Internet Area WG M. McBride
Internet-Draft C. Perkins
Intended status: Standards Track Huawei
Expires: January 7, 2016 July 6, 2015
Multicast Wifi Problem Statement
draft-mcbride-mboned-wifi-mcast-problem-statement-00.txt
Abstract
There have been known issues with IP Multicast, in an 802.11
environment, which have prevented the deployment of multicast in
these wifi environments. The mboned working group would like to
gather the problems of wifi multicast into one problem statement
document so as to offer the community guidance on current
limitations.
Status of This Memo
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This Internet-Draft will expire on January 7, 2016.
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Internet-Draft Multicast Wifi Problem Statement July 2015
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Multicast over WiFi Problems . . . . . . . . . . . . . . . . 2
2.1. Low Reliability . . . . . . . . . . . . . . . . . . . . . 3
2.2. Low Data Rate . . . . . . . . . . . . . . . . . . . . . . 3
2.3. High Interference . . . . . . . . . . . . . . . . . . . . 4
2.4. High Power Consumption . . . . . . . . . . . . . . . . . 4
3. Common remedies to multicast over wifi problems . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Multicast over wifi has been used to low levels of success, usually
to a point of being so negative that multicast over wifi is not
allowed. More applications, such as push to talk in hospitals, video
in enterprises and lectures in Universities, are streaming over wifi.
And many end devices are increasingly using wifi for their
connectivity. To make make multicast over wifi work successfully
they often need to modify the multicast to instead be sent as unicast
in order for it to successfully transmit with useable quality.
Multicast over wifi experiences high packet error rates, no
acknowledgements, and low data rate. This draft reviews these
problems found with multicast over wifi. While this is not a
solutions draft, common workarounds to some of the problems will be
listed, along with the impact of the workarounds.
2. Multicast over WiFi Problems
802.11 is a wireless broadcast medium which works well for unicast.
With multicast, however, there are no ACKs for multicast packets so
there can be a high level of packet error rate (PER) due to lack of
retransmission and because the sender never backs off. It is not
uncommon for there to be a packet loss rate of 5% which is
particularly troublesome for video. Additionally, multicast is
typically sent on a low date rate which makes video particularly
troublesome. Wifi loses many more packets then wired due to
collisions and signal loss. There are also problems because clients
are unable to stay in sleep mode due to the multicast control packets
continuing to unnecessarily wake up those clients and subsequently
reduce energy savings. Video is becoming the dominant content for
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end device applications, with multicast being the most natural method
for applications to transmit video. Unfortunately, multicast, even
though it is a very natural choice for video, incurs a large penalty
over wifi.
One big difference between multicast over wired versus multicast over
wired is that wired links are a fixed transmission rate. Wifi, on
the other hand, has a transmission rate which varies over time
depending upon the clients proximity to the AP. Throughput of video
flows, and the capacity of the broader wifi network, will change and
will impact the ability for QoS solutions to effectively reserve
bandwidth and provide admission control.
The main problems associated with multicast over WiFi are as follows:
o Low Reliability
o Lower Data Rate
o High interference
o High Power Consumption
These points will be elaborated separately in the following
subsections.
2.1. Low Reliability
Because of the lack of acknowledgement for packets from Access Point
to the receivers, it is not possible for the Access Point to know
whether or not a retransmission is needed. Even in the wired
Internet, this characteristic commonly causes undesirably high error
rates, contributing to the relatively slow uptake of multicast
applications even though the protocols have been available for
decades. The situation for wireless links is much worse, and is
quite sensitive to the presence of background traffic.
2.2. Low Data Rate
For wireless stations associated with an Access Points, the necessary
power for good reception can vary from station to station. For
unicast, the goal is to minimize power requirements while maximizing
the data rate to the destination. For multicast, the goal is simply
to maximize the number of receivers that will correctly receive the
multicast packet. For this purpose, generally the Access Point has
to use a much lower data rate at a power level high enough for even
the farthest station to receive the packet. Consequently, the data
rate of a video stream, for instance, would be constrained by the
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environmental considerations of the least reliable receiver
associated with the Access Point.
2.3. High Interference
As mentioned in the previous subsection, multicast transmission to
the stations associated to an Access Point typically proceeds at a
much higher power level than is required for unicat to many of the
receivers. High power levels directly contribute to stronger
interference. The interference due to multicast may extend to
effects inhibiting packet reception at more distant stations that
might even be associated with other Access Points. Moreover, the use
of lower data rates implies that the physical medium will be occupied
for a longer time to transmit a packet than would be required at high
data rates. Thus, the level of interference due to multicast will be
not only higher, but longer in duration.
Depending on the choice of 802.11 technology, and the configured
choice for the base data rate for multicast transmission from the
Access Point, the amount of additional interference can range from a
factor of ten, to a factor thousands for 802.11ac.
2.4. High Power Consumption
802.11 multicast is somewhat incompatible with radio sleep schedules.
One of the characteristics of multicast transmission is that every
station has to be configured to wake up to receive the multicast,
even though the received packet may ultimately be discarded. This
process has a relatively large impact on the power consumption by the
multicast receiver station.
3. Common remedies to multicast over wifi problems
One common solution to the multicast over wifi problem is to convert
the multicast traffic into unicast. This is often referred to as
multicast to unicast (MC2UC). Converting the packets to unicast is
beneficial because unicast packets are acknowledged and retransmitted
as needed to prevent as much loss. The Access Points (AP) is also
able to provide rate limiting as needed. The drawback with this
approach is that the benefit of using multicast is defeated.
Using 802.11n helps provide a more reliable and higher level of
signal-to-noise ratio in a wifi environment over which multicast
packets can be sent. This can provide higher throughput and
reliability.
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4. IANA Considerations
None
5. Security Considerations
None
6. Acknowledgments
The following people have contributed information helpful for the
development of this Internet Draft:
o Dave Taht
o Donald Eastlake
o Marc Mosko
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
Mike McBride
Huawei
2330 Central Expressway
Santa Clara CA 95055
USA
Email: michael.mcbride@huawei.com
Charlie Perkins
Huawei
2330 Central Expressway
Santa Clara CA 95055
USA
Email: charlie.perkins@huawei.com
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