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ATDMA Vs SCDMA | Difference between ATDMA and SCDMA in DOCSIS standard

This page compares ATDMA vs SCDMA used in DOCSIS standard and mentions difference between ATDMA and SCDMA. ATDMA stands for Advanced Time Division Multiple Access and SCDMA stands for Synchronous Code Division Multiple Access.

Introduction: DOCSIS (Data Over Cable Service Interface Specification) has been developed to provide internet access over existing cable network. It is developed by CableLabs along with other companies. DOCSIS standard defines protocol layers to ensure interoperability and quality of the cable modems. There are two techniques used to provide resource sharing among cable modems. The access techniques are ATDMA and SCDMA. They are introduced in DOCSIS standard 2.0. Refer architecture of DOCSIS network >>.


TDMA-Time Division Multiple Access
Figure-1 : TDMA in GSM900

In DOCSIS 1.x standard, TDMA multiplexing is used in Physical layer. Using TDMA all the cable modems transmit on same RF frequency at their individual time slots. This TDMA scheme has been adopted in DOCSIS 2.0 with numerous enhancements which is called as "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. It uses FEC, burst interleaving and 24 tap equalizer in its physical layer.


Figure-2 : CDMA

SCDMA stands for Synchronous Code Division Multiple Access. It has been introduced in DOCSIS 2.0 standard along with ATDMA. SCDMA uses 128 orthogonal codes in order to transmit 128 symbols simultaneously. SCDMA allows multiple cable modems to transmit in the same time slot. SCDMA increases data throughput of the system like ATDMA.

Tabular difference between ATDMA and SCDMA

Following table mentions difference between ATDMA and SCDMA with respect to their use in DOCSIS series of standards.

Full Form Advanced Time Division Multiple Access Synchronous Code Division Multiple Access
Basic access technique TDMA CDMA
Symbol time Shorter Longer than ATDMA
Modules used to combat burst and impulse noise FEC and byte interleaving Time spreading and framing
Resource allocation Multiple cable modems use different time slots at same frequency. Cable modems use different orthogonal codes to transmit symbols simultaneously in same frequency and time slot.
Application where it is most favourable Ingress environment Impulse noise environment
Advantages and disadvantages ATDMA >> SCDMA >>

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