TDMA and CDMA: concept, typical applications and comparison
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II. MEDIA ACCESS TECHNIQUES
Generally speaking, typical scenario of communications networks interaction consists in a bunch of devices trying to communicate with each other by using the same medium. Home wireless networks, cellular networks, Ethernet environments are only a few examples of this setting.
Sharing a medium requires specific procedures or agreements among all network users in order to set rules for effective and efficient communication, these procedures are the so called Media Access Control Protocols. They dictate important aspects of the interaction, some of them are presented as follows:
When and how long can a device access a medium.
How can a device determine weather the communication succeeded or not.
How does a device deal with problems (access conflicts, security issues, etc). [*Data communications and networking, Forouzan, Fifth edition page 326]
Nowadays there are multiple MAC protocols that serves different network approaches and topologies, it is clear that for instance an Ethernet TCP/IP network that is packet based, wired and so forth should have a different sharing medium technique than a 2G voice network that uses free air as communication interface and is mainly voice oriented. Any MAC protocol in general should pursuit some basic guidelines in terms of structure and performance but implementation may be different depending on the network setting and application.
A broad break down of MAC protocols is presented as follows:
CSMA (Carrier sense media access)
CSMA/CD (Carrier sense media access with collision detection)
CSMA/CA (Carrier sense media access with collision avoidance)
TDMA (Time division media access)
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...ium access selected technique. It also uses FDD carriers and was first developed for 900 MHz (GSM 900) band but then extended to 1800 MHz band (GSM 1800). GSM was originally designed as a voice oriented network but later on some new features have been aggregated to support data transmission. General Packet Radio Services (GPRS) and Enhanced Data-rates for Global Evolution (EDGE) showed up accordingly to fulfill market demands in terms of voice reliability and high transmission data rates. As of today GSM is still dominant in current mobile communication market, it has 70% of market share (wikipedia reference) despite new upcoming technologies such as LTE taking prominence these days.
IV. CODE DIVISION MEDIA ACCESS
CDMA is a multiple access technique that allows multiple users to access a medium by assigning each user a unique key also known as spreading code.