VOIP over Wireless
Network
Prof. Anirudha Sahoo
KReSIT
IIT Bombay
IIT Bombay
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
17 July 2016
IIT Bombay
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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-toend)
• Provides a toll bypass path for telephone
calls
• Enables Telephony providers to provide
cheaper service
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VOIP System
PSTN gateway
IP Network
PSTN Network
PSTN gateway
gatekeeper
PBX
PBX
typical
PSTN
system)
(A (A
typical
VOIP
system)
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VOIP System (cont.)
IP Network
CPE router
CPE router
LAN
LAN
SIP proxy
PSTN Gateway
PSTN
IP phone
Soft phone
17 July 2016
(Another VOIP system)
IIT Bombay
IP phone
5
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
17 July 2016
IIT Bombay
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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
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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)
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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
– > 400ms : unacceptable
• Thus packet delay is a very important QoS
parameter in VOIP system for an
acceptable telephone conversation
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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
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Network QoS
• Can be provided by few approaches
–
–
–
–
Engineering the network
IntServ
DiffServ
MPLS-based
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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.
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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)
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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
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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.
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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
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VOIP in Wired Network
RSVP/Diffserv/MPLS/
Engineered Network
PSTN gateway
PBX
IP Network
gatekeeper
PSTN gateway
PBX
(Delay bounded VOIP system)
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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
17 July 2016
IIT Bombay
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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
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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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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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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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DCF (cont.)
CWi  2
k i
1
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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DCF Timing diagram
DIFS
Src
Data
SIFS
Dest
Ack
DIFS
Contention Window
Next MPDU
Others
Defer Access
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Backoff after Defer
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DCF Example
B1 = 25
B1 = 5
wait
data
data
B2 = 20
cw = 31
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wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
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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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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
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VOIP over Wireless (cont.)
IP Network
CPE router
SIP proxy
CPE router
PSTN Gateway
PSTN
Soft phone
Mobile IP phone
Mobile IP phone
(A VOIP over Wireless System)
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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
17 July 2016
IIT Bombay
29
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
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Enhanced DCF
• 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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Access Category (AC)
AC1
AC2
AC3
Backoff
AIFS[1]
BO[1]
Backoff
AIFS[2]
BO[2]
Backoff
AIFS[3]
BO[3]
17 July 2016
AC0
Backoff
AIFS[0]
BO[0]
• 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
Virtual Collision Handler
Transmission
Attempt
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Differentiated Channel
Access
• Each AC contends with
– AIFS[AC] (instead of DIFS) and CWmin[AC],
CWmax[AC] (instead of CWmin, CWmax)
Immediate access when
medium is idle >=
AIFS[AC]+SlotTime
AIFS[AC]
+SlotTime
Busy
Medium
AIFS[AC]
PIFS
SIFS
Contention Window
from [1,1+CWmin[AC]]
Backoff
Window
Next Frame
SlotTime
Defer Access
17 July 2016
Select Slot and decrement backoff
as long as medium stays idle
IIT Bombay
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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
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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
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Distributed Fair Scheduling
(cont.)


Lki  
Bi      scaling _ factor   
i  


• 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
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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
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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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WTRP (cont.)
seq = 1
F
seq=2
A
Seq=3
unknown
seq=4
unknown
seq=5
D
BB
B
A
CC
F
E
D
Transmission range of E
Connectivity table of E
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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
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Blackburst
• Devised with a view to minimizing delay for realtime 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
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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
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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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VoW
RSVP/Diffserv/MPLS/
Engineered network
IP Network
CPE router
CPE router
EDCF/DFS/
WTRP
EDCF/DFS/
WTRP
SIP proxy
PSTN Gateway
PSTN
Mobile IP phone
Soft phone
Mobile IP phone
(Delay bounded VoW system)
17 July 2016
IIT Bombay
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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
17 July 2016
IIT Bombay
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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
17 July 2016
IIT Bombay
46
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)
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Mobility
Internet
Hot Spot B
Hot Spot A
AP
Micro mobility
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AP
AP
Macro mobility
AP
Micro mobility
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Mobility
• Two approaches available:
– Mobile IP
• handoff at network layer
– SIP
• handoff at the application layer
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Handoff using Mobile IP
• 3 Parts of Mobile IP
– Advertising Care-of Addresses
– Registration
– Tunneling
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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
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Registration
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Tunneling
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Handoff using SIP
• Two scenarios
– Pre-call mobility
– Mid-call mobility
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Pre-call mobility
(2)INVITE
Correspondent
node
Home Network
SIP
server
(3) 302 moved
temporarily
(5) 200 OK
(4) INVITE
(1) Registration of
New contact with
registrar
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Visited network
Mobile
node
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Mid-call mobility
Home Network
Correspondent
node
SIP
server
(1) re-INVITE
(2) 200 OK
Visited network
Mobile
node
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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
17 July 2016
IIT Bombay
57
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
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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
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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.
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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.
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