How international traffic leaves the country

Almost all internet traffic between continents and across seas travels through fibre-optic cables laid on the seabed. Countries in Southeast Asia, including Vietnam, rely on a handful of regional systems such as AAG, APG, IA, AAE-1 and ADC to reach Singapore, Hong Kong, Japan and the United States, where many large services host their servers.

Each cable has a fixed capacity. Internet providers buy capacity on several cables and spread traffic across them, with some overland routes through neighbouring countries as backup. As long as enough capacity is available, you never notice which path your data takes.

Cables are damaged surprisingly often, usually by ship anchors, fishing gear or seabed movement. Repairs need a specialised ship, good weather and permits, so a fault can take weeks to fix, and incidents on several cables at once have repeatedly slowed international access in the region.

What a cable problem looks like

The classic sign is an uneven slowdown. Local news sites, banking apps and services cached inside the country still load quickly, while overseas websites, some games, cloud storage, software updates and work tools hosted abroad become slow, especially in the evening peak.

Ping to foreign servers rises sharply, for example from 40 ms to well over 100 ms for Singapore, and you may see packet loss. Video may still play because large platforms keep copies of popular content on caches within the provider network.

If every site is slow, including local ones, or only one device suffers, the cause is much more likely to be your Wi-Fi, router or plan than a cable fault.

  • Domestic sites fast, foreign sites slow.
  • Higher ping and jitter to overseas game servers.
  • Slow downloads from cloud storage and update servers.
  • Worse in the evening, better late at night.
  • Many people on different providers report the same problem at once.

How to tell whether it is the cable or your connection

Start with a wired or close-range speed test to a nearby server. If the result is close to your plan, your line is fine. Then compare ping to a domestic host with ping to a well-known overseas host. A big gap that was not there last week points to an international bottleneck.

Check whether other households and other providers see the same thing. Providers and the press usually announce cable incidents, and community forums fill up quickly with complaints when a major route is affected.

Testing at different times helps too. A problem that disappears after midnight but returns at 8 p.m. is congestion on limited international capacity, not a broken router.

Comparing domestic and overseas latency
ping vnexpress.net        -> 8 ms     (domestic, normal)
ping 1.1.1.1             -> 35 ms    (often served nearby, normal)
ping speedtest.singapore -> 180 ms   (overseas, much higher than usual)
Result: local line fine, international route congested

What actually helps (and what does not)

There is no setting that adds capacity to a damaged cable, but you can avoid the worst routes. Prefer services and servers that are hosted nearby or cached locally. Many games let you choose a region; picking a server that is still reachable on a healthy route can cut lag considerably.

Changing DNS can occasionally help because a different resolver may send you to a different content delivery node, but it does not make the cable faster, and a far-away resolver can make things worse. A VPN sometimes improves a specific route if its server happens to use a less congested path, yet it usually adds latency and can make things slower overall.

Schedule large downloads, backups and updates for off-peak hours, and use mobile data as a temporary backup if your mobile provider uses different international capacity.

  1. Pick game and work servers in the nearest available region.
  2. Download big files late at night.
  3. Try mobile data for urgent overseas work.
  4. Test DNS or VPN changes one at a time and keep them only if a speed test confirms an improvement.
  5. Avoid paying for a faster plan; the bottleneck is outside your home.

Honest limits and expectations

A cable incident is outside the control of both you and, to a large extent, your provider. Providers can reroute traffic and buy temporary capacity, but quality usually returns to normal only after the repair ship finishes its work.

Be wary of apps or services that promise to “bypass” a cable cut. At best they choose a different route, which you can test yourself; at worst they collect your data in exchange for little benefit.

Keeping a simple record of speed and ping results helps you notice when the situation improves and gives you evidence if you need to contact your provider.

Common questions

Why is YouTube fine but my game lags? Large video platforms cache content inside the country, while game servers are often located abroad and need a live, low-latency connection.

Will upgrading my plan help? Rarely during a cable incident. Your access line is not the bottleneck, so a faster package mostly helps domestic traffic.

Is it my router? If local sites and a nearby speed test are fast, your router is very likely fine.