To meet the requirements of almost all potential users, a home networking solution must deliver reliable network connectivity without new wiring. Only the truly adventurous technologist will struggle with the installation of cabling.
However, the home networking user has neither access to support staff nor a comparable budget-witness home-PC price trends. Simplicity is paramount. Most people will expect "access points wherever I need them"-the network must fit the consumer's requirements, not vice versa. And no one will tolerate a painstakingly slow download every time more than one user comes on the network because the value of a network is created by the multiplicity of devices that can be connected, often simultaneously.
In that light, the ac power line is a compelling medium. Power line does not require the additional cost of a 2.4-GHz RF front end, and many access points are available in every room by virtue of the electrical code.
Basically, consumers expect low cost, adequate bandwidth and coverage everywhere. Besides those basic expectations, there are technical requirements upon a home networking solution to support multimedia. That is because even in the home, the days of a data-only network have passed. Any home networking solution must additionally support the isochronous requirements of voice, audio and video.
This quality of service (QoS) support for multimedia must be integral to the design of the medium access control (MAC) and Link layers. A physical layer technology alone cannot provide QoS. In addition, four essential ingredients must be incorporated in the MAC and Link protocols: minimum guaranteed bandwidth, fairness in bandwidth allocations, bounded access latency and short packet handling.
The first element is important because when you start a voice conversation, you expect to be able to complete it. People will not tolerate being cut off because of a temporary lack of network bandwidth. And when it comes to bandwidth allocation, just because I started my download before you decided to make a call doesn't mean that I get to tell you to try again later. Bounded access latency is needed because voice, audio and video traffic need consistent and regular access to the network. Finally, no matter how fast you talk, you still generate only a small amount of digitized bits every 10 to 20 milliseconds. Real-time traffic can result in frequent, but very short packets. The network must not penalize this type of traffic by expanding these short packets to large minimum-sized packets resulting in inefficient utilization of the network.
To arrive at a power line home networking solution, the medium-specific problems must be recognized and then addressed hierarchically. As a consequence of the complex impedances of all the devices and signal reflections, signal attenuation is both time-dependent and frequency-dependent. Similarly, noise varies over both time and frequency. zzzthe frequency and time dependencies are dramatically more severe than for a digital subscriber line (DSL) link.
A wideband spread-spectrum transmission would ensure that some portion of the transmitted spectrum would be received, albeit with significant distortion due to the multiple peaks and valleys of the power line transfer function. Adaptive equalization is required to correct for this distortion. Furthermore, unlike DSL, a network rather than a point-to-point link is required. Multiple nodes must therefore receive any transmission, subject to different distortion at different locations and times. Consequently, all nodes adaptively equalize to a transmitter without any adaptation by the transmitter to a specific receiver.
Because of the time-varying characteristics of the noise and attenuation, the longer the minimum signaling period, the greater the probability of a corrupted transmission. Fast synchronization enables the data to be transmitted in short frames, which in turn maximizes the probability of reception, enables adaptive equalization on a frame basis and serves as the foundation for low-latency isochronous communications.
No matter how robust the physical layer solution, there will be bit errors. Forward error correction (FEC) and an embedded low-level link protocol incorporating automatic repeat request (ARQ) further maximize the probability of recovering any received frame.
With good partitioning between analog and digital signal processing, the physical-layer solution through the error-control coding can be implemented in low-cost silicon in a pure CMOS process.
A token-passing MAC is uniquely suited to the requirements of both reliable transfer of control in a noisy medium and support for multimedia. False synchronization, missed transmissions and near-far problems that are inherent to a noisy medium such as the power line are best addressed with a token-passing deterministic access scheme. On the power line it is difficult to distinguish between a signal and noise. Token passing transfers control of network access deterministically, ensuring only one token holder at any time even in a noisy environment. Since the location of each node is different, each node receives a transmission subject to different distortion and noise. There is thus the possibility that some nodes will miss a transmission that other nodes hear. In token passing, nodes cannot transmit unless they hold the token, so there is no possibility that nodes will transmit during another node's transmission.
The token-passing MAC includes the use of a Token Rotation Time (TRT). The TRT is a fixed value that sets the maximum amount of time a station must wait for the token. This value is chosen to balance the worst-case access latency against network bandwidth being consumed for nonproductive token-passing overhead. The resultant low latency supports multimedia.
When nodes gain access to the network they are limited to their allotted Token Hold Time (THT). The THT is the amount of time a station is allowed to transmit before it must pass the token to the next station. Enforcing a THT ensures that all nodes receive their fair allocation of network bandwidth.
The use of priorities based on traffic type when allocating network access allows delay-sensitive traffic to first gain access to the network and maintain its required bit rate allocation.
Segmentation and reassembly (SAR) is integral to the architecture. Short power line frames are derived from segmentation of the typical packet. Segmentation into short frames ensures that high-priority traffic is not delayed by maximum-size Ethernet data packets as well as adherence to the THTs. Starting with a power-line-optimized physical layer as the foundation, the architecture described here adds forward error correction, ARQ, SAR and a token-passing MAC to yield an "as good as wire" 10-9 BER that looks to the world like an Ethernet NIC. Using a power-line signal processor combined with a low-cost embedded ARM-based controller yields a reliable, multimedia-capable home network.