The need for Edge QAM

The "Edge QAM" came to existence with Switched Digital Video (SDV). Prior to SDV, QAM modulation was used in cable plants to carry all digital cable services. It sits in hub and is used to move video, voice and data from headends to homes. Each QAM, in a rack-mounted metal box, is capable of carrying 38.8 Mbps of downstream data which is equivalent to one analog channel, 2-3 High-Definition (HD) streams, or 10-12 Standard-Definition (SD) digital video streams.

Even though all QAM use the same architecture, each QAM is dedicated to a specific service. For example, one QAM is used to move Web pages to your cable modem, while another one is assigned to move the digital video channels to your HDTV. Likewise, a QAM dedicated to video on demand (VOD) could not be used for Switched Digital Video (SDV). Hence, the cable operators had to purchase QAM based on services they wanted to provide.

This fundamental issue, created the need for building "Edge QAM", which by definition is capable of carrying both VOD and SDV streams. The next generation Edge QAM will be able to carry IP related services, such as data (Internet service) and voice (VoIP service) in addition to VOD and SDV.

The Players

Some of major vendors of Edge QAM are, Scientific Atlanta/Cisco, RGB Spectrum, Arris, BigBand Networks, Harmonic, Motorola.

The cable operator evaluate the Edge QAM based on three objectives, Price, Density and Openness. With respect to price, currently the QAM are in range on $250. The goal is to reduce to price to $25 range. With respect to density, currently one rack-mounted unit can take between 8-24 QAMs. The cable operators expecting new innovation will increase the density which will ultimately bring the price down. With respect to openness, the cable operator are looking for vendors that eventually can adopt standards protocols.

Functionality

In order Edge QAM move VOD and SDV streams to homes, it needs to communicate with two other device in the network. The "Session Manager", which sets up the linkage between VOD/SDV server and Set-Top-Box and acts upon selecting a channel by viewer. The "Resource Manager", which determines which QAMs are supposed to be moving what stuff and to where. Also see SDV Architecture and IPTV - The software behind SDV for more information.

Currently, the devices are tightly coupled, meaning that they work on propriety protocols. Each manufacturer has developed it's own protocol which makes cable operators bound to purchase set of devices from same vendor. New standard is underway to make the communication protocols between devices consistent. This will create competition among vendors to cut the prices, since cable operators will be able to purchase their devices from multiple vendors. A switch from vendor, a resource manager from another and a session manager from yet a different vendor.

About me:
bruce atlasi is a professional computer engineer, skilled in telecomm and datacomm technologies and architecture. He has diverse working experience with many telecomm start-ups and fortune 100 companies, including Cisco Systems, IBM, and Siemens. He regularly blogs on About Hi-Tech site.