D-AMPS

D-AMPS, singketan tina kecap-kecap basa Inggris Digital Advanced Mobile Phone System, nyaéta wangun digital tina téknologi AMPS. D-AMPS mangrupa téknologi telepon mobil generasi kadua (2G). Conto téknologi ieu nyaéta IS-54 jeung IS-136nu dipaké di sakuliah buana Amérika, hususna di Amérika Sarikat jeung Kanada. D-AMPS kasilih ku téknologi GSM/GPRS jeung CDMA2000. D-AMPS mindeng disebut sabagé TDMA, singketan tina time division multiple access, nyaéta téknik multi aksés anu umum dipaké ku réa protokol kaasup GSM, ogé IS-54 jeung IS-136. Sanajan kitu, D-AMPS bersaing jeung GSM sarta sistem-sistem nu dumasar kana code division multiple access (CDMA), sanajan ahirna éléh ku téknologi GSM/GPRS jeung CDMA2000.

D-AMPS maké kanal-kanal AMPS nu aya sahingga ieu téknologi bisa jadi jambatan (cukang) antara téknologi-téknologi analog jeung digital di wilayah-wilayah anu masih ngagunakeun ieu dua téknologi sahingga nawarkeun transisi anu lemes ti sistem analog ka digital. D-AMPS leuwih unggul manan AMPS dina hal kapasitas ku cara ngabagi unggal kanal nu rubakna 30 kHz kana tilu waktu (time slot) sarta mangpetkeun data sora sacara digital, nu ningkatkeun kapasitasna tilu kali lipet dina unggal sélna. D-AMPS ogé leuwih aman lantara scanner analog henteu bisa nyadap sinyal digital. Dina ieu téknologi, sora diénkripsi (disandikeun), sanajan algoritma nu digunakeunana (CMEA) ka dieukeun katohyan henteu pati kuat.  

IS-136 nambahkeun sajumlah fitur kana spesifikasi IS-54, kaasup SMS, circuit switched data (CSD), jeung protokol komprési nu geus dihadéan.  SMS jeung CSD duanana sadia sabagé bagian tina protokol GSM; IS-136 ngagunakeunna dina cara nu ampir sarua.

Jaringan IS-136 nu gedé kaasup di antarana AT&T jeung U.S. Cellular di Amérika Sarikat, jeung Rogers Wireless di Kanada.  AT&T jeung Rogers Wireless geus ngahadéan jaringan IS-136 bogana supaya jadi GSM/GPRS, samentara US Cellular migrasi kana téknologi anu panglobana digunakeun ku manéhna nyaéta CDMA2000.  Rogers Wireless mindahkeun sakabéh frékuénsi IS-136 dina wilayah 1900 MHz dina taun 2003, ogé dina wilayah 800 MHz. Rogers ngaeureunkeun jaringan IS-136 bogana (babarengan jeung AMPS) dina tanggal 31 Méi 2007.

IS-54 mangrupa sistem komunikasi mobil munggaran nu boga standar pikeun kaamanan sarta nu munggaran maké téknologi TDMA.

Panganteur

IS-54 stands for Interim Standard-54, which is a mobile communication standard employing digital technology.  It was standardized by Electronic Industries Alliance (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.

Sajarah
                                     
The evolution of mobile communication has been almost wholly in 3 different géographic regions. The standards that were born in these regions were quite independent. The 3 regions are North America, Europe and Japan. The éarlier 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 Européan 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 Telephones/450 (NMT450) and NMT900 (both in Nordic countries), NMT-F (French version of NMT900), Radiocom 2000 (RC2000) (in France), and Total Access Communication System(TACS) (in the United Kingdom). 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, Européan engineering efforts were divided among the various standards, and the Japanese standards didn’t 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 incréasing 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 méant éach cell site would transmit on different frequencies, allowing many cell sites to be built néar éach other.

However, AMPS had many disadvantages too. Primarily, it didn’t have the potential to support the incréasing demand for mobile communication usage. éach cell site did not have much capacity for carrying higher numbers of calls. It also had a poor security system which allowed péople to stéal a phone's serial code to use for making illegal calls. All of these triggered  the séarch 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 incréased capacity by cutting down voice channels from 30 kHz to 10 kHz. IS-54, on the other hand, incréased capacity by digital méans 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 couldn't access IS-54's new féatures. IS-54 also supported authentication, a help in preventing fraud.

Panerapan téknologi
                                      
IS-54 employs the same 30 kHz channel spacing and frequency bands (824-849 and 869-894 MHz) as AMPS. Capacity was incréased over the preceding analog design by dividing éach 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 éach of the 30 kHz AMPS channels into 3 full-rate TDMA channels, éach of which is capable of supporting a single voice call. Later, éach of these full-rate channels was further sub-divided into two half-rate channels, éach 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. éach frame has six time slots of 6.67-ms duration. éach 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 overhéad; 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 éach of the six time slots making up the frame, thereby allowing éach receiver to synchronize to its own préassigned 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. éach base station has its own préassigned 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 linéar modulation method is the power inefficiency, which translates into a héavier hand-held portable and, even more inconvenient, a shorter time between battery recharges.

IS-54 security féatures is also a matter of interest as it was the first standard to specify some security méasures. IS-54 uses the CAVE (Cellular Authentication, Voice Privacy and Encryption) algorithm for authentication and the CMéa (Cellular Message Encryption Algorithm) for encryption.

The technical specifications can be summarized as below:

Call processing
A conversation's data bits makes up the DATA field. Six slots maké up a complete IS-54 frame. DATA in slots 1 and 4, 2 and 5, and 3 and 6 maké up a voice circuit. DVCC stands for digital verification color code, arcane terminology for a unique 8-bit code value assigned to éach cell. G méans guard time, the period between éach time slot. RSVD stands for reserved. SYNC represents synchronization, a critical TDMA data field. éach 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 (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 éach 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 won’t 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 spéaker. 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 incréased 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 féatures 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, the largest US carrier to support D-AMPS (which it refers to as "TDMA"), had turned down its existing network in order to reléase the spectrum to its GSM and UMTS platforms in 19 wireless markets, which started on May 30, 2007, with other aréas 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 aréas where service was provided by Dobson Communications.  The Dobson TDMA and AMPS network is expected to be shut down by March of 2008.

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.

Rujukan
 History of mobile communication:1990's 
 Paper on CMEA 
 Cingular Wireless: "TDMA Service Sunset"
 Edge Wireless: "TDMA Network Changing to GSM/GPRS"

Tumbu kaluar
 Timeslot structure for IS-136 and IS-54 

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