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Relative Analysis of GSM, CDMA and Wireless

Posted on 2006-06-27
Medium Priority
Last Modified: 2008-02-01

Sometime back myself and friend were discussing about the wireless, when we were travelling back home and seeing the cops speaking on wireless phones. It was totally inaudible with so much noise. My friend was saying that wireless phone is very cheap but very insecure. CDMA phones are very costly but very secure too. GSMs scale up in between them.

I am quite interested in finding out where can be find more information in analysis of the same -- cost wise, infrastructual investment, security etc. I was searching out for fundamental info but could'nt locate one.

Please help.
Question by:deepaknet
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Expert Comment

ID: 16991299
Cell phones using Gm/CDMA technology are wireless too!!

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Author Comment

ID: 16991339
But how does these differ from Wireless phones that cops normally use? There seems to be alot of noise in those wireless phones? Are these private spectrum?

Accepted Solution

hiteshgupta1 earned 150 total points
ID: 16991577
One of the basic things that differentiates GSM (based on TDMA) , CDMA AND WIRELESS PHONES (the one which is used by cops)is the way the carve up bandwidth.
the problem with wireless phones  is that theier signal strength is very less(around 0.25 watts) ,that's why the distance coverd by such phones is limited.
hotspots used by such wireless phones can be open or secure. If a hotspot is open, then anyone with a valid wirelss card(for a praticular frwequcnecy band) can access the hotspot. If it is secure, then the user needs to know a  key  to connect to the hotsopt.(That is all i know about o called wirelss phones)

Now something about CDMA and GSM
The major difference between GSM/TDMA and CDMA is in the way they divide up those signals between multiple users.

GSM/TDMA uses a Time Division method. TDMA, in fact, stands for Time Division Multiple Access. Simply put, this means that each device on the local network is allocated a time slice where it "owns" the bandwidth, and it can send/receive its data.

So lets just pick a number and say there are 30 available time slices in a given cycle. Each phone would then get 1/30th of every cycle that it could send and receive data (aka, voice).

CDMA uses a different method, called Code Division Multiple Access. The specifics of how it breaks the cycle up are beyond me, but how it works out is that the phones only get a slice of the bandwidth cycle when they actually need one. So if you are not talking, and the other person is not talking, nothing is transmitted.

With GSM/TDMA, each phone is transmitting and receiving during its slices of the bandwidth cycle, whether it needs it or not.

Since most coversations are comprised largely of silence, the end result is that CDMA phones have to transmit less data. They don't have to send silence, like GSM/TDMA phones do.

This means a few things. More CDMA calls can be fit into a given amount of frequency spectrum (ie. it is more efficient for the network), less radiation is being created from the phone towards the user (you only get radiation when you are talking, basically), and battery power is conserved since the handset only transmits when it actually has something to send.

There are other differences, too, that I can't get into. For one, it is harder to implement a CDMA network. The tower placement is more difficult. Dealing with hills is more difficult than with GSM/TDMA. Things like that.
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Assisted Solution

arthurjb earned 150 total points
ID: 16992713
In most places the cops use a private radio system, not really a wireless phone.  (Although the technologies overlap and use similar underlying basics.)

In the US at least when cops are using wireless phones, they are using the same system that is availabe to the locals.  There may be priority service attached to the phones, so that the cops get lines to use, when the other users may get a system busy message.

In some places the cops use wireless phones because it is more secure than the police radios.  If using one of the digital modes, the phones are a lot harder to tap than a simple police radio.

Also, in small, poor areas, it may be that the police cannot afford their own radio system, and use the phones instead.  This is very poor management, but if the choice is phone or nothing, then the phone is better than not being able to talk at all.

Contrary to the opinion pushed on us by the industry, old time analog single channel radio is the safest way to communicate in an emergency, since there are no computers and no worrying about sharing resources.  Also a digital conversation may be made totally unuseable due to poor signal, while the old style analog system may be readable...

I am not sure how you determined the "It was totally inaudible with so much noise." statement.  Were you standing near the cop listening to both side of the converstaion, or do you mean that the surrounding area was noisy?  The various systems have similar noise, it is normally the quality of the receiving device that determines the sound.  For example a cheap phone using the built in speaker phone, or "walkie talkie" mode may have a poorer (sometimes refered to as nosier) sound,  while a better phone would sound better.

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Assisted Solution

lherrou earned 75 total points
ID: 16992809

I'm not sure what you are referring to, "wireless phones"? Can you elaborate?

In most countries, police departments use fm-based two-way radio service. There are a number of variants of that, but the most common is the "public-service" frequencies range, similar to the Ham Radio 2-meter band. This is reliable and provides good range (3-5 miles for a hand-held, 20+ miles for a car-mounted radio), but is generally not secure at all. Here in the US, most police departments have switched to other bands, including multi-frequency systems in the 800MHz band, where the radios hop between frequencies to prevent the average listener from recieving more than fragments of any transmission. The up-side of using radios over phones is simply push to talk (no need to know or select the number for a particular unit), broadcast to all units, and no need to rely on a telecomm co for service. Cell phones MUST go though cell towers, and cannot talk direct to one another without the underlying telecomm service.

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Expert Comment

ID: 16992962
>The up-side of using radios over phones is simply push to talk (no need to know or select the number for a particular unit), broadcast to all units, and no need to rely on a telecomm co for service.

This is only true in the old style fm system that you refered to in the begining of your post.

That is why I added my sentence about the industry falicies.  In most metro areas in the US, the publice service folks use trunking radios, which means that every conversation goes through the main system (towers similar to cell towers) and that officiers (and fire fighters etc) can die because the radios can get a "busy signal" or may not be able to communicate with other radios because of technical issues which do not exist with simple fm 2 way radios.....

There were reports of communications failures of the trunking system in NYC during the world Trade Center disaster.  Most of the failures were because of the complicated communication system which would be fine for taxi cabs or busses, but is horrible for emergency commuunications.
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Expert Comment

ID: 16993163

I can't say I disagree on the issues with digital and trunking radio systems. However, conventional systems also have their issues, as anyone who's had to wait on another to finish their verbose message to get through on a conventional system can attest. The best hybrid systems these days allow conventional and simplex operations as well as fully-trunked communications on a single unit - not that complexity isn't an issue, as you say, but a large metro area like NYC simply could not have enough frequencies to operate on a conventional system.

You hadn't posted yet when I started writing my original post, we clearly are saying similar things. :)

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