Wireless Mobile Networking with ANSI-41

502 Pages·2001·2.05 MB·English

by  Snyder R.A.

Source: Wireless Mobile Networking with ANSI-41
1
PART
Introduction
to Wireless
Telecommunications,
Network
Architecture, and
Functions
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Introduction to Wireless Telecommunications, Network Architecture, and Functions
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Source: Wireless Mobile Networking with ANSI-41
1
CHAPTER
Basics of
Wireless
Telecommunications
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Basics of Wireless Telecommunications
4
Part 1: Introduction to Wireless Telecommunications
To begin to understand the ANSI-41 signaling protocol, it is necessary to
understand some basics about wireless telecommunications. This chap-
ter provides a general overview of the most important concepts in wire-
less cellular telecommunications that apply to ANSI-41. For some read-
ers, this will be a high-level review; for others, it may clarify some
misconceptions and provide an overall understanding of cellular wire-
less technology that will be useful in understanding ANSI-41.
What Are Wireless
Telecommunications?
The concept of wireless telecommunications can be viewed from two per-
spectives: the wireless subscriber’s and the wireless network’s. From the
subscriber’s perspective, wireless telecommunications is a service that
allows telephone calls to be made or received while the telephone equip-
ment moved from place to place or while it is in motion. From this per-
spective, the telephone handset (known as the mobile station) is wireless
and affords the ability to be mobile.
From the network’s perspective, wireless telecommunications is a
service provided to end-users. Wireless telecommunications, within the
context of ANSI-41, is a service based on a set of functions internal to
the network known as mobility management. Mobility management
functions enable the network to maintain location and subscriber status
information so that end-users can make and receive calls while they
move from place to place.
Origin of Advanced
Mobile Phone System
Wireless telecommunications can be considered both a system and a
service. The network equipment including antennas, radios, switches,
databases, and all hardware and software within the network, repre-
sents a wireless telecommunications system that provides wireless
telecommunications service to subscribers.
The first wireless telecommunications system based on cellular tech-
nology was (and is) known as AMPS (Advanced Mobile Phone System), a
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Basics of Wireless Telecommunications
5
Chapter 1: Basics of Wireless Telecommunications
technology developed by Bell Laboratories in 1947. The term cellular
refers to a network of small cells or radio transceivers, each providing a
limited range of radio coverage, which are linked by a computer-con-
trolled switching system that manages subscriber mobility and interfaces
to the fixed wire-line telephone network. The technology is based on cel-
lular frequency reuse (described later), providing a high-capacity system
and allowing network access using low-power mobile stations (typically
less than 6 W). The radio transceiver modulation methods used are based
on analog frequency modulation (FM) signals similar to those used for
commercial radio, but at a higher frequency range and lower bandwidth.
The first commercial cellular system in the United States became
operational in Chicago in 1983. However, other countries around the
world provided operational cellular systems several years earlier.
Today, cellular systems based on AMPS technology are implemented
in more than 100 countries. It is interesting to note, however, that there
is no single worldwide standard for the implementation of these cellular
systems. The different systems deployed generally represent differing
radio technologies, each based on the concepts of AMPS. The networking
technologies used to link the cells together are also quite different.
These technologies are considered supplemental to the defining charac-
teristics of AMPS (i.e., cells, frequency reuse, etc.). In fact, AMPS can
operate within a variety of networking schemes.
Some Basic Cellular Concepts
Basic Radio Technology
Cellular radio technology allows a subscriber to originate and receive
telephone calls wherever compatible cellular radio coverage is provided.
Acellis an individual radio coverage area controlled by a radio base sta-
tion (BS) system. Individual calls within a single cell use different fre-
quencies. These frequencies can be reused by other cells, provided that
there is no interference with the other cells. The frequency reuse pat-
tern of the cells is dependent on the distance between the cells and the
radio transmission power.
First-generation radio technologies (AMPS-based) use signals based
on analog FM for speech transmission. Subsequent generations of radio
technology for wireless systems include NAMPS (narrowband AMPS),
which is also based on analog FM, and a variety of sophisticated digital
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Basics of Wireless Telecommunications
6
Part 1: Introduction to Wireless Telecommunications
technologies based on TDMA (time division multiple access) and CDMA
(code division multiple access).
The cellular radio spectrum (range of allowable and available radio
frequencies) used for these cellular system technologies is regulated by
government agencies in different countries. In the United States, cellu-
lar service providers are categorized by one of two sets of non-contiguous
25 MHz radio frequency bandwidths. PCS service providers are catego-
rized by six sets of noncontiguous bandwidths: three sets of 30-MHz
radio frequency bandwidths and three sets of 10-MHz radio frequency
bandwidths.
Cellular A-band and
B-band Carriers
The two sets of bandwidths licensed for cellular radio service are known
as the A-band and the B-band. A-band carriers are cellular service
providers originally termed the nonwire line licensees. These original
licensees are companies that provide cellular service and are not associ-
ated with any local wire line telephone company.
B-band carriers are cellular service providers originally termed the
wire line licensees. These licensees are companies that provide cellular
service and are associated with the local wire line telephone company
(i.e., the original Regional Bell Operating Company or RBOC) in the
area where they provide cellular service.
The concept of A-band and B-band carriers was devised as part of the
Modification of Final Judgment (MFJ) consent decree in 1982 that broke
up the AT&T/Bell system monopoly in 1984. The AMPS technology orig-
inally developed by Bell Laboratories was given up to the seven RBOCs
as part of the compromise to divest them from AT&T. The mandated
provision to allow two cellular service provider licenses in a given geo-
graphic area was designed to provide competition between an independ-
ent cellular carrier and the cellular carrier owned by the local wire line
carrier. Note that the cellular A- and B-band licenses are allowed to sup-
port analog or digital radio technologies. Figure 1.1 depicts the cellular
radio spectrum licenses in use today.
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Basics of Wireless Telecommunications
7
Chapter 1: Basics of Wireless Telecommunications
Phone Transmit 846.5 Base Station Transmit 891.5
824 825 835 845 849 869 870 880 890 894
A band B band 20 MHz A band B band
A'' band 33(31 0- 3M0H KzH)z 33(31 0- 3M0H KzH)z A' bandB' band tbraentwsmeeitn A'' band 33(31 0- 3M0H KzH)z 33(31 0- 3M0H KzH)z A' band B' band
channels channels groups channels channels
(1 MHz) (1.5 MHz) (1.5 MHz) (1 MHz) (1.5 MHz) (1.5 MHz)
33 - 30 KHz 50 - 30 KHz 83 - 30 KHz 33 - 30 KHz 50 - 30 KHz 83 - 30 KHz
channels channels channels channels channels channels
Figure 1.1 Cellular licensed frequencies that are in use today. The A’, B’, and A” bands were originally set
aside for control functions, but can be used for normal traffic.
PCS A–F-band Carriers
In 1994, the U.S. government publicly auctioned six new sets of nation-
wide wireless telecommunications licenses. These licenses had restric-
tions on who could own them different from those of the original A and
B-band licenses. They were no longer limited to what type of carrier
could own them (i.e., local wire line or independent wireless); rather,
restrictions were put on the total number and types of wireless licenses
that a single carrier could own in a given market.
In the U.S., PCS service providers are categorized by one of three sets
of noncontiguous 30-MHz radio frequency bandwidths or one of three
sets of noncontiguous 10 MHz radio frequency bandwidths. Note that
the PCS A- through F-band licenses (see Figure 1.2) are allowed to sup-
port only digital radio technologies, typically operating among cell sizes
of much smaller radii than analogous cellular systems. Figure 1.2
depicts the PCS radio spectrum licenses in use today.
Frequency Reuse
Cellular frequency licenses provide for each mobile station to occupy 60
kHz of bandwidth (30 kHz for transmission and 30 kHz for reception)
within an entire radio frequency (RF) allocation of 25 MHz for each of
the two cellular carriers (A and B) in a given area (i.e., 12.5 MHz for
transmit and 12.5 MHz for receive for each carrier). PCS frequency
licenses provide for each mobile station to occupy 60 kHz of bandwidth
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Basics of Wireless Telecommunications
8
Part 1: Introduction to Wireless Telecommunications
Phone Transmit
1850 1865 1870 1885 1890 1895 1910
A band D band B band E band F band C band
(15 MHz) (5 MHz) (15 MHz) (5 MHz) (5 MHz) (15 MHz)
449 - 30 KHz 165 - 30 KHz 448 - 30 KHz 165 - 30 KHz 165 - 30 KHz 449 - 30 KHz
channels channels channels channels channels channels
20 MHz between
transmit groups
Base Station Transmit
1930 1945 1950 1965 1970 1975 1990
A band D band B band E band F band C band
(15 MHz) (5 MHz) (15 MHz) (5 MHz) (5 MHz) (15 MHz)
449 - 30 KHz 165 - 30 KHz 448 - 30 KHz 165 - 30 KHz 165 - 30 KHz 449 - 30 KHz
channels channels channels channels channels channels
Figure 1.2 PCS licensed frequencies in use today.
(30 kHz for transmission and 30 kHz for reception) within an entire RF
allocation of either 30 MHz (i.e., 15 MHz for transmit and 15 MHz for
receive for each carrier) or 10 MHz (i.e., 5 MHz for transmit and 5 MHz
for receive for each carrier). Note that although the terms cellular and
PCS differentiate categories of frequency bandwidths, PCS systems
employ the same basic technology of cellularsystems.
Cellular systems use a technique known as frequency reuse (see Fig-
ure 1.3). A particular available channel frequency is transmitted from
one base station at a power level that supports communications within a
moderate cell radius around that base station (anywhere from a few
hundred feet to about 50 miles!). Because this transmitted signal power
is controlled to serve only a limited range, the same frequency can be
transmitted simultaneously, or reused, by another base station, provid-
ed there is no interference between it and any other base station using
that same frequency.
Figure 1.3 depicts a typical cellular frequency reuse model using a
seven-cell pattern that provides uniform distances for channel reuse.
In the model, each base station is considered to be located at the cen-
ter of a hexagon, with the hexagons (or cells) labeled A through G repre-
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Basics of Wireless Telecommunications
9
Chapter 1: Basics of Wireless Telecommunications
radius r
FSeigveunre-c e1ll. 3frequency separation distance
reuse pattern.
G r
F B G
A F B
E C A
D E C
D
senting seven channel sets. The frequencies used for the channel sets in
the A cells are the same, as are the frequencies in the B cells, C cells,
etc. There are many possible frequency reuse patterns. Since the total
number of channels for cellular and PCS is fixed, the selection of a reuse
pattern and cell size determine how many subscribers can be supported
in a given service area. Available capacity, however, is much greater
than the actual number of channels accessible in a given cell. This is
due to the nature of end-user calling behavior: not everyone wants to
talk at the same time. A cell can typically serve 10 to 20 times more sub-
scribers than the total number of channels supported.
Digital Radio
Digital radio technologies have been developed for use on the air (or
radio) interface. These can dramatically increase the number of sub-
scribers supported on the range of frequencies used for wireless telecom-
munications systems (note that the analog NAMPS technology also pro-
vides an increase in the number of subscribers supported). The two basic
types of digital technology are time division multiple access (TDMA) and
code division multiple access (CDMA). Many standards exist for the use
of these basic technologies. TDMA is based on the use of time-interleav-
ing of multiple signals to provide an apparently simultaneous transmis-
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Basics of Wireless Telecommunications
10
Part 1: Introduction to Wireless Telecommunications
sion of those signals on a single radio frequency. CDMA is based on a
technique known as direct sequence spread spectrum (DSSS) that digital-
ly codes a base signal and employs different signal encoding patterns and
frequency hopping to redistribute that base signal across a broad range
of frequencies. Logical channels for each signal are created through the
use of unique code sequences.
TDMA and CDMA can provide many advantages over analog-based
systems. Examples are better voice quality, increased capacity, less
noise and interference, and the ability to provide digital services such as
data and messaging. The ANSI-41 networking protocol is designed to
support versions of these newer-generation digital technologies as well
as the original analog systems.
Handoff
Handoff encompasses a set of functions, supported between a mobile
station (MS) and the network, which allows the MS to move from one
cell to another (or one radio channel to another, within, or between cells)
while a call is in progress. The handoff function requires sophisticated
coordination between the network and the MS to transfer the MS
smoothly from one radio channel to another during a call. There are two
types of handoff: intrasystem and intersystem.
Intrasystem handoff (see Figure 1.4) is a handoff between two cells or
radio channels that subtend the same mobile switching center (MSC). In
this case, no coordination is required between MSCs to support the move-
ment of an MS between cells. Intersystem handoff (see Figure 1.5) is a
handoff between two cells subtending two different MSCs. This type of
handoff requires specialized signaling between the two MSCs to coordinate
the movement of the MS between the cells. Since the ANSI-41 protocol is
concerned with intersystem operations, it provides the operations neces-
sary to support intersystem handoff. Intrasystem handoff is not within the
scope of ANSI-41 and is handled via proprietary methods at the MSC.
There are three strategies for performing a handoff: MS controlled,
network controlled, and MS assisted.
These strategies differ mainly in which side of the radio interface
determines when to hand off the MS to another channel. MS-controlled
handoff is a technique where the MS itself continuously monitors the
radio signal’s strength and quality. When predefined criteria are met,
the MS checks the best candidate cell for an available traffic channel
and requests that the handoff occur. Network-controlled handoff is a
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