Column

QoS and VoIP With 3G: Not an Easy Marriage

Network Computing Mobile Observer, April 11, 2007

By Peter Rysavy

There's lots of confusion out there about QoS, VoIP and 3G--these don't
work quite the way many people think. This is an area I've been studying
intently as it pertains to various projects, and after hearing a
Qualcomm rep speak recently on the topic in a technical presentation on
EV-DO Rev A, I thought I would clear up some misconceptions and organize
my own thoughts.

Sprint and Verizon are upgrading their CDMA2000 EV-DO (Evolution-Data
Optimized) networks from Rev 0 to Rev A, and one of the features is QoS
(quality of service). QoS is also available in UMTS (Universal Mobile
Telecommunications System) networks, including Cingular's HSDPA
(High-Speed Downlink Packet Access) network. QoS raises two interesting
questions: What applications need QoS, and what's the best way to make
QoS available as a feature for customers?

A good place to start is VoIP. Many popular VoIP systems are based on
best-efforts approaches only, and as a consequence, voice quality is
hit-or-miss, as anybody who uses Skype extensively can attest to,
especially with international use. The Internet at large does not
support QoS protocols, although there are managed domains that do. This
will change over time, especially as the backbone networks carry more
time-sensitive voice or video traffic. Even in the absence of QoS
mechanisms, wired networks have two huge advantages over wireless. The
first is the ability to easily add more bandwidth. The second is
consistent bandwidth with low error rates on any individual link.
Wireless networks, in comparison, are constrained by highly restrictive
spectrum allocations and must cope with high link level error rates. To
do so, they are highly adaptive, changing the amount of coding and
modulation to provide best-available throughput at any moment. The
result from a QoS point of view is problematic: a constantly varying
amount of available bandwidth per user. So what are the options for 3G?

There are different ways to control QoS. Full QoS control means being
able to control parameters such as throughput, delay, variation in delay
(jitter), error rate and whether packets are sent in sequence. However,
since QoS is ultimately a matter of packet prioritization, a much
simpler QoS implementation would be to designate classes of traffic,
where higher-class traffic gets through before lower-class traffic.

The first way that 3G will seriously use QoS is with 3G networks
employing VoIP. Sure, you can run Skype over 3G today. But actually
running VoIP with the same voice quality and capacity as current
circuit-switched systems is complicated, which explains why nobody has
done it quite yet. Circuit-switched methods are highly optimized, but
most VoIP approaches are not. IP headers alone can kill you, with some
40 bytes of IP, UDP and RTP packet headers of overhead for every 22
bytes of VoIP payload. Techniques such as Robust Header Compression
(RoHC) employed within the network bring this down to 4 bytes. If the
cellular operator then applies full QoS for voice call admission and
bandwidth management, and additionally applies a variety of other
complicated low-level bit twiddling such as dejittering and time
warping, the result is high-quality and high-capacity VoIP equaling and
eventually surpassing current circuit-switched voice. This is on the
road map for both E-DO and UMTS, though EV-DO operators are more highly
motivated to make this work because their EV-DO radio channels currently
support data only, whereas UMTS/HSDPA channels allow simultaneous voice
and data. Note, however, that this application of QoS is purely within
the operator's domain. It could, if the operator chose to do so, be made
available to improve your Skype session. But I see this as unlikely in
the near future.

I think it's likely that 3G operators will leverage QoS for their own
applications, including VoIP, as well as for video services. But as far
as exposing interfaces for customer applications to request parameters
such as a certain amount of bandwidth or delay control, that's a harder
sell. The problem is that with finite capacity, do you really want one
set of customers using up all the bandwidth and thus denying data
service for others? And how much would customers be willing to pay for
such a service? Plus, remember that the amount of radio resource
required to deliver certain bandwidth depends on the users'
instantaneous radio conditions. I know operators are scratching their
heads over this one.

However, the class-of-traffic approach might be easier to monetize, as
well as to manage. Here, the operator could provide higher-priority
traffic for, say, emergency services or critical medical applications.
Now, when all hell is breaking loose somewhere, the police and firemen
using a public system could still be able to communicate. I'm not aware
of any operator offering this yet, but it wouldn't be that difficult for
them, at least technically, to do so.

Interestingly, some companies, not waiting for operators to expose QoS
control, have taken matters into their own hands. For example, NetMotion
Wireless with their Mobility mobile VPN, permits prioritization of
traffic within its own tunnels, allowing VoIP, for instance, to get
through ahead of other applications, and even blocking some
applications, such as Windows Update, when on slower connections. Bottom
line, QoS for wide-area wireless has sort of arrived, but it will be
some time before you see widespread usage.

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