The technology recommendations in Section III, Chapter 1, A Technical Model, represent the Network Technology Planning Committee's current view of the most cost effective, reliable network solutions for schools. However, the authors of this report recognize that many newer technologies will become important during this decade; these may become options to consider as the need for greater speed or other functionality arises.
It has been the intention of the Network Technology Planning Committee to recommend well established, standard technologies within a framework that can be used with newer technologies as well. For example, a school might start with an ethernet network backbone and a leased digital circuit to the school district office. Later, the backbone could be replaced with Fiber Distributed Data Interface (FDDI); the link to the district office might be via a Frame Relay network. In both examples the premise wiring within the school could remain the same. In order to upgrade the network, the fiber for FDDI would need to be added and the network router would need to be replaced. The majority of the wiring and equipment could still be used.
Similarly, if applications were to demand it and sufficient funding were available, all school district and county offices of education could be interconnected with high speed SMDS service instead of leasing point-to-point circuits to Internet service providers. Internet service then would be provided for all schools by redundant SMDS connections to one or more of the statewide Internet service providers. This would, in effect, provide a physical Golden State Education Network backbone. Again, the only changes from the currently recommended plan would be to replace the network router in the district or county office and acquire a different communication service.
The technologies discussed below do not represent an exhaustive list, nor does this appendix address the costs of these technologies.
I. LAN Technologies
Local Area Network (LAN) technologies interconnect computers within a building or cluster of buildings. The characteristics of LAN technologies are: high speed and relatively short range. Most often LAN data transmission capacity is shared among many computers. Transmission media typically is owned and managed by the school.
A. Fiber Distributed Data Interface (FDDI)
FDDI is an industry standard "token ring" LAN technology that operates at 100 megabits per second. As its name suggests, it was designed to be implemented with fiber optic cabling between connection points and to attached computers. Recent industry standards define how to implement FDDI connections over the type of copper wiring recommended in the Technical Model (Section III, Chapter 1).
FDDI can be used as a backbone for interconnecting a large number of separate ethernet LANs. Many types of computers can be connected directly to FDDI as well. It is highly recommended that when mixing FDDI and ethernet or other LAN technologies, network routers be used.
Currently many FDDI products are available from a wide variety of vendors. However, it is worth noting that typical costs are still many times higher than those for ethernet.
B. "Fast ethernet"
Industry standards are being developed for ethernet-like LAN technology that will operate at 100 megabits per second. This technology will operate over the type of copper wiring recommended in the Technical Model (Section III, Chapter 1). However, this technology would require replacement of the adapter cards in all attached computers. It is too early to tell whether this technology will become widely available.
C. "Switched ethernet"
A simple way to gain more communications capacity for individual computers on a standard ethernet LAN is to reduce the number of computers attached to the given segment. The ultimate reduction is to a single computer attached to a "switching hub" that will direct ethernet data packets to or from similarly attached computers or ordinary ethernet segments. This could be very effective, for example, for a set of network file servers exchanging data with a small collection of client computers.
Products based on this technology exist, but are very expensive.
D. Asynchronous Transfer Mode (ATM) Backbones
ATM was developed as a communications technology to support digital data transport at many gigabits per second. Recent products support interconnection of FDDI, ethernet, and other LAN technologies through a similar technique. In effect, this provides a very high speed backbone similar in concept to the switched ethernet mentioned above.
In theory, such a backbone could be extended to individual computers and be connected to wide area ATM networks as well. This would allow local and long distance very high speed connections directly between computers and/or other equipment. The vision of integrated data, voice, and video communications to all network nodes is often predicated on the installation of this technology.
ATM products are beginning to emerge but are extremely expensive. Some futurists predict that a future generation of powerful desktop computers may come with this capability built-in.
E. Wireless LAN Systems
A genre of technology is emerging to support wireless communications to individual desktop or portable computers. The advantages of wireless communications include mobility and inherent extensibility to additional nodes as they appear. The disadvantages include limited range, and low speed or significant drain on the batteries in portable computers.
1.) Infrared
Infrared systems are simple to install and relatively inexpensive. The primary disadvantage of Infrared is the requirement for "line of sight" to each computer. In order to protect the integrity of the technology, bright light sources or sunlight must be kept out of view of the receivers, (very heavy fog can also cause insurmountable interference). Nonetheless, there are some good applications for infrared (e.g., a portable computer station that must be operated in the middle of an indoor stage or auditorium). Infrared communication speeds range from very low (9600 bits per second) through ethernet speeds.
2.) Radio
Various forms of wireless LAN technologies are emerging. Ethernet-like systems supporting a few dozen nodes are available that will cover areas from a few rooms to several buildings, depending on the construction of the buildings. Generally these systems cost three to five times the cost of standard ethernet.
Low speed radio data communications is being built in to new portable computers and personal assistant systems such as Apple Computer's Newton product. Typical communications speeds are 9600 bits per second or less.
II. WAN Technologies
Wide Area Network (WAN) technologies are typically used to interconnect LANs or individual computers over long distances. Most WAN technologies are "point-to-point," however some are actually wide area networks that can transport data among dispersed service points.
Several WAN technologies were mentioned in the recommended Technical Model (Section III, Chapter1). Those technologies were highlighted because they are the most commonly available and cost effective today. Each school district should explore the costs and availability of all communications services that can transport digital data. New services are being or soon will be offered by a number of carriers including, but not limited to: Pacific Bell, Sprint, and MCI. WAN technologies that may become important to schools include the following:
A. Integrated Services Digital Network (ISDN)
ISDN is a telecommunications standard that provides two (2) 64 kilobit per second "bearer (B) channels" for voice or digital data and one (1) 16 kilobit per second "data (D) channel" primarily intended for communication with the telecommunications switching systems. ISDN can be run over most existing telephone wiring, including to individual homes and offices. ISDN service should be available in all metropolitan areas of California by the end of 1994.
By combining both of the B channels, ISDN could be used to link schools to district offices at 128 kilobits per second. Furthermore, such connections could be "dialed" when needed rather than being live all the time [1] . The cost of this service can be much less than similar "switched 56" or Advanced Digital Network (ADN) leased line service.
ISDN also could be used to link individual computers in homes or small satellite locations to central school LANs. Standard asynchronous speeds up to 38,400 bits per second are supported. This allows "modem-like" serial communications, but at much higher speeds. Synchronous communications at the full B channel speed can be used to achieve the greatest data throughput, however this usually will require special software and/or hardware.
The cost of ISDN compatible network equipment is still rather high, but costs are expected to significantly decrease as the availability of ISDN service increases the size of the market.
B. Frame Relay Network Service
Frame Relay is a general purpose packet data transport service that can operate at speeds up to at least 1.5 megabits per second. Network routers can connect to this service and establish communications "directly" to any other router in the same service group. As Frame Relay services become available, school districts should explore the costs of this technology compared to traditional point-to-point circuits.
C. Switched Multimegabit Data Service (SMDS)
Like Frame Relay, SMDS is a general purpose packet data transport service, however SMDS is designed to operate at speeds from 1.5 megabits per second to several hundred megabits per second. SMDS typically is offered with different "classes of service" that guarantee differing aggregate data volumes. For example, a district might install a 45 megabit per second SMDS service with a "class of service" guaranteeing only 10 megabits per second of average aggregate data traffic. This would cost less than a higher class of service, but would still allow occasional full bandwidth data transfers.
SMDS appears to be most cost effective over very wide areas. Within a single county, for example, point-to-point circuits between network routers currently are more cost effective.
D. Asynchronous Transfer Mode (ATM)
ATM is the extremely high speed digital communications technology that underlies SMDS and some of the LAN switching hubs described earlier. In essence, it segments all digital data streams into 48 byte "cells" and transports these cells at gigabit per second speeds to their destinations; once at their destinations the cells are reassembled. ATM is capable of transporting traditional packet computer data as well as real time digitized voice and video. The use of ATM is the subject of much current network research.
E. Infrared Optical Links
For distances up to 1,000 meters, infrared optical transceivers are available which support communication speeds as high as 10 megabits per second. As with the infrared technology described above (Wireless LAN Systems), the reliability of this technology can be affected by sunlight, bright lights, or very heavy fog. Infrared links are useful for crossing roadways or other land over which the school district may not have a "right of way."
F. Spread Spectrum Radio
As mentioned above (Wireless LAN Systems), radio technology can be used to transport data over modest distances. Spread spectrum is a relatively new technology that uses low power, unlicensed frequency bands for data transmission. For WAN purposes, spread spectrum radio can support up to several megabits per second over ranges of up to five miles. Point-to-point links currently support 256 kilobits per second over a distance of ten miles. It is expected that a wide range of data communication systems using this technology will become available within the next several years.
G. Microwave Radio Links
Traditional microwave radios can be used for distances of approximately twenty miles. Speeds of 45 megabits per second are possible, but the cost of the radios and other components for this capability may be on the order of $200,000. Lower speed, shorter range links will cost $15,000 to $40,000.
H. Future Integration of Voice, Data, and Video Communications
It is very likely that emerging technologies will support the transmission of high speed data, voice, and video communications over a common infrastructure. Many researchers and product developers are addressing this goal, but it is too soon to tell which technologies will become important in the school environment.
In the meantime, it is possible today to share bandwidth on high speed communication "trunk circuits" between locations, a scenario which may prove economical in some situations. For example, part of a 45 megabit/second trunk circuit could be dedicated to video transmission, another part used for data transmission, and a third part reserved for standard telephone service.
Another approach that is being explored is the use of community cable television (CATV) systems for the distribution of data and even voice services in addition to television programming. Few community systems are designed for this mode of operation today, however many cable operators are interested in the potential. We recommend some caution in committing to this type of a system without thoroughly understanding its costs and limitations.
A more integrated approach may come from ATM-based communications technologies (see Section II.D. of this chapter, Asynchronous Transfer Mode). ATM can support the isochronous communications required by audio and video, allocating the remaining capacity for packetized data. Theoretically the bandwidth can be allocated dynamically as communications needs change from minute to minute, however research in this area is still somewhat inconclusive.
III. Conclusions
As stated earlier, the recommendations in this document are made with the expectation that they will allow schools to take advantage of future technologies as they become available. Because of the already huge installed base, both Internet technology and ethernet LAN systems are likely to be well supported products for at least the remainder of this decade.
Most components installed in the first phase of building the network recommended in this guide can continue to be used as other components are replaced or upgraded. Therefore, overlaying new technologies on top of the recommended infrastructure will require minimal investment and minimal recabling.
By using the technical model suggested in this guide as the framework for your network design, your investments today will be sound and will pave the way to new communications technologies in the future.
[1] Network equipment can "dial" the number of the remote service port whenever data is available to be sent. If no data arrives within a specified interval, the connection is automatically released.
