https://www.youtube.com/watch?feature=player_embedded&v=P9UZhsmmC7g
CDMA vs. GSM: What's the Difference?
Two basic
technologies in mobile phones, CDMA and GSM represent a gap you can't
cross. They're the reason you can't use many AT&T phones on Verizon's
network and vice versa. But what does CDMA vs. GSM really mean for you?
CDMA (Code Division Multiple Access)
and GSM (Global System for Mobiles) are shorthand for the two major radio
systems used in cell phones. Both acronyms tend to group together a bunch of
technologies run by the same entities. In this story, I'll try to explain who
uses which technology and what the real differences are.
Which
Carriers are CDMA? Which are GSM?
In the U.S., Sprint, Verizon and
U.S. Cellular use CDMA. AT&T and T-Mobile use GSM.
Most of the rest of the world
uses GSM. The global spread of GSM came about because in 1987, Europe
mandated the technology by law, and because GSM comes from an industry
consortium. What we call CDMA, by and large, is owned by chipmaker Qualcomm.
This made it less expensive for third parties to build GSM equipment.
There are several variants and
options carriers can choose, like toppings on their technological ice cream. In
this story we'll focus on U.S. networks.
For call quality, the technology you
use is much less important than the way your carrier has built its network.
There are good and bad CDMA and GSM networks, but there are key differences
between the technologies. Here's what you, as a consumer, need to know.
It's much easier to swap phones on
GSM networks, because GSM carriers put customer information on a removable SIM
card. Take the card out, put it in a different phone, and the new phone now has
your number. What's more, to be considered GSM, a carrier must accept any
GSM-compliant phone. So the GSM carriers don't have total control of the phone
you're using.
That's not the case with CDMA. In
the U.S., CDMA carriers use network-based white lists to verify their
subscribers. That means you can only switch phones with your carrier's
permission, and a carrier doesn't have to accept any particular phone onto its
network. It could, but typically, U.S. carriers choose not to.
Many Sprint and Verizon phones now
have SIM cards, but that isn't because of CDMA. The SIM cards are generally
there for Sprint's and Verizon's 4G LTE networks, because the LTE standard also
uses SIM cards. The phones may also have SIM slots to support foreign GSM
networks as "world phones." But those carriers still use CDMA to
authenticate their phones on their own home networks.
3G CDMA networks (known as
"EV-DO" or "Evolution Data Optimized") also, generally,
can't make voice calls and transmit data at the same time. Once more, that's an
available option (known as "SV-DO" for "Simultaneous Voice and
Data Optimization"), but one that U.S. carriers haven't adopted for their
networks and phones.
On the other hand, all 3G GSM
networks have simultaneous voice and data, because it's a required part of the
spec. (3G GSM is also actually a type of CDMA. I'll explain that later.)
So why did so many U.S. carriers go
with CDMA? Timing. When Verizon's predecessors and Sprint switched from analog
to digital in 1995 and 1996, CDMA was the newest, hottest, fastest technology.
It offered more capacity, better call quality and more potential than the GSM
of the day. GSM caught up, but by then those carriers' paths were set.
It's possible to switch from CDMA to
GSM. Bell and Telus in Canada have done it, to get access to the wider variety
of off-the-shelf GSM phones. But Verizon and Sprint are big enough that they
can get custom phones built for them, so they don't see the need to waste money
switching 3G technologies when they could be building out their 4G networks.
The
Technology Behind CDMA vs. GSM
CDMA and GSM are both multiple
access technologies. They're ways for people to cram multiple phone calls
or Internet connections into one radio channel.
GSM came first. It's a "time
division" system. Calls take turns. Your voice is transformed into digital
data, which is given a channel and a time slot, so three calls on one channel
look like this: 123123123123. On the other end, the receiver listens only to the
assigned time slot and pieces the call back together.
The pulsing of the time division
signal created the notorious "GSM buzz," a buzzing sound whenever you
put a GSM phone near a speaker. That's mostly gone now, because 3G GSM (as I
explain later) isn't a time division technology.
CDMA required a bit more processing
power. It's a "code division" system. Every call's data is encoded
with a unique key, then the calls are all transmitted at once; if you have
calls 1, 2, and 3 in a channel, the channel would just say 66666666. The
receivers each have the unique key to "divide" the combined signal
into its individual calls.
Code division turned out to be a
more powerful and flexible technology, so "3G GSM" is actually a CDMA
technology, called WCDMA (wideband CDMA) or UMTS (Universal Mobile Telephone
System). WCDMA requires wider channels than older CDMA systems, as the name
implies, but it has more data capacity.
Since its inception, GSM has evolved
faster than CDMA. As I mentioned above, WCDMA is considered the 3G version
of GSM technology. To further speed things up, the 3GPP (the GSM governing
body) released extensions called HSPA, which have sped GSM networks up to as
fast as 42Mbps, at least in theory.
Our CDMA networks, meanwhile, are
stuck at 3.6Mbps. While faster CDMA technologies exist, U.S. carriers chose not
to install them and have instead turned to 4G LTE to be more compatible with
global standards.
The
Future is LTE
The CDMA vs. GSM gap will close
eventually as everyone moves to 4G LTE, but that doesn't mean everyone's phones
will be compatible. LTE, or "Long Term Evolution," is the new
globally accepted 4G wireless standard. All of the U.S. carriers are turning it
on. For more, see 3G vs. 4G: What's the Difference?
The problem is, they're turning it
on in different frequency bands, with different 3G backup systems, and even, in
the case of the new Sprint Spark network, using an LTE variant (TD-LTE) that
doesn't work with any other U.S. carrier's phones. There are very few phones
that support all of the carriers' LTE bands.
Verizon has said it aims to start
selling LTE-only phones in 2015, but for now, those will require special
Verizon software to make voice calls, so that move won't make it any easier to
switch carriers with your phone. Even without CDMA, the CDMA philosophy of
carrier control of your phone will remain intact.
A growing number of phones support
all of these standards, but it can be hard to tell which ones. The iPhone 6, the iPhone 6 Plus and the Google
Nexus 6 are the most flexible. iPhone 6 and 6 Plus units from
AT&T, T-Mobile, and Verizon can all be used on all three carriers, but
they lack Sprint's special LTE bands. Sprint iPhones have all the bands,
but Sprint has strict unlocking policies. Nexus 6 phones will technically work
on all four carriers, but Sprint only allows phones purchased from Google or
Sprint on its network.
HTC
One (M8) and Samsung Galaxy S5 phones from Verizon will work
somewhat on AT&T's and T-Mobile's networks, albeit with limited coverage
because while they have CDMA, GSM and LTE, they don't have all the
frequency bands AT&T and T-Mobile use. Variants of those same models sold
by AT&T and T-Mobile won't work on Verizon at all, because they lack the
CDMA radio needed for Verizon. It's a mess.
So what does all of this mean for
you? If you want to switch phones often, use your phone in Europe, or use
imported phones, just go with GSM. Otherwise, pick your carrier based on
coverage and call quality in your area and assume you'll probably need a new
phone if you switch carriers. Our Readers' Choice and Fastest Mobile Networks awards are a great place
to start.
