QoS in Wireless Networks

1
QoS in Wireless Networks
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

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)

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)

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

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.

7
DCF Timing diagram
DIFS
Data
Src
SIFS
Ack
Dest
DIFS
Contention Window
Next MPDU
Others
Backoff after Defer
Defer Access
8
DCF Example
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 lt
  DIFS)
• During PCF mode,
• PC polls each station for data
• After a transmission of a MPDU, move on to the
  next station

10
PCF operation

• TBTT Target Beacon Transmission Time
• Carried in the beacon

• Source IEEE 802.11e Wireless LAN for Quality
  of Service
• Mangold et. al., Proc. European Wireless 2002

11
IEEE 802.11 superframe
Source Quality of Service Schemes for IEEE
802.11 Wireless LANs An Evaluation A.
Lindgreen et. al., Kluwer Academic Publisher,
2003.
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

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
A
A
A
B
B
B
B
I
I
I
I
B
B
B
B
a
a
a
a
F
F
F
F
c
O
c
O
c
O
c
O
S
S
S
S
k

k

k

k





o
o
o
o
0
1
2
3
0
1
2
3
f
f
f
f








f
f
f
f








Virtual Collision Handler
Transmission
Attempt
14
Differentiated Channel Access

• Each AC contends with
• AIFSAC (instead of DIFS) and CWminAC,
  CWmaxAC (instead of CWmin, CWmax)

15
Backoff Procedure

• Similar to 802.11
• Backoff time chosen between 0, CWAC.
• On collision
• CWAC (CWAC1) 2 -1

16
Priority to AC Mapping
Priority Access Category (AC) Designation (Informative)
0 0 Best Effort
1 0 Best Effort
2 0 Best Effort
3 1 Video Probe
4 2 Video
5 2 Video
6 3 Voice
7 3 Voice
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

18
HCF Controlled Channel Access(HCCA)

• HC may function as PC that uses CFP for polled
  data (this mode can be used by both 802.11 and
  802.11e STAs)
• HC may also send QoS CF polls
• But not mandatory since it can send those in CP
  also

19
IEEE802.11e superframe

• Source IEEE 802.11e Wireless LAN for Quality
  of Service
• Mangold et al., Proc. European Wireless 2002.

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

21
Immediate block ack
Source IEEE 802.11e standard document
22
Delayed block ack
Source IEEE 802.11e standard document
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 802.11
• 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

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

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

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)

27
DFS (contd)

• 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

28
DFS (contd)

• 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 threshold80, K180, K20.002,
  ?(990) ?(1000) 147.

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 802.11

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

31
WTRP (cont.)
seq 1 F
seq2 A
Seq3 unknown
seq4 unknown
seq5 D
Connectivity table of E
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.

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 gt
  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

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

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

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. 1353-1368.
37
References

• Schiller J., Mobile Communications - Addison
  Wesley, 2000.
• Benvensite M., et. al., EDCF proposed draft
  text IEEE working document 802.11-01/131r1
  (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 802.11 Wireless LANs An Evaluation
  Mobile Networks and Applications vol. 8, pp
  223-235, Kluwer Academic Publishers, 2003.