Roger Slyk
Director,
Product Marketing
Q. What is an M-CMTS?
A. A Modular CMTS, or M-CMTS, is one of the latest initiatives within DOCSIS® 3.0 standardization to enable the development of a CMTS (cablemodem termination system) where different elements can be implemented as separated physical entities.
Q. Why is it important?
A. Three reasons - lower prices, higher bandwidth, and flexible spectrum management.
Traditional CMTSs were built as a single monolithic entity. Inside a CMTS, you have various technologies - IP switching/routing, DOCSIS MAC (media access control), downstream QAM modulation and RF upconversion, and upstream QPSK/QAM demodulation. In the early development of the CMTS it was beneficial to have all of these technologies living in the same box to accelerate the development and deployment of the first systems. Nowadays, breaking these technologies apart allows operators to leverage economies of scale and share some components across multiple services.
One of the key goals of an M-CMTS is to enable higher data throughput for cable data subscribers. This is achieved by allowing multiple downstream data QAM channels to serve the same service group - commonly known as channel bonding or wideband DOCSIS.
Another cable service makes use of the same type of downstream QAM channel - VOD. DOCSIS 3.0 and the M-CMTS allow VOD Edge QAM channels, with minor modifications, to be alternatively employed as DOCSIS downstream data channels. Furthermore, sophisticated edge resource management will allow operators to flexibly and dynamically repurpose a VOD QAM channel to be a DOCSIS data channel, and vice versa. During the daytime, when VOD utilization is low, a VOD channel could be repurposed to be a data channel to provide, say, bandwidth capacity for commercial services. The widespread deployment of VOD Edge QAMs, and the subsequent rapid decline in per-QAM capital costs, is the catalyst which will enable the widespreadand cost-effective deployment of wideband DOCSIS.
Q. What are some of the technical challenges with specifying an M-CMTS?
A. DOCSIS has some sensitive timing requirements that regulate the flow of downstream and upstream data. When the upstream and downstream components are separated, extra care must be taken to ensure that these timing sensitivities are still respected.
DOCSIS 1.x and 2.0 make the assumption that a single downstream channel is associated with an upstream channel (although that downstream channelmay be shared among many upstream channels). DOCSIS 3.0 adds the requirement that multiple downstream channels be able to associate with a single upstream channel.
The RF behavior of a single downstream channel is defined in the DOCSIS 1.x and 2.0 specifications. DOCSIS 3.0 amends the RF specification to standardize the proper behavior of multiple downstream channels.
The Edge Resource Management Interface (ERMI), the DOCSIS Timing Interface (DTI), the Downstream RF Interface (DRFI), and the Downstream External PHY Interface (DEPI) constitute the new body of DOCSIS 3.0 specifications that will pave the way for multi-vendor M-CMTS component and solution offerings.
With M-CMTS exciting new opportunities are arising to combine the best elements of video and DOCSIS infrastructure technologies in order to enable more and better services over efficient and well managed cable plants.