1. QoS in Wireless Networks
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2. Wireless Network
Wireless networks are better than wired networks with regards to ease of installation and flexibility
But they suffer from lower bandwidth, higher delays and higher bit error
Thus providing QoS over such a network is quite challenging and requires additional measures
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3. IEEE 802.11 network Most widely used WLAN Uses a shared medium
Low medium utilization
Risk of collision
No service differentiation between types of traffic
Has two access methods (MAC)
Distributed Coordinator Function (DCF)
Point Coordinator Function (PCF)
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4. DCF Uses a CSMA/CA algorithm in MAC
Before a data frame is sent, the station senses the medium
If it is idle for at least DCF interframe (DIFS) amount of time, the frame is transmitted
Otherwise a backoff time B (measured in time slots) is chosen randomly in the interval [0, CW)
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5. DCF (cont.)
After medium is detected idle for at least DIFS, the backoff timer is decremented and frame is transmitted when it reaches zero
If medium becomes busy during count down, backoff timer is paused and restarted when medium is idle for DIFS period
If there is a collision, CW is doubled according to
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6. DCF (cont.) Where i = number of retransmissions
k= constant defining minimum CW
A new backoff time is then chosen and the backoff process starts over.
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7. DCF Timing diagram Src Dest Others 12/30/2018 Data Ack Next MPDU
DIFS
Src
Data
SIFS
Dest
Ack
DIFS
Contention Window
Others
Next MPDU
Defer Access
Backoff after Defer
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8. DCF Example cw = 31 data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
cw = 31
B2 = 10
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9. PCF (Point Coordination Function)
Contention-free frame transfer
Single Point Coordinator (PC) controls access to the medium.
AP acts as PC
PC transmits beacon packet when medium is free for PIFS time period
PCF has higher priority than the DCF (PIFS < DIFS)
During PCF mode,
PC polls each station for data
After a transmission of a MPDU, move on to the next station
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10. PCF operation TBTT : Target Beacon Transmission Time
Carried in the beacon
Source : “IEEE e Wireless LAN for Quality of Service” –
Mangold et. al., Proc. European Wireless 2002
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11. IEEE superframe
Source : “Quality of Service Schemes for IEEE Wireless LANs – An
Evaluation – A. Lindgreen et. al., Kluwer Academic Publisher, 2003.
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12. Enhanced Distributed Channel Access (EDCA)
This is HCF Contention Based channel access
Provides service differentiation
Traffic can be classified into 8 different classes
Each station has 4 access categories to provide service differentiation
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13. Access Category (AC) Access category (AC) as a virtual DCF
4 ACs implemented within a QSTA to support 8 user priorities
Multiple ACs contend independently
The winning AC transmits frames
AC0
AC1
AC2
AC3 A B A B A B A B B I a B I a B I B I F F F a F a O c S O c O c O c [ S S S [ k [ [ k [ [ k [ [ k o 1 o 2 o 3 o ] ] f f ] 1 f 2 3 ] f ] ] f f f ] ] f
Virtual Collision Handler
Transmission
Attempt
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14. Differentiated Channel Access
Each AC contends with
AIFS[AC] (instead of DIFS) and CWmin[AC], CWmax[AC] (instead of CWmin, CWmax)
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15. Backoff Procedure Similar to 802.11
Backoff time chosen between [0, CW[AC]].
On collision
CW[AC] = (CW[AC]+1) *2 -1
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16. Priority to AC Mapping Priority Access Category (AC)
Designation (Informative)
Best Effort 1 2 3
Video Probe 4
Video 5 6
Voice 7
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17. HCF Controlled Channel Access(HCCA)
Manages access to wireless medium using HC which has a higher medium access priority (than EDCA) than non-AP STAs.
Two primary differences between PCF and HCCA
Frame exchange can happen both in CP and CFP period
HC grants a polled TXOP with duration specified in a QoS CF Poll frame
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18. HCF Controlled Channel Access(HCCA)
HC may function as PC that uses CFP for polled data (this mode can be used by both and e STAs)
HC may also send QoS CF polls
But not mandatory since it can send those in CP also
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19. IEEE802.11e superframe
Source : “IEEE e Wireless LAN for Quality of Service” –
Mangold et al., Proc. European Wireless 2002.
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20. Block Acknowledgement
Improves channel efficiency by aggregating several Ack into one frame
A bitmap is used to ack a set of MPDUs
Immediate block ack
BlockAckReq is immediately responded with BlockAck
Delayed block ack
Receiver responds with an ACK to BlockAckReq
Then the receiver would send the BlockAck in the next TXOP
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21. Immediate block ack
Source : IEEE e standard document
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22. Delayed block ack
Source : IEEE e standard document
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23. Distributed Fair Scheduling (DFS)
Based on SCFQ (Actually based on WFQ)
Uses a distributed approach for determining the smallest finish tag using backoff interval mechanism of
Backoff interval is chosen such that it is proportional to the finish tag of packet to be transmitted
So packets with smaller finish tag will be assigned smaller backoff interval
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24. Distributed Fair Scheduling (DFS)
On arrival a packet is stamped with start tag
And the finish tag is
Equivalently
fki is the real time when the packet reaches head of the queue
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25. Distributed Fair Scheduling (DFS)
So the finish number of a packet is given by
A back off interval is chosen so that packets with smaller finish number gets lower backoff. Hence the back off is taken as
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26. Distributed Fair Scheduling (cont.)
is a random variable uniformly distribute between [0.9,1.1] to inject randomness into the backoff mechanism
Backoff interval is inversely proportional to weight assigned to a node. Thus node with higher weight is given a higher priority (because of smaller backoff interval)
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27. DFS (cont’d)
For flows with small weight, the duration of the backoff interval can become quite large
Leads to long duration of idle time when nodes are counting down
An exponential mapping scheme is proposed to address this
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28. DFS (cont’d)
When weights of flows are small, backoff interval is reduced
A larger range of linear backoff interval is compressed into smaller exponential range. Hence the likelihood of collisions can increase with exponential case.
For example, for threshold=80, K1=80, K2=0.002, (990) = (1000) = 147.
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29. Wireless Token Ring Protocol
Wireless Token Ring Protocol (WTRP) can support QoS in terms of bounded latency and reserved bandwidth
Efficient, since it reduces the number of retransmissions
Fair in the sense that every station takes a turn to transmit and gives up its right to transmit (by releasing the token) until the next round
Can be implemented on top of
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30. WTRP (cont.)
Successor and predecessor fields of each node in the ring define the ring and the transmission order
Station receives token from predecessor, transmits data and passes the token to the successor.
Sequence number is used to detect any nodes that are part of the ring, but not in the range of a node
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31. WTRP (cont.) F seq=2 A Seq=3 unknown seq=4 seq=5 D seq = 1 A B C D E F
Transmission range of E
seq = 1 F
seq=2 A
Seq=3
unknown
seq=4
seq=5 D
Connectivity table of E
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32. WTRP (cont.)
Implicit acknowledgement is used to monitor successful transmission of token
Timer is used to guard against loss of token (successor might have moved out of range)
Using connectivity table, the ring can be reformed when a node moves out of range
By controlling the token holding time and token rotation time delay of packets can be bounded.
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33. Blackburst
Devised with a view to minimizing delay for real-time traffic
Stations are assigned priority
When a high priority station wants to send a frame
Senses the medium to see if it is idle for tmed time period and then sends its frame
The data nodes (low priority station) use tlong > tmed to sense medium for access
If medium is busy, station waits until channel has been idle for a tmed and then enters a black burst contention period
The station sends a black burst by jamming the channel for a period of time
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34. Blackburst
The length of the black burst is proportional to the amount of time the station has been waiting to access the medium (calculated as a number of black slots)
After transmitting black burst, the station listens to the medium for a short period of time tobs (less than a black slot) to see if some other station is sending a longer black burst (hence has waited longer)
If the medium is idle, then station sends its frame
Otherwise it waits until the medium becomes idle again and enters another black burst contention
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35. Blackburst
After successful transmission of a frame, the station schedules the next access instant tsch seconds in the future.
This has the nice feature that real-time flows will synchronize and share the medium in a TDM fashion
Unless there is a transmission by low priority station when a high priority station accesses the medium, very little blackbursting needs to be done once stations have synchronized
Low priority stations use ordinary DCF access mechanism
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36. Timing diagram
Source: Sobrinho J.L., Krishnakumar A.S., “Quality of Service in
ad hoc carrier sense multiple access networks” – IEEE Journal on
Selected Areas in Communications 17(8) (1999), pp
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37. References
Schiller J., “Mobile Communications” - Addison Wesley, 2000.
Benvensite M., et. al., “EDCF proposed draft text” – IEEE working document /131r1 (2001)
Vaidya N.H., et. al., “Distributed Fair Scheduling in a wireless LAN” – Sixth International Conference on Mobile Computing and Networking, Boston 2000.
Ergen M., et. al., “Wireless Token Ring Protocol” –Proceedings of 8th International Symposium on Computer and Communication 2003.
Lindgren A., et. al., “Quality of Service Schemes for IEEE Wireless LANs – An Evaluation” – Mobile Networks and Applications vol. 8, pp , Kluwer Academic Publishers, 2003.
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