Tuesday, April 28, 2009
Radio Common Carrier (RCC)
Parallel to IMTS in the U.S. until the rollout of cellular AMPS was a competing mobile telephone technology called RCC. An RCC was a common carrier engaged in Public Mobile Service, which also was not the business of providing land line local exchange telephone service. The service from RCC’s was provided from the 1960s until the 1980s when cellular AMPS made RCC equipment obsolete. These systems operated in a regulated environment in competition with the Bell System’s MTS and IMTS. RCC’s handled telephone calls and were operated by private companies and individuals. One impediment of the service was that RCCs were not governed by any single interoperable technical standard. If you had RCC in Omaha, for instance, your phone would not be likely to work in Phoenix. Thus, the concept of “roaming” did not apply which made interstate traveling with RCC extremely difficult. The fact that there was no centralized industry billing database for RCCs contributed to this as well.
Monday, April 27, 2009
Improved Mobile Telephone Service (IMTS)
In the beginning, there was dispatch mobile service. The base operator broadcasted a message to you. Everyone could hear it. You responded. Then they had mobile telephone service. You picked up the phone in your car, the operator responded. You asked for the number you wanted and she/he dialed it and connected you. You had the channel to yourself but others could still tune in. Then came Improved Mobile Telephone Service (IMTS). Now you could dial from your car without using an operator with some assurance of privacy. IMTS was the pre-cellular mobile telephone service enhancement introduced in 1965, which permitted full duplex mobile radio communications, as well as other enhancements. The original Mobile Telephone Service was introduced in 1946.
IMTS can be considered “0 G” (zero G) as it is a pre-cellular VHF/UHF radio system that links directly to the Pubic Switch Telephone Network (PSTN). The original US mobile telephone system included 3 frequency bands, VHF Low (35-44 MHz, 9 channels), VHF High (152-158 MHz, 11 channels), and UHF (454-460 MHz, 12 channels). The low band was prone to network congestion and interference. Cellular networks remedied this problem by decreasing the area covered by one tower and increasing the number of cells in that area.
IMTS technology severely limited the number of subscribers able to use the service. In the 70s and 80s, there were “waiting lists” of up to 3 years for those wishing to have mobile telephone service. This limited the sales of IMTS devices driving the prices up to $2000 - $3000. The limit of customer numbers on MTS and IMTS was the driver for investment in cellular networks. In remote regions, this is not the case. In remote regions, obsolescence is the driver, but the lack of a suitable and affordable alternative has resulted in regulatory obstacles: customers do not want the MTS/IMTS service to be withdrawn. Increasing affordability of satellite service, and government investment in cellular expansion is slowly allowing MTS and IMTS to be removed.
IMTS can be considered “0 G” (zero G) as it is a pre-cellular VHF/UHF radio system that links directly to the Pubic Switch Telephone Network (PSTN). The original US mobile telephone system included 3 frequency bands, VHF Low (35-44 MHz, 9 channels), VHF High (152-158 MHz, 11 channels), and UHF (454-460 MHz, 12 channels). The low band was prone to network congestion and interference. Cellular networks remedied this problem by decreasing the area covered by one tower and increasing the number of cells in that area.
IMTS technology severely limited the number of subscribers able to use the service. In the 70s and 80s, there were “waiting lists” of up to 3 years for those wishing to have mobile telephone service. This limited the sales of IMTS devices driving the prices up to $2000 - $3000. The limit of customer numbers on MTS and IMTS was the driver for investment in cellular networks. In remote regions, this is not the case. In remote regions, obsolescence is the driver, but the lack of a suitable and affordable alternative has resulted in regulatory obstacles: customers do not want the MTS/IMTS service to be withdrawn. Increasing affordability of satellite service, and government investment in cellular expansion is slowly allowing MTS and IMTS to be removed.
Friday, April 24, 2009
Kingsbury Commitment
December 13, 1913 (12-13-13): A letter from Nathan C. Kingsbury, VP AT&T, to the Attorney General of the United States committed AT&T to dispose of its stock in Western Union Telegraph Company. It also promised to provide long distance connection of Bell System lines to independent phone companies (where there was no local competition) and further agreed not to purchase any more independent phone companies, except as approved by the Interstate Commerce Commission which regulated the phone industry at that time.
This letter formalized AT&T monopoly. Although AT&T divested itself of Western Union, the company was allowed to buy market share, as long as it sold an equal number of phones. AT&T agreed to connect its long distance service to independent local carriers, however, it did not agree to interconnect its local services with other local providers. Nor did AT&T agree to any interconnection with independent long-distance carriers.
In the end, AT&T was able to consolidate its hand both on the most profitable urban markets and in long-distance traffic. Between 1921 and 1934, the ICC (Interstate Commerce Commission) approved 271 of the 274 purchase requests made by AT&T. In 1934, the government acted to set AT&T up as a “regulated monopoly” under the jurisdiction of the FCC (Federal Communications Commission) and this was maintained until its divestiture in 1984.
This letter formalized AT&T monopoly. Although AT&T divested itself of Western Union, the company was allowed to buy market share, as long as it sold an equal number of phones. AT&T agreed to connect its long distance service to independent local carriers, however, it did not agree to interconnect its local services with other local providers. Nor did AT&T agree to any interconnection with independent long-distance carriers.
In the end, AT&T was able to consolidate its hand both on the most profitable urban markets and in long-distance traffic. Between 1921 and 1934, the ICC (Interstate Commerce Commission) approved 271 of the 274 purchase requests made by AT&T. In 1934, the government acted to set AT&T up as a “regulated monopoly” under the jurisdiction of the FCC (Federal Communications Commission) and this was maintained until its divestiture in 1984.
Thursday, April 23, 2009
Digital Wireless Standards (Data)
As you can see there are a multitude of wireless cellular and data standards in various countries across the world. After divining into the more “techie” pieces of the wireless spectrum, we will focus on more conceptual definitions and applications.
Japan (PDC, PHS)
PDC: Personal Digital Cellular is the 2G TDMA-based protocols used in Japan, owned by NTT DoCoMo. PDC services operate in the 800 and 1500 MHz bands. PDC is the Japanese equivalent of GSM but is incompatible with other systems. It is operated by NTT DoCoMo, as well as by all the other Japanese operators, but the technology was developed by NTT DoCoMo. Previously known as PHP (Personal HandyPhone) and Japanese Digital Cellular (JDC)
PHS: Personal Handyphone System is the Japanese version of the U.S.’s PCS (Personal Communications Services) with two key differences. It’s not as powerful as PCS. You can’t use a PHS phone in a rapidly moving vehicle, since there is no cell-handoff (i.e. it won’t move you from one cell to another) and thus, if you move outside your cell with PHS, you lose connection. PHS is a perfect mobile phone for pedestrians in high density cities like Tokyo, as long as they don’t move around a lot during the course of a call.
US (CDPD (AMPS), ARDIS, 802.11)
CDPD (AMPS): Cellular Digital Packet Data is a radio technology that supports the transmission of packet data at speeds of up to 19.2Kbps over the existing analog AMPS (Advanced Mobile Phone Service) cellular network, with appropriate CDPD upgrades. The data is structured in packets that are transmitted during pauses in cellular phone conversations, thereby avoiding issues of developing an overlay cellular network for data communications. Estimates suggest that as much as 20%-30% of an AMPS network is idle, even during periods of peak usage. This idle capacity is due to short pauses between the point in time at which you disconnect your circuit-switched cellular telephone conversation and the time when someone else seizes that same radio channel to place a call. Idle capacity also is created when you are “handed-off” from one cell to another as your travel through the area of coverage in your vehicle. While 19.2 Kbps transmission rates are possible, throughput commonly drops to 2.4 Kpbps or so during periods of peak usage.
ARDIS: A public data communications wireless network that allows people carrying hand-held devices to send and receive short data messages. Such messages might be from a sheriff standing in the street searching his department’s data base for unpaid parking tickets. ARDIS network was purchased by Motient Corporation, formerly American Mobile, in 1998. The network is the largest packet data network in the US and provides packet data services using the DataTAC protocol. ARDIS was originally jointly owned by Motorola and IBM. It was an outgrowth of a network originally created for IBM service technicians.
802.11 (b&a are the most important, therefore we will focus on those)
1) 802.11b is now the most common wireless local area network. 802.11b is now installed in offices, airports, coffee shops, hotels, boardrooms, and homes. Many laptops now come with 802.11b wireless transmit and receive electronics built-in. Also known as Wi-Fi, 802.11b is a low power wireless system so the closer you are to a transmitter, the faster it will be. This is roughly what you’ll get at Wireless operating range (indoors):
a. 100 feet at 11 Mbps
b. 165 feet at 5.5 Mbps
c. 230 feet at 2 Mbps
d. 300 feet at 1 Mbps
2) 802.11a is an updated, bigger, better, faster version of 802.11b. The newer 802.11a supports speeds up to 54 Mbps and runs in a 300-Mhz allocation in the 5 Ghz range, which was allocated by the FCC in support of the Unlicensed National Information Infrastructure.
Europe (GSM, HiperLan)
GSM (please refer to past WotD)
HiperLan: High performance radio local area network. Developed by the European Telecommunications Standards Institute (ETSI), HiperLan is a set of WLAN communication standards used chiefly in European countires. HiperLAN is similar to the IEE 802.11 WLAN standards used in the U.S. There are 2 types of HiperLAN:
1) HiperLAN/1: provides communications at up to 20 Mbps in the 5 Ghz band
2) Hiper LAN/2: provides communications at up to 54 Mbps in the 5 Ghz band
According to ETSI, HiperLAN/2 marks a significant milestone in the development of a combined technology for broadband cellular short-range communications and wireless LANs which will provide performance comparable with that of wired LANs.
Japan (PDC, PHS)
PDC: Personal Digital Cellular is the 2G TDMA-based protocols used in Japan, owned by NTT DoCoMo. PDC services operate in the 800 and 1500 MHz bands. PDC is the Japanese equivalent of GSM but is incompatible with other systems. It is operated by NTT DoCoMo, as well as by all the other Japanese operators, but the technology was developed by NTT DoCoMo. Previously known as PHP (Personal HandyPhone) and Japanese Digital Cellular (JDC)
PHS: Personal Handyphone System is the Japanese version of the U.S.’s PCS (Personal Communications Services) with two key differences. It’s not as powerful as PCS. You can’t use a PHS phone in a rapidly moving vehicle, since there is no cell-handoff (i.e. it won’t move you from one cell to another) and thus, if you move outside your cell with PHS, you lose connection. PHS is a perfect mobile phone for pedestrians in high density cities like Tokyo, as long as they don’t move around a lot during the course of a call.
US (CDPD (AMPS), ARDIS, 802.11)
CDPD (AMPS): Cellular Digital Packet Data is a radio technology that supports the transmission of packet data at speeds of up to 19.2Kbps over the existing analog AMPS (Advanced Mobile Phone Service) cellular network, with appropriate CDPD upgrades. The data is structured in packets that are transmitted during pauses in cellular phone conversations, thereby avoiding issues of developing an overlay cellular network for data communications. Estimates suggest that as much as 20%-30% of an AMPS network is idle, even during periods of peak usage. This idle capacity is due to short pauses between the point in time at which you disconnect your circuit-switched cellular telephone conversation and the time when someone else seizes that same radio channel to place a call. Idle capacity also is created when you are “handed-off” from one cell to another as your travel through the area of coverage in your vehicle. While 19.2 Kbps transmission rates are possible, throughput commonly drops to 2.4 Kpbps or so during periods of peak usage.
ARDIS: A public data communications wireless network that allows people carrying hand-held devices to send and receive short data messages. Such messages might be from a sheriff standing in the street searching his department’s data base for unpaid parking tickets. ARDIS network was purchased by Motient Corporation, formerly American Mobile, in 1998. The network is the largest packet data network in the US and provides packet data services using the DataTAC protocol. ARDIS was originally jointly owned by Motorola and IBM. It was an outgrowth of a network originally created for IBM service technicians.
802.11 (b&a are the most important, therefore we will focus on those)
1) 802.11b is now the most common wireless local area network. 802.11b is now installed in offices, airports, coffee shops, hotels, boardrooms, and homes. Many laptops now come with 802.11b wireless transmit and receive electronics built-in. Also known as Wi-Fi, 802.11b is a low power wireless system so the closer you are to a transmitter, the faster it will be. This is roughly what you’ll get at Wireless operating range (indoors):
a. 100 feet at 11 Mbps
b. 165 feet at 5.5 Mbps
c. 230 feet at 2 Mbps
d. 300 feet at 1 Mbps
2) 802.11a is an updated, bigger, better, faster version of 802.11b. The newer 802.11a supports speeds up to 54 Mbps and runs in a 300-Mhz allocation in the 5 Ghz range, which was allocated by the FCC in support of the Unlicensed National Information Infrastructure.
Europe (GSM, HiperLan)
GSM (please refer to past WotD)
HiperLan: High performance radio local area network. Developed by the European Telecommunications Standards Institute (ETSI), HiperLan is a set of WLAN communication standards used chiefly in European countires. HiperLAN is similar to the IEE 802.11 WLAN standards used in the U.S. There are 2 types of HiperLAN:
1) HiperLAN/1: provides communications at up to 20 Mbps in the 5 Ghz band
2) Hiper LAN/2: provides communications at up to 54 Mbps in the 5 Ghz band
According to ETSI, HiperLAN/2 marks a significant milestone in the development of a combined technology for broadband cellular short-range communications and wireless LANs which will provide performance comparable with that of wired LANs.
Tuesday, April 21, 2009
Digital Wireless Standards (PCS)
Digital Wireless Standards (PCS):
Japan (PHS)Personal Handyphone System is the Japanese version of the U.S.’s PCS (Personal Communications Services) with two key differences. It’s not as powerful as PCS. You can’t use a PHS phone in a rapidly moving vehicle, since there is no cell-handoff (i.e. it won’t move you from one cell to another) and thus, if you move outside your cell with PHS, you lose connection. PHS is a perfect mobile phone for pedestrians in high density cities like Tokyo, as long as they don’t move around a lot during the course of a call.
US (IS-136,IS-95, PCS1900, PACS)
IS-136: Also known as DigitalAMPS (D-AMPS). The EIA/TIA Interim Standard which succeed IS-54, and which addresses digital cellular systems employing TDMA (Time Division Multiple Access). IS-136 also specifies a DCCH (Digital Control CHannel) in support of new features controlled by a signaling and control channel between the cell site and the terminal equipment. IS-136 also allows analog AMPS (Advanced Mobile Phone System) to coexist with North America TDMA on the same cellular network, sharing frequency bands and channels, which supports a smooth transition from analog to digital cellular. IS-136 gave rise to a high-tier standard for PCS (Personal Communications Services), developed by a Joint Technical Committee (JTC) comprising representatives from ATIS and the TIA. High-Tier PCS supports fast-moving vehicular traffic, much like traditional cellular.
IS-95: A TIA standard for North American cellular systems based on CDMA (Code Division Multiple Access), and is widely deployed in North America and Asia. IS-95a defines what generally is known as cdmaOne, which supports voice and 14.4 Kbps data rates. IS-95b supports data rates up to 115 Kbps.
PCS1900: A GSM system offering 148 full-duplex voice channels per cell. The system operates in the 1.9 GHz band used in the United States and is now known as GSM 1900.
PACS: Personal Communications Access System is a cellular system providing limited, regional mobility in a given area. It provides mobility between that of a cordless phone and a full-fledged cellular system. Originally developed by Bell Labs in the early 1980’s, PACS is a comprehensive framework for the deployment of PCS and applies to both licensed and unlicensed applications. Now it is approved by the TIA and Exchange Carriers Standards Associations. Today’s currently implemented versions of PCS are “up-banded” versions of the 900 MHz AMPS and GSM cellular standards.
Europe (DCS1800)
Digital Cellular System at 1800 MHz. A GSM standard for cellular mobile telephony established by ETSI (European Telecommunications Standards Institute) for operation at 1800 MHz. In short, DCS 1800 is GSM adopted to the 1800 MHz frequency b and. This means that existing GSM phones won’t be able to talk on the DCS 1800.
Japan (PHS)Personal Handyphone System is the Japanese version of the U.S.’s PCS (Personal Communications Services) with two key differences. It’s not as powerful as PCS. You can’t use a PHS phone in a rapidly moving vehicle, since there is no cell-handoff (i.e. it won’t move you from one cell to another) and thus, if you move outside your cell with PHS, you lose connection. PHS is a perfect mobile phone for pedestrians in high density cities like Tokyo, as long as they don’t move around a lot during the course of a call.
US (IS-136,IS-95, PCS1900, PACS)
IS-136: Also known as DigitalAMPS (D-AMPS). The EIA/TIA Interim Standard which succeed IS-54, and which addresses digital cellular systems employing TDMA (Time Division Multiple Access). IS-136 also specifies a DCCH (Digital Control CHannel) in support of new features controlled by a signaling and control channel between the cell site and the terminal equipment. IS-136 also allows analog AMPS (Advanced Mobile Phone System) to coexist with North America TDMA on the same cellular network, sharing frequency bands and channels, which supports a smooth transition from analog to digital cellular. IS-136 gave rise to a high-tier standard for PCS (Personal Communications Services), developed by a Joint Technical Committee (JTC) comprising representatives from ATIS and the TIA. High-Tier PCS supports fast-moving vehicular traffic, much like traditional cellular.
IS-95: A TIA standard for North American cellular systems based on CDMA (Code Division Multiple Access), and is widely deployed in North America and Asia. IS-95a defines what generally is known as cdmaOne, which supports voice and 14.4 Kbps data rates. IS-95b supports data rates up to 115 Kbps.
PCS1900: A GSM system offering 148 full-duplex voice channels per cell. The system operates in the 1.9 GHz band used in the United States and is now known as GSM 1900.
PACS: Personal Communications Access System is a cellular system providing limited, regional mobility in a given area. It provides mobility between that of a cordless phone and a full-fledged cellular system. Originally developed by Bell Labs in the early 1980’s, PACS is a comprehensive framework for the deployment of PCS and applies to both licensed and unlicensed applications. Now it is approved by the TIA and Exchange Carriers Standards Associations. Today’s currently implemented versions of PCS are “up-banded” versions of the 900 MHz AMPS and GSM cellular standards.
Europe (DCS1800)
Digital Cellular System at 1800 MHz. A GSM standard for cellular mobile telephony established by ETSI (European Telecommunications Standards Institute) for operation at 1800 MHz. In short, DCS 1800 is GSM adopted to the 1800 MHz frequency b and. This means that existing GSM phones won’t be able to talk on the DCS 1800.
Monday, April 13, 2009
Digital Wireless Standards (Cellular):
Japan (PDC)
Personal Digitial Cellular is the 2G TDMA-based protocols used in Japan, owned by NTT DoCoMo. PDC services operate in the 800 and 1500 MHz bands. PDC is the Japanese equivalent of GSM but is incompatible with other systems. It is operated by NTT DoCoMo, as well as by all the other Japanese operators, but the technology was developed by NTT DoCoMo. Previously known as PHP (Personal HandyPhone) and Japanese Digital Cellular (JDC)
US (IS-54, IS-136, IS-95)
IS-54: Interim Standard 54 is the dual mode (analog and digital) standard for cellular phone service in North America. In its analog form, it conforms to the AMPS standard. IS-54 is an EIA/TIA (Electronics Industries Association/Telecommunications Industries Association) standard, developed with the involvement of the CTIA. Since 1995, IS-54 enhancements fall under IS-136.
IS-136: Also known as DigitalAMPS (D-AMPS). The EIA/TIA Interim Standard which succeed IS-54, and which addresses digital cellular systems employing TDMA (Time Division Multiple Access). IS-136 also specifies a DCCH (Digital Control CHannel) in support of new features controlled by a signaling and control channel between the cell site and the terminal equipment. IS-136 also allows analog AMPS (Advanced Mobile Phone System) to coexist with North America TDMA on the same cellular network, sharing frequency bands and channels, which supports a smooth transition from analog to digital cellular. IS-136 gave rise to a high-tier standard for PCS (Personal Communications Services), developed by a Joint Technical Committee (JTC) comprising representatives from ATIS and the TIA. High-Tier PCS supports fast-moving vehicular traffic, much like traditional cellular.
IS-95: A TIA standard for North American cellular systems based on CDMA (Code Division Multiple Access), and is widely deployed in North America and Asia. IS-95a defines what generally is known as cdmaOne, which supports voice and 14.4 Kbps data rates. IS-95b supports data rates up to 115 Kbps.
Europe (GSM)
GSM originally stood for Groupe Speciale Mobile but now is known as Global System for Mobile Communications. It is the standard digital cellular (also called mobile) phone service you will find in Europe, Japan, Australia and elsewhere – a total of 85 countries! Most countries decided to pick a single, standard wireless phone technology years ago, and they settled on GSM. The U.S. refused to settle on a standard and that has resulted in a patchwork of multiple, incompatible technologies. GSM exists in the U.S., and is gaining ground in the U.S. though it uses a different frequency than the system used in Europe and elsewhere. In the U.S. it is used by companies including VoiceStream and AT&T.
*Although this is an abbreviated definition, we will explore GSM in more detail in future WOTD’s.
Personal Digitial Cellular is the 2G TDMA-based protocols used in Japan, owned by NTT DoCoMo. PDC services operate in the 800 and 1500 MHz bands. PDC is the Japanese equivalent of GSM but is incompatible with other systems. It is operated by NTT DoCoMo, as well as by all the other Japanese operators, but the technology was developed by NTT DoCoMo. Previously known as PHP (Personal HandyPhone) and Japanese Digital Cellular (JDC)
US (IS-54, IS-136, IS-95)
IS-54: Interim Standard 54 is the dual mode (analog and digital) standard for cellular phone service in North America. In its analog form, it conforms to the AMPS standard. IS-54 is an EIA/TIA (Electronics Industries Association/Telecommunications Industries Association) standard, developed with the involvement of the CTIA. Since 1995, IS-54 enhancements fall under IS-136.
IS-136: Also known as DigitalAMPS (D-AMPS). The EIA/TIA Interim Standard which succeed IS-54, and which addresses digital cellular systems employing TDMA (Time Division Multiple Access). IS-136 also specifies a DCCH (Digital Control CHannel) in support of new features controlled by a signaling and control channel between the cell site and the terminal equipment. IS-136 also allows analog AMPS (Advanced Mobile Phone System) to coexist with North America TDMA on the same cellular network, sharing frequency bands and channels, which supports a smooth transition from analog to digital cellular. IS-136 gave rise to a high-tier standard for PCS (Personal Communications Services), developed by a Joint Technical Committee (JTC) comprising representatives from ATIS and the TIA. High-Tier PCS supports fast-moving vehicular traffic, much like traditional cellular.
IS-95: A TIA standard for North American cellular systems based on CDMA (Code Division Multiple Access), and is widely deployed in North America and Asia. IS-95a defines what generally is known as cdmaOne, which supports voice and 14.4 Kbps data rates. IS-95b supports data rates up to 115 Kbps.
Europe (GSM)
GSM originally stood for Groupe Speciale Mobile but now is known as Global System for Mobile Communications. It is the standard digital cellular (also called mobile) phone service you will find in Europe, Japan, Australia and elsewhere – a total of 85 countries! Most countries decided to pick a single, standard wireless phone technology years ago, and they settled on GSM. The U.S. refused to settle on a standard and that has resulted in a patchwork of multiple, incompatible technologies. GSM exists in the U.S., and is gaining ground in the U.S. though it uses a different frequency than the system used in Europe and elsewhere. In the U.S. it is used by companies including VoiceStream and AT&T.
*Although this is an abbreviated definition, we will explore GSM in more detail in future WOTD’s.
Wednesday, April 8, 2009
Digital Wireless Standards (Cordless)
Japan (PHS)
Personal Handyphone System is the Japanese version of the U.S.’s PCS (Personal Communications Services) with two key differences. It’s not as powerful as PCS. You can’t use a PHS phone in a rapidly moving vehicle, since there is no cell-handoff (i.e. it won’t move you from one cell to another) and thus, if you move outside your cell with PHS, you lose connection. PHS is a perfect mobile phone for pedestrians in high density cities like Tokyo, as long as they don’t move around a lot during the course of a call.
US (PACS)
Personal Communications Access System is a cellular system providing limited, regional mobility in a given area. It provides mobility between that of a cordless phone and a full-fledged cellular system. Originally developed by Bell Labs in the early 1980’s, PACS is a comprehensive framework for the deployment of PCS and applies to both licensed and unlicensed applications. Now it is approved by the TIA and Exchange Carriers Standards Associations. Today’s currently implemented versions of PCS are “up-banded” versions of the 900 MHz AMPS and GSM cellular standards.
Europe (DECT)
Digital European Cordless Telecommunication is the pan-European wireless standard based on Time Division Multiple Access (TDMA) used for limited-range wireless services. Based on advanced TDMA technology, and used primarily for wireless PBX systems, telepoint and residential cordless telephony today, used for DECT include paging and cordless LANs. DECT frequency is 1800-1900 MHz. Stop press (the old meaning for DECT) was “Digital European Cordless Telephone” but since the DECT standard also has spread to China and South America, the correct definition is now “Digital Enhanced Cordless Telephones”
Personal Handyphone System is the Japanese version of the U.S.’s PCS (Personal Communications Services) with two key differences. It’s not as powerful as PCS. You can’t use a PHS phone in a rapidly moving vehicle, since there is no cell-handoff (i.e. it won’t move you from one cell to another) and thus, if you move outside your cell with PHS, you lose connection. PHS is a perfect mobile phone for pedestrians in high density cities like Tokyo, as long as they don’t move around a lot during the course of a call.
US (PACS)
Personal Communications Access System is a cellular system providing limited, regional mobility in a given area. It provides mobility between that of a cordless phone and a full-fledged cellular system. Originally developed by Bell Labs in the early 1980’s, PACS is a comprehensive framework for the deployment of PCS and applies to both licensed and unlicensed applications. Now it is approved by the TIA and Exchange Carriers Standards Associations. Today’s currently implemented versions of PCS are “up-banded” versions of the 900 MHz AMPS and GSM cellular standards.
Europe (DECT)
Digital European Cordless Telecommunication is the pan-European wireless standard based on Time Division Multiple Access (TDMA) used for limited-range wireless services. Based on advanced TDMA technology, and used primarily for wireless PBX systems, telepoint and residential cordless telephony today, used for DECT include paging and cordless LANs. DECT frequency is 1800-1900 MHz. Stop press (the old meaning for DECT) was “Digital European Cordless Telephone” but since the DECT standard also has spread to China and South America, the correct definition is now “Digital Enhanced Cordless Telephones”
Tuesday, April 7, 2009
Differential Phase Shift Keying (DPSK):
Also called dibit phase shift keying, it is a modulation technique used to improve the efficiency with which the naturally analog electromagnetic waveform is employed to carry digital bits in a digital bitstream. DPSK is a form of “coherent demodulation,” in which the phase of the incoming signal is compared to a replica of the carrier waveform. The carrier waveform (the carrier frequency “carries” the data, and the waveform is characteristic of all electromagnetic energy), is used as a reference point. With DPSK, the carrier waveform reference point serves to record changes in the binary data code. In other words, a “1” in the PSK (Phase Shift Keying) signal is denoted by no change in the DPSK signal, and a “0” is denoted by a change in the DPSK signal.
DPSK works much better than PSK because so many things can foul up the “absolute” value of a signal sent over an Unshielded Twist Pair (UTP) cable pair or over a microwave radio channel. ElectroMagnetic Interference (EMI) of all sorts can cause the “absolute value” of an originating signal to be “questionable” on the receiving end. Assuming some reasonable level of consistency in the impact of such factors from transmitter to receiver, it helps a lot to have a reference point. DPSK does that.
Takeaway: Differential Phase Shift Keying has emerged as the type of phase modulation most likely to reach commercial deployment since Phase Shift Keying (PSK) is not considered practical due to its extremely difficult method of reception required. Today, air-traffic-control radio communications employ the use of DPSK modulation due to its clear performance advantage over alternative modulation schemes and the availability of rather inexpensive DPSK demodulators (to remove the carrier signal so that the data can be interpreted).
DPSK works much better than PSK because so many things can foul up the “absolute” value of a signal sent over an Unshielded Twist Pair (UTP) cable pair or over a microwave radio channel. ElectroMagnetic Interference (EMI) of all sorts can cause the “absolute value” of an originating signal to be “questionable” on the receiving end. Assuming some reasonable level of consistency in the impact of such factors from transmitter to receiver, it helps a lot to have a reference point. DPSK does that.
Takeaway: Differential Phase Shift Keying has emerged as the type of phase modulation most likely to reach commercial deployment since Phase Shift Keying (PSK) is not considered practical due to its extremely difficult method of reception required. Today, air-traffic-control radio communications employ the use of DPSK modulation due to its clear performance advantage over alternative modulation schemes and the availability of rather inexpensive DPSK demodulators (to remove the carrier signal so that the data can be interpreted).
Thursday, April 2, 2009
Frequency Modulation (FM)
Frequency Modulation (FM): A modulation technique in which the carrier frequency is shifted by an amount proportional to the value of the modulating signal. The amplitude of the carrier signals remains constant. The deviation of the carrier frequency determines the signal content of the message. Commercial TV and FM radio use this technique, which is much less sensitive to noise and interference than is amplitude modulation (AM). In the world of modems, digital bit streams can be transmitted over analog facilities through this same technique, whereby a 0 bit might be represented by a high-frequency sine wave (or set of sine waves) and a 1 bit by a low-frequency sine wave (or set of sine waves).
In the image below, notice the top line represents the audio signal being transmitted.
The red line represents Amplitude Modulation with the frequency (space between waves) remaining constant with each modulation of the signal. The deviation of the amplitude determines the message.
The blue line represents Frequency Modulation with the amplitude (height of the waves) remaining constant with each modulation of the signal. The deviation of the frequency determines the message.
Image Source: http://en.wikipedia.org/wiki/Amplitude_modulation
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