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

SN

[1]C

Wm

in[1]C

Wm

ax[1]T

XO

Plim

it[1]

AIF

SN

[2]C

Wm

in[2]C

Wm

ax[2]T

XO

Plim

it[2]

AIF

SN

[4]C

Wm

in[4]C

Wm

ax[4]T

XO

Plim

it[4]

AIF

SN

[3]C

Wm

in[3]C

Wm

ax[3]T

XO

Plim

it[3]

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