Wireless transmission methods help speed ISDN deployment
Jeffrey Fritz
Dick Tracy's famous wrist radio was way ahead of its time as a portable communications device. It allowed Tracy to be anywhere in the city and still stay in contact with the people and resources he needed to do his job. By contrast, digital telecommunications services are anything but portable. Most are lashed by twisted pair to wall-mounted faceplates. The lack of portability reduces flexibility and restricts access. It also creates dependency on outside agencies, like the telephone carriers, to provide service to the faceplate.
As with most digital services, ISDN has been a tethered service requiring physical connections to the fiber- and copper-based telephone network. Consi
dering the wide range of voice, video, and data applications it supports, ISDN's lack of mobility has been extremely constraining. Fortunately, a new form of ISDN, called ISDN Radio, is breaking the copper umbilical cord and offering users unheralded communications freedom.
ISDN Radio comes in two flavors: satellite and radio (see the figure
"ISDN Radio/Satellite Configurations"
). Satellite ISDN is based on VSAT (Very Small Aperture Terminal) technology. VSAT uses transportable satellite link equipment and relatively small uplink/downlink dishes. Connections are made using leased or call-based satellite channels. Radio ISDN uses specialized modems called spread-spectrum modems which distribute the signal over a wide bandwidth, reducing interference and improving security. While satellite ISDN can span continents, radio ISDN's range is limited. It is broadcast primarily via transmitters operating on 1 W or less. The low power restricts the range, depending on antenna height and te
rrain, to a maximum radius of 30 miles.
Quickness Counts
Among ISDN's biggest drawbacks are long installation time, distance limitations, and a lack of ubiquity. ISDN Radio can help resolve each of these flaws. While terrestrial ISDN orders take days or even weeks to process, ISDN Radio equipment can be set up quickly. It's not unusual to have service in place within 24 hours. This makes ISDN Radio especially valuable when unexpected events take place, such as a network outage, an urgent site coming on-line, a network demonstration that was scheduled without advance notice, or a last-minute video conference.
It's also possible to have ISDN Radio service in locations where terrestrial ISDN is not available. Where there is no terrestrial ISDN, ISDN Radio can step in as an extension service. ISDN Radio's transportability can bring ISDN to a non-ISDN location. It can also extend ISDN past the infamous "last mile," which occurs when ISDN is available locally, but the remote s
ite exceeds the 18,000-foot distance from the central office.
That's where ISDN service delivered over a satellite link can help. "Satellites can seamlessly extend ISDN from any ISDN public network to remote locations that do not have access to ISDN terrestrially," says Thomas von Deak of NASA's Lewis Research Center. "This is important because ISDN will form the basis for the first implementation of the NII [National Information Infrastructure] and the GII [Global Information Infrastructure]."
Reaping Other Benefits
ISDN Radio adds more than basic network connectivity. By nature, ISDN Radio is redundant. Connections are not made over the terrestrial telephone network, but through radio or satellite. The local telephone company is either out of the loop entirely, or ancillary to the connection. This makes terrestrial outages of far less consequence. Network administrators who are challenged to keep their networks alive no matter what the situation will find ISDN Radio pa
rticularly attractive. Should a disaster such as earthquake, fire, flood or tornado disrupt terrestrial-based WAN connections, the network manager can call on an ISDN Radio provider to quickly restore services. The company may also choose to have ISDN Radio in hot standby, or even in active service. When the terrestrial connections go down, the ISDN radio links can be pressed into service.
Broadcasters realize that ISDN can provide enhanced audio quality without the need for multiple analog lines or audio frequency shifting equipment. A single BRI (Basic Rate Interface) line without compression can provide 7.5-kHz bandwidth audio. For comparison purposes, 7.5 kHz is equivalent in quality to a decent AM station. Increasing audio bandwidth to 15 kHz, comparable to FM quality, or adding stereo can be achieved with multiple ISDN lines or compression. This makes ISDN ideal for remote broadcasts that sound as if they originated in the studio.
Points to Ponder
ISDN Radio is not
without its disadvantages. It requires extra equipment, some of which is fairly expensive. It takes special know-how to set up, operate, and maintain the service. Satellite time can be expensive, and satellite channels require access to a satellite provider. ISDN Radio is subject to the same limitations as any radio service. Interference and poor signal quality can cause problems. Most importantly, satellite delays adversely affect the quality of ISDN Radio's service.
Satellite links introduce a fair amount of delay (see
"When Timing's Critical"
). If severe enough, delays can garble voice transmissions, scramble video, and collapse WAN connections. For example, a terrestrial ISDN BRI (Basic Rate Interface) delays the signal about 10 milliseconds. An international terrestrial circuit experiences delays of 140 ms. A single satellite hop has a marginal range one-way delay of 260 ms. Bidirectionally, satellite delays can be well over 500 ms. This puts satellite delays in the unac
ceptable range for some applications.
Delays can cause problems for isochronous applications that require audio and video synchronization, or are intolerant of disruptions in information flow. Delays can also cause problems for network applications. If the delay is long enough, the network protocol may assume that the communications link has been lost and time out the session. Even a less drastic network response to delays can cause unnecessary retransmissions, collisions and, in severe cases, broadcast storms. Users considering ISDN Radio for network or time-sensitive applications should take steps to make sure that the technology will work for them.
What the Future Holds
One of the more interesting demonstrations of ISDN Radio technology is NASA's ACTS (Advanced Communications Technology Satellite), which the Space Shuttle Discovery launched on September 12, 1993. The NASA Lewis Research Center in Cleveland, Ohio, manages the satellite; it is a test of digital communic
ations that span the spectrum when it comes to ISDN satellite applications. ACTS provides single hop mesh ISDN that attempts to integrate seamlessly with terrestrial networks. No attempt is being made to use specially modified equipment for the tests. Off-the-shelf ISDN equipment is currently being tested over the ACTS and used in demonstrations to the public. Interestingly, NASA has cross connected ACTS and terrestrial ISDN circuits through a traffic terminal in Cleveland. This allows access to the ACTS system from anywhere an ISDN connection is available.
The large number of companies, universities, and research organizations that use ACTS includes Comsat, the U.S. Army Research Labs, the National Telecommunications and Information Administration, and NIST (National Institute of Standards and Technology). There are several interesting technology examples being tested on ACTS. In one experiment, Corporate Computer Systems, JPL (Jet Propulsion Labs), and CBS Radio are demonstrating ISDN high-quality au
dio transmissions. The North American ISDN Users' Forum has been testing a PC-based multimedia teleconferencing system over a VSAT-transportable link back to the Lewis Research Center, the JPL, and other sites.
One particularly interesting application is a disaster-recovery and communications-augmentation experiment. Ohio University conducted tests to help Huntington Bank recover from a simulated disaster that created a total loss of communications. ACTS was used to transmit financial data such as deposits, account balances, and transfers of funds. The experiment measured the ability to switch over to a backup communications system within an acceptable period of time as well as the economical advantages of using ISDN satellite as a backup system.
Bellcore is conducting experimentation with satellite-based PCS (Personal Communications Services). The goal of this research effort is to demonstrate a satellites' capabilities for enhancing ground-based personal communications voice and data services.
The experiment will determine the ways in which local exchange network providers can interface to wireless service providers and the kinds of services that should be offered.
Finally NIST has connected the ACTS ISDN system to the government's FTS2000 digital communications infrastructure and is investigating interoperability issues between the terrestrial and satellite systems.
Given encouraging results from ACTS and early user successes, ISDN Radio appears worthy of consideration as a vehicle to provide redundant network backup, remote WAN connections, broadcast remotes, or world-wide videoconferencing. If your local service provider gives you a blank stare when you ask for ISDN connections, ISDN Radio could be your answer.
Clearly, ISDN is getting more interesting by the moment. No longer tethered by copper umbilical cords, the freedom to have digital voice, data, and video services at any time and any place is truly exciting. Dick Tracy would have been very much at home with ISDN Radi
o.
Signal delays pose problems for isochronous applications.
Service Delay (ms) Quality of Service
National T-1 Service 1 Acceptable
Terrestrial ISDN 10 Acceptable
National analog service 25 Acceptable
International terrestrial service 140 Acceptable
Single hop satellite 260 Marginal
Bidirectional satellite 520 Unacceptable
illustration_link (36 Kbytes)
Radio and sa
tellite-based ISDN will make the service more readily available.
Jeffrey Fritz is a telecommunications engineer responsible for the design and management of data communications for West Virginia University, including its ISDN applications lab. He is the chair of the North American ISDN Users' Forum Enterprise Network Data Interconnectivity Family. Mr. Fritz also chairs the National Information Infrastructure Working Group. He is the author of
Sensible ISDN Data Networks
(WVU Press, 1992). You can contact him on the Internet at
jfritz@wvnvm.wvnet.edu
or on BIX at
editors@bix.com
.
Tuning In to ISDN
