Packet Switch Stream (PSS): in which country and approximate launch period was PSS first introduced as a public packet-switched data service?

Introduction:
Packet Switch Stream (PSS) was the United Kingdom’s public packet-switched data network service offered by the national telecom operator. Knowing its launch window helps place X.25-era services on a historical timeline relative to earlier US research networks and later commercial offerings.

Given Data / Assumptions:

• PSS refers specifically to the UK service name, not generic packet switching research efforts.
• Early 1960s US efforts (1961) relate to theoretical groundwork, not a UK public PSS service.
• Sources commonly cite the early 1980s, with service becoming available circa 1981.

Concept / Approach:
Distinguish between research milestones (e.g., early US packet-switching theory and ARPA-related work) and the commercial/public rollout branded “PSS” in the UK. The relevant fact is the UK launch timeframe, which aligns with X.25 network deployment and EDI/host access adoption across enterprises in the early 1980s.

Step-by-Step Solution:

Verification / Alternative check:
Contemporary telecom references and archives list British Telecommunications PSS as being introduced around 1981–1982, matching the early-1980s commercialization of X.25 services in Europe.

Why Other Options Are Wrong:

• US in 1961: not PSS; refers to early research timelines, not the UK service.
• UK in 1980: close but typically cited as 1981.
• US in 1961 & UK in 1981: mixed and partly irrelevant; PSS is a UK service.
• None of the above: incorrect because “UK in 1981” fits best.

Common Pitfalls:
Confusing research milestones with commercial service launch; accepting approximate dates without checking the specific branded service and country.

Final Answer:
The UK in 1981.