EXA North and South

Based on 23 RIPE Atlas measurements from GeoCables monitoring infrastructure, March–April 2026.

EXA North and South — sometimes referred to in carrier documents as the EXA system or, in older filings, by the name of its predecessor network — is a 12,200-kilometre transatlantic cable that connects five landing points across the North Atlantic: Halifax in Nova Scotia, Canada; Lynn in Massachusetts, United States; Dublin, Ireland; Coleraine in Northern Ireland; and Southport in northwest England. The cable was placed in service in 2001 and has been operational for twenty-five years — at the upper edge of typical submarine cable design life. It is owned and operated by EXA Infrastructure, the European-American backbone carrier that emerged in 2021 from the carve-out of GTT's transatlantic and European fibre network. Among transatlantic systems still carrying significant commercial traffic, EXA North and South is one of the older active cables on the route.

The figure 12,200 km describes the total wet plant of the system — every kilometre of fibre laid between the five landings, including the branching units that split traffic between the United States, Canada, the Republic of Ireland, and the United Kingdom. No single packet actually travels all of it. A measurement taken between any two of the five landings traverses only the segment of the cable that connects those two points, and the segment lengths vary substantially. The shortest hop on the system is a few hundred kilometres along the British Isles approach; the longest individual segment crosses the deep Atlantic between North America and Europe.

The minimum round-trip we observe between Southport (UK) and Lynn (Massachusetts) is 85.55 ms. The physics floor for the full 12,200-km system is 119.40 ms — that is, the smallest possible round-trip if a packet actually traversed the entire cable. We measure 0.716× of that floor. Below the floor.

Below the floor means a chord, not a loop

This number does not violate physics. It tells us only that the packets we measure do not traverse the full 12,200 km. The Southport-to-Lynn route is a chord across the North Atlantic — a direct submarine segment between the British and American landings — that is much shorter than the system's branched total length. The cable is a multi-landing trunk; any specific point-to-point flow uses a subset of its segments. The 0.716× multiplier is, in this sense, a measurement of how much of the cable the route actually uses: roughly 71.6%, when measured against the floor of the whole system.

Because EXA North and South lands at five sites, it serves several distinct corridors at once: a Halifax–Boston short-haul on the North American coast, a transatlantic Halifax/Lynn–Dublin/Coleraine/Southport main spine, and an internal Irish-British Isles segment between Dublin, Coleraine, and Southport. Each corridor has its own minimum round-trip, and each can sit at a different multiplier of the system's floor. What we measure on the UK–US corridor — the longest and most consequential one — is consistent with a clean, single-segment transit between two landings on opposite sides of the Atlantic.

Stable to within a millisecond

Across 23 measurements between Southport and Lynn, the round-trip averages 86.69 ms, with a maximum of 87.53 ms and a standard deviation of 0.63 ms. That is, every single measurement we have observed lies within a two-millisecond window. For a cable a quarter-century into service, carrying real commercial traffic between the British Isles and North America, that level of stability is what well-engineered transatlantic infrastructure looks like in measurement form.

This consistency is not a property of EXA's cable specifically — it is a property of mature transatlantic peering. Routes between major financial centres on opposite sides of the Atlantic have been optimised relentlessly for two decades. London-to-New York latency is a hard commercial number that drives algorithmic trading flows, transatlantic content delivery, and dollar-pound currency arbitrage. Carriers compete on the third decimal place. Any cable that carries this traffic — and EXA North and South does — is constrained by the same operational discipline. Standard deviations measured in fractions of a millisecond are the baseline expectation, not the achievement.

By contrast, regional cables in less-mature peering markets routinely show standard deviations of 30–100 ms over similar distances. We have measured that pattern on JaKa2LaDeMa (B2JS) in Southeast Asia, where the same kind of inter-carrier handoff produces decades-old behaviour rather than decades-old discipline. The transatlantic corridor — and EXA's piece of it — has long since left that regime behind.

What 0.716× tells us next to other cables

For most cables in our monitoring set, the observed minimum round-trip lands somewhere between 1× and 5× the physics floor. Gondwana-1 between New Caledonia and Sydney measures at 1.081× — essentially at the floor — because it is a single trunk with no realistic alternative path. ARCOS-1 in the Caribbean measures at 0.613× — well below the floor — because traffic chords across the basin via Florida hubs instead of looping around the cable's full ring path.

EXA North and South at 0.716× sits in the same below-the-floor category as ARCOS, but for a different structural reason. ARCOS is a closed ring whose full perimeter is rarely walked; the chord shortens the journey. EXA is a branched trunk whose total length includes segments that simply are not on the path between any given pair of landings. Both numbers describe the same general phenomenon — observed round-trip below the floor of the cable's nominal length — but ARCOS gets there through routing choice, while EXA gets there through cable topology.

What the two have in common is that the system-wide physics floor, calculated against the cable's full advertised length, is not the right baseline for any particular point-to-point flow. The cable as a financial and operational entity is bigger than any single route across it, and a measurement on a single route reveals the geometry of that single route, not the geometry of the whole system.

Twenty-five years of transatlantic continuity

The cable's longevity is itself worth flagging. EXA North and South entered service in 2001, the same year as ARCOS-1, and now runs alongside it as one of the older systems still carrying commercial transatlantic traffic. Several of its contemporaries have been retired. Several of its successors — including newer transatlantic systems like Marea and the more recent Amitié and Anjana cables — have entered service to take pressure off the older cables and to introduce route diversity for cloud-scale operators. EXA North and South continues to carry its share, and the consistency of our measurements suggests the wet plant remains in working order despite its age.

What we measure on EXA North and South — 86 ms to within a millisecond, year after year — is what one of the more mature pieces of transatlantic backbone infrastructure looks like in 2026. The cable carries the kind of traffic that does not tolerate variance. Twenty-five years in, it still does not introduce any.