MoCA 1.1 spec improves HD home video network performance

With its latest release of the MoCA 1.1 specification, the Multimedia over Coax Alliance has been able to achieve increased throughput performance and incorporate a novel approach to enabling quality-of-service over the coax home network. These spec extensions enable in excess of 175 Mbps of data throughput at the MAC layer and with the new Parameterized QoS (PQoS) method, critical traffic flows, like video streams, can be assured the bandwidth they require. These two key enhancements are designed to address the growing need for more bandwidth within the home LAN and ensure that video traffic is never compromised during situations where heavy, but less time-critical, traffic arises.

MoCA background
Designed for multimedia networking within the home, MoCA crafted a standard that specifically considered the requirements necessary to support real-time streaming of entertainment media. Network technologies that are primarily concerned with providing PC connectivity, Internet access and file sharing capability do not necessarily need nor meet the stringent requirements that a multimedia network does.

Rapid consumer adoption of high-definition television is driving the need for throughput large enough to support multiple high-definition video streams. However, a multimedia network must also support low latency, low error rates, quality-of-service and reach all of the home's coax outlets in order to fulfill the expectations of system operators and consumers. The original MoCA 1.0 spec was targeted to achieve approximately 100 Mbps of aggregate MAC throughput. The newest additions to the specification enable MoCA to achieve 175 Mbps with only MAC layer changes. This increase in MAC efficiency is achieved by aggregating multiple Ethernet packets into a single MoCA frame.

With streaming video data, any extended delays in the delivery of data packets can potentially lead to video decoding artifacts that spoil the viewing experience. Thus, the network requires a QoS mechanism that protects video content from the side effects of heavy network usage. The brute force approach to ensuring QoS is to simply increase bandwidth such that data flows never experience a condition of limited bandwidth. However, increasing bandwidth for QoS concerns is not a very efficient use of network resources and is just a stop-gap measure as applications are continually increasing their needs and will eventually consume this additional bandwidth. Relying on just adding more bandwidth also doesn't take into account that simple network file transfers can consume all of the bandwidth in transient bursts. System operators need a QoS mechanism that protects their premium content, yet is flexible enough to work in a network environment that includes QoS and best-effort traffic.

MoCA 1.0 supports a prioritized traffic scheme to enable QoS data flows simultaneous with best effort traffic. The MoCA 1.1 spec takes QoS one step further by introducing parameterized QoS. Whereas prioritized QoS simply ranked data packets by priority relative to one another, parameterized QoS operates by reserving the actual bandwidth required to ensure that peak data rate transfers are always accommodated over the network.

Packet aggregation
Depending on the applications being used, a mixture of small and large packets may be found traversing the home network. Traditional applications such as file downloads, heavily buffered web video and web page refreshes can use large packets that are transferred from the wide area network (WAN) in long back-to-back bursts. Applications that require real-time data flows like interactive gaming typically use small IP packets transmitted at regular intervals. Even the TCP/IP protocol makes use of small packets in the form of ACKs, of which there may be several within a 1 millisecond period.

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Figure 1 shows a simplified example of a MoCA packet aggregated frame.

In addition to the actual payload, MoCA requires transmission of its own control and header information that serve to delineate the start of a packet and describe the contents of its payload. This additional data is comprised of PHY preambles and MAC headers that do not usually vary in size for any given payload length. By improving the ratio of payload to control/header data using packet aggregation, the efficiency of the network's bandwidth can be maximized. Packet aggregation combines multiple Ethernet packets into a single MoCA MAC frame and greatly increases the ratio of useful payload data to header information. Thus, more network bandwidth is used for application data rather than header information, resulting in improved packet throughput.

Additionally, the overall inter-frame gap (IFG) time and number of reservation requests are reduced since packets that have been aggregated no longer need their own individual MoCA frames. For example, a MoCA frame that has six aggregated packets saves 5*10 = 50 microseconds of IFG time.

Next: Throughput results