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Fall Journal

Volume 30, Issue 3

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Channel extension is revolutionizing data processing and telecommunications planning. It fits into the strategic plans of companies planning to reduce operational costs or experiencing economic pressures, and utilities fighting to reduce operational expenses. Channel extension is becoming prevalent in the computing world for three primary reasons: device flexibility, ease of implementation, and speed.

Flexibility is apparent since devices which are local and channel-attached in the IBM (or plug compatible) computer room can now be supported at remote sites. It is not necessary to purchase remote devices to handle the job since local devices can be implemented immediately.

The key characteristic here is “Immediately.” The ease of implementation means local devices can be moved to any site and immediately connected to the channel extender. Local devices/controllers are normally attached to system hardware and software changes are not necessary for network control programs. Planning for implementation only requires channel/device addresses to be defined in the I/O generation, and communication facilities and devices to be installed.

Devices supported at the remote sites perform at speeds comparable to local channel-attached devices. Laser printers run at rated speed and CRT operators experience sub-second response time when given the appropriate bandwidth.

Channel extension is a comprehensive solution to Disaster Recovery needs. Paradyne pioneered channel extension technology 13 years ago. Channel extension products provide viable solutions in disaster recovery for businesses today.


Paradnye’s family of products includes PIXNET tm, PIXNET-XL tm, and PIXNET-FX tm. PIXNET-FX is a fiber channel extender used to remotely place local channel-attached devices at distances less than two miles. The PIXNET and PIXNET-XL products are used when the distance exceeds this two-mile capability of the fiber extender. PIXNET and PIXNET-XL can be used at distances thousands of miles apart. PIXNET provides a single channel connection. It attaches to a host byte channel and supports both batch and interactive devices with communications line speeds up to 64K bps. The more recent PIXNET-XL products provides two host channel connections to either a host byte and/or block channel. It supports batch and interactive devices with T1 communications facilities.

With the higher bandwidth and block channel support, the PIXNET-XL channel extension node supports a larger number of devices (i.e., hundreds of CRTs) and faster devices, such as microfiche, channel-to-channel adapter emulation, tape, and laser printers (i.e., IBM 3800-3 in graphics mode). The exhibit shows a typical point-to-point channel extension installation. Note that a small portion (392K bps) of the bandwidth is allocated for use with the PIXNET-XL channel extender and the remainder of the T1 is used for voice applications.

The PIXNET-XL can also be used in conjunction with PIXNET-FX. This combination create a campus-type environment when additional channel extension functions are required at multiple locations.

Both PIXNET and PIXNET-XL provide Paradyne’s proprietary “Networking,” the ability to perform application and CPU switching through the network without utilizing the CPU. Exhibit 3 illustrates a network utilizing this feature. The devices attached to the channel extension node in New York can easily switch between Host 1 in Chicago and Host 2 in Atlanta. This feature applies to all devices (i.e., CRTs, printers, tapes, check sorters, reader/punch, microfiche). Further detail is provided throughout this article on the usefulness of “Networking” in disaster recovery.

Recovering business functions is the primary goal of a disaster recovery plan. Living in a computerized world means providing immediate recovery for the computer users. They, in turn, will provide the functions required to service customers. Paradyne’s channel extension family provides local users with the necessary connection to a computer hundreds of miles away. These products support a business during outages which involve communications lines, computers, or the total data center.


In an environment which depends on a network of communications facilities day-in and day-out to support a business, the most frequent point of failure is the communications line. It is critical in evaluating disaster recovery needs to plan for the most obvious, as well as the least likely, point of failure. Paradyne’s PIXNET-XL provides a unique feature known as “Multi-Link Protocol” (MLP). This feature supports multiple communications facilities between two nodes as a single transmission group. These facilities can be any combination of digital, analog, microwave, fiber, or satellite, and can be any variety of line speeds. The most basic of these uses six 56K bps circuits defined to the PIXNET-XL node in an MLP. These six individual lines now appear to the node as a 336K bps facility (56K X 6 lines). If one of the lines is lost then the nodes continue to function on 280K bps (56K X 5 lines) and when the line is back in service, the nodes will again utilize the full 336K bps.

The MLP is of greatest benefit when mixing line speeds and types of facilities. Mixing facilities provides more security, as it is unlikely that satellite, digital, and dial service will experience simultaneous outages unless a catastrophic disaster occurs. If this occurs, the T1 facility would be used as the primary data path, while satellite would become the primary path if the T1 is inoperative. If the T1 and satellite are not available, the dial lines will become the data path when activated.

Every plan for disaster backup should provide contingency plans for communications line outages. The MLP function, provided in the Paradyne channel extension nodes, provides a contingency plan that does not require intervention by an operator.


The second most frequent point of failure is the computer. It supports users so they can meet customer needs as an integral part of day-to-day operations. Consider the possibility of water or fire causing an outage for three to five days. Could a company continue to provide quality service to customers?

By utilizing channel extension at a disaster recovery site, either a company-owned or commercial site, the devices can be supported by a remote computer. The channel switch adds to the ease of switching the local devices from the central processing computer to the channel extension nodes. The devices are switched through the channel switch and the PIXNET-XL nodes to the disaster recovery site. The communications facility is then activated and the devices are varied online, enabling the user community to be restored with minimal outage.


The plan becomes quite complex when the data center is demolished by a catastrophe, such as fire, tornado, hurricane, or earthquake. The plan must prepare for the loss of computers, the devices in the computer room, and all files stored in the area, both tape and disks. Channel extension can be used to connect those devices to the disaster recovery center. PIXNET-XL channel extension is used in the wide area while PIXNET-FX is used within the local area. With a backup channel extension node located permanently at the disaster recovery site, the CRT controllers attached to the PIXNET-XL node in New York can be switched to the disaster recovery site host for operation. The CRT devices supported by PIXNET-FX can be channel switched over to PIXNET-XL and these CRTs can be supported by the disaster recovery site as well. This type of “Networking” will switch local devices into operation through the disaster recovery data center within a reasonable time to meet users’ needs.


The best of all disaster recovery plans is to combine all of these features and provide real-time updates to a disaster recovery data center.


As corporations, such as those in the banking industry, face legal obligations to provide immediate recovery of business function, disaster recovery planning quickly is becoming a common boardroom topic. Paradyne’s channel extension product family provides the ability to backup communications facilities, a single computer, or an entire data center. Since the key factors in the successful implementation of a disaster plan are the ease of its implementation and its operation, Paradyne has shown how the “Networking Channel Extender,” with its “Multi-link Protocol,” provides immediate switching to the disaster recovery site with minimal operator interaction. These products are easily implemented in a company’s disaster recovery data center or in a commercial disaster recovery center that may be contracted to supply a “hot” and/or “shell” data center during an outage.

Don’t wait until it’s too late to implement a disaster recovery plan.

This article adapted from Vol. 1 No. 2, p. 24.