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Introduction
With the deployment of IEEE 802.11 networks, supporting of real-time traffic with stringent Quality of Service (QoS) requirements
on these networks becomes critical. We propose new media access schemes, namely Dynamic Point Coordination Function (DPCF) and
modified DPCF (DPCF2). With our new schemes we can improve the total throughput of up to 20% in IEEE 802.11b networks. We can
also achieve a drastic improvement in the average end-to-end delay with mixed VoIP and data traffic which is kept well below
100 ms in heavily loaded traffic conditions with an average value well under 60 ms in normal traffic conditions.
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Current MAC layer functions
- Distributed Coordination Function (DCF)
The DCF is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) channel accessing mechanism. DCF supports
two different transmission schemes. The default scheme is a two-way handshaking mechanism where the destination transmits a
positive acknowledgment upon successful reception of a packet from the sending station. This Ack is needed because the STA
cannot determine if the transmission was successful just by listening to its own transmission. The second scheme is a four-way
handshake mechanism where the sender, before sending any packet reserves the medium by sending a Request To Send (RTS) frame and
waits for a Clear To Send (CTS) from the AP in response to the RTS. Only upon the receiving of the CTS, the STA will start its
transmission. At this point the first scheme will be used. In order to avoid collisions, a backoff mechanism is used by each STA
to access the medium, before sending any packet. In particular, the STA senses the medium for a constant time interval, the
Distributed Interframe Space (DIFS), before starting to decrease its own backoff timer. The station whose backoff timer arrives
to zero first, transmits. DIFS is used when the frame to be transmitted is a data frame. If the frame to be transmitted is an Ack
or a fragment of a previous packet, then the Short Interframe Space (SIFS) is used instead. While the DCF is the fundamental access
method used in IEEE 802.11 networks, it does not support any QoS, making this scheme infeasible for VoIP applications, with its slight
delay constraint.
- Point Coordination Function (PCF)
The PCF is based on a polling scheme. Each STA in the polling list is polled in turn. The Point Coordinator (PC) sends a CF-Poll frame
to each pollable STA, i.e. station that can respond to a CF-Poll in the polling list. The STA responds by sending a Data frame if it has
data to send or a Null packet if it has no data to send at that time. In particular, piggybacking is usually used. If the PC has some
data to send to a particular pollable STA, a Data + CF-Poll frame will be sent to this STA when its turn to be polled arrives and the
STA will respond with a Data + CF-Ack frame if it has data to send or with CF-Ack (no data) if it does not have any data to send at that
time. In an infrastructure network normally the AP acts as the PC. In particular, the PC will gain access to the medium with a higher
priority than other STAs. The PC needs to sense the medium idle for an amount of time equal to Point Interframe Space (PIFS) before gaining
access to the medium, where SIFS < PIFS < DIFS. When a PC is operating, the two access methods alternate, with a CFP followed by a CP. The
PCF controls frame transfers during a CFP while the DCF controls frame transfers during a CP.
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