GSM, CDMA and LTE: A Guide to Mobile Network Standards
March 4, 2013 By Benjamin J. Roethig
If you have a cell phone, there is a hodgepodge of wireless technology behind it. To help (or confuse) the customer, carriers use a generic term like “2G”, “3G”, or “4G”. However, they are not always consistent carrier to carrier, even between carriers using the same technology. I’m going to delve into the alphabet soup to tell you what all this means.
There are two current voice technologies: Global Systems for Mobile communications (GSM) and Code Division Multiple Access (CDMA). While these are the voice standards, they are also informally used as a blanket term to include the cooperating data technology.
GSM: World Heavyweight Champion
GSM was developed by the European Telecommunications Standards Institute to replace the analog networks we knew and didn’t love. The first GSM networks started popping up in the early-to-mid 1990s. In the decade and a half since, it has become the de facto standard when it comes to worldwide communications, with a couple notable exceptions.
While it was not part of the initial specification, modern GSM devices use a Security Identity Module. This little chip card contains the International Mobile Subscriber Identity (IMSA) Number, carrier information, and your contact information. Assuming the phone is not carrier locked, the SIM card slot allows you to easily migrate your phone between carriers.
GSM is the technology behind AT&T and T-Mobile USA (plus some regional carriers) here in the states. Elsewhere, it’s either the dominant or even sole cellular technology present. Its dominance, combined with the SIM slot for using cheap local SIMs, makes ideal technology for travel. GSM-type phones also have the ability to simultaneously connect on voice and data radios which until very recently set it apart from CDMA.
GSM has used a lot of data specifications and unfortunately for you, most of them were grouped together. For 2G data, GSM uses GPRS or enhanced GPRS aka Edge. They were phased out of most metropolitan areas in the U.S. by late 2011, but assuming you can find coverage, Edge is prominent in rural areas with even the occasional GPRS tower. In areas where faster service exists, Edge is typically used as a backup network. 2G data also typically used the same radio band as the GSM voice radio.
3G gets really confusing. Unlike 2G, 3G standards have their own dedicated radio bands for data. The benefit is more bandwidth for faster speeds, the downside is fragmentation which we will get to later. UMTS came first, followed up with HSPA, HSPA+, and dual-carrier HSPA+. What does this mean? Your 3G smartphone can be anywhere from 3.5 Mbps UMTS to a nearly LTE-like 42 Mbps HSPA+. Because of this wide discrepancy, T-Mobile USA and then AT&T started referring to the more advanced versions as “4G”.
CDMA: The American Contender
The other well-travelled cellular technology is CDMA. CDMA was developed by Qualcomm. The CDMA brand name encompasses two different standards: IS-95 aka CDMA One, a 2G digital service, and its modern 3G successor CDMA-2000. As far as i can tell, just about every CDMA carrier is using the later standard. While there is an analog to the SIM card called RUIM, they have not been widely used with carriers in the Americas. In those markets, that data is stored on the phone itself which has made moving your phone between carriers effectively impossible.
“3G” Data is provided by Evolution Data Optimized (EV-DO). EV-DO is technically on its 4th version, Rev. C, but the majority of carriers are using Rev. A. From a technical standpoint, EV-DO has two historical disadvantages compared to UTMS/HSPA data on the GSM path. First off, CDMA phones have not been able to conduct simultaneous voice and data sessions. In other words, if you’re talking, you can’t use cellular data. This has been fixed very recently with a variant called SV-DO. The other is not as easily fixed. EV-DO is reasonably competitive with early UMTS speeds, but hasn’t evolved much beyond that. Because of the less than ideal speeds, CDMA carriers started to look for other options. Sprint has offered Clearwire’s WiMAX network as a “4G” solution. Verizon, on the other hand, started an early transition to LTE.
CDMA has traditionally been dominant in the US and Canada, especially in rural areas and with regional carriers. It also has traction in Japan, India, and the former eastern block states. That said, CDMA’s day is ending. Canada’s CDMA providers, while continuing support for legacy devices, have transitioned to a GSM/HSPA+ network in advance of their LTE transition. US and Japanese CDMA providers are also transitioning to LTE, but keeping the CDMA network.
LTE: The Future is Here
While Long Term Evolution (LTE) is part of the GSM path, it will be the only cellular technology going forward. It has been introduced in data form by most carriers and in widespread use by Verizon. Voice over LTE (VoLTE) will eventually handle your voice calls as well. That being said, LTE is not fully baked by any means and requires some technology advancement to be fully realized. LTE also comes in two different flavors: Frequency-Division Duplexing (FDD) used in the west, and Time-Division Duplexing (TDD).
If you’ve used it on a carrier with sufficient bandwidth, LTE is fast. Unless you’re in Kansas City with Google Fiber, LTE will probably be faster than you home broadband connection. That’s both a blessing and a curse. Since data allotments have not changed with the faster speeds and larger file sizes, you can find yourself with an excessively large phone bill if you’re not not careful. An upgraded version, LTE advanced, will make it even faster.
Once all is said and done, there will be one global LTE based network. However, it faces a major problem… spectrum.
Spectrum and radio technology
With 2G technology, you had basically four radio bands to support data and voice. For 3G, there were 4 data bands, plus additional bands in the Americas and Japan. For LTE there are 27 FDD Bands and an addition 12 TDD bands. Of course all of them won’t be used, but you have carriers currently using three different bands for LTE data alone. That could rise to four or even five bands.
This presents a giant-sized problem and a complete contradiction of LTE’s promise. Current technology allows for pentaband radio technology. With EV-DO and UTMS/HSPA data, that’s a world phone. With LTE, that’s enough to support all the bands of a single carrier. What this means is while fallback 3G networks are worldwide, the chipset makers basically have to craft a custom LTE radio for each individual carrier. For a new class of world phones to come forth, there has to be a leap in radio and antenna technology.
Closing
If you wanted clarity, I’m sorry that just doesn’t come with this subject. It’s a chaotic subject in a very chaotic time. What I do hope you came away with is knowledge in my brief summary. Trust me people, that was the cliff notes. I’ll end with one piece of advice, make sure you know what spectrum your LTE provider has. It could affect your service. In fact, a rundown of LTE spectrum that might even be my next article.
http://geekbeat.tv/a-guide-to-cellular-radios/
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