In Praise of Cables

For most of the last twenty or so years I seem to have started off the year by writing an article claiming that this would finally be the one when wireless data takes off.  It’s nice to see things changing: Wi-Fi is finally starting to move outside internet access for PCs and Phone, Bluetooth Smart is appearing in desirable consumer devices and should trigger an avalanche of connected accessories, and smart metering is bringing ZigBee and Wireless M-Bus into homes as a static PAN.  That doesn’t mean that there are not still massive unexplored opportunities in M2M, but it’s good progress.

Instead of the obvious call for more, I’d like to look back at the many advantages of cables.  As designers rush into wireless, it’s easy to forget what they’re giving up.  Wireless offers new opportunities, but only at the expense of many serious compromises.  In this brave new world of wireless it’s apparent that some people are forgetting those compromises.  In this and the following article I’m going to look at what they are and then address the misconception that wireless standards can be treated in the same way as wired ones, debunking the common misconception that they follow the OSI model.

Once you sign up to the wireless dream, it’s very easy to forget just how good cables are.  Unless you’re pushing through extremely high frequencies (in the range that we’d generally consider to be wireless), or very high throughputs, then cabled range within the home isn’t a problem.  If you want a cable to go farther you just buy another reel of cable.  And for most of the time, throughput’s not much of a problem either.  Compared to wireless standards, cables are fast.  They support throughputs in hundreds of Megabits and Gigabits per second where most wireless standards struggle to get over a few hundred kilobits.

One of the big differences from cables, which many would say is a key advantage of wireless is topology.   This is where wireless becomes non-intuitive, which is both an advantage and a disadvantage.  Topology with cables is easy.  You get the end of the cable, find a mating plug or socket and plug it in.  Instant connectivity – it works.  With wireless you have nothing physical to hold or plug into.  Wireless is cunningly invisible.  To connect it, both ends (and it can be more than two, which is even more complicated) need to start talking to each other.  It’s that dreaded term “pairing”, which is where a large percentage of wireless products fail at the first hurdle.  It also brings us to the next difference – security.

Security is something that you don’t need to bother about with most cables, because the act of plugging them in gives you physical security.  The data that you send stays within the cable and no-one else can access it.  Devotees of spy fiction will point out that you can break in and attach covert listening devices, or even use sensitive radio receivers to listen in to the weak electromagnetic field generated by the data flowing along your cable.  They’re right, but it’s not likely that your neighbour or the local kids will be capable of doing that.  And if they are you’ve probably got more important things to worry about.

With wireless you potentially lose all of that security.  The data you’re sending from your laptop to your router, or your smart meter to your display can be captured by anyone else within range.  (Unlike cables which are strictly point to point, wireless is generally omni-directional.)  That raises two problems – stopping other products masquerading as legitimate device on your network which might send spurious commands, and adds a requirement to encrypt your data, so that if someone else captures some of your transmissions, they can’t decode them.

There is a whole science built up around wireless security, composed of experts who write security algorithms and experts who try to crack them, both for genuine and nefarious reasons.  That’s led to something of a security arms race, where to remain secure ever more complex algorithms and authentication schemes are being developed, which require ever more powerful microprocessors to be built into wireless chips.  These are there just to maintain the security of the cable-replacement link.  Other security, such as the https security the internet uses for credit card purchases has to run on top of these, whether the underlying link is a cable or wireless.

The ever more powerful processors needed to support complex security algorithms bring us to another point of difference – power consumption.  It’s a fallacy to say that cables don’t need power – they have an inherent resistance which needs power to send bits of information, although in most cases it’s very low compared to wireless.  As you send more data, you need to put in processing effort to shape the pulses going down the cable; you also need to terminate the cable to make it behave more like a waveguide, which again needs power.  But you don’t have the overhead of processing for security, nor the fact that wireless is generally transmitting in all directions, even though it’s only receiving in one.  (And anyone who says that MIMO is power efficient has been reading and believing too many marketing manuals.)

Then there’s latency.  It’s an eternal surprise to engineers starting to work with wireless that when you put data into one end of a link it doesn’t immediately pop out of the other.  The IP community is already familiar with the problem, but the magnitude of potential delay can be totally unexpected.  Try listening to a film over a Bluetooth stereo headset to discover what lipsynch is all about.  Latency can often be measured in seconds.

Even when you’ve got your wireless system to work there’s the annoying problem of robustness, which comes down to available spectrum and interference.  That’s not to say that cables can’t suffer from interference.  A well place nuclear explosion can create a very disruptive electromagnetic pulse over a short period, but it’s not something you come across on a daily basis in most domestic environments.  In contrast different wireless standards that share the same limited spectrum get in each other’s way.  Ten years ago, when I first installed Wi-Fi, or 802.11b as it was called then, it worked really well throughout the house and garden.  As all of my neighbours installed it, along with baby monitors and TV sender, the performance degraded to the point where it became worse than useless.  As a result I’ve just wired my house with CAT-5 for Internet access.  It’s a worrying by realistic fact that any in-home wireless system will probably work best on the day you install it, and then get progressively worse as all of your neighbours get one too.

Those are the big physical differences, but there’s a whole host of less tangible ones to consider.  Differences that just don’t exist in the world of cables.  Let’s start with interoperability.  In the cable world that’s mostly down to different plugs and sockets.  Of course there’s fine detail like multiway and coaxial cables and the level of screening, but in most cases you can solder a different connector on and it works.  In contrast there’s absolutely no interoperability between different wireless standards.  Quite often there’s precious little backwards compatibility between the different versions of a single wireless standard.  From a consumer viewpoint that means the product you buy today probably won’t work with the one you bought a few years ago.  With cables there’s pretty good compatibility – at most you need to change the plug or buy an adapter to get several decades of backwards compatibility.  In contrast wireless generally means a complete replacement.  Nice work when you can get it, but not something that endears you to the user.

So why the enthusiasm for wireless?  Mostly because of the Holy Grail of simplicity that it purports to offer.  Simplicity of installation (no wires or holes in walls), simplicity of commissioning (no need to connect cables to inaccessible sockets), simplicity of mobility (you can carry devices around and connect to different things in difference places).  But it’s really difficult getting all of that to work.

To put back the basics of a cable – throughput, range, security, latency ease of connection, all of the things I’ve talked about above, is an immensely difficult task.  You can specify most cables on a single sheet of paper.  In comparison most wireless standards require several thousand pages of technical specification.  And even then each standard is a compromise, making its individual choice of what is important for a specific application.  Each one takes hundreds of man years of effort to write, test and turn into reality. 

Which adds another complication.  To make this complexity work requires some highly talented companies and individuals.  All of them contribute their ideas and patents into a standard.  Which means that all wireless standards involve some form of IP license or certification requirement.  That means you either need to pay money to join the standards group if you want to use it in your products, pay someone a license fee, or pay a test house to certify it before you’re allowed to ship it.  After which you may discover you’re not allowed to export it, as some of the high tech aspects, particularly around security are subject to export restrictions.  Bear in mind that I’m not talking about exotic industrial or military standards here, but everyday consumer standards for products you’d use in the home.  In contrast, anyone can make and ship a cable.

But that Holy Grail of wireless connectivity is strong.  However difficult it may be compared with the simple cable it paints a wonderfully desirable future free of tangles, along with the promise of a new generation of product design where products are no longer islands, but can communicate with each other in that vision of the Internet of Things.

But it’s important to understand the compromises underlying wireless.  To paraphrase the psalm, “and now abide range, throughput, security, these three; but the greatest of these is security”.  Well developed, robust wireless standards are neccessary, because without them we can’t attempt to fulfil these requirements.  And if any one of these three fail, so will customer confidence.  In conclusion, I’m hoping for another year of growth in wireless.   But it’s as well to understand the shifting sands we stand on when we make that wish.