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Advantages of ATDMA | Disadvantages of ATDMA in DOCSIS

This page covers advantages and disadvantages of ATDMA in DOCSIS standard. It mentions benefits or advantages of ATDMA and drawbacks or disadvantages of ATDMA. ATDMA stands for Advanced Time Division Multiple Access.

What are TDMA and FDMA ?

TDMA, FDMA and CDMA are multiple access techniques which helps in resource allocations and their access by multiple subscribers or users on shared basis. FDMA divides allocated frequency spectrum into different carrier frequencies called "channels". The carrier frequencies are separated by channel spacing between them to prevent interference. FDMA allows simultaneous operation by multiple subscribers using different unique frequencies. The figure depicts channel allocation in GSM in both uplink and downlink within its allocated frequency spectrum (890 to 915 MHz in uplink and 935 to 960 MHz in downlink). Total 124 channels are allocated in 25 MHz spectrum with 200 KHz channel spacing between them.

FDMA in GSM900
Figure-1 : FDMA in GSM900

TDMA-Time Division Multiple Access
Figure-2 : TDMA in GSM900

In TDMA scheme, multiple subscriber devices or cable modems share same RF channel in different time slots. As shown in the figure, each GSM channels are assigned eight time slots which are used by eight subscribers. TDMA is also used in DOCSIS network for communication by cable modems with CMTS. Refer architecture of DOCSIS network >>.

What is ATDMA?

In DOCSIS 1.x standard, TDMA multiplexing is used in Physical layer. It allows cable modems to share same RF carrier frequency i.e. channel at different time slots. This TDMA scheme has been adopted in DOCSIS 2.0 and consecutive standards with numerous enhancements which is called by the term "ATDMA". The ATDMA refers to Advanced Time Division Multiple Access.

ATDMA is used in upstream direction with bandwidth up to 6.4 MHz at 5.12 Msym/sec. It uses 8QAM, 32QAM and 64QAM modulation types. It uses FEC, burst interleaving and 24 tap equalizer in its physical layer.

Benefits or advantages of ATDMA

Following are the benefits or advantages of ATDMA:
➨ATDMA allows simultaneous access of cable network by multiple cable modems at the same frequency during different time slots. This increases spectrum efficiency of the DOCSIS system.
➨ATDMA uses FEC and byte interleaving in order to combat impulse noise and burst noise. Hence it makes physical layer more robust.
➨ATDMA uses wide bandwidth compare to TDMA used in DOCSIS 1.x version. Moreover it supports higher order modulation schemes such as 8QAM, 32QAM and 64QAM which helps ATDMA to support higher throughput.

Drawbacks or disadvantages of ATDMA

Following are the drawbacks or disadvantages of ATDMA:
➨It is inferior to SCDMA in impulse noise environment conditions.
➨It does not work with load balancing.

➨Refer advantages and disadvantages of SCDMA >> and difference between ATDMA versus SCDMA >>.

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