But what about wireless?
Within the cabling arena, the trend is toward every application having its own 4-pair cable. And life is very busy for most everyone
Within the cabling arena, the trend is toward every application having its own 4-pair cable. And life is very busy for most everyone. Designers are hired to deal with how to physically support and manage huge cabling infrastructures; cabling manufacturers sell a lot of product; and many installers are busy placing, terminating, and testing all that cabling.
This month's column is dedicated to Arlyn Powell, group editorial director of this magazine, who never misses a chance to ask, "But what about wireless?"
First, a definition: "Wireless" is a data-transmission system designed to provide location-independent network access between computing devices by using radio waves rather than cabling infrastructure.
Sounds scary for those who make their living cabling. But wireless local area networks (LANs) are usually employed as the permanent link between the existing wired network and a group of mobile computers, so they are not exactly "wireless." Rather, they could be considered "less wired."
Faster transmission speeds, lower implementation costs, and increasing mobile-worker productivity are reasons why users are supplementing wired networks with wireless. Early adopters of this technology include manufacturing facilities, warehouses, and retail stores. But acceptance is growing within healthcare facilities, educational institutions, and corporate offices, to include conference rooms and public areas.
Several wireless networking standards exist today, including Bluetooth, HomeRF, and IEEE 802.11 by the Institute of Electrical and Electronics Engineers (IEEE-New York City). Each serves a different purpose. While Bluetooth and HomeRF were designed for short-range links between computing devices and peripherals, IEEE 802.11 is emerging as the primary standard for wireless-network and Internet access.
The IEEE 802 committee has drafted the standards that have driven the LAN industry for the past two decades. In 1997, after seven years of work, the IEEE ratified 802.11 as the standard for wireless LANs. But 802.11 only had provisions for 1- and 2-Mbit/sec data rates-too slow to support most general business requirements.
In September 1999, the IEEE ratified 802.11b (also known as 802.11 High Rate), adding two higher speeds (5.5 and 11 Mbits/sec) to 802.11. With 802.11b, wireless LANs will now be able to achieve wireless performance throughput comparable to that of 10Base-T.
As cabling-system designers, you are familiar with IEEE 802.3 (Ethernet) and 802.5 (Token Ring). But what do you know about IEEE 802.11?
Like all IEEE 802 standards, the 802.11 standards focus on the bottom two levels of the ISO model-the physical layer and data-link layer. The original 802.11 standard defines the basic architecture, features, and services of 802.11b. The 802.11b affects only the physical layer, adding higher data rates and more robust connectivity.
The 802.11 standard identifies two modes-infrastructure mode and ad-hoc mode. In infrastructure mode, the wireless network consists of at least one access point connected to the wired network infrastructure and a set of wireless end stations. In ad-hoc mode (also called peer-to-peer), wireless stations communicate directly without using an access point or any connection to a wired network.
In concept, 802.11 is similar to 802.3, in that both are designed to support multiple users on a shared medium by having the sender sense the medium before accessing it.
In an 802.3 Ethernet LAN, the car rier sense multiple access with collision detection (CSMA/CD) protocol regulates how Ethernet stations establish access to the medium (in this case, cable) and how they detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN.
In an 802.11 wireless LAN, collision detection is not possible, so 802.11 uses carrier sense multiple access with collision avoidance (CSMA/CA). Using the
CSMA/CA protocol, a station wishing to transmit senses the medium (in this case, air). If the station senses no activity, it waits an additional, randomly selected period of time then transmits if the air is still free. If the packet is received intact, the receiving station issues an acknowledgement frame. If the sending station does not detect the acknowledgement frame, it assumes a collision; after waiting another random amount of time, it retransmits the packets. This adds overhead to 802.11 that 802.3 does not have; hence an 802.11 LAN will always have slower performance than an equivalent Ethernet LAN.
The three physical layers originally defined in 802.11 included two spread-spectrum radio techniques and a diffuse infrared specification. The radio-based standards operate within the 2.4-GHz Industrial, Scientific, and Medical (ISM) band. These frequency bands are recognized by international regulatory agencies in the United States, Europe, and Japan, for unlicensed radio operations. As such, 802.11-based products do not require user licensing or special training.
The 802.11 standard specified data rates of 1 and 2 Mbits/sec via radio waves using frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS). FHSS and DSSS are fundamentally different signaling mechanisms and will not interoperate. But 802.11b is based on DSSS, making migration from a 2-Mbit/sec DSSS system to an 11-Mbit/sec system easy. 2-Mbit/sec DSSS systems will be able to coexist with 11-Mbit/sec systems, enabling a smooth transition to the higher-data-rate technology.
Any LAN application, network operating system, or protocol will run on an 802.11-compliant wireless LAN as easily as it will run over Ethernet-just more slowly. Since IEEE 802.11b wireless LANs communicate using radio waves that can penetrate or may reflect around many indoor structures, how much more slowly is primarily determined by the quality of the design.
To establish a wireless LAN, you must install and configure the access points and network-interface cards (NICs). And yes, you likely will need to add cable from the existing network infrastructure to the access points, because they are rarely located near existing outlets. The number of access points depends on the coverage area, number of users, and types of services required.
The single most important part of the installation is placement of the access points to ensure proper coverage. Most manufacturers provide a site-survey tool. Place the access points and use the site-survey tool to record signal strength and quality while roving within the intended coverage area. Once the access points are installed, access points and NICs must be configured. Configuration options vary according to the manufacturer. When you are designing a wireless LAN, the cost of the equipment is just one concern; you must also consider installation and maintenance expenses.
Although all 802.11b-compliant products are based on a single standard, the standard offers no guarantee that access points, NICs, and other equipment from various manufacturers will interoperate.
Interoperability and interference
Understanding that users are not likely to walk around with a pocketful of different NICs, industry leaders have formed the Wireless Ethernet Compatibility Alliance (WECA). WECA's mission is to certify cross-vendor interoperability and compatibility of 802.11b wireless networking products. WECA has created tests to certify interoperability and announced the WiFi (wireless fidelity) standard, which is an awarded seal for those wireless LAN products that have successfully completed prescribed interoperability testing. The WiFi seal provides assurance that products bearing the logo will work together. Many of the industry's leading wireless LAN manufacturers belong to WECA. More information is available at www.wirelessethernet.org.
The WiFi effort is effective to the extent it reaches, but understand its limitations. Not all wireless products are 802.11b-based. For example, both IEEE 802.11 and Bluetooth share common spectrum in the 2.4-GHz ISM band. Both are targeted at the business user.
Now with 11-Mbit/sec data rates possible, IEEE 802.11 DSSS radios can provide a mobile extension to wired networks in large enterprise installations, and even completely replace a cabling infrastructure in small-office and home- office environments. Meanwhile, Bluetooth is becoming essential to the mobile worker and business traveler by facilitating e-mail to a laptop using a cellular telephone, synchronization with palmtop devices, and access to local printers.
It is inevitable that 802.11b and Bluetooth will bump into each other at the office. Currently, studies are underway to see exactly what the effects will be. But it is known that IEEE 802.11b susceptibility to Bluetooth interference increases as the distance between DSSS wireless nodes and DSSS access points increases. For this widespread acceptance of 802.11b to continue, compatibility among the various wireless standards is imperative.
The next generation of 802.11 is on the horizon. Recently, the IEEE standards board approved a new project within 802.11 to increase the data rate of wireless LANs operating in the 2.4-GHz band from the current 11 Mbits/sec to greater than 20 Mbits/sec. Many expect wireless LAN data rates to increase as part of this project. IEEE 802.11 will form a new Task Group for the development of the draft standard P802.11g.
Then there is the entire issue of cordless telephones and radio-frequency lighting controls-neither of which cares whether the users' data transmission makes it through the airwaves. It could get very interesting.
Donna Ballast is a communications analyst at the University of Texas at Austin and a bicsi reg-istered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, the University of Texas, Austin, TX 78713; tel: (512) 471-0112, e-mail: email@example.com.
Is all that RF safe?
Any health effects related to radio-frequency (RF) transmissions correlate to the output power and physical proximity to the transmitter. Federal Communications Commis-sion regulations limit output power of wireless local-area-network systems to less than 100 mW-less than that of a mobile phone. But is it safe? Well, it's safer than your mobile phone.