| 802.11e for QoS |
802.11e for QoS
- the new standard to provide Quality of Service, QoS
for 802.11 Wi-Fi applications
Wi-Fi technology based on the 802.11 standard is now
widespread in its use. Not only is it used to provide real wireless LAN (WLAN)
functionality, but it is also widely used to provide localised mobile
connectivity in terms of "hotspots". A variety of flavours of the IEEE 802.11
are available: 802.11a, 802.11b, 802.11g, and these different standards provide
different data throughput speeds and operate on different bands.
One of the major shortfalls for the developing applications
for Wi-Fi is that it is not possible to allocate a required quality of service
for the particular application. Now with IEEE 802.11e the Quality of Service or
QoS problem is being addressed.
The need for QoS
The issue of Quality of Service, QoS on 802.11 Wi-Fi is of particular importance
in some applications, and accordingly 802.11e is addressing it. For surfing
applications such as internet web browsing of sending emails, delays in
receiving responses or sending data does not have a major impact. It results in
slow downloads, or small delays in emails being sent. While it may have a small
annoyance to the user, there is no real operational impact on the service being
provided. However for applications such as voice or video transmission such as
Voice over IP, VoIP, there is a far greater impact and this creates a much
greater need for 802.11e. Delays, jitter and missing packets result in the
system loosing the data and the service quality becoming poor. Accordingly for
these time sensitive applications it is necessary to be able to prioritise the
traffic. This can only be done by allocating a service priority level to the
packets being sent, and this is now all being addressed by IEEE standard
802.11e.
MAC layer
The way in which data is transmitted and controlled has a major impact on the
way that QoS is achieved. This is largely determined by the way the Medium
Access Control (MAC) layer operates. Within 802.11 there are two options for the
MAC layer. The first is a centralised control scheme that is referred to as the
Point Coordination Function (PCF), and the second is a contention based approach
called Distributed Coordination Function (DCF). Of these few manufacturers of
chips and equipment have implemented PCF and the industry seems to have adopted
the DCF approach.
The PCF mode supports time sensitive traffic flows to some
degree. Wireless Access Points periodically send beacon frames to communicate
network management and identification which is specific to that WLAN. Between
the sending of these frames, PCF splits the time frame into a contention free
period and a contention period. If PCF is enabled on the remote station, it can
transmit data during the contention free polling periods. However the main
reason why this approach has not been widely adopted is because the transmission
times are not predicatable.
The other scheme, DCF uses a scheme called Carrier Sense
Multiple Access with Collision Avoidance (CSMA/CA). Within this scheme the MAC
layer sends instructions for the receiver to look for other carriers
transmitting. If it sees none then it sends its packet after a given interval
and awaits an acknowledgement. If one is not received it then it knows its
packet was not successfully received. It then waits for a given time interval
and also checks the channel before retrying to send its data packet.
In more exact terms the transmitter uses a variety of methods
to determine whether the channel is in use, monitoring the activity looking for
real signals and also determining whether any signals may be expected. This can
be achieved because every packet that is transmitted includes a value indicating
the length of time that transmitting station expects to occupy the channel. This
is noted by any stations that receive the signal, and only when this time has
expired may they consider transmitting.
Once the channel appears to be idle the prospective
transmitting station must wait for a period equal to the DCF Inter-Frame Space
(DIFS). If the channel has been active it must first wait for a time consisting
of the DIFS plus a random number of back off slot times. This is to ensure that
if two stations are waiting to transmit, then they do not both transmit
together, and then repeatedly transmit together.
A time known as a Contention Window (CW) is used for this.
This is a random number of back-off slots. If a transmitter intending to
transmit senses that the channel becomes active, it must wait until the channel
comes free, waiting a random period for the channel to come free, but this time
allowing a longer CW.
While the system works well in preventing stations
transmitting together, the result of using this access system is that if the
network usage level is high, then the time that it takes for data to be
successfully transferred increases. This results in the system appearing to
become slower for the users. In view of this WLANs may not provide a suitable
QoS in their current form for systems where real time data transfer is required.
Introducing QoS
The problem can be addressed by introducing a Quality of Service, QoS identifier
into the system. In this way those applications where a high quality of service
is required can tag their transmissions and take priority over the transmissions
carrying data that does not require immediate transmission and response. In this
way the level of delay and jitter on data such as that used for VoIP and video
may be reduced.
To introduce the QoS identifier, it has been necessary to
develop a new MAC layer and this has been undertaken under the standard IEEE
802.11e. In this the traffic is assigned a priority level prior to transmission.
These are termed User Priority (UP) levels and there are eight in total. Having
done this, the transmitter then prioritises all the data it has to waiting to be
sent by assigning it one of four Access Categories (AC).
In order to achieve the required functions, the re-developed
MAC layer takes on aspects of both the DCF and PCF from the previous MAC layer
alternatives and is termed the Hybrid Coordination Function (HCF). In this the
modified elements of the DCF are termed the Enhanced Distributed Channel Access
(EDCA), while the elements of the PCF are termed the HCF Controlled Channel
Access (HCCA).
EDCA
Of these the EDCA provides a mechanism whereby traffic can be prioritised but it
remains a contention based system and therefore it cannot guarantee a give QoS.
In view of this it is still possible that transmitters with data of a lower
importance could still pre-empt data from another transmitter with data of a
higher importance.
When using EDCA, a new class of interframe space called an
Arbitration Inter Frame Space (AIFS) has been introduced. This is chosen such
that the higher the priority the message, the shorter the AIFS and associated
with this there is also a shorter contention window. The transmitter then gains
access to the channel in the normal way, but in view of the shorter AIFS and
shorter contention window, this means that the higher the chance of it gaining
access to the channel. Although, statistically a higher priority message will
usually gain the channel, this will not always be the case.
HCCA
The HCCA adopts a different technique, using a polling mechanism. Accordingly it
can provide guarantees about the level of service it can provide, and thereby
providing a true Quality of Service level. Using this the transmitter is able to
gain access to a radio channel for a given number of packets, and only after
these have been sent is the channel released.
The control station which is normally the Access Point is
known as the Hybrid Coordinator (HC). It takes control of the channel. Although
it has an IFS, it has what is termed a Point Coordination IFS. As this is
shorter than the DIFS mentioned earlier, it will always gain control of the
channel. Once it has taken control it polls all the stations or transmitters in
the network. To do this it broadcasts as particular frame indicating the start
of polling, and it will poll each station in turn to determine the highest
priority. It will then enable the transmitter with the highest priority data to
transmit, although it will result in longer delays for traffic that has a lower
priority.
Summary
There may still be a number of problems to overcome before QoS is fully
implemented on Wi-Fi. One is the possibility of people "hi-jacking" services
when there is no real need. Nevertheless 802.11e is a major step in the right
direction, and already vendors of WiFi products are adopting the standard. As
such this makes it an important step forward in ensuring that 802.11 Wi-Fi meets
the growing demands being placed upon it.
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