RFATDMA vs SCDMA: DOCSIS Standard Comparison
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This page compares ATDMA vs SCDMA used in the DOCSIS standard and highlights the differences between them.
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 networks. It was developed by CableLabs along with other companies.
The DOCSIS standard defines protocol layers to ensure interoperability and quality of cable modems. There are two techniques used to provide resource sharing among cable modems: ATDMA and SCDMA.
They were introduced in the DOCSIS standard 2.0. Refer to the architecture of DOCSIS network.
Figure-1: TDMA in GSM900
In the DOCSIS 1.x standard, TDMA multiplexing is used in the Physical layer. Using TDMA, all the cable modems transmit on the same RF frequency at their individual time slots.
This TDMA scheme was adopted in DOCSIS 2.0 with numerous enhancements, which is called “ATDMA”. ATDMA refers to Advanced Time Division Multiple Access.
ATDMA is used in the upstream direction with bandwidth up to 6.4 MHz at 5.12 Msym/sec. It uses 8QAM, 32QAM, and 64QAM. It also utilizes FEC, burst interleaving, and a 24-tap equalizer in its physical layer.
Figure-2: CDMA
SCDMA stands for Synchronous Code Division Multiple Access. It was introduced in the 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 the data throughput of the system like ATDMA.
Tabular difference between ATDMA and SCDMA
The following table mentions the difference between ATDMA and SCDMA with respect to their use in the DOCSIS series of standards.
| Features | ATDMA | SCDMA |
|---|---|---|
| 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 the same frequency. | Cable modems use different orthogonal codes to transmit symbols simultaneously in the same frequency and time slot. |
| Application where it is most favourable | Ingress environment | Impulse noise environment |
