Following on from my earlier blog posts using the Vigor 2862 series of "hybrid" routers, I thought I’d put together an update on further refining the potential of the implementation I’ve been developing.
In my earlier posts I was extolling the virtues of the 2862 for resilient communications, and in particular, making use of an LTE (4G) cellular SIM combined with conventional VDSL broadband to achieve a resilient home working setup. LTE results are good when you combine the DrayTek with upgraded internal or external antennas. It’s possible, at some times of the day, to achieve 50Mbps down and 25+Mbps up through LTE alone.
Now, in practice, this is all well and good. You can configure VDSL as the primary bearer, and then fall back to LTE in the event the router starts to experience issues with VDSL. But it’s not as simple as that as this default mode remains clunky to operate.
First off, it’s worth reviewing the WAN > Internet Access options for both the VDSL and WAN connections. Under "WAN Connection Detection" there are a few ways of monitoring the link for up/down state; For VDSL for example, the 2862 can monitor the PPP link or alternatively use ICMP ping to ping a remote target. This is the foundation for resilience in this router, whether you use active/passive or active/active WAN configurations.
The Draytek documentation seems to recommend using ICMP ping if possible, though it notes a 20MB or so daily bandwidth hit from using ICMP in this way (ping per second). ICMP has the advantage of testing both the local bearer connectivity but also the full route out to an Internet destination, so it’s much more comprehensive than PPP or ARP detection. However, that is also its weakness, as it only tests one Internet route that may, for some reason, become temporarily unavailable; of course, the Internet is a distributed routing model (at least in theory, whether in practice ISPs really do that is a separate discussion) that might still be able to route even if one path fails. Also, you need a target IP that you can reliably ping without causing concerns from an unsuspecting system operator.
For each WAN connection, "WAN > General Setup > Active Mode" is the setting that configures the dynamic behaviour of the WAN link. In a simple setup, the VDSL might be configured as "Always On" and the LTE connection as "Failover". In the event the link detection identifies that VDSL has failed, the LTE in this configuration would start and communications would be re-established using the SIM card.
However, this is not the most elegant setup. For one, the SIM card will take 5-10 seconds potentially to start up and become serviceable for data connections. This will inevitably lead to some form of disruption in network connectivity. The second limitation is, especially when you have an unlimited data SIM, is you’re potentially missing out on an additional 20-60Mbps of bandwidth that the SIM card provides. If it spends most of its time inactive, you’re losing this added bandwidth and in addition it’s not really getting tested for the resilience scenario. On top of all of that, the upload speed of LTE is much more symmetric than VDSL, which is asymmetric by design, so the relative boost in upload throughput is significant.
So this got me thinking about how to maximise the benefit from the additional LTE connection. Fortunately it’s possible to develop a more sophisticated setup with the Draytek that really drives home the ROI on both connections and the unit itself.
In this more elaborate setup, I set about achieving the following:
Though LTE latency has significantly reduced compared to HSDPA, depending on the connection it still may a multiple of 4 or 5 times that of VDSL. Because the DrayTek cannot truly bond the bearers to support transparent migration across the WAN links, it means that any source allocated to LTE will also be limited to the bandwidth of LTE for the duration of the session/call. Under heavy contention LTE may reduce in bandwidth significantly, or in the presence of traffic management in the cellular provider or MVNO you might find data rates capped. It is therefore a safer bet that LTE is suited to applications that are slightly more elastic in nature, such as downloads, updates, Sky Q downloads, streaming, etc. It is also susceptible to co-channel interference being a shared medium so throughput can go up and down depending on nearby LTE stations such as handsets or some other kind of interference.
In contrast, VDSL is perhaps less prone to significant contention (though it is, ultimately, contention based), It also benefits in good locations from low latency, so is better suited to applications that are sensitive to latency (UX applications, online gaming, etc.)
Based on this, it’s possible to configure the DrayTek 2862 to make better use of both VDSL and LTE. Reconfiguring the Internet access configuration for each to "Always on" is the first step.
The next step is to configure the DrakTek routing panel under "Routing > Load-Balance/Route Policy". Click each of the index numbers to set the default routing policy for specific protocol/source/destination/port patterns. Then configure the "send via…" section to preferentially select the bearer of first preference. Then select "Failover to" to the alternate WAN link (either LTE or VDSL). Optionally then configure the "Failback" to revert back to the rule in the event the preferential bearer restores service after an outage (though note that this will clear existing sessions immediately). Finally enable the check box adjacent to each rule.
The only other point worth noting on the LB/RP policy is the ordering of rules. I have a final "catch all" all traffic rule at the bottom of the list to preferentially select VDSL over LTE (with failover to LTE). This ensures anything that is not configured explicitly for LTE goes over VDSL. I’ve not experimented with leaving this out yet, which may cause the router to use the "Load Balance Mode" setting under "WAN > General Setup". This should enable me to achieve the goal of combining bandwidth.
The default load-balance mode on the 2862 is according to DrayTek to "makes the routing decision according to the ratio of each WAN's remaining bandwidth, which is calculated by subtracting the bandwidth currently used from the maximum bandwidth". Reviewing the DrayTek documentation, "Auto Weight" looks the best option for LTE as it dynamically calculates recent peak throughput to make the decision on remaining bandwidth.
In summary, with some work on load-balancing rules it’s possible to keep LTE and VDSL up all of the time, use the extra LTE bandwidth alongside VDSL, and have a seamless failover that minimises LTE startup time.
March 12, 2021