2. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 4
VOIP System
IP Network
PSTN gateway
PBX
gatekeeper
PBX
PSTN gateway
(A typical VOIP system)
PSTN Network
(A typical PSTN system)
5. IIT Bombay
2 May 2023 5
VOIP System (cont.)
IP Network
CPE router
SIP proxy
CPE router
(Another VOIP system)
LAN
LAN
IP phone IP phone
PSTN
PSTN Gateway
Soft phone
6. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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
– > 400ms : unacceptable
• Thus packet delay is a very important QoS
parameter in VOIP system for an
acceptable telephone conversation
10. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 11
Network QoS
• Can be provided by few approaches
– Engineering the network
– IntServ
– DiffServ
– MPLS-based
12. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 17
VOIP in Wired Network
IP Network
PSTN gateway
PBX
gatekeeper PBX
PSTN gateway
RSVP/Diffserv/MPLS/
Engineered Network
(Delay bounded VOIP system)
18. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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.
1
2
i
k
i
CW
24. IIT Bombay
2 May 2023 24
DCF Timing diagram
Ack
Data
Next MPDU
Src
Dest
Others
Contention Window
Defer Access Backoff after Defer
DIFS
SIFS
DIFS
25. IIT Bombay
2 May 2023 25
DCF Example
data
wait
B1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervals
at nodes 1 and 2
cw = 31
B2 = 10
26. IIT Bombay
2 May 2023 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 < DIFS)
• During PCF mode,
– PC polls each station for data
– After a transmission of a MPDU, move on to the next
station
27. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 28
VOIP over Wireless (cont.)
IP Network
CPE router
SIP proxy
CPE router
(A VOIP over Wireless System)
Mobile IP phone
Mobile IP phone
PSTN
PSTN Gateway
Soft phone
29. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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
Virtual Collision Handler
Backoff
AIFS[0]
BO[0]
Backoff
AIFS[1]
BO[1]
Backoff
AIFS[2]
BO[2]
Backoff
AIFS[3]
BO[3]
Transmission
Attempt
33. IIT Bombay
2 May 2023 33
Differentiated Channel
Access
• Each AC contends with
– AIFS[AC] (instead of DIFS) and CWmin[AC],
CWmax[AC] (instead of CWmin, CWmax)
Busy
Medium
SIFS
PIFS
AIFS[AC]
Backoff
Window
SlotTime
Defer Access Select Slot and decrement backoff
as long as medium stays idle
AIFS[AC]
+SlotTime
Contention Window
from [1,1+CWmin[AC]]
Immediate access when
medium is idle >=
AIFS[AC]+SlotTime
Next Frame
34. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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
i
k
i
i
L
factor
scaling
B
_
37. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 39
WTRP (cont.)
A
B
B
C
C
B
D
E
F
Transmission range of E
seq = 1 F
seq=2 A
Seq=3 unknown
seq=4 unknown
seq=5 D
Connectivity table of E
40. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 44
VoW
IP Network
CPE router
SIP proxy
CPE router
(Delay bounded VoW system)
Mobile IP phone
Mobile IP phone
PSTN
PSTN Gateway
Soft phone
RSVP/Diffserv/MPLS/
Engineered network
EDCF/DFS/
WTRP
EDCF/DFS/
WTRP
45. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 48
Mobility
Hot Spot A Hot Spot B
Micro mobility Micro mobility
Macro mobility
AP AP
AP AP
Internet
49. IIT Bombay
2 May 2023 49
Mobility
• Two approaches available:
– Mobile IP
• handoff at network layer
– SIP
• handoff at the application layer
50. IIT Bombay
2 May 2023 50
Handoff using Mobile IP
• 3 Parts of Mobile IP
– Advertising Care-of Addresses
– Registration
– Tunneling
51. IIT Bombay
2 May 2023 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
54. IIT Bombay
2 May 2023 54
Handoff using SIP
• Two scenarios
– Pre-call mobility
– Mid-call mobility
55. IIT Bombay
2 May 2023 55
Pre-call mobility
SIP
server
Mobile
node
Correspondent
node
Visited network
(1) Registration of
New contact with
registrar
(2)INVITE
(3) 302 moved
temporarily
(4) INVITE
(5) 200 OK
Home Network
56. IIT Bombay
2 May 2023 56
Mid-call mobility
SIP
server
Mobile
node
Correspondent
node
Visited network
(1) re-INVITE
(2) 200 OK
Home Network
57. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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. IIT Bombay
2 May 2023 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.
