Circuit Switching vs. Packet Switching

The old telephone system (PSTN) uses circuit switching to transmit voice data whereas VoIP uses packet-switching to do so. The difference in the way these two types of switching work is the thing that made VoIP so different and successful.

To understand switching, you need to realize that the network in place between two communicating persons is a complex field of devices and machines, especially if the network is the Internet. Consider a person in Mauritius having a phone conversation with another person on the other side of the globe, say in the US. There are a large number of routers, switches and other kinds of devices that take the data transmitted during the communication from one end to the other.

Switching and routing

Switching and routing are technically two different things, but for the sake of simplicity, let us take switches and routers (which are devices that make switching and routing respectively) as devices doing one job: make a link in the connection and forward data from the source to the destination.

Paths or circuits

The important thing to look for in transmitting information over such a complex network is the path or circuit. The devices making up the path are called nodes. For instance, switches, routers and some other network devices, are nodes.

In circuit-switching, this path is decided upon before the data transmission starts. The system decides on which route to follow, based on a resource-optimizing algorithm, and transmission goes according to the path. For the whole length of the communication session between the two communicating bodies, the route is dedicated and exclusive, and released only when the session terminates.

Packets

To be able to understand packet-switching, you need to know what a packet is. The Internet Protocol (IP) , just like many other protocols, breaks data into chunks and wraps the chunks into structures called packets. Each packet contains, along with the data load, information about the IP address of the source and the destination nodes, sequence numbers and some other control information. A packet can also be called a segment or datagram.

Once they reach their destination, the packets are reassembled to make up the original data again. It is therefore obvious that, to transmit data in packets, it has to be digital data.

In packet-switching, the packets are sent towards the destination irrespective of each other. Each packet has to find its own route to the destination. There is no predetermined path; the decision as to which node to hop to in the next step is taken only when a node is reached. Each packet finds its way using the information it carries, such as the source and destination IP addresses.

As you must have figured it out already, traditional PSTN phone system uses circuit switching while VoIP uses packet switching.

Brief comparison

• Circuit switching is old and expensive, and it is what PSTN uses. Packet switching is more modern.
• When you are making a PSTN call, you are actually renting the lines, with all it implies. See why international calls are expensive? So if you speak for, say 10 minutes, you pay for ten minutes of dedicated line. You normally speak only when your correspondent is silent, and vice versa. Taking also into consideration the amount of time no one speaks, you finally use much less than half 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.
• 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 for a network what a traffic jam is for the road), and hence the delays or even packet loss. This explains the relatively lower quality of VoIP voice compared to PSTN. But you actually have other 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 (unless text - since a comma lost can mean a big difference), packet-switching is finally ideal for VoIP.

Various Switching Techniques

Circuit switching

In telecommunications, a circuit switching network is one that establishes a fixed bandwidth circuit (or channel) between nodes and terminals before the users may communicate, as if the nodes were physically connected with an electrical circuit.

The bit delay is constant during a connection, as opposed to packet switching, where packet queues may cause varying delay. Each circuit cannot be used by other callers until the circuit is released and a new connection is set up. Even if no actual communication is taking place in a dedicated circuit that channel remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle.

Virtual Circuit Switching is a packet switching technology that may emulate circuit switching, in the sense that the connection is established before any packets are transferred, and that packets are delivered in order.

There is a common misunderstanding that circuit switching is used only for connecting voice circuits (analog or digital). The concept of a dedicated path persisting between two communicating parties or nodes can be extended to signal content other than voice. Its advantage is that it provides for non-stop transfer without requiring packets and without most of the overhead traffic usually needed, making maximal and optimal use of available bandwidth. The disadvantage of inflexibility tends to reserve it for specialized applications, particularly with the overwhelming proliferation of internet-related technology.

Examples of circuit switched networks

• Public Switched Telephone Network(PSTN)
• ISDN B-Channel
• Circuit Switched Data(CSD) and High Speed Circuit Switched Data in a cellular system such as GSM
• X.21

Compared to datagram packet switching

Since the first days of the telegraph it has been possible to mutiplex multiple connections over the same physical conductor, but nonetheless each channel on the multiplexed link was either dedicated to one call at a time, or it was idle between calls.

With circuit switching, and virtual circuit switching, a route is reserved from source to destination. The entire message is sent in order so that it does not have to be reassembled at the destination. Circuit switching can be relatively inefficient because capacity is wasted on connections which are set up but are not in continuous use (however momentarily). On the other hand, the connection is immediately available and capacity is guaranteed until the call is disconnected.

Circuit switching contrasts with packet switching which splits traffic data (for instance, digital representation of sound, or computer data) into chunks, called packets, that are routed over a shared network.

Packet switching is the process of segmenting a message/data to be transmitted into several smaller packets. Each packet is labeled with its destination and the number of the packet, precluding the need for a dedicated path to help the packet find its way to its destination. Each is dispatched and many may go via different routes. At the destination, the original message is reassembled in the correct order, based on the packet number. Datagram packet switching networks do not require a circuit to be established and allow many pairs of nodes to communicate almost simultaneously over the same cha

Packet switching

Packet switching is a network communications method that splits data traffic (digital representations of text, sound, or video data) into chunks, called packets, that are then routed over a shared network. To accomplish this, the original message/data is segmented into several smaller packets. Each packet is then labeled with its destination and the number of the packet. This precludes the need for a dedicated path to help the packet find its way to its destination. Each packet is dispatched and may go via different routes. At the destination, the original message/data is reassembled in the correct order, based on the packet number and other statistically determined factors. In each network node, packets are queued or buffered, resulting in variable delay. This contrasts with the other principal paradigm, circuit switching, which sets up a specific circuit with a limited number of constant bit rate and constant delay connections between nodes for exclusive use during the communication session.

Packet mode or packet-oriented communication may be utilized with or without a packet switch, in the latter case directly between two hosts. Examples of that are point-to-point data links, digital video and audio broadcasting or a shared physical medium, such as a bus network, ring network, or hub network.

Message switching

In telecommunication, message switching was the precursor of packet switching, where messages were routed in their entirety, one hop at a time. It was first introduced by Leonard Kleinrock in 1961. Message switching systems are nowadays mostly implemented over packet-switched or circuit-switched data networks.

Examples

Hop-by-hop Telex forwarding and UUCP are examples of message switching systems. E-mail is another example of a message switching system.

When this form of switching is used, no physical path is established in advance in between sender and receiver. Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e. router) then forwarded later at one hop at a time. Each block is received in its entity form, inspected for errors and then forwarded or re-transmitted.

A form of store-and-forward network. Data is transmitted into the network and stored in a switch. The network transfers the data from switch to switch when it is convenient to do so, as such the data is not transferred in real-time. Blocking can not occur, however, long delays can happen. The source and destination terminal need not be compatible, since conversions are done by the message switching networks.

A message switch is “transactional”. It can store data or change its format and bit rate, then convert the data back to their original form or an entirely different form at the receive end. Message switching multiplexes data from different sources onto a common facility.

Store and forward delays

Since message switching stores each message at intermediate nodes in its entirety before forwarding, messages experience an end to end delay which is dependent on the message length, and the number of intermediate nodes. Each additional intermediate node introduces a delay which is at minimum the value of the minimum transmission delay into or out of the node. Note that nodes could have different transmission delays for incoming messages and outgoing messages due to different technology used on the links. The transmission delays are in addition to any propagation delays which will be experienced along the message path.

In a message-switching centre an incoming message is not lost when the required outgoing route is busy. It is stored in a queue with any other messages for the same route and retransmitted when the required circuit becomes free. Message switching is thus an example of a delay system or a queuing system. Message switching is still used for telegraph traffic and a modified form of it, known as packet switching, is used extensively for data communications.

Components Of switching

The Basics of Switches

If you have ever installed a set of cold cathodes, or other special lighting gadgetry, then you most likely have been presented with an optional switch to install along with it to allow you to turn the device off and on instead of it staying on as long as the PC is on. I'm sure there people new to this sort of installation out there so here is a simple guide for hooking up a few types of switches you may encounter while tricking out your own PC with special electronics and/or lighting/cooling devices.

Why bother installing a switch?

The answer to that is really simple... control. A switch allows you to be in control of the state of activity of a component; whether that means a simple off or on state or something that will let you choose a setting among more than just two choices. For a device as simple as a cold cathode you may think, "Why even both with hooking up a switch when I can just let my lighting bling 24/7?" (or while the PC is on).

There times when the glare of an uber-lit PC next to you is not a desirable thing... like when gaming at a LAN party in tournament? When playing in total darkness for gaming ambiance? How about if you have to sleep in the same room with your PC? Plus, lighting fixtures all have a lifespan of use. When you turn off your lighting occasionally, and/or when you go to sleep at night, you are extending the life of that product's usefulness. That way the bling is there when you want it there and not in the way other times.

Then there is, of course, the matter of safety. I have seen first-hand a cold cathode burst into flames and melt it's housing. A switch allowed me to immediately cut juice to the inverter for that cathode and may have prevented serious damage to my other PC components.

Switch Terminology

Several terms are used to describe a switch and some other "need to know" terms:

- Pole - The number of switch contact spots.
- Throw - The number of conducting positions (single or double).
- Way - The number of conducting positions (three or more).
- Momentary - The switch returns to its original position when released. A PC Reset switch is a momentary switch.
- Open - The "off" position (contacts not conducting).
- Closed - The "on" position (contacts conducting; there may be several on positions).
- (+) Positive - The pole that connects to the live current source (usually via the various 5V [usually red wired] or 12V [usually yellow wired] contacts provided by the PC motherboard and/or PSU). Often marked with a "+" symbol, a small triangle or is labeled as pole #1.
- (-) Negative (Ground) - The pole that is the ground connection. Usually marked with a "-" symbol.
- Source, Trigger, Etc. - The pole that a signal/current is sent to when in a particular position. The pole will be labeled differently on various devices. Check it's documentation when available. When not available, simply eliminate the other poles by finding the ones marked with a + or -. If you only find one or the other, then the pole furthest from it is usually the opposite. The pole in the middle will normally be the trigger.

Example: the most simple on-off switch has one set of contacts (single pole) and one switch position which conducts (single throw); ie. SPST (Single Pole Single Throw). The switch has two positions: open (off) and closed (on); but it is called 'single throw' because only one position conducts.

Typical Types of Switches

We'll use four basic switches which you may cross paths with or find a use for someday. We'll use examples from the Frozen CPU Center for an easy reference point. There are many other forms of switches out there but these are the most commonly used types for a PC application.

Rocker Switch - A simple toggle switch that is used to control when a device receives current. This is what most cold cathode kits come with as an optional install method.

Rocker LED Switch - A rocker switch which includes a small LED that is normally lit when in the "On" position.

Military LED Toggle Switch - Very similar to the Rocker LED switch; just in a different form.

Bulgin LED Switch - SP Push-to-Make Switch that is a nice replacement for standard PC Power and Reset buttons.

Note: There are other components which are used as a type of switch (so to speak) which include devices like Rheostats. These devices are hooked up much the same way as the switches described above, except they allow you to "dial down" the juice that is delivered the devices powered through it. This is a popular setup for a system fans because it gives you the ability to turn up or down the speed of your fans to find the right balance of performance and noise for your needs.

Install

I don't feel that there is much need to show examples of the regular rocker switch installed since you really can't mess it up as long as you install the switch on the (+) positive line going to your device. Enough said. Instead, I will show you a VERY easy method for hooking up a rocker LED switch or a Military Toggle switch (using FrozenCPU's excellent custom leads) to turn off/on a device as well as an easy method for replacing your PC's Power and/or Reset buttons.

First, here is an example of using Frozen CPU's EZ Bulgin Cable for extra easy install of LED lit switches. (These are the same cables I personally use.) In order to make your Bulgin switch install as painless as possible you will need the 2-Pin Power LED or HDD LED Cable and the 2-Pin Power Switch cable. You can create your own cables like these but it can be a fairly tedious task. For the work and supplies involved, FrozenCPU's prices for these cables are very fair. Be sure NOT to confuse the two cables, however. The LED cables have a much needed resistor embedded in the cable to make sure that the LED in your switch doesn't get fried with too much current.

Bulgin Vandal Switch Install

Using FrozenCPU's provided diagram of the Bulgin Vandal Switch you can see that the outer polls are for the (+) and (-) current leads and that the inner poles are for the wire leads to turn on whatever devices you are hooking up (the PC power switch in our case). Start by sliding the Spade connectors for the 2-Pin Power Switch cable over the two inner posts. For this application a specific post is not needed for the switch wires. Next, slide the spade connectors for the 2-Pin Power LED cables on the outer poles; MAKE SURE you pay attention to where you are placing these! The black wire should be slid over the (-) pole and the colored (usually yellow [+12v] or red [+5v]) wire should be slid over the (+) pole. Now all you have to do is connect the 2-Pin ends of the wires to the appropriate motherboard pins; Connect the Power Switch cable to the power switch pins and connect the Power LED cable to the power LED pins on the motherboard. Simple! If you would instead like to use your Bulgin switch as a HDD activity indicator you can connect the LED cable to the HDD LED activity pins on the motherboard instead of the Power LED pins.

Uses: Bulgin Vandal switches are momentary so the switch is only "closed" when held down. This works well for a PC power or reset switch which works the same way. This is not a good choice for a switch to turn on a set of fans or lights since once you release the switch, the power will be cut off from these devices.

LED Rocker Switch Install

The LED Rocker Switch is like the Bulgin switch in that the outer poles are the (+) and (-) contacts; but notice that there is only a single center pole. Also notice that the outer poles are not labeled very clear like the Bulgin switch poles were. In the picture below I have circled where the manufacturer has labeled the poles as "1", "2" and "3". When you first look at the switch poles it can be a little confusing at first since the #3 pole is labeled with both a (+) and (-) symbol. A few things usually stay pretty constant when determining where the actual (+) pole is. The (+) pole will usually be labeled as "1", it will have a "+" symbol next to it, or it will have a tiny triangle pointing to it. If you ever get lost, look for one or more of these tell-tale signs. After you determine where the (+) contact is then the (-) contact should be the furthest one from the (+) contact. The reason for this is because if the live (+) and (-) lines touch one another you are in store for a light and sound show (the bad kind) and risk damaging your hardware.

To install this switch as an on/off switch for components you will need these spade connectors in order to ensure a safe and secure contact with the pole. Install is just what you would predict. Attach the (+) and (-) wires to the appropriate poles and connect the device to be turned on to the center pole.

Uses: Unlike the Bulgin switch, the rocker switch is not momentary which makes it a poor choice for a PC power or reset switch but it is an ideal choice for devices you want to be able to turn off and on like cold cathodes or supplemental cooling.

Military LED Toggle Switch Install

You will notice now that you have a basic knowledge of how to hook up an electrical switch that it gets easier and easier to hook them up. This Military LED toggle Switch is a bit of an exception. Using the tips I gave you earlier, I was unable to determine where the (+) line needed to be connected. After some quick googling for "military LED Toggle switch" I found a company diagram showing how to hook this switch up properly. Notice in the picture below that the power line is connected to the #2 pole which is in the middle of the poles on the underside of the switch. The ground (-) wire is connected to the single pole on the side of the switch. Lastly, the #3 pole is used to connect the devices to for on/off control. Note that the #1 pole is not used at all. The lesson here is to always double and triple check your findings before hooking unknown things up. Protect your gear and yourself; do your research first.

Uses: The uses for this switch are the same as the Rocker switches. Since it is not a momentary switch it is not suited for use as a PC power or reset switch.

Creating your own source of power

In closing, here is a simple idea for making your own source of power for your components. Just in case you have more than one Bulgin switch to power or your motherboard pins are not in a convenient location, etc. Take an old pass-through cable or other molex plug cable you are not using and then remove any extra wires besides the main 4 power wires. Then simply snip off the "male" molex plug and you now have access to solder on your own component wires. I have marked the wire voltages for you for easy reference. Red is normally +5V, Yellow is normally +12V and black is normally ground (-). Once you know what power line you need you can even remove the pins you don't need to keep everything as tidy as possible. FrozenCPU has a great little tool for that.

In Closing

Hopefully, those of you that were a bit intimidated by tinkering with your eletronics setup or installing a new PC power switch are now a little more confident with the terminology, process and reasoning behind switches. If you make heavy use of special lighting in your case I highly recommend putting that lighting on its own switch so you can turn them off when you aren't around to enjoy them. It will greatly extend the life of your lighting setup.

Network switch

a NEtwork Switch is a broad and imprecise marketing term for a computer networking device that connects network segmants.

The term commonly refers to a network bridge that processes and routes data at the data link layer (layer 2) of the OSI model. Switches that additionally process data at the Network layer (layer 3) (and above) are often referred to as Layer 3 switches or Multilayer switches.

The term Network switch does not generally encompass unintelligent or passive network devices such as hubs and repeaters

Function

As with hubs, ethernet implementations of network switches support either 10/100 Mbit/s or 10/100/1000 Mbit/s ports Ethernet standards. Large switches may have 10 Gbit/s ports. Switches differ from hubs in that they can have ports of different speed.

The network switch, packet switch (or just switch) plays an integral part in most Ethernet Local Area Networks or LANs. Mid-to-large sized LANs contain a number of linked managed switches. Small office,home office(SOHO) applications typically use a single switch, or an all-purpose converged device such as gateway access to small office/home office broadband services such as DSL router or cable, Wi-Fi router. In most of these cases, the end user device contains a router and components that interface to the particular physical broadband technology, as in the Linksys 8-port and 48-port devices. User devices may also include a telephone interface toVOIP.

In simple terms, in the context of a standard 10/100 Ethernet switch, a switch operates at the data-link layer of the OSI model to create a different collision domain per switch port. This basically says that if you have 4 computers A/B/C/D on 4 switch ports, then A and B can transfer data between them as well as C and D at the same time, and they will never interfere with each others' conversations. That is the basic idea. In the case of a "hub" then they would all have to share the bandwidth, run in half-duplex and there would be collisions and retransmissions. Using a switch is called micro-segmentation - it allows you to have dedicated bandwidth on point to point connections with every computer and to therefore run in full duplex with no collisions.

Role of switches in networks

Network switch is a marketing term rather than a technical one. Switches may operate at one or more OSI layers, including physical,data link,network or transport .A device that operates simultaneously at more than one of these layers is called a multilayer switch, although use of the term is diminishing.

In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, for example ethernet,fibre channel,ATM and 802.11. This connectivity can be at any of the layers mentioned. While Layer 2 functionality is adequate for speed-shifting within one technology, interconnecting technologies such as Ethernet and token ring are easier at Layer 3.

Again, "switch" is principally a marketing term; interconnection of different Layer 3 networks is done by routers. If there are any features that characterize "Layer-3 switches" as opposed to general-purpose routers, it tends to be that they are optimized, in larger switches, for high-density Ethernet connectivity.

In some service provider and other environments where there is a need for much analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules. Some vendors provide firewall network and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.

In other cases, the switch is used to create a "mirror" image of data that can go to an external device. Since most switch port mirroring provides only one mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as packet sniffers.

Switching Elements
Responsible for the transport of data across a network - routing
May support congestion handling or quality of service mechanisms
Examples
routers - IP
switches - ATM or local area networks (e.g., Ethernet-802.3, token ring)
bridges, hubs - local area networks

Protocols
A protocol is a set of conventions that specifies the rules or parameters for communication between two entities (hosts, processes, switching elts, devices, etc).

Protocols specify conventions for:
The type, semantics, and format of information to be conveyed between entities
Establishing/closing connections or sessions
Routing across a network or internetwork
Flow control: speed matching between entities
Reliability: error detection, handling, correction
ABSTRACT
The problem of finding a minimum number of patterns to exercise the logic elements of a combinational switching net is investigated. Throughout, the word “testing” refers to exercising of this kind; or, equivalently, to fault diagnosis where each line of the net can be directly observed. Any set of permanent faults can be selected to test against, examples of which range from “stuck-at” faults (allowing the most economical test) to “any possible fault” (requiring the most complete test). The method used depends upon exact structural analysis rather than upon search algorithms or random pattern generation. The types of results presented appear to be fundamentally new. In particular, the maximum number of patterns required to test any one of an infinite class of nets is frequently found to be finite and extremely small. For example, any nontrivial connected tree of 2-input nand gates can be tested for “any possible fault” by exactly five patterns—no more and no less. The method in brief: Given a set F of switching functions and a set of required inputs for each (collectively denoted T), a “testing” function is defined for each element of F for each positive integer r. If the lines of a net can be mapped to the domain of the testing functions P(T, r) so that the gates perform consistent with these functions, we say the net “accepts” P(T, r)—and then r patterns are sufficient to test the net for T. Only nets in which each logic element is intended to realize the same switching function are discussed here. Trees (nets without fanout) are studied first, and the conditions under which a tree of identical gates “accepts” a partial function on an arbitrary domain is established. Then the common symmetric switching functions are separately investigated to find for each a minimum value of r such that all trees composed solely of the function accept P(T, r) (for various T). In most cases, as in the example given, the number of patterns required to test any such tree is extremely low. The conditions under which all nets (nontrees included) accept a set of partial functions with arbitrary domain are then established. These conditions are rarely met in practice, even where F consists of a single function. However, many subclasses of nets can be identified which require only a few patterns at most (depending on the function and the class of faults selected). These subclasses often contain nets of arbitrary size and complexity, and frequently consist of exactly those nets for which a related graph can be “colored” (i.e., h-node colored for some particular h) in the classical graph-theoretic sense. For example, any net of 2-input nand gates can be tested by five patterns if one of its related graphs is 2-colorable and another one is 3-colorable (!). The detailed results and methods used to obtain them are summarized, and in conclusion coloring problems and test construction are commented upon.
REFERENCES
Note: OCR errors may be found in this Reference List extracted from the full text article. ACM has opted to expose the complete List rather than only correct and linked references.

1
CHANG, H.Y., MANNING, E., AND METZE, G. Fault Diagnosis of Digital Systems Wiley- Interscience, New York, 1970.

2
HARARY, F. Graph Theory Addison-Wesley, Reading, Mass., 1969.

3
HORNBUCKLE, GARY D., AND SPXNN, R.N. Diagnosis of single-gate failuresin combinational circuits. IEEE Trans. EC-18, 3 (Mar. 1969), 216-220.

4
Kenneth E. Iverson, A Programming Language., John Wiley & Sons, 1962

5
KAUTZ, W H. Fault testing and diagnosis in combinational digital circuits. IEEE Trans. EC.17, 4 (Apr. 1968), 352-366.

6
MALING, K., AND ALLEN, E.L. A computer organization and programming system for automated maintenance IEEE Trans. EC-IP, 5 (Dec. 1963), 887-895.

7
OaE, O. The Four-Color Graph Problem. Academic Press, New York, 1967.

8
POWELL, T. J A procedure for selecting d~agnostic tests IEEE Trans. EC-18, 2 (Feb. 1969), 168-175.
9
Donald L. Richards, Efficient Exercising of Switching Elements in Combinatorial Nets, Journal of the ACM (JACM), v.20 n.2, p.320-332, April 1973 [doi>10.1145/321752.321763]

10
RICHARDS, DONALD L. Test size and optimum detection in switching nets J. ACM (to appear).

11
ROTH, J.P. Diagnosis of automata failures: A calculus and a method. IBM J. Res. Devel. 10, 4 (July 1966), 278-291.

12
ROTH, J.P., BOURICIUS, W. G, AND SCHNEIDER, P.R. Programmed algorithms to compute tests to detect and distinguish between failures in logical circuits. IEEE Trans. EC-I6, 5 (Oct 1967), 567-580. {In addmon, consult IEEE Trans. EC-20, 11 (Nov. 1971).}
CITED BY

Donald L. Richards, Efficient Exercising of Switching Elements in Combinatorial Nets, Journal of the ACM (JACM), v.20 n.2, p.320-332, April 1973
INDEX TERMS
Primary Classification: G. Mathematics of Computing G.2 DISCRETE MATHEMATICS
Additional Classification: B. Hardware B.8 Performance and Reliability
General Terms: Design, Measurement, Performance, Reliability, Theory, Verification
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