Consumers expect their TV viewing to be nearly as reliable as their electricity. Turn on the TV, and the program runs without interruption. As we move into the era of networked home entertainment, consumers will expect nothing less. Room-to-room streaming of high-definition (HD) content is one of the most desired capabilities for today's digital home.

For cable and content providers, the flawless delivery of entertainment services is their livelihood, and they must ensure that their customers receive reliable services.

Figure 1: Isochronous support, fundamental to TDMA networks, is required for the streaming of multimedia A/V content with QoS.

Failure results in customer service calls and the potential loss of customers to competitors. In recent years, the industry has pursued whole-home entertainment networks using existing networking technologies.

The demands of home network delivery
Existing networking technologies have been great for bulk file transfers, Internet access, e-mail or networking printers. However, these have proven largely ineffective when it comes to reliably distributing A/V digital content with the QoS that whole-home entertainment networks require.

Streaming content such as HD entertainment throughout the home imposes requirements that these legacy networking technologies were not designed to support. Among these requirements are substantial bandwidth, guaranteed QoS, low latency (minimal delays) and low jitter (differences in the latency from packet to packet). Additional features of an ideal home entertainment network include:

The ability to use existing wires in our homes or go wireless when possible;

A set of protocols and applications that operate across the network and provide true plug and play;

A user experience that mimics the relative simplicity of your entertainment system (a remote control you point at the TV), not a keyboard/ mouse/PC that only the IT guru in the home can use.

Figure 2: On an asynchronous network, new activity on the network is like new cars entering the freeway—nobody knows how bad traffic might get.

Pulse~LINK has developed CWave technology to support these requirements. The CWave UWB chipset enables the seamless distribution of interactive HD content, multichannel audio and high-speed data throughout the home over both wired and wireless media.

Because the same chipset is used for both wired and wireless communications, uniform guaranteed QoS and bandwidth reservations are maintained as entertainment data traverses the home network between differing devices and data transport media.

Networking technologies commonly promote ever-increasing data rates in their quest to support home entertainment networks, but the media access control (MAC) is seldom mentioned. Without a MAC that can guarantee QoS, real-world entertainment networks will "fall apart," resulting in customer service calls and product returns that create unwanted financial burden for service providers and manufacturers.

Figure 3: TDMA networks are comparable to a rail system that is managed with reservations and deterministic schedules.

CWave brings more bandwidth to home entertainment networks, but also leverages the IEEE 802.15.3b MAC, designed from the ground up to bring guaranteed QoS to IP-based networks.

This TDMA-based standard shares the same protocol stack as Ethernet and Wi-Fi so that it seamlessly interfaces with IP-based networks - but with important differences.

MACs serve as "traffic cops" between devices on a network. The MAC data communication protocol sub-layer is part of the seven-layer Open Systems Interconnection (OSI) model corresponding to the data link layer at L2.

It provides addressing and channel access control mechanisms, making it possible for multiple network nodes to communicate within a multipoint network embodied in any whole home entertainment network.

The MAC sub-layer interfaces to the logical link control (LLC) sub-layer at the very top of L2. The output of the LLC into the L3 network layer is the common convergence point where the network stacks for wireless, Ethernet, coax and power line become common.

The MAC provides logical control of how and when the network node is given physical access to the actual network transport medium. This is what makes it possible for several network nodes to be connected to the same physical medium and to cooperatively share access to it.

The ability for a MAC to provide access to the physical transport medium within tightly bounded limits of time (isochronous) is what determines whether a particular MAC is capable of guaranteeing the QoS required for a whole-home entertainment network.

A MAC that can't guarantee access to the PHY layer transport media within tightly bounded time limits is considered to be a "best effort" type of network.

CSMA, TDMA
The predominant MAC architectures used in home networking technologies today largely support two channel access schemes known as carrier sense multiple access (CSMA) and time division multiple access (TDMA).

CSMA enables the Ethernet and Wi-Fi technologies found in our homes and offices today. These were designed for network functions such as bulk file transfer, e-mail, simple Internet access and printer sharing.

Because of their legacy use in our homes and offices, there are attempts to modify CSMA networks to support multimedia networking by adding "prioritization" (parameterization).

These result in better performance for multimedia over such networks, but do not address two fundamental issues and thus fall short of "guaranteed" QoS.

The first fundamental issue is the contention-based access to the physical transport media. Because the CSMA scheme is not scheduled, QoS (delivery time, jitter) is not "guaranteed" due to delays associated with collisions or collision-avoidance protocols. The second issue is the connectionless, prioritized transmission of data, which relies on intermediate nodes to have enough resource and intelligence to get the data delivered to the endpoint.

Alternately, TDMA provides a suitable foundation for multimedia networks. TDMA guarantees QoS within tightly bounded limits of time and supports isochronous data streams such as those required by multichannel audio and HDTV. TDMA is fundamental to the IEEE 802.15.3b MAC, which is used by CWave.

The QoS requirements of entertainment networks differ from networks for e-mail, file sharing and printing. In an entertainment network, if the data does not arrive on time, every time, the TV screen goes blank, and displays errors such as video artifacts or dropped audio. The delivery of data within tightly bounded limits of time is referred to as "deterministic" or "isochronous."

The other basic method for transporting data over a network is asynchronous. CSMA networks are asynchronous. Think of an asynchronous network as similar to the highway system. It may have been designed to allow you to drive at high speeds, but it depends on traffic conditions and doesn't guarantee that you will always be able to do so.

On an asynchronous network, new activity on the network is like a new car entering the freeway - nobody knows how bad traffic might get.

Prioritization parameters
The "fix" for multimedia networking on CSMA networks has been to add "prioritization" and "parameters." Think of these methods as adding a siren on your car and a traffic cop at the entry to the freeway to let only approved traffic through - delivery may improve in heavy traffic, but a jam still leaves you slowing down or stopping entirely.

An alternate solution for CSMA networks is to add buffering in the form of memory or a hard drive. Think of it as hitting the highway long before you need to be home so that no matter how bad the traffic, you'll get home before you need to be there.

While this can work, it adds cost and latency to the multimedia network and can complicate realtime applications such as interactive gaming or channel changing for TVs and STBs, since the buffer must be flushed and refilled with each new channel.

TDMA networks are isochronous, comparable to a rail system. You can reserve a seat on the 9:15 train and know exactly when you will arrive at your destination. It is a managed system with reservations and deterministic schedules. The IEEE 802.15.3b MAC is capable of supporting deterministic isochronous data streams.

A network capable of supporting "guaranteed" delivery of network traffic in a deterministic manner typically eliminates the need for buffering or hard drives. Isochronous support, fundamental to TDMA networks, is required for the streaming of multimedia A/V content with QoS.

A whole-home entertainment network requires the rethinking of what we have traditionally found to be acceptable for "best effort" networks of the past. Streaming HD content around the home raises the bar substantially, requiring increased data throughputs coupled with higher levels of QoS.

The good news is that there is now a ratified IEEE standard for IP-based networks fundamentally designed from the ground up to bring deterministic QoS necessary for whole-home entertainment networking - the IEEE 802.15.3b standard.

John Santhoff is Founder andChief Technology Officer Pulse~LINK Inc.