Why replace the modem supplied by eir?¶
Eir (formerly known as eircom) provides their customers with F2000 or F3000 modem/router/access point units for all their customers. It supports all types of eir services as ADSL2/VDSL2/Ethernet connections. The ADSL2 mode is used in the "up to 24Mb" and VDSL2 is used in the "up to 100Mb" offerings. The Ethernet routing is used if you are lucky enough to get the Fiber-to-the-Home (FTTH) service.
The F-series modems are just rebranded units made by different manufacturers. The F2000 modem is the Huawei HG659b and the newest F3000 is a Sagemcom unit.
So why is it worth considering a replacement? In the HomeLab environment most users would like to have full control over their network by using an enterprise-grade router and a separate access point solution. The only part missing is the modem that allows bridging the copper telephone line and the router.
What capabilities should the replacement modem have?¶
There are several capabilities the modem should have so the Internet connection is the best possible in terms of speed and quality. The other features that are desirable do not directly impact the Internet connection but are nice-to-haves for the IT enthusiast.
In order to properly utilize a third party ethernet router as an edge device in the network the DSL modem has to support the bridge mode. This mode allows establishing OSI Layer 2 bridge between the DSL connection and the WAN port on the modem.
The only alternative to the bridge mode is static NAT configuration for the IPv4 and DHCP-PD server running on the modem device for IPv6 but both are very rare specialised features not present on any devices that I researched.
Fortunately the bridge mode is widely supported and all decent modems (including the ones provided by eir) have that functionality.
In order to be able to utilize this configuration the router has to support PPPoE protocol which is implemented on both pfSense and Ubiquiti EdgeRouter devices.
Advanced VDSL standards support¶
In order for the modem to work properly it has to support a range of ADSL/VDSL standards that are utilized on the network.
The ADSL2 is quite old standard and all recent modems should support all variants of it. eir has deployed the newest ADSL2 variant ITU G.992.5 Annex M that allows for speeds up to 24Mb/s downstream.
The VDSL2 standard is much more recent and can provide speeds up to 300Mb/s on the good quality lines. It is based around profiles that specify the frequency ranges used for both upstream and downstream communication.
In their deployments eir uses 17a profile that supports up to 100Mb/s downstream speeds on a good quality line. As the newer 30a and 30b profiles are not utilized across the eir network it is not necessary for the modem to support them.
It is important that the modem supports the G.Vector (ITU-T G.993.5) standard as it reduces the crosstalk interference and has a potential to significantly increase the connection speeds.
Noise and crosstalk resistance¶
The VDSL speed connection negotiation, called TRAINING, is a complicated process during which the modem and the exchange settle on the bandwidth allocation across the frequency range. As different modem chipsets have different sensitivity and noise resistance the resulting connection speed can vary significantly between them.
Even if the modem supports vectoring the implementation can vary between chipsets that can result in different crosstalk resistance. The F2000 modem supplied by eir uses the Broadcom BCM63168 chipset that is highly recommended in the DSL community.
I have used a modem based on the Lantiq chipset and my connection speeds become unstable as soon as more premises are connected. Connecting the eir supplied F2000 modem improved my speeds by 15% proving BCM63168's exceptional crosstalk resistance in comparison to Lantiq.
Baby Jumbo frames¶
The next feature important for the HomeLab environment is the DSL Baby Jumbo frames support. As the DSL uses PPPoE connection protocol there is 8 byte per-packet overhead applied. It means that the MTU inside PPPoE connection is reduced to 1492 bytes which limits maximum usable bandwidth by decreasing the payload to header ratio for the connection.
The mechanisms implemented in TCP protocol should allow the connection to be working with reduced MTU, but unfortunately many administrators often misconfigure their firewalls forcing users with the reduced MTU on their connection to apply MSS clamping, which is not the ideal solution.
Fortunately eir supports increasing the MTU on the PPPoE connection to 1508 bytes allowing for the bigger packet sizes. Some of the all-in-one modem/router appliances support the larger MTU when used in router mode, but not in the bridge mode. I am providing workarounds for some appliances in the last section of this post.
To utilize the increased MTU size the router has to support the 1500 byte MTU PPPoE connection and the 1508 MTU link on the interface connected to the modem.
It is desirable to monitor DSL connection parameters at all times. It allows to spot any intermittent problems and makes it possible to evaluate the parameters of the connection over the longer period.
From my experience with eir, it is crucial to be able to prove the connection speeds achievable in the past in order to force them to admit that the line deteriorated.
The golden standard for monitoring of network devices is the SNMP protocol. It allows to probe the modem for connection parameters and it allows to use any monitoring tool that supports it.
Alternative solution is to use telnet or ssh service to log in to the modem and parse the status command output periodically. There are applications written by enthusiasts that can collect the data from different modem models. This method however is less widely supported across monitoring solutions and that's why it is not ideal.
Which modem meets all the requirements?¶
As some of the features are not mentioned in product specifications it is hard to research all modems available on the market. After hours of checking specifications and manuals I can recommend a handful of products that meet all of the above requirements.
The F2000/F3000 (and the older F1000) modems do support the bridge mode required to achieve basic connectivity. Unfortunately the eir firmware locked down the ssh and telnet connectivity to the unit and also there is no support for SNMP.
The Baby Jumbo frames are supported when the device is used in router mode but it fallbacks to reduced 1492 MTU size when used as a L2 bridge.
As it uses BCM63168 chipset its connection parameters are very good, especially that eir also tends to use BDM chips in their exchanges so using modems based on chipset from the same manufacturer gives the best possible compatibility for speed negotiation.
I would use the device provided by eir only as a backup and not for the main operation. Complete lack of monitoring capabilities and no 1500-byte MTU support make it unsuitable for use with third party routers.
Draytek Vigor 130¶
Draytek Vigor 130 is the only purpose made VDSL modem on the market. It provides bridge mode connectivity and can't act as a router. The support for the increased 1508 MTU is built in and available in the standard firmware.
Given the lack of WLAN and router capabilities the device is somewhat expensive. It was designed as a modem and provides exactly that - just a modem that supports 17a VDSL profile and G.Vector for reduced crosstalk interference. Unfortunately as it is based on the Lantiq chipset the connection speeds established with eir exchange might be lower than the ones available when using Broadcom-based devices. Also the crosstalk resistance seems to be much worse.
The device supports telnet connectivity for monitoring but there is no SNMP support.
I have used Vigor 130 for many years on eir's ADSL and VDSL connections. Unfortunately in the last few months my connection parameters started to drop significantly when more houses were connected to the same exchange. After some testing it was clear that Broadcom based devices achieve much better speeds and I was forced to look for alternative modem.
Zyxel has a wide range of routers based on the Broadcom chipsets in their offer but some of them are using the low cost BCM63381 which is designed for budget modems and might be struggling at higher speeds. For the best performance modem based on the BCM63168 chipset should be used.
Zyxel doesn't make any pure modem devices, so the additional router/WLAN capabilities will be turned off when the device is configured as a bridge. It is not an ideal solution but it doesn't impact the functionality in any way.
The cheapest and the least advanced model is VMG1312-B10A. It is also based on the desirable BCM63168 chipset but only offers Fast Ethernet connection to the router. There are reports online that the Ethernet controller used in this model might struggle at speeds above 80Mb/s. Otherwise this unit has the best price point and also smaller form factor makes it easier to deploy in the networking cabinet.
The VMG8x25-B10A series has an additional noise input filter built in that in certain circumstances might lead to better performance. Unfortunately the additional filter doesn't guarantee better connection parameters.
There are two models in VMG8x25-B10A series: VMG8325-B10A and VMG8825-B10A. The only difference between them seems to be WLAN capabilities which are not going to be used when the unit acts as a bridge. Both are no longer sold by Zyxel so the availability of the new devices is low.
Zyxel VMG3925-B10B is the new router/access point based on the same BCM63168 chipset as used in the VMG3925-B10B series. It lacks the additional input noise filtering but otherwise the functionality is identical as on the previous Zyxel models.
Unfortunately none of the Zyxel units support increased 1508 byte MTU size on the standard firmware. In order to take advantage of that the alternative image has to be used. For VMG1312-B10A and VMG8x25-B10A series routers it can be found on this GitHub repository. For the newest VMG3925-B10B model the same developer posted alternative firmware on the Kitz forum, this is the direct link to the DropBox location.
Monitoring using the SNMP protocol is available on Zyxel devices but there seems to be no support for DSL/VDSL MIB so the connection statistics can only be checked using ssh/telnet or fetched by http connection to port 8000. The http monitoring is a feature added in unofficial firmware that also enables 1500 MTU PPPoE connection.
There are more devices based on the Broadcom chipsets available from different manufacturers however I was unable to find any devices with 1508 byte MTU support. There is a list of modem-router units based on the Broadcom chipsets available on Kitz forum.
If you have any questions or you are aware of any other modems that meet all the requirements in this post please let me know in the comments.