Introducing circuit and packet switching
"Thanks to more capable electronics for handhelds, communications companies are scrambling to deploy so called 2.5G (for generation 2.5) networks more attuned to the world of data. In earlier networks, whether analog or digital, each call creates a circuit that reserves a channel between two parties for the entire session. The 2.5G devices are the first to use Internet-style packet switched networks; they send bursts of data only when needed. Because these devices don't hog an entire circuit, they can be "always on."
John Ueland, writing in the article 'Internet Everywhere', from the September/October issue of MIT's Technology Review (external link).
There's much talk about the coming mobile internet, about how people will have a wireless, always on connection to the web. How will that come about? Two words: packet switching, a fundamental, elemental change between how wireless was delivered in the past and how it will be presented in the future.
Conventional cellular radio and landline telephony use circuit switching. Services like Cellular Digital Packet Data or CDPD, by contrast, employ packet switching. General Packet Radio Service GRPS, Bluetooth, and some aspects of 3G, also use packet switching.
Circuit switching dominates the public switched telephone network or PSTN. Network resources set up calls over the most efficient route. That might mean a call from New York to San Francisco goes through switching centers in San Diego, Chicago, and Saint Louis. But no matter how convoluted the route, that path or circuit stays the same throughout the call. Got it? One call, one circuit. It's like having a dedicated railroad track with only one train, your call, permitted on the track at a time.
Before contrasting circuit switching with packet switching, let's talk about some digital basics. Voice and data from the local loop goes digital once it hits a telephone switch. Traffic between American telephone offices is nearly all digital, you know, 1s and 0s. Bits. That includes most circuit switched traffic, like we just discussed. All these bits get packaged into small groups called packets, frames, blocks, or cells. TCP/IP, X.25, ATM, frame relay, pick your packet switched technology, all traffic gets put into one form of packet or another. But simply packetizing data does not mean a call is packet switched.
TDMA and CDMA in wireless, T-Carrier and SONET in wireline networks, are transmission methods, transport mechanisms that carry information from one point to another across the telephone network. They packetize data but do not in general switch that data. According to their own protocol or standard, they package up data sent to them in the most efficient way possible, without interfering in switching. If my laptop is connected to the internet over a cellular modem, for example, then I am using TCP/IP to surf the net, while the modem may be using TDMA or CDMA to actually transport the call. Read more in the Ericsson quote below, where voice over the internet is sent using wideband CDMA. I don't mean to confuse anyone here, I just want to point out the difference between packets in switching and packets in certain transmission technologies. If you really want to get confused, know that some packet technologies like TCP/IP combine elements of both transmission and switching. But stay with the discussion.
Packet switching dominates data networks like the internet. A data call or communication from San Francisco to New York is handled much differently than with circuit switching. With circuit, all packets go directly to the receiver in an orderly fashion, one after another on a single track. Like the train we mentioned before, hauling one boxcar after another. With packet switching routers determine a path for each packet or boxcar on the fly, dynamically, ordering them about to use any railroad track available to get to the destination. Other packets from other calls race upon these circuits as well, making the most use of each track or path, quite unlike the circuit switched calls that occupy a single single path to the exclusion of all others.
Upon getting to their destination, the individual packets get put back into order by a packet assembler. That's because the different routes practically ensures that packets will arrive at different times. This approach is acceptable when calling up a web page or downloading a file, since a tiny delay is hardly noticed. But one notices even the tiniest delay with voice. This point is really important. Circuit switching guarantees the best sounding call because all packets go in order. No delay. Delays in packet switching for voice causes cause voice quality to fall apart, as anyone who has talked over the internet can tell you.
Program elements in the broadcast domain are continuous signals in real time. IT in broadcasting deals with compressed media files transferred at greater than real time rates. Continuous essence streams do not require buffering.
Gigabit means one billion bits per second. In the IT realm, this is a theoretical number where the number may or may not specify a certain bandwidth. What transfer rate really takes place on a so-called gig pipe? Is it 500Mb/s or 700Mb/s? What about collisions? In other words, it may not really be a gig’s worth of bandwidth. On the other hand, as of 1997, the SMPTE 292 standard specifies sustained, real-time transfer rates of 1.5Gig HD signals as commonplace. For broadcasters, 1.5Gb/s means 1.5Gb/s.
On the other hand, IP-based file transfers give one the freedom to route any signal anywhere. Packets can arrive out of sequence or be delayed and then be reassembled at the other end. Unfortunately, video signal transfers are intolerant of any variable delays. To improve video delivery, the IT world is moving toward switched circuits and away from routers. This is exactly the kind of composite and SDI routing that has been done for years in a broadcast infrastructure. It seems that video router methodologies are where the IT network world is finally heading.
There may be a dozen or so essence transfers, video or audio, moving around the broadcast infrastructure at one time. How will this traffic be segmented? If file transfers are at four times real time, will it only take six hours to move a day’s worth of programming? On the negative side, one hour down means four hours of real time content lost.
As technology gets better, voice over packet switched networks will get better. But it will be a work in progress. Sending voice over packet switched networks in a cellular radio context, though, will be very difficult. Ericsson is confident about the 'air interface' as the following shows but, again, the problem gets worked on, adjusted, but it will never get completely solved:
"Recently, Ericsson and Japan Telecom . . . successfully completed the world's first field trial of Voice-over-IP [using] wideband CDMA. The field trial results prove that voice can be efficiently transported over an IP-based mobile network. This includes the cellular air-interface, to mobile terminals, with full quality of voice service as well as full quality of other service features such as data, without loss of capacity. . . The field trial was conducted in July and August, 2000 with Japan Telecom at its network center in Chiba, Japan. . . 'The trend in today's telecoms industry is towards 'all-IP' transport networks," says Håkan Eriksson, Vice President and General Manger, Ericsson Research. "Operators want to be able to use the same network for all services; data, voice and video. The field trial conducted together with Japan Telecom has proven that it is possible to transport voice over an IP-based mobile network, without compromising quality or system performance."
Some companies like Caspian Networks (external link) are developing router like devices which will recognize packet types and prioritize accordingly, thus speeding up packet delivery and reducing lag time with voice and video. As Josh McHugh wrote about Caspian's optical IP superswitch in the May 2001 Wired, saying "It can identify packet types (voice, text, video, et cetera) and priorities, allowing it to determine one packet's relation to others, and expedite traffic in a way that's impossible today. For example, the Aperio will recognize all portions of a video stream and label them as a part of a greater whole so they can be more efficiently slotted and moved to their ultimate destination." We shall see.
Packet switched networks exist for the data communication needs of education, business, and government throughout the United States. These networks rely on telephone lines, of course, but the circuits are so arranged that they retain a permanent connection with their customers. The Public Data Network or Packet Switched Network, stands as the data counterpart to the Public Switched Telephone Network. I used to dial a local number to access Delphi, a now defunct internet service provider. Compu$erve and Plodigy used the same telephone number. All three used the same packet network, which you accessed when your computer dialed and logged in. An identification nmber directed your traffic to the right ISP, no matter where in the country it was. If you logged out but did not hang up the modem, you could enter numbers at the prompt on your screen and connect to, among other services, the NASA packet switching network. But I wander from the point I wanted to make.
I will probably get sued but I wanted you to see this nice graphic from Warner Brothers and Packet Video (external link). Packet Video is promising video clips at 60Kbs over conventional circuit switched cellular radio channels, indeed, they say they are platform independent, that is, their technology will work over whatever radio technology a carrier is using.
Unlike circuit switching, no one call takes up an entire channel for an entire session. Bits get sent only when traffic goes on, when people actually speak. During pauses in a conversation a channel gets filled with pieces of other conversations. Because your call doesn't hog an entire circuit the telephone system can permit an always on connection. You might pay a flat monthly charge or by the bandwidth or bits you actually use. Whether wireless operators can afford to do so is difficult to decide. Too many customers means building many more expensive cell sites. Even if technology permits we may stay with a per minute charge.
If packet switching is so efficient, why hasn't the landline public switched telephone network converted to it? The answer is time and money. Replacing circuit switched switches with packet switches accross the country would be a monumental task, requiring billions of dollars over years and years. The legacy of circuit switching will be around for quite a long time, following us far into the new century. Still, traffic engineers must think about changing, with lengthy dial up calls to the internet placing huge demands on switches that were never planned for, circuits now tied up longer than ever imagined. But change has to come at some point, and the internet's traffic now motivates engineers to move toward a unified switching method in the PSTN. As Bell Labs puts it "Telecommunications companies and Internet providers view these new problems as opportunities to move from separate voice and data networks to converged packet-switched voice and data networks."
DSL and ASDL and cable modem connections will either speed or retard this transistion; a local telephone company directs this broadband traffic to a packet switch, bypassing the existing local, circuit based switch. As broadband users increase call holding times should decrease, as dial up modems are taken out of service. The local switch should not be as overwhelemed as many currently are. A telco may then decide to delay a transistion to packet switching.
While the PSTN creeps towards convergence, many telecom companies are looking at placing calls over packet switched local area networks the internet. John Quain notes in the October,2000 Computer Shopper that GTE is partners with Dialpad.com (external link), a net based service allowing computer to landline telephone calls, while AT&T owns 30 percent of Net2Phone (external link), which permits free computer to computer calls. This is voice over internet protocol technology, or VoIP (Jade Clayton's quick article internal link). Calls sound poor at times, reminding me of short wave. But free is good, especially if you are an American who needs to talk with another computer user in New Zealand. Panasonic will soon debut a cordless phone with a Net2Phone button, push it before making a call and the cordless will place the connection over the net, with no need for a computer. Call setup may take a while, of course, but Panasonic hopes a 3.9 cent a minute toll charge to anywhere in the country will mollify users. I'm not so sure. Quain also says Netscape's 6.0 browser has Net2Phone built in but does not say if there is a Macintosh version. A complete lack of Mac compatible VoIP systems has prevented me from playing with this technology.
Call quality differs from the PSTN for many reasons: slow speed internet connections, feedback from poor microphone placement, low grade transmitters and receivers. Companies using packet switching to place voice calls over their high speed local and wide area networks don't suffer from these problems as much. Quain says companies like 3Com market systems to small firms which funnel inbound calls to the packet switch for a company. Once packetized the call goes directly to whatever phone number was being dialed. This eliminates the traditional office switch and allows software, not hardware, to enable features like conferencing and call forwarding. Even video conferencing if the number being dialed at the office is to a computer and not a desk telephone. That's simpler than it sounds.
When a call comes into your computer over such a system a graphic or an image comes up, saying you have a call. An keypad image lets you point and click on the numbers to make a call. Your computer or the one for the company enables voice mail and stores telephone directories. A company with a packet based switch will alow you to eventually store all of your e-mail and pages and faxes and voice calls on a single computer which also acts as your phone. See where convergence is taking us? And how getting away from circuit switching will help? The drive toward unified packet switching will enable a brand new future for the public telephone system.
Some people say that Bell System engineers had good ideas for developing packet switching for voice traffic on the PSTN but I will have to do more research to confirm this. The following article, written by George Gilder, gives some clues but no specific references or dates. But for now, knowing the difference between circuit switching and packet switching will, I hope, make understanding the new wireless data services a little easier.