Technology In Practice
E-Definitions With Mark Tamminga
Packet Traffic: Riding Cheek by Jowl on the Net
Circuit switching "Mr. Watson, come here. I want to see you." These unassuming words, spoken by Alexander Graham Bell into his "harmonic telegraph" in 1867, launched the telephone age.
The approach Bell took is known as circuit-switched telephony. Circuit switching is the foundation of the plain old telephone service (or POTS, as the phone guys like to say). It is simply the opening of a dedicated line (a circuit) between callers. The pathway between callers stays constant and committed until the callers hang up. If the call lasts for 20 minutes, an hour or all day, it doesn’t matter. The line is yours the whole time and no one else can use it.
In the time of the ancients, a human operator used a switchboard to make an actual physical connection between two callers. This has long since been replaced, first by mechanical and then by digital switches, but the same simple idea of a dedicated connection for every call still underlies the present public phone system.
Simple isn’t always best. Dedicated circuits consume significant resources and the POTS can only handle so many simultaneous calls before all the lines are used up. Just try calling home on Mother’s Day. That busy signal isn’t necessarily Mom talking to someone else; it may be just an overwhelmed phone system.
Packet switching In the late 1960s, Arpanet, the predecessor of the Internet, was busy linking government and academic computer sites using a cool new trick called packet switching. The designers had also looked at the phone system as a possible model, but they knew that circuit switching was unacceptably vulnerable. It was like building a railroad track between the source and destination that permitted only one train at a time. One well-placed sabotage event on the tracks and, poof, no more connection. Permanently.
Instead, Arpanet employed a technique that involved chopping up the data stream into small packets containing not only a snippet of the data but also the source and destination address. Each packet is like a car that knows where it came from and where it is going. The packets are launched separately and told to do their best to find their own way to their targets. Once there, all the packets are reassembled again into a meaningful data streamÑan e-mail, picture, video, request for a Web site, whatever.
Packet switching is what now underpins the Internet. It is the reason why the expression the "Internet routes around damage" makes sense. If the best way between source and destination is clogged, packets simply veer over to the second-best route, and so on, until, by hook or by crook, they get to where they want to go.
In addition to being disaster-resistant, packet switching is much more efficient than circuit switching. With no open dedicated circuit required, packets are ideal for balancing the load on the system as a whole. Packets from different sources headed off to different targets can ride cheek by jowl over the same lines.
VOIP You’ll have noticed by now that you don’t pay long-distance charges for touring the Louvre over the Web or sending e-mail to Indonesia. Your little packets of data go where you send them for no more than your monthly ISP charge.
This fact hasn’t escaped the attention of software designers, either, who have set about replacing the circuit-switched phone system with a packet-switched system running over the Internet. Voice Over Internet Protocol, or VOIP, is based on the fact that your voice is just data that, like any other kind of data, can be reduced to 1s and 0s. Your computer can cut this binary information into little packets and send them anywhere in the world over the Internet. If it’s done right, you suddenly have free long-distance calling, a prospect that has phone companies worldwide in a state of low-grade panic.
Quality of service So why are we still stuck with old-school circuit-switched phones? Mainly because VOIP call quality generally sucks. If you chop your voice signals into data packets and send them helter-skelter across the packet-switched Internet, the packets are almost certainly going to arrive out of order. Not a problem if you’re sending an e-mail, because the order is corrected fairly quickly–but not usually quickly enough for a normal voice conversation.
Voice conversations are "time critical" and packet switching, by its very nature, has a built-in latency. IP only assures data delivery. It makes no promises about timeliness. A delay of more than half a second in voice traffic is noticeable and annoying. And then there’s the problem of lost packets. Happens all the time on the Internet. Not a problem for e-mail, because a replacement packet is sent quickly enough and the whole message is reconstructed only when all the packets have been accounted for. But lost packets make a mess of a voice conversation, because there’s just not enough time to fish for a replacement before the lost packet becomes noticeable in the form of lousy sound quality.
Does that mean circuit-switched telephony wins because of its inherently high quality of service–even if it is inefficient? Not likely. Packet switching is so much easier on the whole communications infrastructure that tremendous effort is being put into converting to high-quality packet-switched telephony. As bandwidth increases and the quality problems are slowly ironed out, you’ll begin to see new communication services as your phone becomes your computer and your computer becomes your phone.
Mark Tamminga (firstname.lastname@example.org) practices law and fiddles with software at Gowling Lafleur Henderson LLP in Toronto.