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
18 September 2018

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
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4. VOIP System (A typical PSTN system) (A typical VOIP system) IP Network
PSTN gateway
IP Network
PSTN gateway
PSTN Network
gatekeeper
PBX
PBX
(A typical PSTN system)
(A typical VOIP system)
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5. VOIP System (cont.) (Another VOIP system) IP Network CPE router
LAN
LAN
PSTN Gateway
SIP proxy
PSTN
Soft phone
IP phone
IP phone
(Another VOIP system)
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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
18 September 2018

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
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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)
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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
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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
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11. Network QoS Can be provided by few approaches Engineering the network
IntServ
DiffServ
MPLS-based
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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.
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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)
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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
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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.
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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
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17. VOIP in Wired Network (Delay bounded VOIP system) RSVP/Diffserv/MPLS/
Engineered Network
IP Network
PSTN gateway
PSTN gateway
gatekeeper
PBX
PBX
(Delay bounded VOIP system)
18 September 2018

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
18 September 2018

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
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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)
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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)
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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
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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.
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24. DCF Timing diagram Src Dest Others 18 September 2018 Data Ack
DIFS
Src
Data
SIFS
Dest
Ack
DIFS
Contention Window
Others
Next MPDU
Defer Access
Backoff after Defer
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25. 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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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
18 September 2018

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 products do not have PCF implementation
Delay can be large when too many stations have data to send in CFP
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28. VOIP over Wireless (cont.)
IP Network
CPE router
CPE router
PSTN Gateway
SIP proxy
PSTN
Soft phone
Mobile IP phone
Mobile IP phone
(A VOIP over Wireless System)
18 September 2018

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
18 September 2018

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
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31. 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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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 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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33. 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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34. 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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35. Distributed Fair Scheduling (DFS)
Based on SCFQ
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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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
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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
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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
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39. 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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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
18 September 2018

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
18 September 2018

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
18 September 2018

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
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44. VoW (Delay bounded VoW system) 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)
18 September 2018

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
18 September 2018

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
18 September 2018

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)
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48. Mobility Internet Hot Spot B Hot Spot A AP AP AP AP Micro mobility
Macro mobility
Micro mobility
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49. Mobility Two approaches available: Mobile IP SIP
handoff at network layer
SIP
handoff at the application layer
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50. Handoff using Mobile IP
3 Parts of Mobile IP
Advertising Care-of Addresses
Registration
Tunneling
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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
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52. Registration
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53. Tunneling
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54. Handoff using SIP Two scenarios Pre-call mobility Mid-call mobility
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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
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56. Mid-call mobility Correspondent Home Network node SIP server
(2) 200 OK
(1) re-INVITE
Visited network
Mobile
node
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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
18 September 2018

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 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
18 September 2018

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
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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 /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.
18 September 2018

61. References
Sobrinho J.L., Krishnakumar A.S., “Real-time Traffic over the IEEE Medium Access Control Layer” – Bell Labs Technical Journal (1996), pp
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
Perkins C.E, “Mobile IP Tutorials”,
Schulzrinne H., Wedland E., “Application-layer mobility using SIP” – ACM SIGMOBILE Mobile Computing and Communications Review, vol. 4, no. 3, July 2000, pp
18 September 2018