The document is a technical report from an APL Users Meeting held in Toronto in September 1978. It describes the design and implementation of a data communications system specifically built for APL users at I. P. Sharp Associates. Here’s an overview of its key points:
Background and Context
- Historical Evolution: The system originally started as a TDM (Time Division Multiplexing) network in the summer of 1969. By the mid-1970s, as the network grew (including a trans-Atlantic link), there was a shift toward a packet-switched architecture.
- Primary Goal: The main objective was to provide efficient and reliable APL service to remote low-speed terminals, while retaining as many advantages of the old system as possible.
User Interface and Connection Process
- Simple Initial Connection: Users make a single input that specifies both the terminal type and the destination APL machine. Once connected, the network operates transparently as a simple forwarding agent—directly channeling output from APL to the terminal and vice versa.
- Terminal Identification: The system uses specific keystrokes (like the right parenthesis) to identify terminal characteristics. This information is used for terminal control and helps in printing legible error messages.
Transition from TDM to Packet-Switched Technology
- TDM Drawbacks:
- Error Sensitivity: Telephone lines used with TDM technology introduced noticeable transmission errors, which were directly visible to the user.
- Fixed Bandwidth Issues: Traditional TDM systems had fixed allocations per port, limiting flexibility especially when accommodating different terminal speeds.
- Packet-Switching Advantages and Drawbacks:
- Dynamic Bandwidth Allocation: Unlike TDM, packet switching allocates bandwidth on demand. This allows a larger number of ports to be installed, taking advantage of the fact that not all ports are active simultaneously.
- Overhead Concerns: Packet systems add extra bits (addressing, sequencing, redundancy checks) to each block of data. Although this increases overhead compared to TDM, it improves overall network flexibility and efficiency.
- Delay Tradeoffs: Packet length directly affects transmission delay. For instance, with a 256-bit maximum packet and a 2400 bps link, delays become noticeable, requiring a balance between efficiency and response time.
Addressing, Routing, and Call Management
- Virtual Call Network: Instead of a datagram network, the system employs a virtual call network. In this model:
- A unique call number is assigned to each connection.
- Routing is managed as fixed point-to-point paths, which simplifies the management of control information and ensures that both endpoints are properly notified in case of link failure.
- Routing Considerations: The report discusses the importance of having reciprocal routes (the route from A to B being the reverse of B to A) to facilitate efficient packet delivery and proper termination of calls upon failures.
Link Management and APL Interface
- Handling Errors: The system employs selective retransmission of only the corrupted packet instead of the entire packet pipeline, which minimizes delay but requires additional measures (like interend sequence numbering) to maintain the correct order of packets.
- Frontend Computer Role: A dedicated frontend computer is introduced between the APL machine and the network. This machine:
- Adjusts the rate of data transfer to ensure compatibility between APL’s output speed and the network’s capacity.
- Handles dynamic allocation of communication channels, simplifying the connection process and allowing APL to continue operating with minimal modifications.
Conclusion
The report concludes that the transition from TDM to packet-switched technology has been achieved with minimal changes to the APL system itself. The key benefits include improved reliability and the capacity to serve multiple geographically dispersed locations, albeit with some compromises in response time.
This document provides a detailed look into early network design challenges and the innovative solutions developed to support interactive computing for APL users during that era.