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air interface 空中接口


Air interface
Question: In no more than 1000 words explain what is meant by ‘ air
interface’ in relation to wireless communication. Explain its function in the communication process and briefly explain two of the standards used for the air interface with cellular mobile phones

Answer:
Air interface is the connection between the User Terminal and Radio Access network. In other words, air interface is used to connect the mobile phone to base station in the wireless telecommunication. The air interface in mobile is the modulation part and the antenna which the information is modulated and sent by antenna. The antenna sends the information using electromagnetic signal to air. And the receiver part in the base station is also belongedto the air interface. Air interface provide the standard of wireless transmission between mobile phone and base station. It stipulates the frequency, channel bandwidth and coding method in wireless communication. In GSM digital cellular network, air interface is divided into two kinds. One is Physical channel and the other one is Logic channel. Every time-slot is a physical channel. While, there are two logic channels in GSM air interface—Traffic channel (TCH) and Control channel (CTH). TCH

is always transmit voice and date information.For further, TCH can be divided into full-speed TCH and half-speed TCH.“Um” interface is the wireless which is the most important interface in this system. As same as the air interface, “Um” interface is between the mobile station and the base transceiver station. The physical layer of Um interface is built on wireless channel and this layer can divide logic channel then produce different logic channels. In GSM system, it uses the three layers protocol which includes physical layer, date link layer and application layer. The physical layeris the bottomlayer in protocol and provides the all function that physical medium will need to transmit bit stream. Date link layer can make sure to send massage correctly andrecognize the single calling. In the GSM system, the wireless interface on physical layer is TDMA frame and on the date link layer is LAPDm protocol. In the application layer, the communication information is divided into radio resource management, mobile management and call control according to the step and function. When a call begins, the system will build a radio resource connection first, it can manage the physical and logical channel. In the normal situation, only the radio resource connection built, the mobile management can be connected. Then construct the call control connection. After that, the calling path can be connection successfully. When a call finished, the system starts to eliminate telecommunication link. The calling control and mobile management connection will be

firstly eliminate when one side hang up the phone. Then release the radio resource connection to release the channel they used. The next air interface is used inCDMA. The name of air interface used in CDMA is also named “Um”. In this module, it also definesthe physical layer structure and the signaling in wireless interface which to make sure the communication can connect successful and better. The followings are the rules and specialties of Um interface in CDMA. (1) The abilities of channel structure and access capability. (2) MS-BSS communication protocol, (3) The character of preserve and operation. (4) Operation peculiarity In the CDMA, the air interface is divided into three layers. Layer 1 is the physical layer which is the bottom layer in the structure. In physical layer, high layer information and air wireless signal can convert each other through physical channel. Beside, this layer also can process different kinds of physical channel. Such as date coding/decoding and modulate/demodulate. Then is date link layer which is the second layer that contains LAC (Link Access Control layer) and MAC (Media Access Control layer). LAC is mainly to make sure the signaling can transmit reliable. And MAC is to multiplex operations on every logic channel and classify the service level. Last is layer 3 which is on the top. In this layer, it includes signaling layer ’s structure, the function and format of the

signaling control. In the MAC sub-layer, it defines a Best Effort Delivery function. It means that Radio Link Protocol provide the high quality reliable to make sure the information can transmit on it safely. In this layer, it also contain the function of control multiplexandQoS (Quality of Service). It is defined as dealing by management to coordinate the conflict between contradictory operations and manage the proper priority for input requirement. All these are to make sure following the QoS level rules. Beside, MAC can convert the map between logic channel and physical channel.

Reference: 1. http://www.pcmag.com/encyclopedia/term/37626/air-interfac e 2. http://searchmobilecomputing.techtarget.com/definition/air-in terface 3. http://www.artechhouse.com/uploads/public/documents/cha pters/hei-ch07.pdf 4. https://zh.scribd.com/doc/104483865/An-Introduction-to-CDM A-Air-Interface


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