There are a lot of misconceptions when it comes to cell phone networks and network standards. Here we'll try to wade through them.
First, let me clarify some widely misused terms: TDMA and CDMA. In the US, the terms "TDMA" is often used for D-AMPS networks. The term "CDMA" is used for cdmaOne and CDMA2000 networks. Simply calling these "TDMA" and "CDMA" is wrong, and should be avoided.
Multiple Access methods
The major difference between mobile standards of different generations is the Multiple Access system - the way the networks fit multiple users onto a single slice of radio bandwidth. The follow methods have been used/are in use:
FDMA Frequency Division Multiple Access
Each handset is assigned its own frequency to broadcast on. This method has been around since Marconis time, and is incredibly easy to implement. It's also incredibly inefficient. Used by FM and AM radio, and the analog mobile standards (NMT, AMPs).
TDMA Time Division Multiple AccessA frequency is split up into a number of short "time slots", which different mobile terminals can subscribe to (when they make a call). This is much more efficient than FDMA, and pretty easy/cheap to implement. Used in the first digital systems, such as GSM and D-AMPS.
CDMA Code Division Multiple AccessEach mobile terminal is assigned a unique "code". Then it transmits its data at the same time on the same frequency as all the other terminals - this signal will look like noise. The base station then uses math voodoo to isolate that signal using the code. In order for the math voodoo to work, all the handsets have to transmit at exactly the same power level, and they have to continually (millions of times per second) adjust their power level after atmospherical conditions (range, obstacles, etc). CDMA is incredibly efficient, and has many pros. It's also very space-age and tricky to implement, let alone implement in a power-efficient and compacy manner. CDMA is used in cmdaOne and the 3G systems: CDMA2000 and UMTS.
The multiple access type has a big impact on the behavior of a cell when it comes to capacity and coverage:
- In TDMA systems, there's obviously a limited amount of timeslots per frequency (it's 8 timeslots in GSM). If all those timeslots are full, on all the frequencies available to an operator, that means that the cell can no longer handle any more calls until someone hangs up, and you'll just get "call refused".
- TDMA cell size is limited by the timeslot length. The time between the subscription of a timeslot and the timeslot itself cannot be too long, or the handset will have missed its opportunity to send by the time it gets to "OK you got the timeslot" signal. In GSM this means a maximum cell size of 35 km.
- In CDMA systems, there's no limit to the codes. Instead, what happens when too many terminals are transmitting at once is that the signals from far away terminals are drowned out by close terminals. This makes the cell blind to the terminals at the edges of its range, and the cell size gets smaller. When there are less terminals/less traffic in range, the cell gets bigger again. This behavior is called "breathing". One pro to this behavior is that cells can overlap to a larger extent than in TDMA. Then, when a CDMA cell gets smaller, its adjacent ones can "pick up the slack" by handling the terminals that the overworked one can no longer reach.
In practical terms, what this means is that CDMA can make more efficient use of radio space, and it better handles an unusually heavy load (at for example a sporting event).
Mobile standards
Now for the fun (and sometimes religious) part. Mobile standards!
NMT
Scandinavian analog system, in use from 1981, the first mobile system with international roaming (between sweden, denmark, norway). Had digital data and text messaging capabilites. Uses FDMA.
AMPS
American analog system, introduced in 1984. Uses FDMA.
D-AMPS
Early american digital system, from 1990. Often referred to as "TDMA". Used, not surprisingly, TDMA.
GSM
Originally european (standardized by the EU/CEPT) digital system, introduced in 1991. Envisioned as a pan-european digital phone standard, it's now now used by over 2 billion subscribers worldwide, in over 210 countries (there are ~240 countries total). The roaming capabilites also mean that you can take your local GSM handset, jump on a plane to Russia, pick up your phone, and it'll work perfectly (albeit more expensively).
One main feature of GSM is the SIM card. This is a smartcard which contains your subscriber identity, a crypto key (shared secret) and can store phone book contacts and SMS messages. Using the SIM card, you can easily switch phones by simply moving your card from one phone to another.
A phone can be locked to only accept SIM of a certain operator, this is called SIM-locking. Such a locked phone can often be unlocked using a code, or by using a special cable. Some operators will unlock your phone (for a fee). The reverse is never done (although it is technically feasable) - you can always use an unlocked phone with any SIM and on any network. This means you can import futuristic unlocked European or Japanese GSM-compatible phones and stick your SIM in it and it will work perfectly.
GSM originally had limited data capabilites - you could use one call channel as a data channel and achieve 9.6 Kbit/s rates, and later combine multiple channels (at a multiple cost...). With the introduction of GPRS, packet data is now supported. GPRS data is sent over any free timeslots, and so the maximum speed depends on how many timeslots are currently free. The maximum speed is 80/20 kbit/s down/up using 4 timeslots (theoretically 8 timeslots could be used, but no networks/handsets allow this). EDGE introduces a more efficient encoding to GPRS, increasting that maximum speed to 236 kbit/s.
GSM uses four different frequencies, depending on where you live. The original European standard frequencies are 900 MHz and 1800 MHz (a network can use one or both - lower frequencies have higher penetration), and these frequencies are used in most of the world. When GSM made it to some countries (notably the USA/Canada), the standard frequencies were already in use by other systems, and they settled on the alternative frequencies of 850 MHz and 1900 MHz.
To make handsets compatible with multiple networks and countries, they've been made multiple-band. "Dual-band" means it supports both 900/1800 or 850/1900. "Triple-band" means it'll support the native bands of a country, plus one of the foreign bands. "Quad-band" means it supports all GSM bands used, and can thus be used worldwide.
cdmaOne (IS-95)
American digital standard, created by Qualcomm and finalized in 1995. Usually called (together with cdma2000) "CDMA". Mainly used in the USA, Canada and South Korea. Being replaced with cdma2000. Does not use SIM cards (see cdma2000).
iDEN
iDEN is a proprietary system cooked up by Motorola and used by a handful of operators in the americas and middle east. iDEN is seen as a dead end and will eventually die out, its only redeeming feature being instantaneous push-to-talk. Does not use SIM cards (see cdma2000). It is a TDMA system.
cdma2000
American 3G standard, an evolution of cmdaOne. Used mainly in the USA, Canada, Japan and South Korea. Data speeds with 1xRTT: 144 kbit/s, 1xEV-DO Rev. 0: 2.5/0.15 Mbit up/down, 1xEV-DO Rev. A: 3.1/1.8 Mbit up/down. Rev. A is currently only used in Japan.
cdmaOne/cdma2000 implementations do not use SIM cards (although it is in the standard as an option). When you get a phone, its ESN has to be registered on the network. Operators will virtually never register the ESN of a phone they didn't sell themselves, meaning that you will have to buy your phone from the operator, and thus you're limited to their selection. Even if you find a super-high-tech Korean or Japanese cmda2000 phone, Verizon will not let you use it.
UMTS
Global 3G standard, an evolution of GSM. Often referred to as simply "3G". UMTS uses many parts of the GSM standard, but replaces the air interface with a CDMA system with a higher data rate. The close relationship with GSM means that they can share a core network and provide seamless handover. UMTS is the upgrade path for virtually all GSM operators.
One feature that's highly advertised in UMTS, and is supported by a majority of handsets is video calling.
The data speed is 384 kbit/s. With HSDPA (High-Speed Downlink Packet Access) which is currently being rolled out, download speeds of 2-10 Mbit are possible.
One feature that's highly advertised in UMTS, and is supported by a majority of handsets is video calling.
The data speed is 384 kbit/s. With HSDPA (High-Speed Downlink Packet Access) which is currently being rolled out, download speeds of 2-10 Mbit are possible.
Generations
Something else which has caused a deal of confusion are the terms "2G" and "3G". Here are some quick definitions:
0G
Very early systems that were essentially glorified walkie-talkies.
1GAnalog systems: NMT, AMPS.
2GDigital systems: GSM, D-AMPS, cmdaOne
2.5GFaster digital systems: GPRS, HSCSD
2.75GNear-3G speeds: EDGE, CDMA2000 1xRTT.
3GHigh-speed data: W-CDMA/UMTS, CDMA2000 1xEV-DO.
3.5GHigher-speed download data: HSDPA
3.75GHigher-speed upload data: HSUPA
4GUnknown yet. One suggestion is seamless VoIP roaming between 2G, 3G, WiFi, WiMax, Bluetooth, etc.
How about coverage? Voice quality?
Coverage and voice quality is entirely up to your provider. All the network types are capable of equal coverage, equal voice quality and equal lack of "dropped calls", it all depends on the operator - how many base stations they have (so that they have coverage), and if they've spread them too thin (so that they get overloaded).
Coverage is also a very local thing. Some operator may cover 99% the country, but if that remaining 1% is your house, the service will be pretty useless to you. So if you ask for advice, be sure to ask it of friends and co-workers, people who frequent the same places you do.
Another thing to consider when it comes to coverage, is that GSM has worldwide coverage. Whereas cmdaOne/cmda2000 work in a handful of countries (mainly in the americas), GSM works in over 200 countries, and most providers have roaming agreements with virtually all of them. Even if you don't want to pay exhorbant international roaming rates, you can pick up a cheap local pay-as-you-go SIM card and pop in your own phone and then pay local rates.
GSM coverage:
Quick reference for americans
Cingular: GSM 850/1900
T-mobile: GSM 1900 (roaming on Cingular 850)
Verizon: cdma2000
Sprint: cdma2000
Nextel: iDEN
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