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Multi-access Channels and Random Access Channels
Multi-access Channels
Multi-access channels are network channels that allow several transmitters to communicate with a common receiver via a shared channel. These channels are also called multiple access (MAC) channels. The network channel may be a single cable or optical fiber connecting multiple nodes, or a portion of the wireless spectrum.
Random Access Channels
Random-access channels (RACH) are shared channels used in wireless communication systems to allow mobile devices for call set-up. RACH also handles bursty data transmission. RACH is popularly used by GSM mobiles for transmission over cellular networks.
Multiple Access Schemes
Both multi-access channels and random access channels require a scheme to allow multiple users to gain access to the shared channel and use it simultaneously. The popular schemes are FDMA, TDMA and CDMA.
Frequency Division Multiple Access (FDMA) – In FDMA, the total bandwidth of the channel is divided to a set of frequency bands that do not overlap. Each of these bands is a carrier of a different signal that is generated and modulated by a user. The frequency bands are separated from one another by strips of unused frequencies called the guard bands, to prevent overlapping of signals.
Time Division Multiple Access (FDMA) – In TDMA, the channel is divided into time slots. Each user is allotted an input time slot. During transmission, the users are allotted the time slots in a round-robin basis.
Code Division Multiple Access (FDMA) – CDMA is a digital technique that uses spread spectrum communication. In spread spectrum communications, a narrowband signal is spread over a larger band of frequency or across multiple channels via division. It does not constrict bandwidth’s digital signals or frequencies. It is less susceptible to interference, thus providing better data communication capability and a more secured private line.
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