Cellular Technology Explained

Posted by Sam Churchill on

Gizmodo explains Why Your Call Dropped:

Towers each carry calls placed within a certain radius. In an open, rural environment, this radius can be a few kilometers. In a city, it can be well under 500 yards.

These areas of coverage generally overlap, so that there’s nowhere a phone can be where it doesn’t have a tower to talk to. A phone keeps track of these cells, as they’re called, and notes how many are strong enough to place a call on. When one fades, in theory, the phone will have another to which it can hand off the call.

But these areas aren’t the same size as one another, or even a consistent size. They fluctuate wildly, due to a phenomenon call “cell breathing.”

On just about any 3G network, carriers transmit voice signals with CDMA, or code division multiple access. (Yep, this includes HSPA 3G, which is often referred to as GSM.) What this means is that multiple phones can transmit over the same radio frequencies, and their signals are differentiated by code. (Disclaimer: this is a brutal simplification.) As one network engineer told me, sharing a cell tower is like sharing a room with a bunch of people that speak different languages. Different people can hold concurrent conversations, but everyone can understand what they need to—their brains block out the rest of the conversations, because to them, it’s all just gibberish anyway.

Just like in this shared room, though, as a tower gets more crowded, the volume starts to rise. The more everyone speaks, the louder one has to talk to be understood. Likewise, the more people that are using a cell tower, the more power each phone needs to be “heard” by the tower. This actually results in a contraction of the cell’s coverage area.

In other words, the more people using a tower at once, the less its range.

…So while that grid of cells in theory leaves no spot uncovered, in reality these vibrating fields of coverage have strange shapes that are difficult to calculate, and subject to constant change.


GSM uses Time Division Multiple Access (TDMA) slots rather than Code Division Multiple Access (CDMA) channels, which allows several signals to share the same channel (separared by “codes”. UMTS (3G) uses elements of both.

The GSM Association estimates that GSM is used by 1.5 billion people across more than 212 countries, some 80% of the global mobile market. GSM networks operate in a number of different frequency ranges. AT&T, in the United States, uses the 850 MHz and 1900 MHz (PCS) bands while T-Mobile uses PCS and 1700/2100 (AWS) bands. With GSM, frequency is divided into timeslots for individual calls. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. Handoff can be problematic between cells if slots are not available.

In 4G standards the CDMA spread spectrum radio technology is abandoned and replaced by OFDMA. Orthogonal Frequency-Division Multiple Access (OFDMA) uses a variety of techniques to avoid interference from neighboring cells, using different carrier permutations and a variety of coding to achieve ruggedness and handoff.

Google now allows porting your existing mobile number to Google Voice. This means you can make your cellular phone number, your Google Voice number. That way it can ring any phone you want — or even your computer. Google Voice uses the internet to transport calls between cities.

Porting your number to Google Voice costs $20 and is usually completed within 24 hours. You may incur additional charges, including early termination fees, from your wireless carrier.

After porting your number to Google Voice your mobile service plan will be cancelled, and there are a couple of steps that you’ll have to take to continue making and receiving calls on your mobile device. For more detailed instructions on how Number Porting works and to find tips for making the process as smooth as possible, visit the Google Voice Help Center.

Google Voice provisions a U.S. phone number, chosen by the user from available numbers in selected area codes, free of charge to each user account. Inbound calls to this number are forwarded to other phone numbers of the subscriber. Outbound calls may be placed to domestic and international destinations from any of a user’s configured telephones. Google, via Gmail provides free PC-to-PC voice (and video) calling worldwide by using the internet. Google Voice calls are free, but they still use your cell minutes if you’re using a cell phone.

Google Voice gives you one number that can ring you anywhere, explains PC Magazine. It also gives you a free voicemail box and lets you make cheap international calls. All calls and text messages within the U.S. and Canada are free in Gmail—in both directions.

By contrast, Generic Access Network can support WiFi calling with a UMA-enabled dual-mode handset. This is different than Skype or Google Voice over Wi-Fi because they only work on Wi-Fi. With UMA, if there is Wi-Fi available, your cellular phone may be able to attach and use Wi-Fi. When on Wi-Fi, the call goes through the WiFi-enabled mobile phone, to your wireless WiFi router or other broadband link. The advantage is you can use less cellular minutes at home and may get better indoor coverage.

Wikipedia explains Handoff and soft handoff in cellular networks. How Stuff Works has more on celluar telephony.

Posted by Sam Churchill on Tuesday, January 25th, 2011 at 2:02 pm .

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