Introduction
I spoke with Ed Frank, VP of Engineering, home networking business unit, Broadcom, and got a thorough grounding in home networking. Here’s why you should know about this topic.
The vision for home networking is quite simple. The following image, courtesy of Broadcom, should be self-explanatory. I do have to voice some caution. More home networking visions play to a North American audience that is gadget rich, and info-rich, and media rich. I am not convinced that the rest of the world is going to be as enthusiastic about the possibilities of home networking, but then again, it has yet to take off in the US, either. However, after giving it some thought, I can see why home networking has attracted so much attention in the last two years, and why you can find home networking products on the shelves of computer stores.
Broadband anytime – a vision of the networked home that I can buy into. Courtesy of Broadcom.
Unfortunately, most home networking products that you find on the market today are geared primarily to multiple PC households. I say, unfortunately because, I think the biggest stumbling block for home networking is that fact that everyone is using the term “networking”. Can’t think of anything more frightening and labor intensive than networking. Nevertheless, I have two home networking systems that I am looking over, Diamond’s HomeFree wireless set-up, and 3Com’s HomeConnect which uses Broadcom’s chipsets. I’ll have to get into them in more detail at some future point, but you can investigate Diamond’s home networking site www.thedigitaldreamhome.com, or 3Com’s Connected Home site, both excellent resources for the uninitiated.
Mr. Frank
Back to Mr. Frank at Broadcom, a very engaging and enthusiastic proponent of home networking. Mr. Frank was kind enough to tutor me on the issues of home networking. Here’s Mr. Frank’s official bio:
Ed Frank has more than 20 years of development experience in advanced information technology. Prior to co-founding Epigram, now part of Broadcom Corp, Ed was co-founder and Vice President of Engineering for NetPower, Inc. a leading vendor of high-performance Windows NT workstations and servers. Before Netpower, Ed was a Distinguished Engineer at Sun Microsystems responsible for principal development of the Green project, which created Java, and co-architect of the SPARCstation 10 multiprocessor workstation. Ed has a Ph.D. in Computer Science from Carnegie-Mellon University where he was a Hertz Fellow. He holds MSEE and BSEE degrees from Stanford University and holds more than a dozen patents.
Ed Frank of Broadcom
Home networking isn’t just about multiple PCs, in fact, more importantly, it’s about a whole bunch of other services, included telephony, and streaming audio and video. I can buy into something that deals with distribution of media around the house, more than sharing an Internet connection on my PC, or sharing PCs in general. There are two reasons why I say that:
- I don’t think the PC is going to be preeminent, or dominant, Internet access device in the home in the next three to five years.
- I would dearly love to be able to have a simple and easy to install means of networking my audio and video equipment around the home, more than my PCs.
Mr. Frank co-founded a company called Epigram in 1996 with Jack Holloway. There vision for the company was not PC-centric, but focused on all possible digital devices having a connectivity component. As Mr. Frank tells it, “When Jack Holloway and I started Epigram the central idea was how do you get a billion things on to the Internet. The notion of connectivity is not about PCs to the Net, but everything digital. We recognized that phone wire was the ideal media because, there is a fair bit of it around, and with enough signal processing we can make it a high bandwidth channel.”
Broadcom
Broadcom acquired Eipgram, and used the company as the foundation of its home networking strategy. Not a bad idea because, Epigram had both technology, and was found a member of Home Phoneline Networking Alliance. Both Broadcom, and the HomePNA, as it is often referred to, have good sites which you can browse for background information. Also, at the HomePNA site you will find links to other home networking companies and technologies. There are basically three types of home network; one that uses phone wiring, one that proposes to use the power lines in the home, and one that is wireless. My concern in this article is to look at some of the underlying technology issues of home networking by examining the InsideLine technology found in Broadcom’s iLine10 chipsets. Therefore, my focus is, primarily, phoneline technology, but the techniques deployed here will be probably apply equally to other transmission media. I just think that phoneline technologies are going to be more cost effective to implement for some time to come.
Okay, so we have a broad picture of home networking as being a means to distribute broadband services through the home. I had asked Mr. Frank what the key criteria for a home network are, in his opinion. He gave me the following listing:
- A home network must be robust. You plug into it; it works.
- It must be high performance because, it is ultimately about broadband services coming into the home. Presently, Broadcom’s InsideLine suports 10 Mbit/sec networks, but the aim is for 100 Mbit/sec.
- Quality of Service (QoS). You need low level QoS that allows streaming audio, video, and telephony over the same piece of wire that Internet access is occurring. QoS is largely independent of performance, it’s more about latency. In a home network you have to make sure that there is bounded latency
Interestingly enough, InsideLine, like many home networking technologies, starts with Ethernet. Mr. Frank says, “Our technology is the moral equivalent of Ethernet. However, traditional Ethernet doesn’t have QoS. We carry Ethernet frames, but we carry them over existing home telephone wiring which is a different type of wiring compared with a network in an office. We don’t want people to run new wires, but Ethernet networks are usually built to order.”
As an aside, here’s some background on Ethernet
The Ethernet standard is enshrined in the IEEE 802.3 standard which defines an Ethernet data packet format, the cabling to be used, and the maximum distance for the network. The Ethernet packet contains a preamble of 8 bytes, followed by a destination address, which can be multiple, as well as single, nodes, the source address, a type field to determine which type format the data is using, a data field that holds a maximum of 1500 bytes, and a minimum of 46 bytes, and a Frame Check Sequence that ensures the data has arrived safely. The maximum length of cable is 500 meters, or 1500 ft., and specified a 50 Ohm coaxial baseband cable capable of sending data at 10 Mbit/sec. Obviously, this kind of rigid structure can’t be enforced, or predicted in a home network using pre-existing phone wiring.
It is the QoS, and robustness of a home network that differentiates it from a traditional Ethernet network. Every wire or communication channel has a frequency response. The corollary of that is that every physical communication channel has an impact on the amplitude of any signal across it. In Ethernet LANs, Twisted Pair Category Five cables have a very flat channel response which means that at a range of frequencies the impact of the channel is the same. The problem with a phone line is that it has very deep knolls, points where effectively no signal gets through at frequencies that can’t be predicted. For example, as the temperature in your house changes or someone picks up a phone, the frequency response of your phone wires changes.
So, let’s look at your typical dial-up modem to get a handle on how the phone line is handled in this simple circumstance. You already know that when you turn on a modem it starts to make weird noses. The noises it is making are assessing the channel response, and the term to use for this assessment is training. It is the modem’s way of figuring out the channel response. The problem is that it takes a long time to do, which is not a problem in a point to point network with your modem, but in a home network there are multiple nodes at play. You have lots of devices connected at different points to the network, and therefore, in a home network how can each device know the channel response of every device connected?
This is the coolest thing about home networking technology. It is a technology that is, in many ways, an evolution of Ethernet, to take into account that we live in a world that’s a mish-mash of wires. Granted, we could argue that wireless technology can take care of some of the problems we all have with bad wiring, but the techniques of training home networked devices are fundamentally the same for wires, and without wires. The channel just happens to be thin air in one case, and a pair of wires in another. It’s still a communications channel subject to the same rules as all communications channels. Check out this glossary of telecommunications terms for some background technical information. It’s a great resource.
Signal Quality
In any communications system, the frequency response of the channel determines the quality of the signal on the channel. In any system where the characteristics of the channel cannot be known, or they may vary, as in the case of a home phoneline network, you want to use a filter to compensate for the changes in the frequency response. Filters that adapt for imperfect channel transmissions are called equalizers. Equalizers come in many shapes or forms, but in general they are digital signal processors, or in simpler terms, very high-speed math machines that can take a distorted signal in at one end, and extract the binary data intact at the other end, having done a whole bunch of number crunching in-between.
Broadcom’s InsideLine technology uses Quadrature Amplitude Modulation (QAM) and a patented variation of its own, called Frequency-Diverse QAM (FD-QAM) to transmit digital data reliably on a severely degraded home network. QAM is a generalization of the amplitude modulation (AM) used in radio. AM radio transmits a carrier signal that is modulated by the analog signal that is the sound we hear. QAM does the same for digital bits with a high peak corresponding to a 1 bit, and a lower peak, a 0. At the receiver, a threshold is set which determines what is a 1 and what is a 0. Broadom’s FD-QAM adds an additional function, known as an adaptive equalizer, to the receiver to correct distortions caused by the home wiring. In a home network the combination of devices crate many reflected copies of a transmitted waveform. These reflected waveforms interfere with each other like waves in a pool, causing errors by changing the amplitude of a signal dramatically. This is also called inter-symbol interference (ISI), and the higher the rate of transmission, the bit rate, the more ISI. Adaptive equalization means that the equalizer at the receiver has to adapt for every packet of data it receives.
This image might help you to visualize the problem of a home networks frequency response. As you can see there are deep knolls where the signal is so badly attenuated as to degrade to nothing. This is unpredictable, therefore, while this image represents the response of the channel from one room to the next, it will vary depending on the devices connected, or the presence of mismatched stubs.
Equalization
Mr. Franks put it like this, “Every time we send a packet of data, we precede the frame with a preamble which allows the receiver of the packet to decide the channel response of the channel. Equalization correct for things the channel does.”
The key thing added is that it is done per packet. Every packet re-trains the equalizer at the receiver specifically for that packet. A very simple statement that covers many patents and a lot of hard work. The voice band modem comparison is that every device has a modem, and every modem is being retrained thousands of times a second.
Finally, on the issue of QoS InsideLine divides traffic into classes of services, and the network determines that one class of traffic gets higher priority than another. Telephony, especially Voice over IP, wants to be the highest priority because, it wants to have the lowest latency. You cannot go around dropping video frames, but when it comes to file transfers, the communications can be uncontrolled. So, within the packet structure, InsideLine technology also ensures QoS.
Equalization, and filtering, and unpredictable communication channels are nothing new, but to deliver a workable solution to consumers, at a price that can enable it in all manner of digital devices, that’s tough. That’s why, now, I am a believer in home networking. I think it’s fascinating stuff. Reminds me of how much we take for granted our telephones. We expect to be connected on command, with no problem, despite the fact that we are dealing with billions of devices covering the globe on a network that is more complex, and heterogeneous than any other known to man. So, while some parts of the computer industry wrestle with getting broadband that last mile of wire to the home, using everything from xDSL to cable television, and satellite, home networking developers are already planning ways in which to get the stuff around the house. I hope to cover more home networking technology and products in the future.
