When people use walkie-talkie radio, only one person can talk about time (do people say to press the button). This is because the walkie-talkies only use one radio frequency communication - a form of communication known as:
Simple: use of walkie-talkies, you have to press the button, talk one direction.
Of course this is not how the phone. Mobile phones can simultaneously achieve two-way transmission of data - a situation known as a full-duplex (ie two or more data streams can be sent, it is known as recycling):
Duplex: Let the same time, two-way data transmission. Communication channel from the base station, mobile unit known as downstream, and from the mobile device back to the communication base station is called. How to achieve full duplex communication? Well, there are two possible ways, we will now consider: TDD and FDD. TDD and FDD
Wireless duplex traditionally been required to implement two bands: one for the uplink and downlink (the band structure is known as paired band-band). This technique is called frequency division duplex or FDD. Both parts are separated by "guard zone" contains two signal isolation:
Duplex communication can be achieved when, instead of frequency. In this method, the prior and subsequent run on the same frequency, but they move very quickly: one moment to the uplink channel to send a signal that the next time send a signal to the downlink channel.Because it is moving very quickly but it seems to be a channel, in both uplink and downlink at the same time to act. This is called time division duplex or TDD. Area Development Department will take time, rather than the reverse flow protection between transmission and reception frequency bands.
Symmetric and asymmetric transmission transmission
Transmission of data is symmetric if the information in the downlink and uplink data transmission rate is the same data. This may be a case of voice transmission - the same amount of data transmission in both directions. However, Internet or broadcast data (such as video streaming), it is possible to send more data from the server to the mobile unit. 
Floppy Drive is not suitable for asymmetric transfer applications that use the same frequency band uplink and downlink (wasting valuable spectrum). On the other hand, the department does not have a fixed structure, a flexible bandwidth allocation is suitable for non-symmetric applications such as Internet . For example, TDD may be configured to provide downstream (primarily data transmission direction), and 64 kbps to 384 kbps upstream (such as traffic, in particular including information and confirmation request).
Ten macro, micro-cell and Pico-cellular
3G networks can be classified in a tiered approach:
Macro-cells - the largest coverage, for example, that the whole city.
Micro-cells - the coverage, for example, the city center.
Pico-cell - the smallest coverage, for example, a "hot spot at the hotel or the airport."
TDD mode does not allow transmission over long distances (deferred payment will uplink and downlink between the interference). For this purpose, TDD mode can only lead in the environment so that the small propagation delay (Pico cell). As explained in the transmission of symmetric and asymmetric transmission in the previous section, TDD mode suitable for Pico-cellular Internet data transmission efficiency.
We must consider how the phone can send and receive calls at the same time (through the uplink and downlink). Now we will check how many users can reuse the same channel (ie, share a channel without interference from another user), an ability known as multi-site. 3G technology is basically two competing technologies to achieve Multiple Access: TDMA and CDMA.
TDMA is Time Division Multiple Access. This can be split into smaller time slots, the radio frequency. Each caller group is given a specific period of transmission. Similar quick-change, each channel has the exclusive right to use the caller impression.
CDMA is Code Division Multiple Access. CDMA works is to give each user a unique code. Of all user signals can be scattered over a wide frequency range. For each user transmission frequency is not fixed but can vary in different bands. Recipient of the sender's unique knowledge of the code can extract the correct signal regardless of frequency.
This spread is very wide-band technique called spread-spectrum signals. Extending the benefits is that resistance to interference - if a block of frequency noise sources, the signal can still be sent to another frequency. Spread-spectrum signal, it is difficult to destroy, it is not surprising that this technology has been used for developing military applications.
Finally, we consider another advanced technology was originally developed by the military and the 3G is to be applied: the development of packet switching.
Circuit switching and packet switching
Traditional voice communication link will need the physical path to the user at both ends of the connection lines, and this road will remain open until the conversation ended. This relationship, let them exclusive access to a direct connection to the transmitter and the receiver of this method is called circuit switching.
Modern network technology is very different from this traditional model because it uses packet data. Packet data information is:
Chopped into pieces (packets)
In view of the recipient address,
Mixing and other data from other sources
With all the other data transmission lines,
Reorganization of the other end.
Packet-switching network is divided into separate "packets as a puzzle," telephone conversation, these are fragments reassembled to reconstruct the original conversation. Packet data first as the basis of Internet technology.
Most of the content is reserved for packet data transmission. This section is called the payload.Under normal circumstances, each transfer of data chopped into the same size payload. The package began as a small area called a header. Header is very important because the packet header contains the address of the recipient. This means that mobile phone users from different data packets can be mixed in the same transmission channel, and is positioned at the other end. It is no longer constant, exclusive between the sender and the receiver needs a direct line. Packet data is added to the channel if there are some things to send, the user is only charged for the amount of data transmitted. For example, to read, when a small article, users will only pay for what is sent or received. But both the sender and receiver access to communication channels that are "always" impression.
Below are data packets only added to the channel, the channel has a hole, so the speed can not guarantee a fact. The resulting delays caused by the voice packet network transmission problems because the web pages can load very slowly.
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