Digital AMPS
Encyclopedia
IS-54 and IS-136 are second-generation (2G
) mobile phone systems, known as Digital AMPS (D-AMPS). It was once prevalent throughout the Americas
, particularly in the United States
and Canada
. D-AMPS is considered end-of-life, and existing networks have mostly been replaced by GSM
/GPRS
or CDMA2000
technologies.
This system is most often referred to as TDMA. That name is based on the acronym for time division multiple access
, a common multiple access technique which is used by multiple protocols, including GSM, as well as in IS-54 and IS-136. However, D-AMPS has been competing against GSM and systems based on code division multiple access
(CDMA) for adoption by the network carriers, although it is now being phased out in favor of GSM/GPRS and CDMA2000 technology.
D-AMPS uses existing AMPS
channels and allows for smooth transition between digital and analog systems in the same area. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots (hence time division) and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals. Calls were encrypted, although the algorithm used (CMEA
) was later found to be weak.
IS-136 added a number of features to the original IS-54 specification, including text messaging
, circuit switched data
(CSD), and an improved compression protocol. SMS and CSD were both available as part of the GSM protocol, and IS-136 implemented them in a nearly identical fashion.
Former large IS-136 networks included AT&T in the United States, and Rogers Wireless
in Canada. AT&T and Rogers Wireless have upgraded their existing IS-136 networks to GSM/GPRS. Rogers Wireless removed all 1900 MHz IS-136 in 2003, and has done the same with its 800 MHz spectrum as the equipment failed. Rogers deactivated its IS-136 network (along with AMPS) on May 31, 2007. AT&T soon followed in February 2008, shutting down both TDMA and AMPS.
Alltel
, who primarily uses CDMA2000
technology but acquired a TDMA network from Western Wireless, shut down its TDMA and AMPS networks in September 2008. US Cellular, which now also primarily uses CDMA2000
technology, shut down its TDMA network in February 2009.
IS-54 is the first mobile communication system which had provision for security, and the first to employ TDMA technology.
(EIA) and Telecommunications Industry Association
(TIA) together. It later became an American National Standard when it got approved by the American National Standards Institute
(ANSI). When an interim standard becomes an American National Standard, the IS designator is dropped. The ANSI designation of IS-54 is ANSI/TIA/EIA-627, but this standard is still popularly referred to as IS-54.
IS-54 maintains compatibility with Advanced Mobile Phone System
(AMPS) in many ways. It is a digital extension of AMPS and so it is also quite widely known as Digital AMPS (D-AMPS). Another name for IS-54 is United States Digital Cellular (USDC). But sometimes D-AMPS and USDC also refers to the other prominent interim standard IS-136.
The evolution of mobile communication has been almost wholly in 3 different geographic regions. The standards that were born in these regions were quite independent. The 3 regions are North America
, Europe
and Japan
. The earlier mobile or wireless technologies were wholly analog and are collectively known as 1st Generation (1G
) technologies. In Japan, the 1G standards were Nippon Telegraph and Telephone
(NTT) and the high capacity version of it (Hicap
). The European systems were not common and the ‘European Union’ viewpoint that is visible in the later technologies was absent. Various 1G standards that were in use in Europe include C-Netz
(in Germany and Austria), Comviq (in Sweden), Nordic Mobile Telephone
s/450 (NMT450) and NMT900 (both in Nordic countries), NMT-F (French version of NMT900), Radiocom 2000 (RC2000) (in France), and TACS
(Total Access Communication System) (in the United Kingdom
and Ireland
). North American standards were Advanced Mobile Phone System
(AMPS) and Narrow-band AMPS (N-AMPS).
Out of the 1G standards, the most successful was the AMPS system. Despite the Nordic countries
' cooperation, European engineering efforts were divided among the various standards, and the Japanese standards did not get much attention. Developed by Bell Labs
in the 1970s and first used commercially in the United States
in 1983, AMPS operates in the 800 MHz band in the United States and is the most widely distributed analog cellular standard. (The 1900 MHz PCS
band, established in 1994, is for digital operation only.) The success of AMPS kick-started the mobile age in the North America.
The market showed an increasing demand because it had higher capacity and mobility than the then existing mobile communication standards. For instance, the Bell Labs system in the 1970s could carry only 12 calls at a time throughout all of New York City
. AMPS used Frequency Division Multiple Access FDMA which meant each cell site would transmit on different frequencies, allowing many cell sites to be built near each other.
However, AMPS had many disadvantages too. Primarily, it did not have the potential to support the increasing demand for mobile communication usage. Each cell site did not have much capacity for carrying higher numbers of calls. It also had a poor security system which allowed people to steal a phone's serial code to use for making illegal calls. All of these triggered the search for a more capable system.
The quest resulted in IS-54, the first American 2G standard. In March 1990, the North American cellular network incorporated the IS-54B standard, the first North American dual mode digital cellular standard. This standard won over Motorola
's Narrowband AMPS or N-AMPS, an analog scheme that increased capacity by cutting down voice channels from 30 kHz to 10 kHz. IS-54, on the other hand, increased capacity by digital means using TDMA
protocols. This method separates calls by time, placing parts of individual conversations on the same frequency, one after the next. TDMA tripled call capacity.
Using IS-54, a cellular carrier could convert any of its system's analog
voice channels to digital
. A dual mode phone uses digital channels where available and defaults to regular AMPS where they are not. IS-54 was, in fact, backward compatible with analog cellular and indeed co-exists on the same radio channels as AMPS. No analog customers were left behind; they simply could not access IS-54's new features. IS-54 also supported authentication
, a help in preventing fraud.
IS-54 employs the same 30 kHz channel spacing and frequency bands (824-849 and 869-894 MHz) as AMPS. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals.
The IS-54 standard specifies 84 control channels, 42 of which are shared with AMPS. To maintain compatibility with the existing AMPS cellular telephone system, the primary forward and reverse control channels in IS-54 cellular systems use the same signaling techniques and modulation scheme (binary FSK) as AMPS. An AMPS/IS-54 infrastructure can support use of either analog AMPS phones or D-AMPS phones.
The access method used for IS-54 is Time Division Multiple Access (TDMA), which was the first U.S. digital standard to be developed. It was adopted by the TIA in 1992. TDMA subdivides each of the 30 kHz AMPS channels into 3 full-rate TDMA channels, each of which is capable of supporting a single voice call. Later, each of these full-rate channels was further sub-divided into two half-rate channels, each of which, with the necessary coding and compression, could also support a voice call. Thus, TDMA could provide 3 to 6 times the capacity of AMPS traffic channels. Time Division Multiple Access or TDMA was initially defined by the IS-54 standard and is now specified in the IS-13x series of specifications of the EIA/TIA.
The channel transmission bit rate for digitally modulating the carrier is 48.6 kbit/s. Each frame has six time slots of 6.67-ms duration. Each time slot carries 324 bits of information, of which 260 bits are for the 13-kbit/s full-rate traffic data. The other 64 bits are overhead; 28 of these are for synchronization, and they contain a specific bit sequence known by all receivers to establish frame alignment. Also, as with GSM, the known sequence acts as a training pattern to initialize an adaptive equalizer.
The IS-54 system has different synchronization sequences for each of the six time slots making up the frame, thereby allowing each receiver to synchronize to its own preassigned time slots. An additional 12 bits in every time slot are for the SACCH (i.e., system control information). The digital verification color code (DVCC) is the equivalent of the supervisory audio tone used in the AMPS system. There are 256 different 8-bit color codes, which are protected by a (12, 8, 3) Hamming code. Each base station has its own preassigned color code, so any incoming interfering signals from distant cells can be ignored.
The modulation scheme for IS-54 is 7C/4 differential quaternary phase shift keying (DQPSK), otherwise known as differential 7t/4 4-PSK or π/4 DQPSK. This technique allows a bit rate of 48.6 kbit/s with 30 kHz channel spacing, to give a bandwidth efficiency of 1.62 bit/s/Hz. This value is 20% better than GSM. The major disadvantage with this type of linear modulation method is the power inefficiency, which translates into a heavier hand-held portable and, even more inconvenient, a shorter time between battery recharges.
IS-54 security features is also a matter of interest as it was the first standard to specify some security measures. IS-54 uses the CAVE (Cellular Authentication, Voice Privacy and Encryption) algorithm for authentication and the CMEA (Cellular Message Encryption Algorithm) for encryption.
The technical specifications can be summarized as below:
Time slots for the mobile-to-base direction are constructed differently from the base-to-mobile direction. They essentially carry the same information but are arranged differently. Notice that the mobile-to-base direction has a 6-bit ramp time to enable its transmitter time to get up to full power, and a 6-bit guard band during which nothing is transmitted. These 12 extra bits in the base-to-mobile direction are reserved for future use.
Once a call comes in the mobile switches to a different pair of frequencies; a voice radio channel which the system carrier has made analog or digital. This pair carries the call. If an IS-54 signal is detected it gets assigned a digital traffic channel if one is available. The fast associated channel or FACCH performs handoffs during the call, with no need for the mobile to go back to the control channel. In case of high noise FACCH, embedded within the digital traffic channel overrides the voice payload, degrading speech quality to convey control information. The purpose is to maintain connectivity. The slow associated control channel or SACCH does not perform handoffs but conveys things like signal strength information to the base station.
The IS-54 speech coder uses the technique called vector sum excited linear prediction
(VSELP) coding. This is a special type of speech coder within a large class known as code-excited linear prediction
(CELP) coders. The speech coding rate of 7.95 kbit/s achieves a reconstructed speech quality similar to that of the analog AMPS system using frequency modulation. The 7.95-kbit/s signal is then passed through a channel coder that loads the bit rate up to 13 kbit/s. The new half-rate coding standard reduces the overall bit rate for each call to 6.5 kbit/s, and should provide comparable quality to the 13-kbit/s rate. This half-rate gives a channel capacity six times that of analog AMPS.
A dual-mode cellular phone consists of the following:
On May 31, 2007 Rogers Wireless
decommissioned its D-AMPS and AMPS networks and moved the remaining customers on these older networks onto its GSM network.
Alltel
has completed their shutdown of their D-AMPS and AMPS networks in September 2008.
2G
2G is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland by Radiolinja in 1991...
) mobile phone systems, known as Digital AMPS (D-AMPS). It was once prevalent throughout the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
, particularly in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. D-AMPS is considered end-of-life, and existing networks have mostly been replaced by GSM
Global System for Mobile Communications
GSM , is a standard set developed by the European Telecommunications Standards Institute to describe technologies for second generation digital cellular networks...
/GPRS
General Packet Radio Service
General packet radio service is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications . GPRS was originally standardized by European Telecommunications Standards Institute in response to the earlier CDPD and i-mode...
or CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
technologies.
This system is most often referred to as TDMA. That name is based on the acronym for time division multiple access
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, a common multiple access technique which is used by multiple protocols, including GSM, as well as in IS-54 and IS-136. However, D-AMPS has been competing against GSM and systems based on code division multiple access
Code division multiple access
Code division multiple access is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000 and WCDMA , which are often referred to as simply CDMA, and use CDMA as an underlying channel access...
(CDMA) for adoption by the network carriers, although it is now being phased out in favor of GSM/GPRS and CDMA2000 technology.
D-AMPS uses existing AMPS
Advanced Mobile Phone System
Advanced Mobile Phone System was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas in 1983, Israel in 1986, and Australia in 1987. It was the primary analog mobile phone system in North America through the 1980s and into the 2000s...
channels and allows for smooth transition between digital and analog systems in the same area. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots (hence time division) and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals. Calls were encrypted, although the algorithm used (CMEA
Cellular Message Encryption Algorithm
In cryptography, the Cellular Message Encryption Algorithm is a block cipher which was used for securing mobile phones in the United States. CMEA is one of four cryptographic primitives specified in a Telecommunications Industry Association standard, and is designed to encrypt the control...
) was later found to be weak.
IS-136 added a number of features to the original IS-54 specification, including text messaging
Short message service
Short Message Service is a text messaging service component of phone, web, or mobile communication systems, using standardized communications protocols that allow the exchange of short text messages between fixed line or mobile phone devices...
, circuit switched data
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
(CSD), and an improved compression protocol. SMS and CSD were both available as part of the GSM protocol, and IS-136 implemented them in a nearly identical fashion.
Former large IS-136 networks included AT&T in the United States, and Rogers Wireless
Rogers Wireless
Rogers Wireless is a wireless telecommunications provider offering mobile phone and data services throughout Canada using Global System for Mobile Communications and Universal Mobile Telecommunications System technology. It is a wholly owned subsidiary of Rogers Communications...
in Canada. AT&T and Rogers Wireless have upgraded their existing IS-136 networks to GSM/GPRS. Rogers Wireless removed all 1900 MHz IS-136 in 2003, and has done the same with its 800 MHz spectrum as the equipment failed. Rogers deactivated its IS-136 network (along with AMPS) on May 31, 2007. AT&T soon followed in February 2008, shutting down both TDMA and AMPS.
Alltel
Alltel
Alltel Corporation is a wireless service provider, primarily based in the United States. Before an acquisition by Verizon Wireless, it served 34 states. After the merger, Alltel continues to serve six states, mostly in rural areas...
, who primarily uses CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
technology but acquired a TDMA network from Western Wireless, shut down its TDMA and AMPS networks in September 2008. US Cellular, which now also primarily uses CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
technology, shut down its TDMA network in February 2009.
IS-54 is the first mobile communication system which had provision for security, and the first to employ TDMA technology.
Introduction
IS-54 stands for Interim Standard-54, which is a mobile communication standard employing digital technology. It was standardized by Electronic Industries AllianceElectronic Industries Alliance
The Electronic Industries Alliance was a standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They developed standards to ensure the equipment of different manufacturers was compatible and interchangeable...
(EIA) and Telecommunications Industry Association
Telecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies...
(TIA) together. It later became an American National Standard when it got approved by the American National Standards Institute
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
(ANSI). When an interim standard becomes an American National Standard, the IS designator is dropped. The ANSI designation of IS-54 is ANSI/TIA/EIA-627, but this standard is still popularly referred to as IS-54.
IS-54 maintains compatibility with Advanced Mobile Phone System
Advanced Mobile Phone System
Advanced Mobile Phone System was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas in 1983, Israel in 1986, and Australia in 1987. It was the primary analog mobile phone system in North America through the 1980s and into the 2000s...
(AMPS) in many ways. It is a digital extension of AMPS and so it is also quite widely known as Digital AMPS (D-AMPS). Another name for IS-54 is United States Digital Cellular (USDC). But sometimes D-AMPS and USDC also refers to the other prominent interim standard IS-136.
History
The evolution of mobile communication has been almost wholly in 3 different geographic regions. The standards that were born in these regions were quite independent. The 3 regions are North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. The earlier mobile or wireless technologies were wholly analog and are collectively known as 1st Generation (1G
1G
1G refers to the first-generation of wireless telephone technology, mobile telecommunications. These are the analog telecommunications standards that were introduced in the 1980s and continued until being replaced by 2G digital telecommunications...
) technologies. In Japan, the 1G standards were Nippon Telegraph and Telephone
Nippon Telegraph and Telephone
, commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. Ranked the 31st in Fortune Global 500, NTT is the largest telecommunications company in Asia, and the second-largest in the world in terms of revenue....
(NTT) and the high capacity version of it (Hicap
Hicap
Hicap is a mobile technology developed by Nippon Telegraph and Telephone as a higher capacity alternative to their NTT mobile solution.Hicap uses a 25 kHz carrier and utilizes FDMA to separate different calls from each other.-External links:...
). The European systems were not common and the ‘European Union’ viewpoint that is visible in the later technologies was absent. Various 1G standards that were in use in Europe include C-Netz
C-Netz
The Radio Telephone Network C , was a first generation analog cellular phone system deployed and operated in Germany by DeTeMobil...
(in Germany and Austria), Comviq (in Sweden), Nordic Mobile Telephone
Nordic Mobile Telephone
NMT is the first fully automatic cellular phone system...
s/450 (NMT450) and NMT900 (both in Nordic countries), NMT-F (French version of NMT900), Radiocom 2000 (RC2000) (in France), and TACS
Total Access Communication System
Total Access Communication System and ETACS are mostly-obsolete variants of AMPS which were used in some European countries . TACS was also used in Japan under the name Japanese Total Access Communication . It was also used in Hong Kong...
(Total Access Communication System) (in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
and Ireland
Republic of Ireland
Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
). North American standards were Advanced Mobile Phone System
Advanced Mobile Phone System
Advanced Mobile Phone System was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas in 1983, Israel in 1986, and Australia in 1987. It was the primary analog mobile phone system in North America through the 1980s and into the 2000s...
(AMPS) and Narrow-band AMPS (N-AMPS).
Out of the 1G standards, the most successful was the AMPS system. Despite the Nordic countries
Nordic countries
The Nordic countries make up a region in Northern Europe and the North Atlantic which consists of Denmark, Finland, Iceland, Norway and Sweden and their associated territories, the Faroe Islands, Greenland and Åland...
' cooperation, European engineering efforts were divided among the various standards, and the Japanese standards did not get much attention. Developed by Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
in the 1970s and first used commercially in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
in 1983, AMPS operates in the 800 MHz band in the United States and is the most widely distributed analog cellular standard. (The 1900 MHz PCS
Personal Communications Service
At the most basic level Personal Communications Service or PCS describes a set of wireless communications capabilities that allows some combination of terminal mobility, personal mobility, and service profile management...
band, established in 1994, is for digital operation only.) The success of AMPS kick-started the mobile age in the North America.
The market showed an increasing demand because it had higher capacity and mobility than the then existing mobile communication standards. For instance, the Bell Labs system in the 1970s could carry only 12 calls at a time throughout all of New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
. AMPS used Frequency Division Multiple Access FDMA which meant each cell site would transmit on different frequencies, allowing many cell sites to be built near each other.
However, AMPS had many disadvantages too. Primarily, it did not have the potential to support the increasing demand for mobile communication usage. Each cell site did not have much capacity for carrying higher numbers of calls. It also had a poor security system which allowed people to steal a phone's serial code to use for making illegal calls. All of these triggered the search for a more capable system.
The quest resulted in IS-54, the first American 2G standard. In March 1990, the North American cellular network incorporated the IS-54B standard, the first North American dual mode digital cellular standard. This standard won over Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
's Narrowband AMPS or N-AMPS, an analog scheme that increased capacity by cutting down voice channels from 30 kHz to 10 kHz. IS-54, on the other hand, increased capacity by digital means using TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
protocols. This method separates calls by time, placing parts of individual conversations on the same frequency, one after the next. TDMA tripled call capacity.
Using IS-54, a cellular carrier could convert any of its system's analog
Analogue electronics
Analogue electronics are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two different levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represents the signal...
voice channels to digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
. A dual mode phone uses digital channels where available and defaults to regular AMPS where they are not. IS-54 was, in fact, backward compatible with analog cellular and indeed co-exists on the same radio channels as AMPS. No analog customers were left behind; they simply could not access IS-54's new features. IS-54 also supported authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
, a help in preventing fraud.
Technology specifications
IS-54 employs the same 30 kHz channel spacing and frequency bands (824-849 and 869-894 MHz) as AMPS. Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots and digitally compressing the voice data, yielding three times the call capacity in a single cell. A digital system also made calls more secure because analog scanners could not access digital signals.
The IS-54 standard specifies 84 control channels, 42 of which are shared with AMPS. To maintain compatibility with the existing AMPS cellular telephone system, the primary forward and reverse control channels in IS-54 cellular systems use the same signaling techniques and modulation scheme (binary FSK) as AMPS. An AMPS/IS-54 infrastructure can support use of either analog AMPS phones or D-AMPS phones.
The access method used for IS-54 is Time Division Multiple Access (TDMA), which was the first U.S. digital standard to be developed. It was adopted by the TIA in 1992. TDMA subdivides each of the 30 kHz AMPS channels into 3 full-rate TDMA channels, each of which is capable of supporting a single voice call. Later, each of these full-rate channels was further sub-divided into two half-rate channels, each of which, with the necessary coding and compression, could also support a voice call. Thus, TDMA could provide 3 to 6 times the capacity of AMPS traffic channels. Time Division Multiple Access or TDMA was initially defined by the IS-54 standard and is now specified in the IS-13x series of specifications of the EIA/TIA.
The channel transmission bit rate for digitally modulating the carrier is 48.6 kbit/s. Each frame has six time slots of 6.67-ms duration. Each time slot carries 324 bits of information, of which 260 bits are for the 13-kbit/s full-rate traffic data. The other 64 bits are overhead; 28 of these are for synchronization, and they contain a specific bit sequence known by all receivers to establish frame alignment. Also, as with GSM, the known sequence acts as a training pattern to initialize an adaptive equalizer.
The IS-54 system has different synchronization sequences for each of the six time slots making up the frame, thereby allowing each receiver to synchronize to its own preassigned time slots. An additional 12 bits in every time slot are for the SACCH (i.e., system control information). The digital verification color code (DVCC) is the equivalent of the supervisory audio tone used in the AMPS system. There are 256 different 8-bit color codes, which are protected by a (12, 8, 3) Hamming code. Each base station has its own preassigned color code, so any incoming interfering signals from distant cells can be ignored.
The modulation scheme for IS-54 is 7C/4 differential quaternary phase shift keying (DQPSK), otherwise known as differential 7t/4 4-PSK or π/4 DQPSK. This technique allows a bit rate of 48.6 kbit/s with 30 kHz channel spacing, to give a bandwidth efficiency of 1.62 bit/s/Hz. This value is 20% better than GSM. The major disadvantage with this type of linear modulation method is the power inefficiency, which translates into a heavier hand-held portable and, even more inconvenient, a shorter time between battery recharges.
IS-54 security features is also a matter of interest as it was the first standard to specify some security measures. IS-54 uses the CAVE (Cellular Authentication, Voice Privacy and Encryption) algorithm for authentication and the CMEA (Cellular Message Encryption Algorithm) for encryption.
The technical specifications can be summarized as below:
| Mobile Frequency Range | Rx: 869-894 MHz; Tx: 824-849 MHz |
| Multiple Access Method | TDMA/FDM |
| Duplex Method | FDD |
| Number of Channels | 832 (3 users per channel) |
| Channel Spacing/Bandwidth | 30 kHz |
| Modulation | π/4 DQPSK |
| Channel Bit Rate | 48.6 kbit/s |
| Spectrum Efficiency | 1.62 bit/s/Hz |
| Equalizer | Unspecified |
| Interleaving | 2 slot interleaver |
Call processing
A conversation's data bits makes up the DATA field. Six slots make up a complete IS-54 frame. DATA in slots 1 and 4, 2 and 5, and 3 and 6 make up a voice circuit. DVCC stands for digital verification color code, arcane terminology for a unique 8-bit code value assigned to each cell. G means guard time, the period between each time slot. RSVD stands for reserved. SYNC represents synchronization, a critical TDMA data field. Each slot in every frame must be synchronized against all others and a master clock for everything to work.Time slots for the mobile-to-base direction are constructed differently from the base-to-mobile direction. They essentially carry the same information but are arranged differently. Notice that the mobile-to-base direction has a 6-bit ramp time to enable its transmitter time to get up to full power, and a 6-bit guard band during which nothing is transmitted. These 12 extra bits in the base-to-mobile direction are reserved for future use.
Once a call comes in the mobile switches to a different pair of frequencies; a voice radio channel which the system carrier has made analog or digital. This pair carries the call. If an IS-54 signal is detected it gets assigned a digital traffic channel if one is available. The fast associated channel or FACCH performs handoffs during the call, with no need for the mobile to go back to the control channel. In case of high noise FACCH, embedded within the digital traffic channel overrides the voice payload, degrading speech quality to convey control information. The purpose is to maintain connectivity. The slow associated control channel or SACCH does not perform handoffs but conveys things like signal strength information to the base station.
The IS-54 speech coder uses the technique called vector sum excited linear prediction
Vector Sum Excited Linear Prediction
Vector sum excited linear prediction is a speech coding method used in several cellular standards. The VSELP algorithm is an analysis-by-synthesis coding technique and belongs to the class of speech coding algorithms known as CELP .Variations of this codec have been used in several 2G cellular...
(VSELP) coding. This is a special type of speech coder within a large class known as code-excited linear prediction
Code Excited Linear Prediction
Code-excited linear prediction is a speech coding algorithm originally proposed by M.R. Schroeder and B.S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algorithms, such as residual-excited linear prediction and linear predictive coding vocoders...
(CELP) coders. The speech coding rate of 7.95 kbit/s achieves a reconstructed speech quality similar to that of the analog AMPS system using frequency modulation. The 7.95-kbit/s signal is then passed through a channel coder that loads the bit rate up to 13 kbit/s. The new half-rate coding standard reduces the overall bit rate for each call to 6.5 kbit/s, and should provide comparable quality to the 13-kbit/s rate. This half-rate gives a channel capacity six times that of analog AMPS.
System example
The discussion of a communication system will not be complete without the explanation of a system example. A dual-mode cellular phone as specified by the IS-54 standard is explained. A dual-mode phone is capable of operating in an analog-only cell or a dual-mode cell. Both the transmitter and the receiver support both analog FM and digital time division multiple access (TDMA) schemes. Digital transmission is preferred, so when a cellular system has digital capability, the mobile unit is assigned a digital channel first. If no digital channels are available, the cellular system will assign an analog channel. The transmitter converts the audio signal to a radio frequency (RF), and the receiver converts an RF signal to an audio signal. The antenna focuses and converts RF energy for reception and transmission into free space. The control panel serves as an input/output mechanism for the end user; it supports a keypad, a display, a microphone, and a speaker. The coordinator synchronizes the transmission and receives functions of the mobile unit.A dual-mode cellular phone consists of the following:
- Transmitter
- Antenna assembly
- Receiver
- Control panel
- Coordinator
Successor technologies
By 1993 American cellular was again running out of capacity, despite a wide movement to IS-54. The American cellular business continued booming. Subscribers grew from one and a half million customers in 1988 to more than thirteen million subscribers in 1993. Room existed for other technologies to cater to the growing market. The technologies that followed IS-54 stuck to the digital backbone laid down by it.IS-136
A pragmatic effort was launched to improve IS-54 that eventually added an extra channel to the IS-54 hybrid design. Unlike IS-54, IS-136 utilizes time division multiplexing for both voice and control channel transmissions. Digital control channel allows residential and in-building coverage, dramatically increased battery standby time, several messaging applications, over the air activation and expanded data applications. IS-136 systems needed to support millions of AMPS phones, most of which were designed and manufactured before IS-54 and IS-136 were considered. IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD), and an improved compression protocol. IS-136 TDMA traffic channels use π/4-DQPSK modulation at a 24.3-kbaud channel rate and gives an effective 48.6 kbit/s data rate across the six time slots comprising one frame in the 30 kHz channel.Sunset for D-AMPS in the US and Canada
AT&T Mobility, the largest US carrier to support D-AMPS (which it refers to as "TDMA"), had turned down its existing network in order to release the spectrum to its GSM and UMTS platforms in 19 wireless markets, which started on May 30, 2007, with other areas that followed in June and July. The TDMA network in these markets operated on the 1900 MHz frequency and did not coexist with an AMPS network. Service on the remaining 850 MHz TDMA markets was discontinued along with AMPS service on February 18, 2008, except for in areas where service was provided by Dobson Communications. The Dobson TDMA and AMPS network was shut down March 1, 2008.On May 31, 2007 Rogers Wireless
Rogers Wireless
Rogers Wireless is a wireless telecommunications provider offering mobile phone and data services throughout Canada using Global System for Mobile Communications and Universal Mobile Telecommunications System technology. It is a wholly owned subsidiary of Rogers Communications...
decommissioned its D-AMPS and AMPS networks and moved the remaining customers on these older networks onto its GSM network.
Alltel
Alltel
Alltel Corporation is a wireless service provider, primarily based in the United States. Before an acquisition by Verizon Wireless, it served 34 states. After the merger, Alltel continues to serve six states, mostly in rural areas...
has completed their shutdown of their D-AMPS and AMPS networks in September 2008.