Network Switch – The Basics

The network switch plays an integral role in enterprise and home networking, yet many people confuse what the purpose of the equipment is, and how it differs from a router. I decided to write this blog post to explain the basics of the switch – from different types, to vendors for purchasing them.

A brief overview of network switches

A network switch is a type of computer networking hardware that bridges network segments. It is sometimes referred to as a packet switch or simply a switch. The switch plays an important component in most local area networks (LAN), including mid-to-large enterprise networks which utilize several linked managed switches.

A switch is far less sophisticated than a router. Although routers and switches look fairly similar in appearance, routers differ substantially in their internal components.

Types of network switches

Unmanaged Switches: This is typically the least expensive type of switch, most often found in homes or small offices. They are very simple, employing plug and play technology, lacking any specific configuration options

Managed Switches: Managed Switches provide optional configuration options and allow for a great variety of functionality. There are several ways to operate these switches, from utilizing a remote tool like Simple Network Management Protocol (SNMP), to accessing the switch via a command line interface like Telnet.

  • Smart Switches: Smart switches differ from fully managed switches in that they only allow a specific set of modifications and functionality. Because users can only configure basics settings, they are often cheaper than the fully managed breed. Some basic functions often found on a smart switch are turning some particular port range on or off, link speed and duplex settings and priority settings for ports
  • Enterprise Managed Switches: Enterprise switches are the more configurable and expensive version of managed switches. They are most often found in enterprise networks among several other switches. They are more efficient for large business where accessing a central administration module can save time and money. Some advanced functions for enterprise switches are VLAN settings, link aggregation and port mirroring.

Buying switches

There are several brand name switch manufacturers that provide competing and differentiated products, including Cisco, 3Com, and Alcatel. While switches can be purchased out of the box from online retailers, one way to save money is to find a used switch from an online reseller. A business purchaser can often save thousands of dollars purchasing used cisco or other brand name network hardware.

If you do decide to go the route of an online reseller, be sure to check for several qualifying factors to make sure they are a good fit. One factor is a good warranty, as it is always a risk to buy used equipment. Another is significant discounts (at least 50%) off of retail pricing. The third factor I recommend seeking in an online network hardware vendor is good customer support. The ability to speak to a human being for help with your purchase is underrated.

I hope this ‘basic switch support’ post helps out those that are confused or looking for a way to purchase a switch.

The Truth About Current Switching and Measurement Systems

Unless signal differences are taken into account, they can degrade signal integrity and affect total test system performance

The differences among the types of signals that a test system’s switching hardware must handle are not always well understood. But if these differences are not taken into account in switch system design, they can degrade signal integrity and affect overall test system performance.

When designing a measurement system, selection of the switch is as critical as the selection of system instrumentation or the design of the test interface. The intended application must be thoroughly considered, and the switch selected must meet the requirements of the application. Careful attention to detail and to the basic principles of measurement can help ensure greater system accuracy and performance.

Voltage vs. current switching
Voltage sources can usually provide a compliance current up to the programmed voltage. As a result, the typical default condition of a voltage switch is open (in other words, drawing very little current or having high impedance).

Current switching, however, usually requires the default configuration to be a complete circuit. This means the current needs a complete path until switched. Typically, the switching component (that is, the relay) is a normally closed relay, or the HI and LO terminals are shorted in the default condition. A variety of switching topologies are suitable for use in current switching applications: scanner, multiplex, and matrix switching. The scan configuration or scanner is the simplest arrangement of relays in a switch system. It can be thought of as a multiple position selector switch.

Like the scan configuration, multiplex switching can be used to connect one instrument to multiple devices (1:N) or to connect multiple instruments to a single device (N:1). However, the multiplex configuration is much more flexible than the scan configuration. Unlike the scan configuration, multiplex switching enables making multiple simultaneous connections and also permits either sequential or nonsequential switch closures.

The matrix switch configuration is the most versatile because it can connect multiple inputs to multiple outputs. A matrix is useful when connections must be made between several signal sources and a multi-pin device, such as an integrated circuit or a resistor network.

Typical current concerns
Most current measurement applications demand that all current paths be continuous, even when a particular current signal is not connected to the ammeter. To accomplish this, switch cards designed for current switching often use SPDT or Form C relays.

Note that the current will be interrupted briefly when the Form C relay is actuated. This could cause problems when used with high-speed logic or other circuits sensitive to a momentary break in the current flow. Such a problem can be overcome by using a switch card such as those used with the Series 3700 switch system/multimeter with a pair of Form A isolated switches to provide a make-before-break connection.

High-current considerations
When designing a switching circuit for high current (> 1 A), pay particular attention to the maximum current, maximum voltage, and VA specifications of the switch cards and relays. Also, it is important to choose a switch card or relay with low contact resistance to avoid excessive heating, which can cause contacts to weld together and thereby lead to contact failure. Contact heating is caused by I2R power dissipation.

High-current switching can be used for either switching a power supply to multiple loads or for switching an ammeter to multiple sources. When a power supply is switched to multiple loads using a multiplexer scanner card, the power supply will output 1 A to each of four loads. This does not present a problem when only one channel is closed at a time, but when all four channels are closed, the power supply will output 4 A through the common path.

Unfortunately, even though the maximum current of a particular channel is specified at 1 A, the common path on the switch card may not be able to tolerate 4 A. This is not usually specified for a switch card, but the limitation is usually a function of the trace width and connector ratings. One way to avoid this problem is to use a switch card with independent (isolated) relays and to make connections with wires rated to carry the total current.

Low-current considerations
When switching currents of 1 micro-A or less, special techniques must be used to minimize sources of interference such as offset currents, leakage currents, electrostatic interference, triboelectric currents, and electromechanical currents. The interference might come from the switch card itself, the connecting cables, or the test fixturing.

Offset currents are spurious currents generated by a switching card even though no signals are applied. They are usually caused by galvanic sources on the switch card. Offset current is especially significant when measuring low currents if the magnitude of the offset is comparable to that of the current being measured.

Leakage current is an error current that flows through insulators when a voltage is applied. It can be found on the switch card, in cabling, and in test fixtures. Even high-resistance paths between low-current conductors and nearby voltage sources can generate significant leakage currents.

To reduce these effects, always use a switch card with high channel isolation and use the guard capability of the measurement instrument. Another method to help reduce leakage current is to keep the switch card clean. Dirt, body oils, and the like will create a lower-resistance path and allow leakage currents to flow.

To reduce leakage currents in the text fixturing, always use good quality insulators such as Teflon and polyethylene; avoid materials such as nylon and phenolics, which can absorb moisture, affecting their insulating performance.

Shielding is required because high-impedance circuitry is susceptible to picking up spurious radiated noise. Relay contacts should be shielded from the coil to minimize induced noise from the relay power supply. The device under test and interconnect cabling should also be shielded to prevent noise pickup. All shields should be connected to circuit LO.

Triboelectric currents are generated by charges created by friction between a conductor and an insulator, such as between the conductor and the insulation of a coaxial cable. The friction can be reduced by using special low-noise cables with conductive coating (such as graphite) and by securing the interconnect cabling to minimize movement.

Electrochemical currents are generated by galvanic battery action that results from contamination and humidity. Cleansing joints and surfaces thoroughly to remove electrolytic residues (which include PC etchants, body salts, and processing chemicals) will minimize these parasitic battery effects.

Settling time
When a relay opens or closes, a charge transfer on the order of picocoulombs occurs, which causes a current pulse in the circuit. This charge transfer is due to the mechanical release or closure of the contacts, the contact-to-coil capacitance, and the stray capacitance between signal and relay drive lines. After a relay is closed, it is important to allow sufficient settling time before taking a measurement. This can be as long as several seconds, depending on the relay.

If a step voltage is applied to the circuit, a transient current is generated. This current will gradually decay to a steady-state value. The time needed to reach the steady value (or settling time) can be used to determine the proper measurement-delay time.

Cold, hot, and safe
The term “cold switching” indicates that a switch is activated with no signal applied. Therefore, no current will flow when the switch is closed and no current will be interrupted when the switch is opened. When hot switching, voltage is present and current will flow the instant the contacts close. When the switch is opened, this current will be interrupted and can cause arcing.

Cold switching lets power be applied to the device under test in a controlled manner. Its primary advantage is longer switch life than with hot switching. Cold switching also eliminates arcing at the relay contacts and any RFI it might cause.

Hot switching might be necessary if close control must be exercised in the period between the application of power and the making of the measurement. For example, hot switching is typically used where digital logic is involved, because devices might change state if power is interrupted even for a moment. With relatively large relays, hot switching should also be performed every so often to ensure good contact closure. The connection might not be reliable without the “wetting” action produced by current flow through the contacts.

Many electrical test systems can produce hazardous levels of power. These high power levels make operator protection a priority. Some protection methods include:

– Designing test fixtures that prevent operator contact with hazardous circuits
– Double-insulating all electrical connections that an operator could touch
– Using high-reliability fail-safe interlock switches that disconnect power sources when a test fixture is opened
– Providing proper training to all users so that they understand potential hazards and know how to protect themselves from injury

Understanding 3-Way and 4-Way Switches

Do you have a room that has two or more entrances and you would like to have a switch at each entrance to control a light? Or more commonly, did your electrician set one up for you and now you have replaced a broken switch but can’t get it to work right? Fear not, some simple diagrams should help sort it out.

First of all, there are 2-way, 3-way, and 4-way switches; each with a different purpose. Two-way switches are most common and only have two terminals in addition to the ground screw. These are very simple in nature and simply either break or complete the circuit to turn a light on or off.

If you have two switches that control a light, you must use 3-way switches (more on 4-ways later if you have more than 2 switches). Usually we see people get in trouble when they want to replace one switch with a dimmer. The dimmer in this situation must be a three way and the three wires must go to the correct terminals. (at the end of this article we explain how to look at a switch and most of the time get the hook-up correct. When all else fails, use a continuity tester.) Three-way switches will have 3 terminals in addition to the ground screw. One hot(usually black) wire either comes from the power panel into the switch or one hot wire exits the switch and goes to the light. In between the switches are two wires called travelers. These are considered switched hot wires and can be typically black, red, or sometimes a white wire has black tape wrapped around it at each end to designate it as a hot and not a neutral wire.

Four way switches are used when you have 3 or more switches to control a light. As can be seen in the diagrams below, there is always a 3-way switch at the start and end of the circuit, with 1 or more 4-ways in between. The 4-way switches simply have the two travelers coming in and then going out to the next switch down the line.

Let’s take a look a some diagrams to understand how the circuit and switches work.

First here is an example of a 3-way switch setup. Light is off as there is no path for the hot.

Switch 1 Switch 2

—- —- OFF

Hot | / 2|——-+2 | ——-

—-|1 | | 1|—-| Light |

| 3|——-+3 / | ——-

—- —- Neutral|


Switch 2 is moved, Light is ON as there is now a path for the hot.

Switch 1 Switch 2

—- —- ON

Hot | / 2|——-+2 | ——-

—-|1 | | 1|—–| Light |

| 3|——-+3 | ——-

—- —- Neutral |


Either moving Switch 1 or Switch 2 will break the hot. And from the Off state, either Switch will make the connection.

Now for an example of a 4-way switch setup. The 4-way switch must be in-between the 3-ways.

3-way 4-way 3-way

Switch 1 Switch 2 Switch 3

—- —- —- ON

Hot | / 2|—–|1–3|—–|2 | ——-

—-|1 | | | | 1|—–| Light |

| 3|—–|2–4|—–|3 | ——-

—- —- —- Neutral |


Moving either switch 1 or 3 like before will turn the light off.

3-way 4-way 3-way

Switch 1 Switch 2 Switch 3

—- —– —- OFF

Hot | / 2|—–|1 3|—–|2 | ——-

—-|1 | | X | | 1|—–| Light |

| 3|—–|2 4|—–|3 | ——-

—- —– —- Neutral |


Switch 2 either connects 1to3 and 2to4 like shown or when it is flipped it cross connects 1to4 and 2to3. So in the case shown, if switch 2 was flipped, the path would go from switch 1 1-2, then switch 2 1to4, but would stop a switch 3 since there is no path and thus the light goes off.

You may add additional 4-way switches into the middle of the wiring. The 3-way switches must always be at the beginning and at the end of the circuit.

The wires in-between the the 3-way switches are called travelers. If you are pulling wire through conduit, best to use different colored wire like blue and orange. If you are using Romex, I prefer to use the 4 wire version which has a ground, a white neutral, and a black and a red. Little more expensive but from a safety perspective I prefer not to wrap a black piece of tape around the white wire to mark it as a hot.

So use the red and the black for your travelers between switches. On the 3-way switches, you cannot just connect the hot to one of the terminals and the travelers to the remaining two. Look at a diagram on the switch or most times there is a single terminal on the top or bottom for the in/out hot and then two terminals (one on each side) at the other end are for the travelers. Connecting the hot to the side that has one terminal and the travelers to the side that has two terminals is usually NOT the way to do it.

Same goes for the travelers in and out of a 4-way switch. The in is typically both sides of the top of the switch and the out goes on both side of the bottom of the switch. You can verify what is right or wrong using a simple continuity buzzer and comparing the results against the diagram above.

If you didn’t get the wires on the correct terminals, then you will find that it sometimes takes flipping two of the switches to get the light to turn on or off. With the proper wiring, any single switch that is flipped should cause the light to go on or off.

And please remember to turn the power off first. BZZZT sounds or arc welding your switch is not a good thing.