The Internet Encyclopedia (Volume 3)

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PCS STANDARDS 821

because fewer base stations mean lower cost (Dorman,

2001; Geier, 2001; Wasi, n.d.).

Voice Coding

All digital systems need to encode the analog waveforms

of speech into a bitstream. The program used for this is

called a codec (coder/decoder), often embedded within a

special chip called a digital signal processor. Many differ-

ent codecs are used in cell phones. The aim is to produce

the lowest possible bit rate while maintaining acceptable

sound quality. Because computing power is increasing

continuously, new phones and networks are capable of

using more advanced compression technology.

Waveform Coding

The simplest way to digitize a sound signal is to sample

the waveform at regular intervals. This is called pulse code

modulation (PCM) and is used by the DC and PSTN codec.

A shorter interval will result in more accurate sampling

but a higher bit rate.

Vocoding

Instead of sending an actual signal, a vocoder calcu-

lates how speech was produced and sends only the rel-

evant pitch and tone information plus a description of

the sender’s mouth movements and vocal tract. A decoder

then synthesizes a voice. Vocoders are used in a very low

data rate situation. They can reach data rates as low as

1 Kbps.

Hybrid Codecs

Most codecs use a mixture of waveform and vocoding,

based on synthesized speech with some PCM information.

The precise bit rate depends on the quality of the sound

and on how much processing power is available. For a

mobile phone, the limiting factor is usually the need to

compress and decompress in real time, using a battery-

operated device. In any codec, there is a tradeoff between

bandwidth and speech quality. Cellular networks typically

use a codec between 5 and 13 Kbps.

PCS STANDARDS

PCS is any digital system that provides high-quality voice

and narrowband data. It is the second generation of wire-

less technology, between analog cell phone and broadband

mobile multimedia.

Three main categories of PCS are in use today. The first

is the digital cell phones, which offer high-quality voice

and limited data, the second category of PCS concentrates

on data only, and the third category uses noncellular tech-

nologies in emergency services and large businesses. It

works without a base station and supports automatic con-

ference calls.

The largest category of PCS is used for digital cell

phone networks. Although these were all designed primar-

ily for voice, they can also support data transmission at

varying rate. Most PCS systems are based on TDMA (time

division multiple access), because FDMA (frequency di-

vision multiple access) is too wasteful of bandwidth, and

CDMA (code division multiple access) had not yet been

invented at the time they were standardized. The PCS sys-

tem centers on spectral efficiency by how much capacity

a system can squeeze out of its allocated frequencies.

GSM (Global System for Mobile

Communication)

Global system for mobile communication is an open, non-

proprietary system that is constantly evolving. One of its

great strengths is the international roaming capability. It

is used by more than half of all mobile phones. It gives

consumers seamless and the same standardized contact

ability in more than 170 countries. GSM satellite roam-

ing has extended service access to areas where terrestrial

coverage is not available.

What is technically distinctive about the technology is

that GSM differs from first generation wireless systems in

that it uses digital technology and time division multiple

access transmission methods. Voice is digitally encoded

via a unique encoder, which emulates the characteristics

of human speech. This method of transmission permits a

very efficient data rate/information content ratio.

The system operates on four different frequency

bands. It was originally designed for frequencies around

900 MHz, to reuse the spectrum intended for Europe’s

analog TACS networks. It was later adapted to 1,800 MHz,

licensed in Europe specifically for GSM, and then to

1,900 MHz, used in America for several different digital

networks. Like other digital cellular technologies, GSM

encodes data into waves using a form of phase modula-

tion, a system that uses the different parts of a waveform

to represent information. The precise type is known as

GMSK, which achieves a symbol rate and data rate of

270.8 Kbps in each of its 200-KHz channels.

HSCSD (High-Speed Circuit-Switched Data)

HSCSD (high-speed circuit-switched data) is a very sim-

ple upgrade to GSM that gives each user more than one

time slot in the multiplex. All HSCSD-capable networks

use the enhanced data codec, so that each channel allows

a rate of 14.4 Kbps. The standard allows up to four of

these to be tied together, for a maximum of 57.6 Kbps. Be-

cause of the overheating problem, most HSCSD devices

are asymmetric, allowing greater download than upload

speed.

GPRS (General Packet Radio Service)

GPRS (general packet radio service) is the one most pop-

ular among operators of all the wireless Internet schemes.

Designed for data, it promises to give every user a perma-

nent and high-capacity connection to the Internet. GPRS

represents a major step forward in mobile networks. Its

key improvement is packet switching, which for most data

application is more efficient than circuit switching. Packet

switching uses bandwidth only when needed, freeing up

gaps in the data stream for other uses. Because of this

greater efficiency, the full specification calls for terminal

scapable of using all eight time slots at once. The plan

is that GPRS networks will eventually carry voice over

packet, using a variable rate codec so the data can be

transmitted during gaps in conversation.