Cell phone technologies?

[Deeko]

What are the real differences between the types of signals sent for TDMA, CDMA, and GSM cell phone services? What makes one superior to another?

Another question that has always bugged me is how do the towers actually locate the phone? Obviously every phone is equipped with an ESN; is the phone constantly receiving every signal the towers send, and filtering all but the correct one, or is the phone in constant communication with the tower, thus knowing what data to receive?

 

[Mark R]

The basic difference between TDMA and CDMA is described in their names (Time/Code division multiple access).

In TDMA multiple phones are able to operate 'simultaneously' on the same frequency because they transmit short bursts in a round-robin fashion - careful timing systems ensure that the phones transmit their signals so that they arrive at the tower in their specific time slot. GSM uses TDMA type transmission where 8 phones can share one physical frequency - Each phone transmits for 577 us, with a repetition time of 4.62 ms.

CDMA uses spread spectrum modulation. The signal is broadcast by 'spreading' it over a very wide frequency band - this is done by mixing a low-bitrate data stream, with a high-bitrate pseudo-random bitstream (or code). To receive the signal, you need to know the code used to transmit it. Multiple different devices can transmit on the same frequency band by using different codes.

I'm only familiar with GSM, so I can't give much more information about the other systems, other than what I've stated above. Technically there's not much to choose from between a TDMA system and CDMA. Possibly, it's slightly easier to implement packet based networks with CDMA.

Perhaps one could say GSM is superior because it is more popular (in terms of number of subscribers, it out numbers all other systems put together by 3 to 1).

In GSM, the phone will transmit it's ID whenever it first logs into the network (when it switches on) or after it has been out of coverage for a long period. Additionally, it will transmit its ID occasionally (perhaps every 15-20 mins), whenever it has network coverage and is otherwise idle. The network keeps a record of where a phone was last seen, and in the event of an incoming call, will first try that cell. If there is no reply then the network will progressively ping neighbouring cells until the phone is found. If you switch a phone off, the phone will transmit a specific 'switching off' message, so the network does not waste resources looking for it.

On GSM there is a dedicated 'public' channel which is used to initiate communications between phones and the network. This channel is continuously monitored by the phone for messages directed to it. Once the network/phone start communicating the exchange is normally transferred onto a dedicated channel chosen by the network (a typical phone tower can handle hundreds of different channels, the phone needs only to listen to 1 or 2 at a time).

 

[rectifire]

A couple more things you might like to know are that TDMA and CDMA use ESN's. Therefore, in order to switch your service to another phone (say you just bought a new phone), you must call up customer service and have them switch over your service to the ESN of the new phone.

GSM on the other hand uses a small computer chip (called a SIM - Subscriber Identification Module) to store your information. This is also often called a smart chip. The SIM contains the information that identifies your account on the service provider's network........not the phone itself. Therefore, the phone itself becomes more like a removable shell that can be replaced at will. Heck, you could have a different phone for every day of the week. Just swap the SIM card into the new phone every day. However, GSM phones themselves DO have something comparable to an ESN. It's called the IMEI - International Mobile Equipment Identifier. It's mainly used by service providers to ban stolen phones from their network.

Another more interesting tidbit is that TDMA and GSM cell phones can only be transmitting and receiving from one tower at once. Once the signal gets weak, you are automatically handed off to the next tower with a stronger signal........however, your phone cannot remain in contact with both towers at once. If the new tower is busy or hasn't the capacity to handle your call.....you will be dropped.
CDMA phones on the other hand, can remain in contact with several towers at once....so that if the new tower(s) can't handle your call at the moment, the other tower(s) will continue your transmission. The difference between the two is often referred to as a hard or soft handoff, as one kind of handoff must happen instantly, and the other kind of handoff can occur gradually.

Because of the nature of CDMA technology, it has certain advantages in network capacity and data transmission capability that outstrip TDMA/GSM. Because of this, it's been widely recognized that CDMA is a "technically" superior cell phone technology. For example, there is no technical capacity limit for a CDMA tower. However, the more users a CDMA tower supports, the more call quality, coverage area, and service degrades for all users of that tower. On the other hand, TDMA and GSM only have a certain number of timeslots available to support subsciber phone calls. Once all those timeslots and available frequencies are used up.........that's it, no more phone calls possible. However, call quality and coverage area stay the same for all users of on that tower.....even at full capacity.

As you can see, just because CDMA is "technically" superior, this doesn't necessarily translate into better service. Both kinds of technologies have their advantages. What really affects call quality, coverage, and service is how the provider manages/builds/maintains their network.

One last thing I wanted to mention is the maximum distance possible from the tower varies according to the type of signal technology used. For example, with CDMA, there is no technical distance limit of your phone away from the tower. As long as you can get a signal and the tower can get a signal from you, you can make a phone call........even if the tower is 100 miles away. TDMA and GSM however, have distance limits because of the time slotting involved in their signal transmission. This is because after a certain distance, the signal recieved from your cell phone will arrive too late to fit in it's proper timeslot. Even though the signals travel at the speed of light, the network is alternating timeslots in the micro and millisecond range. After a certain distance, even the speed of light is not fast enough to get the signal back in time to fit in the proper timeslot!! The limit for TDMA is somewhere around 75 miles away from a tower. With GSM, the limit is quite a bit less at somewhere around 25 miles away from a tower, due to it's super fast time slotting compared to TDMA. Hence you can see why a GSM network has more capacity than an equivalent TDMA network.....because it timeslots are shorter.

Again, the way a provider builds and maintains their network is the number one factor in relation to good cell phone service.......not the type of cell phone technology used.

Hope this was informative and didn't confuse anybody.

 

[gsaldivar]

How Cell Phones Work

How GSM Works

 

[AlexWade]

The last time I was in Europe, I called my GSM phone from a land-line. You could hear a pinging noise as Cingular was trying to locate my cell-phone. It was very quick and less than 10 seconds, the phone began ringing. That was 2 years ago. Last year, I tried the same thing and it took even less time.

My parents went to Ukraine (which, by the way, has coverage that puts us to shame) and it took only about 1 second to find the phone and connect.

The point of all this is that since in Europe there is only 1 standard, GSM has become very mature. European GSM has many more features than US GSM.

Also, most US GSM phones are locked to a particular carrier. That means, you have to unlock it to switch to a different carrier. But, if you are switching phones on the same carrier, you just swap cards and you are done. SIM cards are what make GSM hands-down better. And Nextel uses SIM cards for their iDen standard. The SIM cards work the same, but phones are not interchangable. iDen is built on the 800 MHz Analog range, even though it is digital.