VOIP over Wireless Network

1
VOIP over Wireless Network

• Prof. Anirudha Sahoo
• KReSIT
• IIT Bombay

2
Outline

• Primer on Voice over IP System
• QoS in VOIP
• Primer on Wireless LAN (802.11)
• Different approaches to VOIP over wireless
  network
• Mobility Issues
• Summary

3
Voice Over IP (VOIP)

• Transmission of digitized voice in packet network
  (e.g. IP, ATM, Frame Relay)
• Enables telephone conversation to be carried over
  IP network (in part or end-to-end)
• Provides a toll bypass path for telephone calls
• Enables Telephony providers to provide cheaper
  service

4
VOIP System
PBX
PBX
(A typical PSTN system)
(A typical VOIP system)
5
VOIP System (cont.)
IP Network
CPE router
CPE router
LAN
LAN
PSTN Gateway
SIP proxy
PSTN
Soft phone
IP phone
IP phone
(Another VOIP system)
6
Outline

• Primer on Voice over IP System
• QoS in VOIP
• Primer on Wireless LAN (802.11)
• Different approaches to VOIP over wireless
  network
• Mobility Issues
• Summary

7
QoS in VOIP

• VOIP applications (e.g. telephone call) are real
  time in nature
• So they require QoS from the underlying system
• Many factors determine voice quality
• Choice of codec
• Delay
• Jitter
• Packet loss

8
Delay

• VOIP packet can experience delay at various point
  on its path
• Encoding delay in the codec (algorithmic
  processing) (17ms) (for G729 codec)
• Packetization/Depacketization delay (20ms)
• Access (up) link transmission delay
• Delay in the backbone network
• Access (down) link transmission delay
• Jitter buffer delay (10 60ms)
• Decoder delay in codec (at the receiver) (2ms)
• Playout delay (0.5ms)

9
Delay (cont.)

• ITU-T G.114 recommends the following one-way
  delay time limits
• 0 150 ms acceptable for most user apps
• 150 400 ms acceptable for international
  connections
• gt 400ms unacceptable
• Thus packet delay is a very important QoS
  parameter in VOIP system for an acceptable
  telephone conversation

10
Delay (cont.)

• From the breakdown of end-to-end delay it is
  clear that some delays are unavoidable
• Delay in the network is the component that can be
  controlled
• Network QoS

11
Network QoS

• Can be provided by few approaches
• Engineering the network
• IntServ
• DiffServ
• MPLS-based

12
Network QoS Engineering the network

• Set aside separate resources for voice flows
• Priority queuing at the routers for voice packets
• Weighted Fair Queueing with high weight for voice
• Policing traffic so that some percentage of bw is
  reserved for voice traffic.

13
VOIP QoS Intserv

• RSVP is the protocol of choice for providing QoS
  under IntServ architecture
• Uses a separate reservation phase to allocate
  resources for voice calls
• Guaranteed service model used in RSVP can provide
  delay guarantee to voice call
• Has scalability problem and large overhead
• Hence only suitable for an enterprise network
  (e.g. intranet)

14
VOIP QoS Diffserv

• Diffserv was developed to circumvent some of the
  problems in Intserv
• Achieves scalability by providing differentiated
  service to aggregate traffic
• Packets carry the PHB (Per Hop Behavior) info. in
  the header (DS field)
• Resources are provisioned for particular Class of
  Service by the ISP
• Policing and Shaping is done at the edge of the
  network to check for conformance (with SLA)
• Thus appropriately classifying voice packets will
  provide QoS to voice calls

15
VOIP QoS MPLS

• Use MPLS to achieve traffic engineering
• Use RSVP-TE to reserve resources as well as
  provide explicit routing
• CR-LDP can also be used to engineer traffic by
  providing explicit route
• DiffServ can also be combined with MPLS to map
  DiffServ Behavior Aggregates (BA) to LSPs.

16
VOIP QoS Summary

• So there are architectures and mechanisms
  available to provide QoS for VOIP applications in
  a wired network so that the delay constraint of
  such applications can be met

17
VOIP in Wired Network
RSVP/Diffserv/MPLS/ Engineered Network
IP Network
PSTN gateway
PSTN gateway
gatekeeper
PBX
PBX
(Delay bounded VOIP system)

18
Outline

• Primer on Voice over IP System
• QoS in VOIP
• Primer on Wireless LAN (802.11)
• Different approaches to VOIP over wireless
  network
• Mobility Issues
• Summary

19
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 running VOIP application over such a network
  is quite challenging and requires additional
  measures

20
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)

21
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)

22
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

23
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.

24
DCF Timing diagram
DIFS
Data
Src
SIFS
Ack
Dest
DIFS
Contention Window
Next MPDU
Others
Backoff after Defer
Defer Access
25
DCF Example
26
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

27
VOIP over Wireless (VoW)

• Since VOIP requires bounded delay it is obvious
  that DCF is not suitable for VOIP traffic (since
  it is contention based, it cannot provide any
  deterministic delay bound)
• PCF, being polling based, can provide delay
  bound, hence is a good candidate for VOIP
• But most 802.11 products do not have PCF
  implementation
• Delay can be large when too many stations have
  data to send in CFP

28
VOIP over Wireless (cont.)
IP Network
CPE router
CPE router
PSTN Gateway
SIP proxy
PSTN
Mobile IP phone
Soft phone
Mobile IP phone
(A VOIP over Wireless System)
29
Outline

• Primer on Voice over IP System
• QoS in VOIP
• Primer on Wireless LAN (802.11)
• Different approaches to VOIP over wireless
  network
• Mobility Issues
• Summary

30
VOIP over Wireless (cont.)

• Various mechanisms can be used to provide delay
  bounds for VOIP communication
• Enhanced DCF (EDCF)
• Distributed Fair Scheduling
• Wireless Token ring
• Blackburst

31
Enhanced DCF

• Provides service differentiation
• Traffic can be classified into 8 different
  classes
• Each station has 4 access categories to provide
  service differentiation

32
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
33
Differentiated Channel Access

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

34
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
35
Distributed Fair Scheduling (DFS)

• Based on SCFQ
• 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

36
Distributed Fair Scheduling (cont.)

• 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)
• VOIP application can use the scheme to achieve
  better QoS by availing priority over data traffic

37
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

38
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

39
WTRP (cont.)
seq 1 F
seq2 A
Seq3 unknown
seq4 unknown
seq5 D
Connectivity table of E
40
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.
• Hence WTRP can be used for VOIP applications

41
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 PIFS
  time period and then sends its frame
• If medium is busy, station waits until channel
  has been idle for a PIFS and then enters a black
  burst contention period
• The station sends a black burst by jamming the
  channel for a period of time

42
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
  (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

43
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

44
VoW
RSVP/Diffserv/MPLS/ Engineered network
IP Network
CPE router
CPE router
EDCF/DFS/ WTRP
EDCF/DFS/ WTRP
PSTN Gateway
SIP proxy
PSTN
Mobile IP phone
Soft phone
Mobile IP phone
(Delay bounded VoW system)
45
VoW (cont.)

• Since end-to-end delay of a VOIP call is
  important, in the VoW system it is necessary to
  budget the delay appropriately across the various
  components (e.g. wired network, wireless LAN) in
  the path of the call
• Calls have to be admitted carefully so that
  end-to-end delay is within acceptable limit

46
Outline

• Primer on Voice over IP System
• QoS in VOIP
• Primer on Wireless LAN (802.11)
• Different approaches to VOIP over wireless
  network
• Mobility Issues
• Summary

47
Mobility

• Mobility adds complexity to VOIP connections
• Need to have fast and smooth handoff
• Can be of two types
• Micro mobility
• Mobile station (MS) moves within a domain,
  usually within an enterprise
• Can quickly connect to the new AP (300ms) (link
  layer handoff)
• Macro mobility
• MS moves into a different domain (e.g. moves from
  one hotspot to another and the two hotspots are
  managed by different ISPs)

48
Mobility
Internet
Hot Spot B
Hot Spot A
AP
AP
AP
AP
Micro mobility
Macro mobility
Micro mobility
49
Mobility

• Two approaches available
• Mobile IP
• handoff at network layer
• SIP
• handoff at the application layer

50
Handoff using Mobile IP

• 3 Parts of Mobile IP
• Advertising Care-of Addresses
• Registration
• Tunneling

51
Mobile IP

• A mobility agent is either a foreign agent or a
  home agent or both
• Mobility agents broadcast agent advertisements
  (periodically)
• Mobile hosts can solicit for an advertisement
• Advertisements contain
• mobility agent address
• care-of addresses
• lifetime

52
Registration
53
Tunneling
54
Handoff using SIP

• Two scenarios
• Pre-call mobility
• Mid-call mobility

55
Pre-call mobility
(2)INVITE
Correspondent node
Home Network
(3) 302 moved temporarily
SIP server
(5) 200 OK
(4) INVITE
(1) Registration of New contact with registrar
Visited network
Mobile node
56
Mid-call mobility
Correspondent node
Home Network
SIP server
(2) 200 OK
(1) re-INVITE
Visited network
Mobile node
57
Outline

• Primer on Voice over IP System
• QoS in VOIP
• Primer on Wireless LAN (802.11)
• Different approaches to VOIP over wireless
  network
• Mobility Issues
• Summary

58
Summary

• VOIP applications require QoS
• Delay is the most important QoS parameter
• Wired networks have mechanisms available to
  provide QoS (RSVP, Diffserv, MPLS)
• Wireless LAN such as 802.11 does not have
  implementation that can support VOIP
  communication adequately
• EDCF (802.11e), DFS, WTRP and blackburst are few
  mechanisms that can be used to facilitate VOIP
  communication in wireless LANs

59
Summary (cont.)

• Handoff can be handled
• By Mobile IP
• By SIP
• Delay has to be budgeted properly and calls have
  to be admitted carefully so that end-to-end delay
  bounds are within the acceptable limit

60
References

• Goode B., Voice over Internet Protocol Proc.
  of IEEE, vol. 90, no. 9, Septmember 2002.
• 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.

61
References

• Sobrinho J.L., Krishnakumar A.S., Real-time
  Traffic over the IEEE802.11 Medium Access Control
  Layer Bell Labs Technical Journal (1996), pp.
  172-187.
• 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.
• Perkins C.E, Mobile IP Tutorials,
  http//www.computer.org/internet/v2n1/perkins.htm
  r30
• Schulzrinne H., Wedland E., Application-layer
  mobility using SIP ACM SIGMOBILE Mobile
  Computing and Communications Review, vol. 4, no.
  3, July 2000, pp. 47-57.