TDMA - Time Division Multiple Access
Why use TDMA?
TDMA (Time Division Multiple Access), is used in the largest available networks in the world. It is a digital communication method allowing many users to access a single communication channel. TDMA is aimed at dealing with multiple access to the same communication medium. Each individual user is given a unique time slot within the defined communication channel. This methodology increases the efficiency of transmission by allowing multiple users simultaneous access to a time slot. A significant benefit is TDMA can be easily adapted to transmission of data as well as voice communication. And the reason for choosing TDMA for all these standards is that it enables some vital features for system operation in an advanced cellular or PCS environment.
TDMA offers the ability to carry date rates of 64 kbps to 120 MBPS, which enables options of communication such as fax, voiceband data, sms, as well as bandwidth intensive apps. TDMA allows the mobile device to have extended battery life, since the cellular device is only transmitting a portion of the time during conversations. In addition, TDMA is the most cost effective technology for upgrading an analog system to digital (http://www.iec.org/online/tutorials/tdma/topic04.html).
How it works
It’s necessary for TDMA to rely upon that fact that the audio signal has been digitized. These signals are divided into a number of milliseconds and distributed into time slots. TDMA is also the access technique used in the European digital cellular standard, GSM, and the Japanese digital standard, personal digital cellular (PDC). A single channel can carry all four conversations if each conversation is divided into relatively short fragments, is assigned a time slot, and is transmitted in synchronized timed. For instance, if there are four people: Jan, Tom, Bill, and Bob making calls, each would be assigned a time slot on a single channel. However, if Bob was no longer using his phone, his time slot would still be used.
Pros & Cons
TDMA can be wasteful of bandwidth because time slots are allocated to specific conversations whether or not anyone was speaking at the given moment. There is an enhanced version however, EDTMA, which attempts to correct this problem. Unlike TDMA which waits to determine whether a subscriber is transmitting, ETDMA assigns subscribers using a dynamic method. The data is sent through pauses which normal speech contains. If the subscriber has something they would like to transmit, it is placed as one bit in the buffer queue. The system then scans the buffer and notices the user has something to transmit, allocating the bandwidth accordingly. However, if there is nothing to transmit, it goes to the next subscriber. This well proven multiple access technique can be 10 times more efficient as analog transmission of TDMA.
Cell Phone Providers
There are several cell phone companies competing to sell their phones and advertise their network coverage area. They will primarily be competing within two categories: TDMA and CDMA. The pros and cons of TDMA have been mentioned, and the companies that offer TDMA are the following: AT&T, Cingular, Nextel, T-Mobile. The companies that support CDMA are: ALLTEL, Amp'd Mobile, Cricket Wireless, ESPN, Quest, Sprint, Verizon, Virgin Mobile. As we can see, more companies are supporting CDMA. The question that arises is, what makes the two so different?
TDMA Vs. CDMA
TDMA is better for international plans and debately has better battery life. CDMA claims it has better battery life and coverage, however, Cingular, in the US, is the largest TDMA carrier. All of this started however ever since CDMA was introduced in 1989, and the wireless world has been in debate over merits of TDMA and CDMA. Those who are for CDMA have claimed that its technology has bandwidth efficiency of up to 13 times that of TDMA and between 20 to 40 times that of analog transmission.
Those who favor TDMA point out that there has been no successful major trial of CDMA technology that supports the capacities it claims. Not to mention, theoretical improvements in bandwidth efficiency claimed CDMA is now being approached by enhancements to TDMA technology. TDMA's evolution allows capacity increases of 20 to 40 fold over analog in the near future. CDMA is a very expensive technology that needs $300,000 per base station, compared to $80,000 for TDMA. Lastly, TDMA is the proven leader as the most economical digital migration path for existing AMPS networks. No one has the final word in this debate, however, it is evident that TDMA will remain the dominant technology in the wireless market.