VoD Transport

The popularity of Video on Demand (VoD) is growing and is now a key differential for MSOs. From a bandwidth perspective, VoD is an interesting service insofar as the traffic flow is primarily downstream while upstream traffic is limited to simple control signals from set-top boxes.

The key building blocks in a VoD system are content servers, edge QAMs, and set-top boxes. While the set-top boxes obviously need to be located at the end-user premises, the location of the servers and edge QAMs is discretionary.

There are three main architectures which can be deployed:

•   Centralized servers and edge QAMs at the headend


•   Distributiion of both servers and edge QAMs at the hub


•   Hybrid architecture with servers at the headend and edge QAMs at the hub

Each approach has its own relative merits.

Centralized architecture:

•   All equipment is at a fully-manned facility, making this approach operationally efficient.


•   The equipment can be shared across the entire serving area, optimizing the usage of the servers and edge QAMs.


•   However, precious RF bandwidth (and fibers) are needed to transport from the edge QAMs to the field.

Distributed architecture:

•   All key equipment is local to the end-user, but requires a large (probably un-manned) hub location, consuming valuable real-estate.


•   The equipment services a limited serving area, resulting in inefficient scaling and potentially expensive sparing.


•   Bandwidth usage is efficient, with limited extra bandwidth consumed from the headend to the hub.

Hybrid architecture:

•   Content servers are at a staffed location and these expensive servers are shared across a larger subscriber base, optimizing usage.


•   Interconnection from the headend to the hub can be implemented using cost-effective uni-directional Gigabit Ethernet, using the IP interfaces of both the servers and the edge QAMs.

A hybrid architecture is the optimum implementation; it is scalable, with up to 40 wavelengths supported on one fiber today, and cost-effective to deploy. This architecture also lends itself to the emerging modular CMTS and universal edge QAM technologies.

Figure 1. Optimized VoD architecture

Click on diagram to enlarge

Aurora's DX3515 Digital Transponder provides a simple means to transport the optical bit stream from the server to edge QAM devices. With up to 40 wavelengths supported, this is a very fiber-efficient solution. Indeed, armed with our wide range of passives, multiplexers, special purpose demultiplexers, and EDFAs, MSOs have a system which can reach up to 200 km. And for even greater distances, a re-transmit or dispersion compensation module can be deployed.

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