Physical configuration

The physical links involved in this project will consist of the following (see Fig. 2)

• A T1 circuit has been installed between the JAC and UKIRT, with a continuation from UKIRT to JCMT. A T1 circuit consists of 24 DS0 channels, each capable of passing 64 kbit/s. This circuit will be used for internal data and voice communications. The T1 circuit is bridged to a local area network (LAN) at each site running Digital Equipment Corporation's (DEC) Local Area Transport (LAT) protocol over baseband coaxial cable (Ethernet). This configuration effectively places all three sites onto the same LAN, allowing for fast data transfer between them.
• A dedicated 56 kbit/s data line has been installed between Hilo and Edinburgh. This service is being provided by GTE/Hawaiian Telephone. In order to maximize data throughput while providing voice communications, it was decided to use voice compression techniques. The equipment we have purchased will allow us to compress a voice channel down to a 2400 bit/s bandwidth, so we will only have to give up one 4800 bit/s for voice. This will allow use of the remaining 48 kbit/s for data, at a bandwidth of 38.4 kbit/s, with the possibility of adding another voice channel at a later date. The allocation for voice will increase the transfer time for a 20 Mbyte image to approximately 7 minutes, which was also felt to be an adequate rate. The voice channels as well as data channels are time multiplexed onto the 56 kbit/s line and separated out at each end. The data channels are then passed via a DEC X25router and used for direct DECnet connections to the computers in the LAVC (using DDCMP).