|
 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.
 |
Blog updates via RSS
Technology 
