transfer, but restrict it to local calls and
tie trunks or to a particular class of service.
Tie Trunks and Networking
Organizations operating multiple PBXs have two alternatives for interconnecting
them, tie trunks and networking. Tie trunks can be analog or digital, terminating on
444 PART 4 Customer Premise Systems
the trunk side of the PBX. Analog tie trunks are rare today because most organizations
have both voice and data networks running over digital circuits. If VoIP is not
used in the PBX, voice and data can share a T1/E1 by splitting the line through an
add-drop multiplexer as shown in Figure 25-4. This configuration avoids the QoS
issues of VoIP, but it does not allocate bandwidth dynamically. If the bandwidth balance
is not optimum, the multiplexer must be reconfigured. Nevertheless, it is an
inexpensive way of sharing a digital line with legacy equipment.
Tie trunks are generic, and can be set up between PBXs of different manufacture.
Signaling is usually E&M and it supports no feature transparency beyond call
origination and termination. A call into one PBX can terminate to a station in the
other PBX, and users can transfer calls across the tie lines, but sharing a voice mail
requires QSIG networking. If tie trunks terminate in a single location they are often
accessed by dialing a digit, such as 8, which connects them to the distant PBX. Many
multi-PBX organizations have a separate dialing plan for each system plus a single
organization-wide dialing plan. The PBX then is then programmed to provide the
translations necessary to reach the distant number over the tie trunk network. This
feature is called uniform dial plan. To avoid the need for users to understand the dialing
plan, many organizations use the PBX’s ARS to dial the necessary codes. Users
dial the number, and the PBX selects the route and dials any additional digits.
Most companies network their PBXs to obtain feature transparency.
Proprietary networking protocols may be used between PBXs of the same manufacture,
or, as discussed in Chapter 24, QSIG may be used between otherwise
incompatible products. Networked PBXs provide service equivalent to a central
CHAPTER 25 TDM Customer-Premise Switching Systems 445
T1/E1 line
Ehernet
Add-drop multiplexer
Router
PBX
T1/E1
F I G U R E 25-4
Voice-Data Line Sharing with an Add-Drop Multiplexer
PBX and RSU except that each PBX contains a separate database and is inherently
survivable. If the link to the main PBX is lost, features are lost, but local switching
is unaffected.
Distributed Switching
Many PBXs offer remote switching units. An RSU extends line and sometimes trunk
interfaces over T1/E1 lines to secondary locations. The main advantage of an RSU
as compared to networked PBXs is that the remote stations are in every respect
equivalent to those in the host. All configuration and management are centrally controlled,
in contrast to networked PBXs, each of which is separately administered. All
processing is in the central unit and connects to the remote over an umbilical. If the
remote is in a separate calling area it may be equipped with local trunks.
On the down side, if the umbilical fails, the remote is dead. Some remotes
have survivability features that permit limited stand-alone operation. A survivable
remote typically has a copy of the line and trunk database so the system
operates with reduced capabilities. If incoming calls terminate on the central unit
during normal operation, they will be lost during emergency conditions.
Remote units have several advantages compared to networked PBXs:
_ Only one processor and software set are needed. This is usually less
expensive than maintaining separate systems.
_ Administration is from the host. All database changes are made
on the host switch.
_ Wiring costs are reduced on a campus. It is often less costly to install
a remote than to cable from the central site.
_ Total feature transparency is achieved. Users in the remote location
share the same voice mail, numbering plan, and trunks as the central
site, and have access to exactly the same features.
Administration
All PBXs are administered from a maintenance and administration terminal
(MAT). In some products a dumb terminal connects directly to the PBX through a
serial port, but many current PBXs provide Ethernet connections to a PC. The
MAT terminal is used to enter station and trunk translations in a command language
that is unique to the product. Most products offer an optional PC-based
program that allows the administrator to make changes on a graphic screen and
upload them to the switch. The same terminal and associated printer are used to
display and diagnose maintenance messages that indicate hardware or trunking
faults. The same terminal is used to collect statistics such as processor activity, and
line, trunk, and feature usage. Most PBXs support remote administration either
over Ethernet or dialup to a remote maintenance port.
446 PART 4 Customer Premise Systems
The maintenance and administration process is designed to be closed and
proprietary. It is kept out of the hands of users as one of the ways the configuration
is controlled to support management’s objectives. The process is complex
enough that training in factory-authorized schools is required to become certified
in administering the system.
Redundancy
Modern TDM PBXs are inherently reliable. Total failures are rare, but not unheard
of. Reliability can be increased through redundancy. Some products offer redundant
processors while others offer redundant switching matrices, power supplies,
and common equipment items. With full redundancy and uninterruptible power
supplies, TDM PBXs can achieve reliability in the order of central offices.
Emergency Communications
Because PBX stations are tied to a location, it is easy to convey location information
to an emergency center. Fixed station locations are changed only through
order activity, which gives the administrator an opportunity to update the emergency
database. This is in contrast to IP stations, which may change location without
management’s involvement. Systems employing remotes that do not have
separate trunks, must take precautions to ensure that the true location of the
reporting station is transmitted to the PSAP.
Computer–Telephony Interface
Most PBXs provide an open interface for limited call control from an external
processor. The physical interface and the command language are proprietary to
the manufacturer, and allow application programmers to connect the PBX to a
standard application program interface (API). The most common are telephony
API (TAPI), which is for connecting a PC running MS Windows to telephone services
and telephony server API (TSAPI), which is a server-based interface. CTI is
discussed in Chapter 27.
Wireless Capability
Many organizations have classes of users who must roam the building. Wireless
systems allow use of the telephone anywhere in a building or within a restricted
range on a campus. Two types of wireless systems are available. One type plugs
into analog ports on the PBX, and gives the user capabilities of analog telephones.
Proprietary wireless systems provide the features of digital telephones including
multi-line capability and button access to features. Wireless phones are discussed
in Chapter 21.
CHAPTER 25 TDM Customer-Premise Switching Systems 447
TDM CPE Application Issues
Nearly every business that has more than a handful of stations is in the market for a
key system, hybrid, PBX, or its central office counterpart Centrex. PBXs are economical
for some small businesses that need features such as restriction, networking, and
ARS that are beyond the capabilities that most key systems provide. Very large businesses
may use central office switching systems of a size that rivals many metropolitan
public networks. Between these two extremes lie hundreds of thousands of PBXs.
PBX Standards
PBXs, almost by definition, are proprietary in their internal switching and features,
but trunk interfaces are standard T1/E1 or analog trunks that are covered in
ANSI/TIA/EIA-464-C. This document is a detailed set of standards for transmission,
signaling, framing, and private network synchronization. It describes digital
and analog interfaces to a local central office in detail. The interfaces are so well
documented that incompatibility is practically non-existent.
QSIG, as discussed in Chapter 24, is an ISO standard for networking PBXs of
disparate manufacture. SMDI, also discussed in Chapter 24, is a Telcordia
Technical Reference TSR-TSY-000283, Interface Between Customer Premises
Equipment; Simplified Message Desk and Switching System. Both of these are open
interfaces for connecting peripherals to proprietary PBXs. The TAPI and TSAPI
standards for connecting CTI peripherals are open standards, but the protocols are
unique to each switch, which publishes its own APIs.
Evaluation Considerations
In choosing a key system or PBX it is important that you understand exactly what
you want it to do. The variety of key and hybrid systems on the market is so vast
that managers must carefully evaluate their requirements before selecting a system.
The differences between systems are often subtle, and differences in function
and support are not apparent until you have lived with the system for several
months. This makes it important to check references carefully. Considerations that
are important in some applications will have no importance in others and the
buyer should weigh them accordingly.
The first consideration in selecting any office phone system is to determine
how many station and line or trunk ports are needed initially and to accommodate
growth. The following are some general rules, but be aware that exceptions are
many, and product lines are changing constantly, which may invalidate some of
these distinctions:
_ If more than 24 central office trunks are required, favor a PBX or a hybrid.
_ If fewer than eight central office trunks are required, favor a key system
unless the system will grow significantly.
448 PART 4 Customer Premise Systems
_ If the system will never grow beyond three or four lines and about
eight stations and if PBX features are not needed, consider a KSU-less
system.
_ If ACD or voice processing is required, favor a PBX or a hybrid, with
a PBX providing superior features.
_ If half the total system traffic is intercom, favor a PBX or, depending
on size, a hybrid.
Line and Trunk Interfaces
Every system must conform to the standard EIA-464 interface to a local telephone
central office and must be registered with the FCC for network connection. In
addition, interfaces such as these should be considered:
_ PRI and BRI interfaces
_ Computer–telephony integration interface
_ QSIG interface
_ T1/E1 interface to external trunk groups or to internal devices such
as remote access servers
_ IP line and trunk interfaces (Chapter 26)
A key consideration in evaluating any system is the type of terminals it supports.
All systems have, at a minimum, a two-wire station interface to a standard
analog DTMF telephone, either directly or through an analog adapter. Ordinary
telephones are the least expensive terminals and because of the quantities of stations
involved in a large PBX, inability to use standard telephones can add significantly
to the cost. The standard analog telephone falls short as a user device in
most offices, but it is usually preferable in residential facilities such as hospitals,
hotels, and dormitories.
Proprietary telephones are the most practical way of accessing integrated
key telephone features. A proprietary terminal makes some features, such as call
pickup and transfer, easier to use by assigning features to buttons to avoid the
switch hook flashes and special codes required with analog telephones.
These features, among others discussed in Chapter 24, should be considered
in evaluating a PBX or key system terminal interface:
_ Proprietary or nonproprietary telephone interface
_ Number of lines and characters on the telephone set display
_ Station conductor loop range (in a campus environment)
_ Integrated key telephone system features
_ Message waiting or nonmessage waiting analog line card
_ Availability of BRI interface
CHAPTER 25 TDM Customer-Premise Switching Systems 449
Voice Mail
Voice mail evaluation is discussed in Chapter 28. Once available only in PBXs and
some hybrids, voice mail is now one of the most desired features among users
purchasing new key systems.
Wireless Capability
Many organizations need wireless to enable some users to roam all or part of a
building. Wireless systems that support analog telephones can be used with any
PBX or key system, but the ability to use proprietary telephones may be important
if button access to features is required.
ISDN Compatibility
PRI capability is an essential feature for all PBXs and many hybrids. Key systems
and hybrids may support BRI toward the central office. Determine whether ISDN
compatibility is likely to be required within the life of the system, and if so,
whether the system can be purchased or retrofitted to interface either BRI or PRI
lines. If ISDN is not used, ADSI may furnish equivalent service on key systems and
hybrids. BRI line interfaces are needed in many PBXs to support video conferencing.
Administrative Interface
All PBXs have some form of administrative interface through a terminal or
attached PC. The ease of use of this interface differs significantly among products.
The most difficult products to use have a command-driven terminal interface. At
the other end of the spectrum are systems with graphical user interfaces that allow
users to make point-and-click changes. Key systems and hybrids are administered
by plugging a laptop computer into a serial interface or in some systems a telephone
can be used for system setup.
The ease of changing classes of service and telephone numbers is an important
evaluation consideration. If an easy-to-use maintenance terminal can control
these, it is possible to add, remove, and move stations and change features such
as restrictions without using a trained technician.
The degree to which a PBX can diagnose its own trouble and direct a
technician to the source of trouble is important in controlling maintenance
expense. It is also important that a system has remote diagnostic capability so the
manufacturer’s technical assistance center can access the system over a dial-up port.
Cost
The initial purchase price of a PBX or key system is only part of the total lifetime
cost of the system. All systems require maintenance and administration, and the
method of accomplishing them can be significantly different among products. As
with all types of telecommunications apparatus, the failure rate and the cost of
restoring failed equipment are critical and difficult to evaluate. The most effective
way to evaluate them is by reviewing the experience of other users.
450 PART 4 Customer Premise Systems
Installation cost is another important factor. One factor is the method of programming
the station options in the processor. Some systems provide such
options as toll call restriction, system speed calling, and other such features in an
external database that can be uploaded to the switch.
Maintenance costs may be significant over the life of the system. The best
way to evaluate maintenance cost is to request a quotation on a maintenance contract,
which most vendors offer. Cost savings are possible with systems that provide
internal diagnostic capability. Virtually all PBXs provide remote diagnostic
capability so the vendor can diagnose the system over an ordinary telephone line.
These features can offer cost savings in hybrids, but are less important in key
systems.
Power Failure Protection
During power outages, a PBX or key system is inoperative unless battery backup
or a UPS is provided. Some systems include emergency battery supplies, while
others are inoperative until power is restored. Lacking battery backup, the system
should at least maintain its system memory during power failures.
The system should include a power-failure transfer system that connects
incoming lines to ordinary telephone sets so calls can be handled during power
outages. The method of restarting the system after a power failure is also important
because of the time required to get the system restarted. Some systems use
nonvolatile memory that does not lose data when power fails. Other systems
reload the database from a backup tape or disk, which results in a delay before the
system can be used following restoral.
Key System Considerations
Capacity
Key telephone systems should be purchased with a view toward long-term
growth in central office lines and stations. This specified size figure is the capacity
of the cabinet, and further expansion may be expensive or impossible. Some
systems can grow by adding another cabinet, but it also may be necessary to
replace the power supply and main control module. With some systems it is possible
to move major components, such as line and station cards, to a larger cabinet
to increase capacity. Most key systems use plug-in circuit cards. These are less
costly than wired systems, which must be purchased at their ultimate size. The
number of internal or intercom call paths also should be considered.
Station Equipment Interfaces
Many key telephone systems support only a proprietary station interface so
analog telephones cannot be used. The lack of a single-line interface is not
a disadvantage in many applications, but some companies need to connect
modems or facsimile machines to key system ports. An important feature for
CHAPTER 25 TDM Customer-Premise Switching Systems 451
many users is upward compatibility of telephone sets and line and trunk cards
across the manufacturer’s entire product line. This capability reduces the cost of
converting from a key system to a PBX and enables users to keep their instruments,
which not only reduces cost, but also minimizes retraining.
Key Service Unit versus KSU-Less Systems
Some systems support from two to four lines without a KSU. For small systems
these can be effective, providing many of the capabilities of a key telephone system
without the need for a central unit. KSU-less systems have disadvantages,
however, which make them inappropriate for many installations. First, they have
limited capacity, so they are usable only for small locations and cannot grow.
Second, they usually lack intercom paths, on-hook voice announcing, and other
features that are essential in a multiroom office.
Centrex Compatibility
Key systems are often used behind Centrex. Many Centrex features cannot be activated
unless the key system is Centrex compatible. For example, call transfer
requires a switch hook flash to get second dial tone. To make a key system Centrex
compatible, it must have a special button to flash the central office line. Many key
systems are provided with buttons to make them directly compatible with
Centrex features.
Number of Intercom Paths
A nonblocking switching network is one that provides as many links through the
network as there are input and output ports. For example, one popular key system
has capacity for 24 central office trunks, 61 stations, and eight intercom lines.
The system provides 32 transmission paths, which support calls to and from all 24
central office trunks. The eight intercom paths limit intrasystem conversations to
eight pairs of stations. A nonblocking network provides enough paths for all line
and trunk ports to be connected simultaneously. In this system, if all central office
trunks are connected, of the remaining 37 stations, only eight pairs can be in
conversation over the intercom paths. Although this system is not nonblocking, it
meets an important test of having sufficient paths to handle all central office
trunks and intercom lines.
PBX Considerations
Universal-Shelf Architecture
Universal-shelf architecture permits various types of line and trunk cards to be
installed in any slot. Lacking this feature, slots are dedicated to a particular type
of card. It is, therefore, possible to have spare slot capacity in the PBX but have no
room for cards of the desired type. Check to determine if all port cards are universal
or whether some specialized types require dedicated slots.
452 PART 4 Customer Premise Systems
Switch Network
A key evaluation consideration is whether the switch network is blocking or nonblocking.
Blocking networks are acceptable, but may require additional administrative
effort to keep them in balance. Also, consider the number of BHCAs the
PBX is capable of supporting to determine if the processor limits the capacity of
the system. Some manufacturers use the term “virtually nonblocking” to indicate
that there are nearly as many time slots as stations.
Redundancy
Organizations that cannot tolerate PBX outages can improve reliability by purchasing
redundant systems. Several levels of redundancy are available. The lowest
level provides redundant processors. Higher reliability can be achieved with
redundant power supplies and switching networks. Even with redundancy failures
will still occur, but reliability should be much higher than with a nonredundant
system.
Application Programming Interface
An open architecture interface is important for future computer–telephony applications
that will be appearing in the next few years. Determine if the PBX has such
an interface, and if the standards are readily available to developers. Consider that
outside developers will apply the greatest amount of development effort to the
most popular PBXs.

Chapter 19

Satellite Communications

The telecommunications industry experienced some spectacular failures around
the end of the twentieth century and the start of the twenty-first, but few could
compare to satellite companies Iridium and Teledesic. Iridium opened for business
in late 1998 with a series of low Earth orbit (LEO) satellites, which delivered
phone connectivity anywhere on the face of the Earth without the delay of a geosynchronous
satellite. Led by Motorola, Iridium spent some $5 billion, but did not
attract enough subscribers to make it pay. In 2000, Iridium filed for bankruptcy
and was on the verge of destroying its satellites until a group of private investors
acquired them for $25 million. With lower rates and a tolerable financial structure,
the service remains alive through its network of 66 satellites.
Teledesic did not get quite that far. Its original plans called for a network of
840 satellites. This dropped to 288 satellites, then in February 2002 Teledesic
announced that it was planning 20 medium Earth orbit (MEO) satellites. In July
2003 the company gave up its frequency slots and put the venture on hold.
A merger with another MEO company, ICO, was considered and dropped. ICO
had raised $3.1 billion before filing for Chapter 11 protection in 1999. A group of
investors acquired control of ICO’s assets for $1.2 billion. The network is not yet
in service at this writing.
Satellite communications had its beginning in 1962 with AT&T’s launch of
its Telstar 1. Orbiting the Earth in about 2 h, Telstar was visible from the Earth
station for less than half an hour, as the antennas followed its track across
the sky. Although Telstar demonstrated that the technology was feasible, loworbiting
satellites were abandoned in favor of geosynchronous satellites (GEO).
Orbiting the equator at an altitude of 22,239 miles (35,790 km) GEO satellites
appear stationary to the Earth station.
The LEO was still a good idea, however, because sending a radio signal up
to a repeater station that far from the Earth results in a round-trip delay of about
327
270 ms. Most data protocols can work around this kind of latency, but for voice
many people find the delay disconcerting. The growth of fiber optics has captured
the voice market except for those countries where nothing but satellite is available.
Although the delay makes satellites less than ideal for voice communications,
they have plenty of applications that cannot readily be filled by any other
alternative. Communications satellites are used for global positioning, communications
with ships at sea, telemetering data from trucks in transit, and for many
other applications where the user is either moving or in a remote area or both.
Another application, direct broadcast television, was a long time in coming, but
now high-quality TV can be received, even in remote locations. Very small aperture
terminal (VSAT) enables users to mount small antennas on rooftops to run
a multitude of applications such as point-of-sale, which need low bandwidth
facilities distributed over a wide range.
SATELLITE TECHNOLOGY
Figure 19-1 shows the relative position of the three orbit classes—LEO, MEO, and
GEO. At geosynchronous orbit, the satellite travels at the same speed as the Earth’s
rate of spin, so the orbiting vehicle remains at a fixed position with relation to
the Earth station. From geosynchronous orbit, three satellites can theoretically
cover the Earth’s surface, except for the polar regions, with each satellite subtending
a radio beam 17° wide. The portion of the Earth’s surface that a satellite
328 PART 3 Transmission Technologies
F I G U R E 19-1
Communication Satellite Orbits
Satellite dish
Satellite dish
GEO: Altitude
35,790 km
MEO: Altitude
10,400 km
LEO: Altitude
700-1400 km
Earth's Surface
illuminates is called its footprint. MEO and LEO satellites have the advantage of
low delay, but at the expense of needing more satellites and tracking antennas to
provide the same coverage.
Satellites fall into three general categories—domestic, regional, and international.
Domestic satellites carry traffic within one country. Regional satellites
span a geographical area, such as Europe, and international satellites are intended
for intercontinental traffic. Although undersea fiber-optic systems carry most of
the international voice traffic, international television is still a large and growing
market for satellites. International satellite communications are provided by
Intelsat Ltd, which became a private company in 2001 following 37 years as an
intergovernmental organization. Intelsat operates more than 20 satellites and
serves about 150 countries at this writing.
As Table 19-1 shows, the frequencies available for communication satellites are
limited. The lower frequency is always used from the satellite to the ground because
Earth stations’ transmitting power can overcome the greater path loss of the higher
frequency, but solar battery capacity limits satellite output power. The 4- and 6-GHz
C-band frequencies are the most desirable from a transmission standpoint because
they are the least susceptible to rain absorption. Satellites share the C-band frequencies
with common carrier terrestrial microwave, requiring close coordination of spacing
and antenna positioning to prevent interference. Interference between satellites
and between terrestrial microwave and satellites is prevented by using highly directional
antennas. Currently, satellites are spaced about the equator at 2° intervals.
The Ku-band of frequencies has come into more general use as the C-band
becomes congested. K-band frequencies are exclusive to satellites, allowing users
to construct Earth stations almost anywhere, even in metropolitan areas where
congestion often precludes placing C-band Earth stations. The primary disadvantage
of the Ku-band is rain attenuation, which results in lower reliability. With
identical 2° spacing for both C and Ku-bands, most satellites carry transponders
for both bands.
Ka-band satellites are becoming more attractive as the lower frequencies
become congested. Although the higher frequency of Ka-band means a higher
probability of fading, it is possible to use smaller antennas and inexpensive Earth
stations, which offset fading to some degree. Although considerable bandwidth
CHAPTER 19 Satellite Communications 329
TA B L E 19-1
Principal Communication Satellite Frequency Bands
Band Uplink Downlink
C 5925 to 6425 MHz 3700 to 4200 MHz
Ku 14.0 to 14.5 GHz 11.7 to 12.2 GHz
Ka 27.5 to 31.0 GHz 17.7 to 21.2 GHz
is available in Ka and higher frequencies, further development is needed before
these come into general use.
Satellites have several advantages over terrestrial communications. These
include:
_ The receiving station can be mobile, which offers coverage from a single
satellite.
_ Within the coverage range of a single satellite, circuit cost is independent
of distance.
_ Impairments that accumulate on a per-hop basis on terrestrial
microwave circuits are avoided with satellites because the Earth-stationto-
Earth-station path is a single hop through a satellite repeater.
_ Sparsely populated or inaccessible areas can be covered by a satellite
signal, providing high-quality communications service to areas that
are otherwise difficult or impossible to reach.
_ Coverage is also independent of terrain and other obstacles that may
block terrestrial communications.
_ Earth stations can verify their own data transmission accuracy by listening
to the return signal from the satellite.
_ Because satellites broadcast a signal, they can cover wide areas.
_ Large amounts of bandwidth are available over satellite circuits, making
voice, video, and high-speed data circuits available.
_ The satellite signal can be brought directly to the end user, bypassing
the local telephone facilities that are expensive and limit bandwidth.
_ Multipath reflections that impair terrestrial microwave communications
have little effect on satellite radio paths.
Satellites are not without limitations, however. The greatest drawback is the
lack of frequencies. If higher frequencies can be developed with reliable paths,
plenty of spectrum is available, but atmospheric limitations may prevent their use
for commercial-grade telecommunications service. Other limitations include:
_ Multihop satellite connections impose delay that is detrimental to voice
communications and is generally avoided.
_ Path loss is high (about 200 dB) from Earth to satellite.
_ Rain absorption affects path loss, particularly at higher microwave
frequencies.
_ Frequency crowding in the C-band is high with potential for interference
between satellites and terrestrial microwave operating on the same
frequency.
The rapid growth of fiber-optic systems has had an adverse effect on satellites’
share of the telecommunications market, but the technology shows no signs of
330 PART 3 Transmission Technologies
dying. Though the satellites’ market share may be dropping, the traffic carried by
communications satellites continues to increase and will do so in the future.
Satellite Systems
A satellite circuit has five elements as shown in Figure 19-2: two terrestrial links,
an uplink, a downlink, and a satellite repeater. The satellite has six subsystems
described below:
_ physical structure
_ transponder
_ attitude control apparatus
_ power supply
_ telemetry equipment
_ station-keeping equipment
Physical Structure
The size of communications satellites has been steadily increasing since the launch of
the first commercial satellite in 1965. Size is limited by the capacity of launch vehicles
and the need to carry enough solar batteries and fuel to keep the system alive for its
design life of 5 to 10 years. Advances in space science such as launch vehicles that
can carry greater payloads are making larger satellites technically feasible. A large
physical size is desirable. Not only must the satellite contain the radio and support
equipment, but it must also provide a platform for large antennas to obtain the high
gain needed to overcome the path loss between the Earth station and the satellite.
Transponders
A transponder is a radio relay station on board the satellite. Transponders are technically
complex, but their functions are identical with those of terrestrial microwave
CHAPTER 19 Satellite Communications 331
F I G U R E 19-2
Communication Satellite System
Earth Station
Earth Station
Terrestrial
Link Terrestrial
Link
Uplink Downlink
radio relay stations. A receiving antenna picks up the signal from the Earth station
and amplifies it with a low noise amplifier (LNA), which boosts the received signal.
The LNA output is amplified and converted to the downlink frequency. The downlink
signal is applied to a high-power amplifier, using a traveling wave tube or
solid-state amplifier as the output device. The output signal couples to the downlink
or transmitting antenna. Solid-state amplifiers are popular because of their high
reliability. Most satellites carry multiple transponders, each with a bandwidth of
36 to 72 MHz. For example, Echostar 9 (also known as Telstar 13 and Intelsat
Americas 13), which was launched in August 2003, has 2 Ka-, 32 Ku-, and 24 C-band
transponders. It covers the United States from 121° west longitude and operates
with 120 W of power.
Attitude Control Apparatus
Satellites must be stabilized to prevent them from tumbling through space and to
keep antennas precisely aligned toward Earth. Satellite stabilization is achieved
by two methods. A spin-stabilized satellite rotates on its axis at about 100 rpm.
The antenna is despun at the same speed to provide constant positioning and
polarization toward Earth. The second method is three-axis stabilization, which
consists of a gyroscopic stabilizer inside the vehicle. Accelerometers sense any
change in position in all axes and fire positioning rockets to keep the satellite at a
constant attitude.
Power Supply
Satellites are powered by solar batteries. Power is conserved by turning off
unused equipment with signals from the Earth. On spin-stabilized satellites, the
cells mount outside the unit so that one-third of the cells always face the sun.
Three-axis stabilized satellites have cells mounted on solar panels that extend like
wings from the satellite body. Solar cell life is a major factor that limits the working
life of a satellite. Solar bombardment gradually weakens the cell output until
the power supply can no longer power the on-board equipment.
A nickel–cadmium battery supply is also kept on board most GEO satellites
to power the equipment during solar eclipses, which occur during two 45-day
periods for about an hour per day. The eclipses also cause wide temperature
changes that the on-board equipment must withstand.
Telemetry Equipment
A satellite contains telemetry equipment to monitor its position and attitude and
to initiate correction of any deviation from its assigned station. Through telemetry
equipment, the Earth control station initiates changes to keep the satellite at its
assigned longitude and inclination toward Earth. Telemetry also monitors the
received signal strength and adjusts the receiver gain to keep the uplink and
downlink paths balanced.
332 PART 3 Transmission Technologies
Station-Keeping Equipment
Small rockets are installed on GEO vehicles to keep them on station. When the satellite
drifts, rockets fire to return it to position. The tasks that keep the satellite on position
are called station-keeping activities. The fuel required for station keeping is the
factor, with solar cell life that limits the design life of the satellite. Moveable antennas
track MEO and LEO satellites, so station-keeping equipment is not required.
EARTH STATION TECHNOLOGY
Earth stations vary from simple, inexpensive, receive-only stations that individual
consumers can purchase to elaborate two-way communications stations such as
the one in Figure 19-3 that offer commercial satellite service. An Earth station
includes microwave radio relay equipment, terminating multiplex equipment,
and a satellite communications controller. The Earth stations for MEO and LEO
satellites link to the PSTN for dial-up telephone service.
Radio Relay Equipment
The radio relay equipment used in an Earth station is similar to terrestrial
microwave equipment except that the transmitter output power is considerably
CHAPTER 19 Satellite Communications 333
F I G U R E 19-3
Satellite Earth Station on Majuro (Photo by Author)
higher. In addition, antennas up to 30 m in diameter in GEO Earth stations provide
the narrow beam width required to concentrate power on the targeted satellite.
Because the Earth station’s characteristics are more easily controllable than
the satellite’s and because power is not limited on Earth as it is in space, the Earth
station plays the major role in overcoming the path loss between the satellite and
Earth. Path loss for GEO satellites ranges from about 197 dB at 4 GHz to about
210 dB at 12 GHz. In addition, the higher the frequency, the greater the loss from
rainfall absorption. Therefore the uplink always operates at the higher frequency
where higher transmitter output power can overcome absorption, while the lower
frequency is reserved for the downlink.
GEO antennas are adjustable to compensate for slight deviations in satellite
positioning. Antennas at commercial stations are normally adjusted automatically
by motor drives, while inexpensive antennas are adjusted manually as needed.
Thirty-meter antennas provide an extremely narrow beam width, with half-power
points 0.1° wide. LEO and MEO antennas are moveable to track the satellite in
its orbit.
Satellite Communications Control
A satellite communications controller (SCC) apportions the satellite’s bandwidth,
processes signals for satellite transmission, and interconnects the Earth station
microwave equipment to terrestrial circuits. The SCC formats the received signals
into a single integrated bit stream in a digital satellite system or combines FDM
signals into an analog FM signal. The multiplex interface of an Earth station is conventional.
Satellite circuits use either analog or digital modulation, with interfaces
to frequency division and time division terrestrial circuits.
Access Control
Satellites employ several techniques to increase the traffic-carrying capacity and
provide access to that capacity. Some FDMA satellites divide the transponder
capacity into multiple frequency segments between endpoints. One disadvantage
of FDMA is that users are assigned a fixed amount of bandwidth that cannot
be adjusted rapidly or easily assigned to other users when it is idle. In addition,
the guard bands between channels use part of the capacity.
TDMA time-shares the total transponder capacity. Earth stations transmit
only when permitted by the access protocol. When the Earth station receives
permission to transmit, it is allotted the total bandwidth of the transponder for the
duration of the station’s assigned time slot. A master station controls the access
or the Earth station listens to which station transmitted last and sends its burst in a
preassigned sequence. Each Earth station receives all transmissions but decodes
only those addressed to it. TDMA provides priority to stations with more traffic to
334 PART 3 Transmission Technologies
transmit by assigning those stations more time slots than it assigns to low-priority
stations. Therefore, a station with a growing amount of traffic can be allotted a
greater share of total transmission time.
Demand-assigned multiple access (DAMA) is an alternative to preassigned
multiple access. DAMA equipment keeps a record of idle radio channels or time
slots. Channels are assigned on demand by one of the three methods—polling,
random access with central control, and random access with distributed control.
Control messages are sent over a separate terrestrial channel or contained in a
control field in the transmitted frame.
Signal Processing
The SCC conditions the signals for transmission between the terrestrial and satellite
links. The type of signal conditioning depends on the service provider and may
include voice compression, echo cancellation, forward error correction, and digital
speech interpolation to avoid transmitting the silent periods of a voice signal.
GEO SATELLITE TRANSMISSION
Much of the previous discussion is of only academic interest to those who use
satellite services. However, satellite circuits and terrestrial circuits have different
transmission characteristics.
Satellite Delay
The quarter-second delay between two Earth stations is noticeable in voice
communications circuits, but most people become accustomed to it and accept it as
normal if the circuit is confined to one satellite hop but delay affects many data protocols.
TCP/IP, for example, sees a delay in packet acknowledgement as a sign of
congestion or packet loss and may force retransmission of packets that have actually
been correctly received. Furthermore, TCP has a slow-start characteristic that sets
its packet acknowledgement window at a single packet and gradually opens it.
Therefore, performance depends on the transfer rate and the round-trip delay.
Performance problems begin to occur when TCP/IP operates over what the
industry calls a “long fat pipe.” For example, a T1 satellite channel has a bandwidth
delay product of 100 outstanding TCP segments of 1200 bytes each, which
requires a large buffer to contain unacknowledged packets. RFC 1323 discusses
recommended TCP modifications to improve performance on satellite circuits.
One solution is “spoofing,” in which the Earth stations acknowledge the packets
to the endpoints and communicate between themselves in the satellite link with a
protocol that is not delay susceptible. Satellite circuits are unsatisfactory for VoIP
because the combined delays are well outside the recommended range.
CHAPTER 19 Satellite Communications 335
Rain Absorption
Rain absorption has a dual detrimental effect on satellite communications. Heavy
rains increase the path loss and may change the signal polarization enough to
impair the cross-polarization discrimination ability of the receiving antennas.
Rain absorption can be countered by these methods:
_ choosing Earth station locations where heavy rain is least likely
_ designing sufficient margin into the path to enable the circuits to tolerate
the effects of rain
_ locating a diversity Earth station at a sufficient distance from the main
station
Technical considerations may limit the first two options. Transmit power
and antenna gain from the satellite can be increased only within the limits dictated
by the size of the satellite and the transmit power available. Locations with low
precipitation cannot always deliver service where required. These considerations
mandate the use of Earth station diversity at higher frequencies.
Sun Transit Outage
During the spring and fall equinoxes for periods of about 10 min/day for 10 days,
the sun is positioned directly behind the satellite and focuses a considerable
amount of high-energy radiation directly on the Earth station antenna. This solar
radiation causes a high noise level that renders the circuits unusable during this
time. If the outage cannot be tolerated, traffic must be rerouted through a backup
satellite.
Carrier-to-Noise Ratio
Satellite transmission quality is based on carrier-to-noise ratio, which is analogous
to signal-to-noise ratio on terrestrial circuits. The ratio is relatively easy to
improve on the uplink portion of the satellite circuit because transmitter output
power and antenna gain can be increased to offset noise. On the downlink portion
of a circuit, the effective isotropic radiated power (EIRP), which is a measurement
of the transmitter output power that is concentrated into the downlink footprint,
can be increased only within the size and power limits of the satellite or by using
spot beams to concentrate signal strength.
REPRESENTATIVE SATELLITE SERVICES
In this section, three different types of services are discussed to illustrate the
versatility of communications satellites. LEO is a new service that avoids the delay
inherent with other satellite services. Maritime radio service is an excellent example
336 PART 3 Transmission Technologies
of a service that cannot be provided in any other feasible way: communication with
aircraft and ships at sea. VSAT replaces conventional terrestrial communications
and offers the advantage of bringing signals directly to the user without requiring
the last link in a communications path—the local telephone loop—that is often
expensive and bandwidth limiting.
Low Earth Orbiting Satellite
Iridium launched commercial voice, fax, paging, and narrow-band data service in
2001, using a network of 66 operational satellites plus six in-orbit backups. The
system operates at an altitude of 780 km (485 miles). Its Earth station links and
intersatellite links operate in the Ka-band. Voice is digitized and compressed to
2.4 Kbps. Dial-up data can be transmitted at a maximum speed of 2.4 Kbps, with
Internet access at 10 Kbps. Short messages of up to 160 characters can be received
on the telephone. In addition, Iridium offers short-burst data message service,
which supports messages up to 1960 bytes.
Satellites are connected via intersatellite links to the four nearest neighbors
and to an Earth station. Calls can be connected to landline telephones or to other
satellite phones. The satellites use the L-band (1616 to 1626.5 MHz) for communication
with the subscriber terminals. Subscribers can receive and place telephone
calls and pages while roaming anywhere in the world that the service is
authorized.
International Maritime Satellite Service
Inmarsat began life in 1979 as an intergovernmental organization. In 1999, it was
converted to private ownership with headquarters in Britain. The company provides
phone, fax, and data communications to more than 287,000 ship, vehicle, aircraft,
and other mobile users through a network of nine satellites. Inmarsat provides
service for all types of ocean-going vessels ranging from merchant ships to yachts.
The system has a network of coastal Earth stations that can communicate with
ships at sea. The ship’s Earth station mounts above decks and automatically stays
in position with satellite tracking equipment.
Inmarsat offers a variety of fleet services, providing both ISDN and mobile
packet data service (MPDS). MPDS charges by packet volume and not for the
time spent online, which makes it convenient for real-time access. The service can
be used for video conferencing, store-and-forward video, remote monitoring,
chart and weather updates, telemedicine, and distress and safety signals.
In the past, the principal methods of communication from ships were telex
and high-frequency radio, which were unreliable and expensive. Now data
circuits are replacing those modes of communication. Voice circuits replace the
high-frequency ship-to-shore radio that often suffered from poor reliability. Ship
locations can be monitored precisely through polling equipment. Distress calls
CHAPTER 19 Satellite Communications 337
can be received and rebroadcast to ships that are in the vicinity but out of radio
range. Broadcasts such as storm warnings can be made to all ships in an area.
Through their I-4 satellites, Inmarsat provides Broadband Global Area
Network (BGAN) service, which supports up to 432 Kbps for Internet access,
mobile multimedia, and similar applications. BGAN is also compatible with thirdgeneration
cellular service.
Inmarsat is the wholesaler of satellite airtime, which is sold by hundreds of
partners. In addition, Inmarsat provides satellite services to commercial airliners
and corporate jets. Inmarsat’s Swift64 offers Mobile ISDN- and IP-based MPDS
connectivity at 64 Kbps. United Airlines has announced the use of Swift64 to
provide video surveillance of the secured flight deck via satellite. Scandinavian
Airlines System is offering high-speed Internet access on several of its long-range
aircraft. The availability of satellite services for Internet access will become
increasingly common in the future.
Very Small Aperture Terminal
VSATs are named for the size of the transmitting antennas, which are much
smaller than those used in conventional Earth stations. VSAT antennas are
normally 1.8 m (6 ft) or less in diameter, which makes them easy to conceal on
rooftops and in areas with zoning restrictions. A VSAT network is star-connected
with a hub at the center and dedicated lines running to the host computer as
shown conceptually in Figure 19-4. The hub has a larger antenna, often 4 to 11 m
in diameter, aimed at the satellite. Hubs are complex and expensive, so only the
largest organizations can justify a privately owned hub. Usually, the VSAT vendor
338 PART 3 Transmission Technologies
F I G U R E 19-4
A VSAT System
Earth Station
VSAT Terminal
VSAT Terminal
VSAT Terminal
VSAT Master
Station
Server
owns the hub, or one organization owns it and shares it with others. Not only is a
shared hub more cost-effective for most companies, but it also relieves the company
of the necessity of managing the hub, which may require one or two people
per 100 nodes. Generally, a privately owned hub is feasible only when 200 or more
remote stations share the service.
The hubs control demand assignment to the satellite and monitor and diagnose
network performance. Demand is allocated in one of the four ways—pure aloha,
slotted aloha, TDMA, or spread spectrum. The first three methods generally are used
on Ku-band and the last on C-band. Pure aloha is an inefficient method of regulating
access. Stations transmit at will, and when their transmissions collide they must
retransmit. Slotted aloha is somewhat more efficient in that stations can transmit only
during allotted time slots. TDMA and spread spectrum are the most effective ways
of allocating access. VSAT provides bandwidth as high as T1/E1 and as low as the
customer needs to go. It is used for voice, video, and data transmission.
The remote station has an antenna and a receiving unit, which is about the
size of a personal computer base unit. The receiving unit contains a modulator/
demodulator, a packet assembler/disassembler, and a communication controller.
The remote transmitter operates with an output power of about 1 W. The receiver
uses a low-noise amplifier.
The primary application for VSAT is data, although it can also carry voice
and video. Typically, C-band VSATs carry 9.6 Kbps data, and Ku-band VSATs
carry 56 or 64 Kbps data; some systems carry a full or fractional T1/E1. Most
applications are two-way interactive. The primary advantage of VSAT is its ability
to support multiple locations. For a few locations, the terrestrial link from the host
computer to the hub plus the investment in remote stations may make VSAT
prohibitively expensive. As the number of remote sites increases, however, VSAT
becomes more attractive.
SATELLITE APPLICATION ISSUES
In one sense, satellite applications diminish as terrestrial and undersea fiber-optic
circuits become more plentiful and economical, but satellite services are still
uniquely suited for many applications. The heavy investments the major providers
are making show that they expect demand to remain healthy.
Satellite Service Evaluation Considerations
Satellite space vehicle evaluation criteria are complex, technical, and of interest
only to designers, owners, and manufacturers of satellites and on-board equipment.
Therefore, this discussion omits these criteria. Likewise, common carrier
Earth station equipment evaluations are omitted from this discussion. Evaluation
criteria discussed in Chapter 18 on microwave equipment generally apply to satellite
services except that multipath fading is not a significant problem in satellite
CHAPTER 19 Satellite Communications 339
services. In addition, alarm and control systems in terrestrial microwave are
different from those used in satellite systems.
The following factors should be considered in evaluating satellite services
and privately owned Earth station equipment.
Availability
Circuit availability is a function of path and equipment reliability. To the user of
capacity over a carrier-owned Earth station, equipment reliability is a secondary
consideration. The important issue is circuit reliability measured as percent errorfree
seconds in digital services and percent availability within specified noise
limits for analog services.
These same availability criteria apply with privately owned Earth stations,
but the carrier can quote availability based only on path reliability. Equipment
availability depends on MTBF and MTTR and must be included in the reliability
calculation. The frequency and duration of any expected outages because of
solar radiation or solar eclipse should be evaluated. Availability figures of 99.5 to
99.9% are typical.
Data Bandwidths
Satellite carriers typically provide transponder bandwidth in 128 Kbps segments.
The number of stations that can be supported in this bandwidth depends on the
amount of activity. VSAT is generally cost-competitive with frame relay in large
networks and offers a similar type of service, but with somewhat lower throughput
for a given bandwidth of the access facility. The applications of VSAT are
similar to those for frame relay with certain exceptions. First, the cost of a frame
relay access channel varies with the distance from the carrier’s point-of-presence.
VSAT is not distance-sensitive except for the cost of a backhaul circuit to carry
data from the Earth station to the customer’s site. The backhaul circuit can be a
point-to-point circuit, IP, or frame relay.
Access Method
Satellite carriers employ several techniques to increase the information-carrying
capacity of the space vehicle. Techniques such as DAMA can result in congestion
during peak load periods and the possibility that Earth station buffer capacity
can be exceeded or access to the system blocked. Users should determine what
methods the carrier uses to apportion access, whether blockage is possible, and
whether transmission performance will meet objectives.
Transmission Performance
The carrier’s BER, loss, noise, echo, envelope delay, and absolute delay objectives
should be evaluated. To support TCP/IP, the carrier should provide spoofing.
Except for absolute delay, which cannot be reduced except by using terrestrial
340 PART 3 Transmission Technologies
facilities to limit the number of satellite hops, satellite transmission evaluation
should be similar to terrestrial circuits.
Earth Station Equipment
Earth station equipment is evaluated against the following criteria:
_ equipment reliability
_ support for specific protocols such as TCP/IP
_ technical criteria, such as antenna gain, transmitter power, and receiver
sensitivity, that provide a sufficiently reliable path to meet availability
objectives
_ antenna positioning and tracking equipment that is automatically
or manually adjustable to compensate for positional variation in the
satellite
_ physical structure that can withstand the wind velocity and ice-loading
effects for the locale
_ the availability of radome or deicing equipment to ensure operation
during snow and icing conditions
Network Management Capability
Network management is important in VSAT networks where many Earth stations
are under the control of a single hub. The service provider should be able to
reconfigure the network rapidly from a central location. Monitoring and control
equipment should be able to diagnose problems and detect degradations before
hard faults occur. The network management package should collect statistics
on network use and provide information for predicting when growth additions
will be required. Determine whether the network provider can service all network
components including routers.

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