Does your femtocell like tomato?

Tomato VoIP (Voice over IP) services can suffer from bad press where they compete for bandwidth with your PC or other data applications on your network. Voice is particularly intolerant to delay - if a packet isn't received in time then it's discarded causing silence. Some codecs are tolerant of packet loss, and this one reason why Skype is perceived to work better than many other VoIP systems.

An important bottleneck is near the start of its journey, where voice traffic from your femtocell competes with other devices to send data through your broadband connection. Where a typical DSL broadband connection is used, the uplink capacity is typically only 5% to 10% of the downlink, but voice calls use bandwidth more symetrically than data services. This increases the chances of packet collision, and delays.

 

Traffic prioritisation for femtocells is the solution

The solution to this specific problem is to prioritise the voice traffic above other sources within the home network. DSL broadband modems with built in VoIP ports automatically prioritise the VoIP traffic above other external data sources, solving this problem. Where femtocells are incorporated with DSL broadband modems, this same technique is also used with similar results.

Office VoIP phones solve this problem by inserting themselves between office computers and the switched ethernet connection to each desk. Early femtocells are more likely to be standalone units which connect into a router port using ethernet. Many basic routers do not have the capability to prioritise the traffic on individual ports.

There are two features here:

  • Simple prioritisation of packets from one port above others. Your VoIP port (or femtocell) always gets to the front of the transmit queue in your router, and so never has to wait.
  • Bandwidth allocation between ports. More complex than above, setting maximum limits for each port, so that no device can "hog" the capacity and block out all other users.

What's Tomato got to do with it?

The Open Source community has come to the rescue by providing a free software package to replace the firmware inside several types of common router/modem boxes. Specifically those based on the Broadsoft chipset, such as found in D-Link. The project, called Tomato, includes configuration capabilities to define the capacity allocated to each port, thus allowing your femtocell to have its fair share of capacity.

We can also find this QoS prioritisation feature in more recent and/or upmarket broadband modems. Zyxel, for example, include a Multimedia Bandwidth Management (MBM) feature in theirs. Linksys and D-Link also advertise QoS management in the more recent products. Generally speaking, these features are not well described in their sales literature, so its difficult to make a detailed comparison.

How does QoS prioritisation work?

Where the router knows which physical port is used for VoIP, it can identify the UDP port range the VoIP traffic is being transmitted on. Typically, packets are tagged to IP Predence 5 (if not already tagged by the VoIP device) or diffserv code point EF. This will ensure that the VoIP traffic is prioritised "OUT" from the router over and above lower priority traffic.

If you mark your traffic, it's about local prioritisation to ensure that in your LAN (if you have one) and on the egress (your port to the ISP) that traffic that has a higher class of service, such as voice, is put on the wire ahead of lower class traffic.

What do ISP's do with QoS prioritisation markings

Most if not all ISP will not touch the markings, this includes doing nothing when they receive the packets. This is because when it comes to data plane (user/customer traffic) they just pass this as FIFO (first in first out) traffic.

Most ISPs don't act on those markings today, but in future many/most will do. Instead, ISP's prioritise traffic based on factors such as the subscription plan of the user - those wanting higher quality/preference should pay for it. When and in what form they will also prioritise on the QoS markings will be at their discretion.

Some are looking to deploy subscriber management tools to identify and are considering downgrading VoIP traffic that is not using their own service as it is not making them revenue..... and then preserve or guarantee the marking/bandwidth allocation for their own service offerings or for a premium price.

This is the basis of the "net neutrality" debate, where regulators are mandating that service providers can only impose the same QoS prioritisation schemes across all subscribers and not differentiate between their own and their competitor's.

You might not have noticed - up to now

Whilst not an essential capability - many VoIP applications may never notice a problem - those busy households with multiple users of the femtocell with IPTV will start to need these features. Here in the UK, a very popular service is streaming live and recent programs from the BBC using their iPlayer. It's said to be taking up to 10% of all wireline broadband capacity in the UK today. This heavy load would interfere with VoIP and/or femtocell traffic unless a router with this capability is used.

I believe this issue may also affect dual-mode UMA phones connected using WiFi, such as T-Mobile US Hotspot@Home, and could benefit from the same solution.

Where the network provider supplies both the femtocell and the wired broadband line, then they often provide the WiFi router and can manage/configure it and/or replace/upgrade it if required. Few problems have been reported about QoS with UMA, perhaps because Orange supply and configure the WiFi router used. And perhaps because IPTV is still in its infancy.

And perhaps it won't be a problem by the time femtocells are mainstream

Two pieces of the jigsaw need to be in place:

  • domestic routers capable of prioritising and marking femtocell traffic
  • ISP broadband networks capable of acting on those markings, especially prioritising voice calls 

In practice, this may have been solved by the time femtocells are widely deployed. Early adopters of femtocells are likely to have fairly recent modem/router equipment and/or be more aware of potential issues. By the time femtocells become mainstream, the population of routers including QoS will be high. Operators also have the choice to ship a complete modem/router/WiFi/femtocell integrated unit, which avoids configuration and setup problems albeit at higher initial cost.

For enterprise deployments, where there may be substantial amounts of voice and data traffic sharing a network, a QoS strategy will be important. Much can be learnt and applied from existing wired office VoIP systems in these cases.

We should also expect ISPs to have developed the capability of prioritising suitably marked traffic in the next few years. Whether they will charge extra for this privilege (even if done fairly on the same basis for their own retail customers), remains to be seen.

But for those of us with domestic femtocells who like tomatoes, the Open Source community can solve another problem.

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