Digital AMPS
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Template:Table Mobile phone standards IS-54 and IS-136 are second-generation (2G) mobile phone systems, known as Digital AMPS (D-AMPS). It is used throughout the Americas, particularly in the United States and Canada. D-AMPS is considered end-of-life, and existing networks are in the process of being replaced by GSM/GPRS and CDMA2000 technologies. Although most often referred to as TDMA, this is an incorrect reference to the protocols. IS-54 and IS-136 do, however, use Time Division Multiple Access as an air interface.
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 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. [1]
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.
Large IS-136 networks include Cingular Wireless and U.S. Cellular in the United States, and Rogers Wireless in Canada. Cingular (who purchased AT&T Wireless in October 2004) and Rogers Wireless have upgraded their existing IS-136 networks to GSM/GPRS, while US Celluar is migrating most of their network to CDMA2000. Rogers Wireless removed all 1900 MHz IS-136 in 2003, and is slowly doing the same with their 800 MHz spectrum as the equipment fails. It is not unusual for a Rogers D-AMPS customer to have coverage in a specific location one day, and then none in that same area the next.
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Introduction
IS-54 stands for Interim Standard-54, which is a mobile communication standard employing digital technology. Standards, in general establish the compatibility and performance requirements for technology. Standards assist manufacturers, service providers, and end users to build buy and use products that provide mutually agreed upon features, functionality, and performance. Standards also facilitate the interoperability of products built by different manufacturers. With standards, the service provider and end user have more choices in products than would otherwise, and manufacturers have access to market than otherwise. IS-54 was standardized by Electronics Industries Association (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. Still, this standard is popularly called 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.
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 led by the United States of 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), Comvik (in Sweden), Nordic Mobile Telephones/450 (NMT450), NMT900 (both in Nordic countries), NMT-F (French version of NMT900), Radiocom 2000 (RC2000)(in France), Total Access Communications System(TACS)(in the United Kingdom). And the North American standards were AMPS and Narrow-band AMPS (N-AMPS). Out of the 1G standards the most successful was the AMPS system. Despite the Nordic countries cooperation, the European intellect got divided to 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 and 1900 MHz band in the United States and is the most widely distributed analog cellular standard. AMPS was undoubtedly, the leader of the 1G mibile communication systems. The success of AMPS kicked start the mobile age in the North America.
The markets 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 in the whole city of New York. 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 it had many disadvantages too. Primarily it didn’t have the potential to support the increasing demand for mobile communication usage. Each cell site did not have much capacity for carrying higher number 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 these triggered off the search for a more able 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 10KHz. IS-54 on the other hand increased capacity by digital means: sampling, digitizing, and then multiplexing conversations, a technique called TDMA or time division multiple access. This method separates calls by time, placing parts of individual conversations on the same frequency, one after the next. It tripled call capacity. Using IS-54, a cellular carrier could convert any of its systems' 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 features. IS-54 also supported authentication, 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 is 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 kb/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-kb/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 pre assigned 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 pre assigned 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 kb/s with 30-kHz channel spacing, to give a bandwidth efficiency of 1.62 b/s/Hz. This value is 20 percent 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 kHz; Tx: 824-849 kHz |
| Multiple Access Method | TDMA/FDM |
| Duplex Method | FDD |
| Number of Channels | 832 (3 users per channel) |
| Channel Spacing | 30Khz |
| Modulation | DQPSK |
| Channel Bit Rate | 48.6Kb |
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 (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 kb/s achieves a reconstructed speech quality similar to that of the analog AMPS system using frequency modulation. The 7.95-kb/s signal is then passed through a channel coder that loads the bit rate up to 13 kb/s. The new half-rate coding standard reduces the overall bit rate for each call to 6.5 kb/s, and should provide comparable quality to the 13-kb/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 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. Some of the major successor technologies include:
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 kbps data rate across the six time slots comprising one frame in the 30-kHz channel.
IS-95
As the demand for mobile communication went up still, the need for a more sophisticated system arose. In 1994 Qualcomm, Inc. proposed a cellular system and standard based on spread spectrum technology to increase capacity. It was and still is called IS-95. It uses the AMPS protocol as a default, but in normal operation operates quite differently than analog cellular or the more advanced IS-54. IS-95 is the first Code Division Multiple Access (CDMA)-based digital cellular standard. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95. Unlike TDMA, all radios can be active all the time, because network capacity does not directly limit the number of active radios. Since larger numbers of phones can be served by smaller numbers of cell-sites, CDMA-based standards have a significant economic advantage over TDMA-based standards, or the oldest cellular standards that used frequency-division multiplexing.
IS-2000
IS-95 is now being supplanted by IS-2000 (CDMA2000), a later CDMA-based standard. It is used in the USA, South Korea, Canada, Mexico, Israel, Australia, Venezuela and China. CDMA2000 is a family of 3G mobile telecommunications standards and is an incompatible competitor of the other major 3G standard W-CDMA (UMTS). CDMA2000 is one of the approved radio interfaces for the ITU's IMT-2000 standard and a successor to 2G CDMA (IS-95, branded cdmaOne). It is standardized by 3GPP2. CDMA2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA) in the United States, not a generic term like CDMA. There are 4 major versions of CDMA2000, namely:
- CDMA2000 1x
- CDMA2000 1xEV-DO (1x Evolution-Data Optimized)
- CDMA2000 1xEV-DV (1x Evolution-Data and Voice)
- CDMA2000 3x
Conclusion
IS-54 has many firsts. It was the first ever mobile communication system to incorporate the potential of digital electronics to itself. Though the European counterpart of IS-54, the Global System for Mobile communication (GSM) proved to be more robust, the credit of being the first digital (though partially) communication technique belongs to IS-54. IS-54 was the first mobile communication system which had some provision for security. Then IS-54 was the first standard to employ TDMA technology, though the usage was as an air interface. Mobile communication, being a technology catering to un-quenchable needs, don’t give much space to older standards. But they have been the base for the newer highly sophisticated systems. Among the old and almost obsolete technologies belonging to 1G and earlier-2G, and even among the whole mobile commutation standards domain, IS-54 has a distinguished position.