Saturday, April 21, 2007

Circuit Switching vs. Packet Switching: What’s the huge difference anyway?

Circuit Switching vs. Packet Switching
What’s the huge difference anyway?

First off, let me explain switches in general. A switched network goes through a switch instead of a router. Most networks are actually headed toward flat switches on VLANs instead of routers. A router can handle the work of a switch, but much of IT today is going toward flat switched networks. So when we’re talking about circuit switching or packet switching, we are more and more talking about doing it on a switch.

Now in my last two articles, I’ve explained the differences between circuit switching networks and packet switching networks. In principle, circuit switching and packet switching both are used in high-capacity networks. Circuit switching establishes a direct point-to-point connection is made, like in a telephone call. The dedicated line cannot be used by anyone else while it’s already in use by two other users. Packet switching doesn’t require the direct line of contact, and uses any available network connections to route data packets (data, voice, video, etc.) through different routes until it reaches its destination where the packets are reassembled to its original message.


Circuit switching –
1) Ideal when data must be transmitted quickly, arrive in sequencing order, and at a constant arrival rate. Ideally, it is used for transmitting real-time data, such as audio and video.
2) Network resources are static.
3) Dominates the public switched telephone network or PSTN

Packet Switching –
1) More efficient and robust for data that is burst in its nature, and can withstand delays in transmission, such as e-mail messages, and Web pages.
2) Uses communication lines that are not dedicated to passing messages from the source to the destination. Different messages can use the same network resources within the same time period.
3) Dominates data networks like the Internet.

The difference in real-world situations:

Packet switching is acceptable when calling up a web page or downloading a file, since a tiny delay is hardly noticed. These tiny delays are very noticeable with voice, though. This point is really important. Circuit switching guarantees the best sounding call because all packets go in order without delay. Delays in packet switching for voice causes cause voice quality to fall apart, as anyone who has used VoIP can tell you.

Bottom line: circuit switching is more reliable than packet switching. When you have a circuit dedicated for a session, you are sure to get all information across. When you use a circuit which is open for other services, then there is a big possibility of congestion (which is like a traffic jam in a network), and hence the delays or even packet loss. This explains the relatively lower quality of VoIP voice compared to PSTN.

Even so, there are protocols giving a helping hand in making packet-switching techniques to make connections more reliable. An example is the TCP protocol. Since voice is to some extent tolerant to some packet loss, packet switching is ideal for VoIP.

When you are making a PSTN call, you are actually renting the lines. This explains why international calls are expensive. Expensive enough for many people to sacrifice quality for cost efficiency. You pay for each and every minute you spend CONNECTED on a dedicated line. In a conversation, you take turn speaking. Plus, there are those moments where there is silence. Ultimately, you’re only using less than half the time of what you are paying for. With VoIP, you actually can use a network or circuit, even if there are other people using it at the same time. There is no circuit dedication. The cost is shared.

Future of circuit and packet switching for telephony:

Packet switching is getting better with improved VoIP technologies, but it may never replace the dominance of circuit switching in PSTN. Replacing circuit switched switches with packet switches across the country would be a monumental task, requiring billions of dollars over years and years. Plus, lengthy calls over the Internet place huge demands on switches that were never planned for, tying up circuits longer than ever imagined. Change is probably going to come at some point, and the Internet's traffic now motivates engineers to move toward a unified switching method in the PSTN.

While the PSTN creeps towards convergence, many telecom companies are looking at placing calls over packet switched local area networks the Internet. A company with a packet based switch will allow you to eventually store all of your e-mails, pages, faxes, and voice calls on a single computer which also acts as your phone. Convergence would enable us to access all these features. Software, not hardware, would be used to utilize features like conferencing and call forwarding; or even video conferencing if the number dialed at the office is to a computer and not to a desk telephone. The drive toward unified packet switching will enable a brand new future for the public telephone system.