Providing QoS in Ad Hoc Networks with Distributed Resource
Reservation
IEEE802.11e and extensions
Ulf Körner and Ali Hamidian
The Goal
• To provide QoS guarantees to WLANs operating in ad hoc mode– by allowing stations to reserve resources
(medium time)– by distributing the existing admission control
and scheduling algorithms
• Example of application area: gaming
No QoS in IEEE 802.11
• Today’s WLANs do not offer any QoS– usually not a big
problem if you just surf the Internet
– bad voice/video quality if you use e.g. Skype or MSN messenger
802.11 MAC & its QoS Limitations
• 802.11 has two medium access methods:– distributed coordination function (DCF)
• All data flows have the same priority
– point coordination function (PCF)• Not possible for stations to send QoS requirements to the AP• Unknown transmission time of the polled stations
• 802.11e introduces:– hybrid coordination function (HCF)
• enhanced distributed channel access (EDCA)• HCF controlled channel access (HCCA)
Hybrid Coordination Function (HCF)
• transmission opportunity (TXOP): A bounded time interval during which a station may transmit multiple frames– Solves the PCF problem with unknown transmission times
• traffic specification (TSPEC): Contains information about the QoS expectation of a traffic stream (frame size, service interval, data rate, burst size, delay bound, etc.)– Solves the PCF problem with the inability to send QoS
needs
Enhanced Distributed Channel Access (EDCA)
• Contention-based• “Enhanced DCF”• access category (AC):
Each station has four ACs (”transmission queues”). Each AC contends for TXOPs independently of the other ACs
• Service differentiation is realized by varying – Different parameters
AIF
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virtual collision handler
Background [1] Best effort [2] Video [3] Voice [4]
mapping to AC
HCF Controlled Channel Access (HCCA)
• Contention-free• “Enhanced PCF”• Medium access controlled by a QoS access
point (QAP)• HCCA allows stations with QoS traffic to reserve
TXOPs using TSPECs
Motivation of our Work: QoS Limitations in 802.11e
• Problem with EDCA– Random medium access & no distributed admission
control => not possible to guarantee QoS
• Problem with HCCA– Centralized infrastructure requirement => HCCA not
useful in ad hoc networks
• We need a solution which is– Deterministic (unlike EDCA)
• Remove the random medium access delays
– Distributed (unlike HCCA)• Remove the need of an access point
EDCA with Resource Reservation (EDCA/RR)
• distributed admission control and scheduling
• possibility to reserve TXOPs for deterministic and contention-free medium access
EDCA/RR Operation
Similar to EDCA as long as LP frames (AC_Background and AC_BestEffort) are sent
EDCA/RR Operation
When a HP frame (AC_Video and AC_Voice) reaches the MAC sublayer, the source checks whether its new stream can be admitted
EDCA/RR Operation
If admission control OK: • schedule the new
stream• broadcast ADDTS
request containing TSPEC
• wait for ADDTS response
ADDTS request
EDCA/RR Operation
Once all ADDTS responses are received by the source, it waits until its first reserved TXOP at service start time & starts transmitting
ADDTS response
EDCA/RR Operation
deterministic and contention-free medium access: the source has now reserved TXOPs every scheduled service interval (SI)
HP data frames
Results
• EDCA/RR implementation in ns-2 based on an enhanced 802.11/802.11e implementation
• EDCA vs. EDCA/RR• Stationary behaviour: How is the average end-
to-end delay of a HP-stream affected when the number of LP streams increases?
Throughput: EDCA
1 LP-stream and 4 HP-streams each started 10 s apart.
ad hoc network
Throughput: EDCA
1 LP-stream and 4 HP-streams each started 10 s apart.
ad hoc network
1 LP stream
Throughput: EDCA
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 1 HP stream
Throughput: EDCA
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 2 HP streams
Throughput: EDCA
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 3 HP streams
Throughput: EDCA
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 4 HP streams
Throughput: EDCA/RR
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
Throughput: EDCA/RR
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream
Throughput: EDCA/RR
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 1 admitted HP stream
Throughput: EDCA/RR
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 2 admitted HP streams
Throughput: EDCA/RR
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 3 admitted HP streams
Throughput: EDCA/RR
ad hoc network
1 LP-stream and 4 HP-streams each started 10 s apart.
1 LP stream + 3 admitted HP streams +1 rejected HP stream
Throughput: EDCA vs. EDCA/RR
EDCA EDCA/RR
End
Average End-to-End Delay
- 1 HP source- 150 simulation runs! - simulation time: 200 s
Problem due to Hidden Stations
• The hidden station C doesn’t receive A’s ADDTS request so it can start sending just before A’s TXOP starts! ==> no QoS guarantees!
Solving the Hidden Station Problem
• The TSPEC is included in the ADDTS response so when B sends an ADDTS response to A, C hears that message and learns about A’s reservation
• In addition:
Send RTS_TSPEC and CTS_TSPEC in the beginning of each TXOP
Results - 0 % packet error
nbr of LP-streams
average end-to-end delay (ms)
99 % confidence interval (ms)
EDCA EDCA/RR EDCA EDCA/RR
0 0.69 12.33 (0.69,0.69) (12.13,12.53)
1 6.21 12.22 (6.20,6.22) (12.02,12.42)
2 11.17 12.27 (11.14,11.19) (12.08,12.47)
3 13.93 12.22 (13.90,13.96) (12.01,12.42)
4 17.12 12.38 (17.08,17.16) (12.19,12.57)
5 20.51 12.25 (20.46,20.56) (12.06,12.45)
Results - 5 % packet error
nbr of LP-streams
average end-to-end delay (ms)
99 % confidence interval (ms)
EDCA EDCA/RR EDCA EDCA/RR
0 0.99 12.55 (0.99,0.99) (12.37,12.73)
1 4.68 12.44 (4.68,4.69) (12.27,12.61)
2 5.25 12.54 (5.24,5.25) (12.35,12.73)
3 5.59 12.34 (5.58,5.60) (12.16,12.52)
4 5.92 12.64 (5.91,5.93) (12.45,12.82)
5 6.28 12.53 (6.27,6.29) (12.34,12.72)
Results - 0 % packet error
nbr of LP-streams
jitter (10-6 s2) C2[d]
EDCA EDCA/RR EDCA EDCA/RR
0 0.02 48 0.05 0.32
1 40 48 1.04 0.32
2 180 48 1.45 0.32
3 276 48 1.42 0.32
4 406 49 1.38 0.32
5 577 49 1.37 0.32
Multi-hop Resource Reservation
1) A: if traffic is admitted, send RREQ-ADDTSRequest
2) B: if traffic is admitted, send RREQ-ADDTSRequest
3) C: if traffic is admitted, schedule traffic and send RREP-ADDTSResponse
4) B: schedule traffic and send RREP-ADDTSResponse
5) A: schedule traffic and send data
AODV + EDCA/RR
Summary
• EDCA/RR – is a MAC scheme with distributed admission
control and scheduling– allows stations to reserve TXOPs for
deterministic and contention-free medium access