Networking > Wireless

RS-232, also known as RS-232C and TIA/EIA-232-E, is a group of electrical, functional, and mechanical specifications for serial interfaces between computers, terminals, and peripherals. The RS-232 standard was developed by the Electrical Industries Association (EIA), and defines requirements for connecting data communications equipment (DCE)—modems, converters, etc.—and data terminal equipment (DTE)—computers, controllers, etc.) devices. RS-232 transmits data at speeds up to 115 Kbps and over distances up to 50 feet (15.2 m).

The standard, which is functionally equivalent to ITU V.24/V.28, specifies the workings of the interface, circuitry, and connector pinning. Both sync and async binary data transmission fall under RS-232. Although RS-232 is sometimes still used to transmit data from PCs to peripheral devices, the most common uses today are for network console ports and for industrial devices.

Even though RS-232 is a “standard,” you can’t necessarily expect seamless communication between two RS-232 devices. Why? Because different devices have different circuitry or pinning, and different wires may be designated to perform different functions.

The typical RS-232 connector is DB25, but some PCs and other data communication devices have DB9 connectors and many newer devices have RJ-45 RS-232 ports. To connect 9-pin PC ports or RJ-45 to devices with 25-pin connectors, you will require a simple adapter cable.

Although sharing printers is one of the primary reasons for installing networks, networks didn’t manage this chore very well until the print server was developed. A print server is a specialized network device that enables network users to share one or more printers. It accepts print jobs from users and manages these jobs on each printer. Print servers are easy to install, use, and maintain.

Typically a print server is a freestanding device that’s connected between the network and the printer; some print servers are so tiny they sit directly on the parallel port of the printer. And some newer printers are even equipped with embedded or internal print servers, so you don’t need to purchase a separate print server at all—just connect the printer to the network. Most print servers support multiple protocols and operating systems. You can also get print servers with multiple ports to connect more than one printer.

When a print job arrives at a print server, it’s entered into a queue where it waits until the desired printer becomes available. Print jobs are normally served in a first-in, first-out order, however many print servers enable users to prioritize print jobs so important jobs can be moved to the front of the queue.

For a print server to manage this entire lineup of print jobs, it must have a place to store them—either in memory (a buffer) or, if the buffer isn’t large enough, sometimes on a hard disk. When a print job arrives at the print server, it sends the job to the printer immediately if the printer is free. Because the print job usually arrives faster than the printer can handle it, data waiting to be printed is stored temporarily in the buffer. If the print job is very large or if multiple print jobs start arriving, the buffer fills up. In this case, the print job is often spooled to a hard disk.

• It’s great for communicating in harsh climates or in areas where it’s expensive to run cable. Wireless solutions are well suited for use in military applications, farming, refineries, mining, construction, and field research.
• Because sometimes you just can’t run wire, like in historic buildings or hazmat areas.
• When it’s physically or legally impossible to support conventional hard-wired RS-232 communications, wireless networking may be your only answer.
• It gives you quick, temporary connections at trade shows, and fast reconfigurations—even troubleshooting or remote field testing.
• It provides reliable disaster relief when all else fails! Count on wireless networks to maintain mission-critical links when disaster strikes.
• It’s more affordable, more reliable, and faster than ever before.
• Best of all, no FCC licensing required!

What is PoE?
The seemingly universal network connection, twisted-pair Ethernet cable, has another role to play, providing electrical power to low-wattage electrical devices. Power over Ethernet (PoE) was ratified by the Institute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for low-level power delivery—roughly 13 watts at 48 VDC—over twisted-pair Ethernet cable to PoE-enabled devices such as IP telephones, wireless access points, Web cameras, and audio speakers.

Recently, the basic 802.3af standard was joined by the IEEE 802.3at PoE standard, ratified on September 11, 2009, which supplies up to 25 watts to larger, more power-hungry devices. 802.3at is backwards compatible with 802.3af.

How does PoE work?
The way it works is simple. Ethernet cable that meets CAT5 (or better) standards consists of four twisted pairs of cable, and PoE sends power over these pairs to PoE-enabled devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair.

When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.

Basic structure.
There are two types of devices involved in PoE configurations: Power Sourcing Equipment (PSE) and Powered Devices (PD).

PSEs, which include end-span and mid-span devices, provide power to PDs over the Ethernet cable. An end-span device is often a PoE-enabled network switch that’s designed to supply power directly to the cable from each port. The setup would look something like this:

End-span device → Ethernet with power

A mid-span device is inserted between a non-PoE device and the network, and it supplies power from that juncture. Here is a rough schematic of that setup:

Non-PoE switch → Ethernet without PoE → Mid-span device → Ethernet with power

Power injectors, a third type of PSE, supply power to a specific point on the network while the other network segments remain without power.

PDs are pieces of equipment like surveillance cameras, sensors, wireless access points, and any other devices that operate on PoE.

PoE applications and benefits.
• Use one set of twisted-pair wires for both data and low-wattage appliances.
• In addition to the applications noted above, PoE also works well for video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point-of-information systems.
• Save money by eliminating the need to run electrical wiring.
• Easily move an appliance with minimal disruption.
• If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.

Using a network camera with a built-in Web server, you can check images, control the camera remotely, and store and save images—all without having to use a PC.

Unlike USB cameras, network cameras can be installed on the ceiling, on a wall, or even outdoors if they have wireless capability. USB cameras, by contrast, can be only placed near a PC and have no wireless capability. What’s more, there’s no limit to the number of network cameras you can install, but it’s often difficult to install multiple USB cameras.

Network cameras also provide a lot more flexibility than USB cameras do when it comes to monitoring. You can check network cameras from a PC, a cell phone that can connect to the Internet, or even a TV. In contrast, you can only monitor USB cameras from a PC.

Small offices, mid-sized businesses, and homes are ideal environments for network cameras. Businesses will find them very useful for facility and parking lot surveillance and two-way communication with off-site branch offices (which cuts down on travel time for routine visits). Entertainment establishments can also use network cameras. Their CCD sensors enable security staff to clearly see patrons on dance floors and at bars, and managers are able to see employees and keep them on their toes.

Network cameras are also great for keeping an eye on children, pets, and your home itself from virtually anywhere. Some network camera systems can be programmed to send an image to an office PC or cell phone when a change is detected in a user’s house. Wireless camera monitoring systems can even enable viewers to check out their entire house on a TV screen.