An Enhanced Alternative to the IEEE 802.11e MAC Scheme

Aravind Velayutham

& J. Morris Chang

2003

Reporter : 馮士銓

Outline

IEEE 802.11 MAC IEEE 802.11e MAC Problems with the 802.11e Enhanced 802.11 Simulation Summary

IEEE 802.11 MEDIUM ACCESS CONTROL (DCF)

DCF based on CSMA/CA– CAMA (Carries Sense Multiple Access)

STA transmit data after detecting that there is no other transmission

– CA (Collision Avoidance) Transmit data after wait random backoff time

All stations have equal probability to access the channel.

DCF has no guarantees for queuing delays so it is not optimal for time-bounded applications.

IEEE 802.11 MAC (PCF)

PCF uses a point coordinator (PC) which periodically polls stations giving them the opportunity to transmit frames and thus avoiding any contention for the channel.

Superframe– Contention Free Period (CFP)

Use polling

– Contention Period (CP) Use DCF

PCF (Cont.)

A superframe includes a CP of a minimum length that allows at least one data packet delivery under DCF.

TBTT (target beacon transmission time )– PC generates a beacon frame at regular beacon f

rame intervals.

End of CFP– Receive CF-End control frame.

Problems with PCF

The beacon frame can get delayed affecting the time allocated to time-bounded traffic.– The transmission of the beacon by the PC

depends on whether the medium is idle at the time of TBTT.

The duration of the transmission that happens after the polling is not under the control of the PC.

Enhanced distributed coordination function (EDCF)

The EDCF is operative only during the CP. The various streams are classified into Traffic Categ

ories (TCs).– Arbitration Inter Frame Space (AIFS) and Contention Windo

w (CW) based on TC. During the CP STA gets its TXOP either when the m

edium is determined to be available under the EDCF rules or when STA receives a QoS CF-Poll frame from the HC.

– defined a starting time and a maximum duration. EDCF provides no guarantees to the real-time traffic.

Hybrid coordination function (HCF)

The HCF is operative during both the CP and CFP durations.

Controlled contention– STAs send update information to the HC.– Update information

This include which STAs need to be polled, polling time and duration of transmissions.

HCF

Problems with the 802.11e

Controlled contention scheme– This is a passive process where a change in

allocation requirement cannot be transmitted immediately.

Problems with EDCF– At high loads, there are a high number of

collisions even for flows with high priority.

Problems with the 802.11e (Cont.)

Per-station priority– This is not necessarily true in real life

applications. Because all flows from STA have the same priority level.

– This decreases the throughput of the TCP flows at STA.

Because in TCP during congestion avoidance phase, a source waits for a new ACK before generating a new packet.

E-802.11 (1/4)

Each packet is attached with the request for future allocation.

The upstream TCP data and downstream TCP-ACK flows are having the specified priority.

Use the More Data field to indicate that the specific flow needs to be polled during the next schedule.

HC uses a scheduling mechanism to come up with a polling schedule for each Contention Free Period.

Frame control field of IEEE 802.11

E-802.11 (2/4)

CFP-Schedule is transmitted at the start of each CFP after the transmission of the beacon frame.

CFP-Schedule specifies– The end of the CF period– STAs and their TXOPs

The end of the CFP the HC sends CP-Schedule which just specifies the stations that can contend for transmission during the Contention Period.

E-802.11

E-802.11 (3/3)

That are specified in the CP-Schedule frame can transmit frames after normal DCF contention mechanisms.

Any new stations that join the Basic Service Segment (BSS) of the HC will issue, along with the association request, any requirements for priority traffic.

Even STA want to change their status from non-real time traffic to real time traffic can attach a request in the packet sent during the CP.

Simulation

Simulate different priority Use Ns-2 A flow with priority level 1 has greater priority than

flows with priority 2.

Average Drop Rate using UDP with priorities

Average Throughput using UDP with priorities

Average Throughput using TCP with priorities

Throughput of UDP flows with varied offered loads

Summary

E-802.11 removes the EDCF used in IEEE 802.11e.– Because it was found that the real-time traffic is

not guaranteed any allocation during EDCF. E-802.11 uses a complete polling scheme for

real-time traffic and contention access for best effort traffic.

The performance of the E-802.11 was found to be much better than the IEEE 802.11e scheme.

English

The sending station will send the request for a slot in the next schedule in the transmitted packet itself.