Telephone Company Speeds Graphics Communications, Cuts Its Transmission Costs, with Data-Compression.
Before the recent phone company divestititures, Con Tel was the third largest independent telephone company in the US, servicing mostly rural communities in 37 states plus the Caribbean. In recent years, it has diversified into information services, business systems, construction and network services.
The RDS group is developing a graphics-based information system which is expected to eventually link most of the local offices of the company. Communicating with central computers over leased telephone lines, the local offices obtain data on high-resolution graphics video workstations and get hard-copies through associated printers and plotters.
Compression Equipment Increases System's Efficiency
According to Randy Ross, manager of RDS implementation, data-compression equipment has enabled the group to double the number of high-resolution graphics terminals on a single line, as well as linking both synchronous and asynchronous devices over the same 9600-b/s line.
The RDS groulp is using DEC VAX 11-780 and 11-785 mini-computers with VMS operating system, INFORMAP-II graphics-application package and OPIS-3 information-management software from Synercom Corporation. The graphics worstations are Tekronix 4109 and Synercom dual-screen GWS-III and GWS-IV.
"We had been using a single synchronous link for each remote graphics terminal and a separate asynchronous line for printers and plotters," Ross reports. "The cost per workstation was significant, so we began looking for ways to reduce the communications cost. The early data-compression devices on the market helped some, but we still had to have separate async lines for the plotters. Then Symplex Communications brought out their Datamizer SDC-4 compression and multiplexer unit, which allowed us to connect both sync and async devices to the same line, running at different baud rates."
Ross didn't estimate how much data-compression actually speeds up the rate of transmission, but points out that Con Tel now has two grphics terminals, running at 9600-b/s synchronous, as well as a text terminal and a plotter connected asynchronously to the same 9600-b/s line.
Graphics Performance Is Up, Cost Is Down
"The performance of the graphics terminal is noticeably better, even with all that equipment running, that we got from a single graphics station before. And with a single four-wire line from Missouri to California costing between $1200 and $1400 a month, the savings are real," he reports.
The data-compression units permit up to four input connections, either synchronous or asynchronous, which they statistically multiplex into a single full-duplex communications link using a standard modem. the inputs can be running at different baud rates and protocols in combiniations that add up to about 19,200 b/s. This data stream is then compressed through a four-level auto-adaptive coding, called SCC Tabling, into a tansmission rate of about 9600 b/s. At the other end of the communications line, a second compression system decodes the signal from its modem and separates it into the original number of paths at their respective rates.
The four-level compression algorithm includes a level using modified Huffman Encoding, a scheme that substitutes an eight-bit character code with a shorter code. The data-compression unit automatically maintains a table of frequently encountered data components, at both character and block levels, and compresses these components, as well as stripping off header and tial coding. The table is based on the actual data stream being handled, not a fixed list.
The multiplexer also permits a user to overcome one of the problems often limiting communication rates--some computers do not operate efficiently at I/O rates greater than 9600 b/s. This could result in actual transmission rates that are less than double, since the compression unit has to ait for the computer. However, with the I/O split into two to four channels and connected to separate ports of the data-compression unit, the effective I/O rate can be increased greatly. The unit can multiplex a maximum aggregate input of 38,400 b/s, as long as no one input is faster than 9600 b/s.
Channels Can Be Assigned Different Priorities
The data-compressor's channels can be assigned different priorities, which allows a user to set all connected devices to 9600 b/s and still ensure that the most important device gets the communication time it needs. A separate control port gives an operator on-line control over the communication process and permits the operator to retrieve network statistics, isolate problems within the network, and program the parameters of the compression unit.
Diagnostic routines are built into the data-compressor and front-panel status indicators let the operator monitor the trnsmission performance of the CPU, the terminal, the modem and the data-compression units. According to Ross, maintenance on the compression equipment has been practically nil.
His group currently has three communications links using data-compression and is in the process of installing more.
"We have plans for quite a few more," he says.
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|Date:||Oct 1, 1985|
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