Second Generation Wireless Technologies (2G)

 
Second Generation Wireless Technologies (2G)
Second generation (2G) wireless technology encompasses the majority of present technologies
and will provide support for future communications systems (i.e., 2.5G, 3G and 4G).
The following sections explore some of these technologies including Personal Communications
Systems (PCS), Global System for Mobile Communications (GSM), Code Division
Multiple Access (CDMA), Time Division Multiple Access (TDMA) and Orthogonal Frequency
Division Multiplexing (OFDM).
10.3.1 Personal Communications Services (PCS)
Most 2G systems we discuss are designed using digital signal transmission (versus analog
signal transmission), which is one of the main differences between 1G and 2G networks.
Digital signals differ from their analog ancestors in form and transmission properties. Digital
signals have a more structured, defined form based on binary coding. Binary coding
uses combinations of the digits 0 and 1 to represent any signal transmission.
The sequence of 0s and 1s combine to form a signal’s unique pattern for each transmission
sent over the network. The base station that receives the signal stores the original
coding pattern of 0s and 1s before processing the signal. This allows the base station to
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reconstruct the signal to duplicate the original signal sent (Fig. 10.3). The base station then
transmits the new signal to its next destination. Although the new signal is almost an exact
replica of the original transmission, translation errors can still occur. The error rate of digital
transmission is reduced as compared to analog systems, improving efficiency. The
quality of the reproduced digital signal is far greater than that for analog signals.
Fig. 10.3 Digital signal transmission and reproduction.
Personal Communication Service (PCS) was introduced as one of the first digital services
in the United States in 1995. It provides an alternative method to analog communications
by using the digital frequency spectrum (frequency spectrum of communications
channels specifically assigned to digital networks), allowing for greater user mobility and
communications efficiency. PCS networks are structured similar to analog networks, with
cells and base stations used to complete communications. PCS breaks down the larger cell
areas used by analog-based networks to form microcells, Microcells, also known as picocells,
allow more cells to inhabit one geographic area, thus providing better network cov-
Base Station
Original Signal Reproduced Signal
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erage by reducing interference and increasing the chance that phone calls will be completed
clearly and without interruption (Fig. 10.4).5
Fig. 10.4 Cell divided into microcells for PCS communications.
There are two types of PCS, narrowband and broadband. Narrowband PCS is used for
two-way paging, credit-card verification, location services, GPS, voice paging and text
paging. Broadband PCS is used in portable computer communications. Both are important
to PCS system and wireless communications. For more information visit www.backofficesystems.
com/tips/hardware/PCS/HTM. Location service and GPS are discussed
in Chapter 3, Location-Identifying Technologies and Location-Based Services
while paging and laptop communications are examined in Chapter 9, Wireless Communications
Technologies (Part I).
Currently, PCS is used in many mobile services networks, such as Sprint’s Sprint
PCSSM service (see Sprint PCS Feature). Sprint’s wireless service is discussed in Chapter
9, Wireless Communications Technologies (Part I). GSM (a technology discussed later in
this section) is one form of a PCS network. Packet Data Cellular (PDC) technology in
Japan uses PCS in conjuction with other network technologies.
Sprint PCS: PCS Mobile Service Network
Sprint PCS (sprintpcs.com) is a network that offers an array of wireless solutions
for both business and personal applications. Code Division Multiple Access (CDMA)
is the main technology driving the PCS network (discussed in Section 10.3.3).
Cell Network
Cells Microcells
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10.3.2 Global System for Mobile Communications (GSM)
The Global System for Mobile Communications (GSM) is a type of PCS digital cellular
communications network. GSM offers data speeds of 9.6-14.4Kbps as compared to other
2G technologies which only offer 9.6Kbps.6
m-Fact 10.1
According to the GSM Association, GSM subscribers worldwide will reach 462 million by
the end of 2001.7
10.1
GSM differs from other technologies described in this chapter because it is considered
a roaming technology—the coverage area of GSM-based networks allows users to complete
wireless communications almost anywhere in the world. Most network providers
establish user calling areas that specify the area or region in which users makes most of
their mobile calls (also known as the home calling area). For example, in the United States
some users can call anywhere in the country for the same amount of money while others
are limited to specific regions. For global communications, roaming areas are established
for users outside their home calling areas. GSM networks can be found worldwide that
Sprint’s Wireless Web Browser, Wireless Web Connection and Wireless Web Messaging
allow users to access information on the Internet through their cell phones.
Sprint’s UP.Browser™ enables users to view Web sites. These sites include Yahoo!
(www.yahoo.com), Amazon.com, Ameritrade (www.ameritrade.com),
MapQuest.com, AOL (www.aol.com), CNN.com, eBay (www.ebay.com) and
others. (Readers with Shockwave software can view a demonstration of the
UP.Browser by visiting www.sprintpcs.com/wireless/
wwbrowsing.html. Shockwave software is available free for download at
www.shockwave.com.)
The Sprint Wireless Web Connection Kit allows customers to use their PCS phones
as modems to connect their laptops to the Internet over the Sprint PCS network. Users
can browse the Web, send and receive e-mail and access electronic schedules. Through
Sprint Wireless Web Updates and Yahoo!, users can view weather information, stock
quotes, current news and forwarded e-mail messages, all delivered directly to their PCS
phones.
Sprint PCS has partnered with several software companies to provide additional
features to corporate users. Customers can use their Sprint PCS phones to connect to
their corporate e-mail accounts, contact lists and calendars. Using Sprint PCS enabled
with Seibel wireless applications (www.seibel.com), salespeople can access
important customer information, check the status of an order or respond to customer
service requests. Sprint PCS phones with PeopleSoft Mobile Company Directory
(www.peoplesoft.com) allow employees to search their companies’ global directories
wirelessly.
Sprint PCS: PCS Mobile Service Network (Cont.)
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allows users to make calls outside their home countries but it usually cost more and coverage
may or may not be reliable. Network developers are undecided between technologies
that offer global-roaming capabilities or always-on connectivity (users are always connected
to the network and are able to receive and send information without re-establishing
a dial-up connection for each transmission).
GSM offers Short Messaging Service (SMS) which allows mobile phones to receive
text messages similar to the paging system first used in the United States. SMS can support
messages140–160 characters in length and has become a popular form of wireless data
communications. SMS is discussed in Chapter 9, Wireless Communications Technologies
(Part I).
The main benefit of GSM-enabled devices is that they can be used in almost any part
of the world. GSM has been accepted as the standard for mobile communications in most
areas of Europe and Asia. In addition, many believe that 2.5G technologies like GPRS and
EDGE (Sections 10.4.2 and 10.4.3 respectively) may enhance GSM to make it one of the
main technologies supporting 3G network development in the future.8
10.3.3 Code Division Multiple Access (CDMA) and CdmaOne™
Code Division Multiple Access (CDMA) technology is one of the more popular standards
both in the United States and internationally. It is used with a variety of networks including
PCS and TDMA. CDMA is a 2G technology standard that is expanding to be a foundation
for 3G networks worldwide.
CDMA uses spread-spectrum technology which allows users to share the same radio
frequencies for wireless communications. Spread-spectrum technology breaks up signals
into smaller segments and spreads them over the entire bandwidth. This allows a larger
volume of users to share the same band and can provide three times as much bandwidth
capacity over any other digital system and up to 20 times that of an analog platform.9 In
addition, each CDMA transmission is assigned to a specific channel, giving the transmission
use of the entire bandwidth within that channel. This reduces the possibility that a connection
will be broken. Spread spectrum technology is also discussed in Chapter 9,
Wireless Communications Technologies (Part I).
CDMA uses packet-switched data transmission technology. Packet-switched data is
data sent and received in packets—groups of data bundled together transmitted over a network.
The packets can be sent over one channel on the network and be received by a different
channel. Packets can be re-routed to other channels if there is congestion. In addition,
packets can be routed around multiple channels to find the most efficient path to complete
the transmission without tying up a channel for an extended period of time. Once a packet
is sent, the channel is free to send or receive more packets for this or other users, increasing
this technology’s effectiveness.
To ensure security, CDMA technology assigns a unique code to each wireless transmission
on the network. The code pertains only to the packet being sent and received and
is not used by any other transmission on the network. The CDMA platform uses less power
to transmit signals which increases talk time and battery life on wireless devices. Interference
and background noise are less common in CDMA than with other systems. CDMA
networks take less time to build than wireline networks or other types of wireless networks
because they require fewer cell sites.10 Security issues are examined in Chapter 6, Secu-
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rity; wireline and wireless networks are discussed in Chapter 9, Wireless Communications
Technologies (Part I).
CdmaOne™ commonly refers to the family of technologies and standards associated
with CDMA used in 2G networks. The Telecommunications Industry Association (TIA), a
voluntary body regulating communications standards in the United States, and the International
Telecommunications Union (ITU), regulate and set standards for international communications.
They both categorize CdmaOne family members as part of the IS-95
standards group, which describes the regulations regarding CDMA technology for 2G systems.
Currently there are two categories of the IS-95 standards group, A and B. Both standards
operate on different network carriers and have different data transmission speeds.11
CDMA is one of the next building blocks for faster and more efficient cellular technologies
in 2.5G and 3G networks because this technology is expandable beyond its current
capabilities. Qualcomm (www.qualcomm.com), one of CDMA’s biggest developers,
has over 75 communication manufacturers worldwide working to expand CDMA technology.
12 In addition, companies like Ericsson (www.ericsson.com) and NTT
DoCoMo are also expanding CDMA resources. These new areas of expansion include WCDMA,
CDMA2000 and BREW which are discussed in later sections of this chapter.
10.3.4 Time Division Multiple Access (TDMA)
Time Division Multiple Access (TDMA) is technology used worldwide with GSM, CDMA
and other network technologies. TDMA is popular in North America, South America and
in some parts of Europe and Asia. TDMA takes multiple calls and assigns each call to a
different time slot on the same radio frequency. Two different technologies are used by
TDMA—Frequency-Division Duplex (FDD) and Time-Division Duplex (TDD). FDD uses
two separate frequency bands. Each station within the frequency bands is assigned two specific
time slots, one for sending and one for receiving signals from the base station. Usually
these slots are staggered (slots do not transmit on the same time schedule) so that each station
can both send and receive messages effectively.
TDD uses one frequency band and all stations take turns communicating over the band
at different times.13 Each station is assigned a time slot and the base station transmits signals
according to the assignments until all transmissions are complete. Once this process is
complete, the base station starts the process over again, sending and receiving signals based
on the time-slot assignments. Both FDD and TDD allow for multiple calls to use the same
frequency simultaneously without interfering with one another. The downside to TDMA is
that it transmits signals at specific time intervals whether or not there is a user is ready to
transmit over the network. This makes TDMA inefficient at times and wastes bandwidth
capacity on unnecessary transmissions. For more information visit www.iec.org/
tutorials/tdma.
10.3.5 Orthogonal Frequency Division Multiplexing (OFDM)
Orthogonal Frequency Division Multiplexing (OFDM) was first used for military and radio
communications in the mid-1960s. Today, it is a new area of development for wireless
communications technologies. We mentioned in Chapter 9, Wireless Communications
Technologies (Part I) that OFDM has become a new technology used in WLAN development.
However, it does have other uses in the development of communications networks.
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OFDM is based on Frequency Division Multiplexing (FDM). FDM divides frequency
channels on the spectrum into smaller channels, or bands, which can be accessed by more
than one person. The system then gives each new channel a different frequency on which
to operate.14 However, when multiple frequencies are used, interference problems occur
due to limited bandwidth. Limited bandwidth is a challenge for those competing for space
on the global spectrum and companies cannot afford to implement transmission methods
that use this space inefficiently.
OFDM improves on the limitations FDM by changing the signal shape and compacting
more signals in one area of bandwidth to reduce the amount of spectrum in use. In addition,
OFDM uses multiple carriers to transmit signals, limiting interference, delay and signal
destruction.15 OFDM can transmit both data and voice and supports speeds of 2–6Mbps.
OFDM, still used for military purposes, is also used for radio and TV broadcast and
DSL connections. Current wireless carriers developing OFDM technology in their networks
include ADB and AT&T Wireless.16 OFDM development is supported by the
OFDM Forum, established in December 1999 (www.ofdm-forum.com). OFDM is used
in the development of new technologies for Wireless LANs and other networks as they
move toward 3G systems and possibly even 4G. This development is in the form of Wideband
OFDM (W-OFDM) and the standards surrounding its development include the
IEEE’s 802.11a and HiperLAN/2 (both are discussed in further detail in Chapter 9, Wireless
Communications Technologies Part I).