27-10-2012, 06:08 PM
A wide area network (WAN)
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NETWORK
A wide area network (WAN) is a communications network utilizing devices such as telephone lines, satellite dishes, antennas, and microwaves to span a larger geographic area than can be covered by a local area network (LAN). Typically, a WAN consists of two or more LANs. A metropolitan area network, or MAN, is a term sometimes used to describe a system that falls somewhere between a WAN and LAN and combines some of the features of both.
If cost were no issue, schools would build their WANs using fiber optic cable, which is capable of transmitting data at a billion or more bits per second (1 Gb/s). Fiber cable provides the fastest, most reliable, and secure way to transmit computer data. But it is often an expensive approach.
To connect their buildings when the Internet was in its infancy, school districts used basic telephone lines costing about $30 per building per month and offering modest speeds of up to 56 kb/s. Communications between buildings were usually limited to electronic text transmissions of administrative data, not graphics. The relatively low-speed connection was not a major issue, and the connection between computers in limited areas, such as offices and computer labs, was generally provided by LANs.
As demand for network access grew following the rapid deployment of educational technologies and Internet access in classrooms, so too did the need for higher capacity and transmission speed, known as bandwidth. Consequently, schools installed services such as ISDN (Integrated Services Digital Network) at 128 kb/s and T1 leased lines at 1.5 Mb/s. These have been expensive options, with a T1 line, for example, costing upwards of $300 per month per building and as much as $2,000 per month for service to remote locations. And, because these lines had to be shared by many users with an attendant reduction in speed for each additional person using the system, speeds at the desktop could be anything but impressive.
Today, with computers in nearly every classroom, Internet access for every computer, and computer labs placing more demands on network resources than ever before, inter-building connectivity has become a hot issue, with schools scrambling for all the bandwidth they can get. But when examining such expensive options as T3 lines that offer speeds of up to 45 Mb/s and dedicated fiber connections costing thousands of dollars per month to lease, many school districts have been seriously viewing the wireless option. Its numerous advantages include high bandwidth, zero ongoing connection costs, reasonable periodic maintenance costs, no trenching, no line leasing, and no need to obtain easements to traverse public rights-of-way.
THE WIRELESS WAN
Wireless WANs are hardly new. They have been utilized since the mid 1980s when microwave transmissions were beamed about by complex and powerful transmitting units that required Federal Communications Commission (FCC) licenses and radios and antennas costing $40,000 to $50,000. Today, wireless systems can deliver up to 100 Mb/s speeds at 40 miles' distance, and speeds are increasing.
Wireless systems gained a foothold for widespread use with the adoption of a standard, IEEE 802.11, in 1997 by the Institute of Electrical and Electronics Engineers that established clear guidelines that helped wireless vendors begin to produce standardized, interoperable products. IEEE 802.11 established 1 Mb/s and 2 Mb/s wireless data transmission rates, but these speeds were inadequate for most business requirements, and wireless LAN and WAN systems were slow to be adopted.
The 1997 technology also was hampered by high costs, with wireless network adapters typically costing $400—twice what they cost in 2001. The earlier products also provided little security, permitting nearly anyone to tap into a private wireless network.
Recognizing the critical need for faster data transmission and improved security, IEEE ratified what it called the 802.11b high rate standard, which permits transmissions at 5.5 and 11 Mb/s. The revised standard fostered explosive growth in wireless LANs in the business community and, more recently, in schools because it promised wireless transmission speeds rivaling the wired Ethernet. In practice, however, IEEE 802.11b only permitted a data transmission rate of about 7 Mb/s— still impressive, but insufficient to transfer data from, for instance, a digital video disk or a video file.
IEEE has recently approved the 802.11g standard, which permits transmission rates of 54 Mb/s and makes wireless wide area networks more viable than ever. This standard is backwardly compatible with the existing 802.11b standard, and it will usher in many new products that harness the increased speed.
For schools, the primary advantage of IEEE 802.11b and subsequent standards is fast communication between buildings with no recurring costs other than periodic maintenance expenses. That means installation costs can be amortized within a year or two in most cases by eliminating fees to telephone companies.
Wireless installations enjoy another advantage over cable in that they represent a one-time capital cost that can benefit a school district for many years. This scenario is far more likely to attract grant money than the situation where a school is making recurring payments to a service provider—typically the telephone company—for leased lines.
LINE OF SIGHT LIMITATION
A major limitation of wireless WANs is that they require a clear line of sight between the transmitters and receivers that provide the communications link between buildings. While installations exist where signals can traverse short distances through such obstructions as heavy foliage, in general wireless transmitters and receivers must be located so that no obstructions exist between them.
What does this mean for a school district that has several buildings but none offering obstruction-free signal paths? The problem is not insurmountable. Moderate obstructions often can be overcome by using transmitting towers. In some cases, school districts have employed existing structures, such as light poles, to surmount problematic transmission conditions caused by obstructions between buildings.
INTERFERENCE, SECURITY, AND RELIABLITY
If other cost-effective, high speed wireless products operating on the licensed 5 GHz frequency band become available, schools will be able to transmit data more securely over their wireless WANs. Experts are debating the value of the 5 GHz products for building-to-building connectivity. Of note are concerns over licensing issues and the shorter transmission distances possible on this frequency compared to the two lower frequencies—900 MHz and 2.4 GHz—presently authorized by the FCC for unlicensed microwave transmission. Most wireless WANs operate on the 2.4 GHz band.
While unlicensed use frees a school district from the red tape of government approvals, it also opens up the possibility of interference caused by others using the same, free, unlicensed frequency. To solve this problem, wireless equipment manufacturers such as Lucent Tech-nologies, Cisco Systems, Inc., 3Com Corp., and Symbol Technologies, Inc. rely on the IEEE 802.11 family of standards, which include communications protocols that let users on a particular wireless system communicate freely while blocking unauthorized users. This eliminates most interference problems.
But there is still the security issue to consider. Present wireless WAN security technology offers two levels of encryption: 40 bit and 128 bit. Typically, 40-bit encryption should be sufficient to prevent unauthorized entry. For greater security, experts recommend 128-bit encryption.
Encrypting signals that traverse the wireless link itself is just one of several layers of security that a school should provide for its computer networks. Other appropriate security methods such as password authentication, firewalls, and virtual private network solutions can greatly reduce security risks for users and for LAN-based resources networked over a wireless WAN.
Wireless systems sometimes can be considered more reliable than leased lines because all the wireless equipment belongs to the owner and remains on the owner's property. Leased lines may experience problems caused by power failures or other interruptions, including unscheduled repairs and maintenance by utility companies. Microwave transmissions can travel up to 25 miles without significant signal degradation and can withstand most weather conditions, including wind, rain, and snow, although they are susceptible to heavy fog. But severe wind might dislodge an antenna, and caked ice and snow can degrade signals. Antennas must also be protected by lightning arrestors to avoid severe damage and consequent loss of service.
Though areas that experience severe weather may be unsuitable for wireless connectivity, weather is generally not a significant factor. An installation at the Bronx Zoo in New York City survived the relatively harsh winter of 2000 without fault. Arizona's Vail School District connects 15 buildings wirelessly, and the district's technology director Billy Martin said the system withstands the Southwest's infamous desert storms without a problem.
THE WIRELESS WAN-IS IT FOR YOU
School districts with multiple buildings within a few miles of each other but located outside of dense urban environments are good candidates for wireless WANs. Unless special arrangements are made with local utilities or other institutions to provide connectivity via fiber, no other system delivers the high bandwidth connectivity between buildings that wireless offers.
Other factors to consider beyond building proximity are terrain and visual obstructions. Wireless works best when terrain is relatively flat. Hills and valleys may destroy clear lines of sight between buildings. Such was the case at Saugus Union School District in Santa Clarita, California, which considered and rejected a wireless solution, despite the recurring fees it had to pay Pacific Bell. In the words of James Klein, the school district's director of information services, "We would have preferred to avoid recurring costs altogether by using wireless links for the wide area network; however, this solution would have required clear line of sight between the radios, which was not possible in the canyon where our school district is located."
Wireless WAN solutions can work in concert with wired network backbones. These hybrid solutions may take many forms and provide satisfactory performance, giving schools the benefits of a wired network backbone and the flexibility of wireless connectivity. For example, a school district may choose a wireless system to connect buildings up to 25 miles apart that have a clear line of sight and use leased or fiber lines to connect buildings that don't.
The thousands of mobile classrooms and temporary buildings in this country offer a strong case for wireless building-to-building connectivity. Many of these facilities lack network capability, and school districts are loath to spend money to install network cabling in such temporary structures. Moreover, burying cable between structures and main buildings is often too impractical or too costly.
This was the case with the Wake County school system in North Carolina's Research Triangle Park region, which decided to wirelessly connect its mobile classrooms to its district network. "Our first initiative was to provide connectivity to our mobile classrooms, which number about 525 here," said Vass Johnson, director of networking. "We already have a fiber network throughout the district, but every year the facilities department must relocate a host of mobile units to meet changing needs at many of the schools. We were constantly reinstalling the fiber connections to keep the desktops and laptops in these classrooms connected with the campus LAN. With wireless, there's no need because the mobile classrooms are always connected. We save a lot of money by not having to make these adjustments every year."