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IIT Bombay
VOIP over Wireless VOIP over Wireless NetworkNetwork
Prof. Anirudha SahooProf. Anirudha SahooKReSITKReSIT
IIT BombayIIT Bombay
April 10, 2023 2IIT Bombay
OutlineOutline• 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
April 10, 2023 3IIT Bombay
Voice Over IP (VOIP)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
April 10, 2023 4IIT Bombay
VOIP SystemVOIP System
IP NetworkPSTN gateway
PBXgatekeeper
PBX
PSTN gateway
(A typical VOIP system)
PSTN Network
(A typical PSTN system)
April 10, 2023 5IIT Bombay
VOIP System (cont.)VOIP System (cont.)
IP NetworkCPE router
SIP proxy
CPE router
(Another VOIP system)
LANLAN
IP phone IP phone
PSTN
PSTN Gateway
Soft phone
April 10, 2023 6IIT Bombay
OutlineOutline• 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
April 10, 2023 7IIT Bombay
QoS in VOIPQoS 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
April 10, 2023 8IIT Bombay
DelayDelay• 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)
April 10, 2023 9IIT Bombay
Delay (cont.)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
April 10, 2023 10IIT Bombay
Delay (cont.)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
April 10, 2023 11IIT Bombay
Network QoSNetwork QoS• Can be provided by few approaches
– Engineering the network– IntServ– DiffServ– MPLS-based
April 10, 2023 12IIT Bombay
Network QoS : Engineering Network QoS : Engineering the networkthe 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.
April 10, 2023 13IIT Bombay
VOIP QoS : IntservVOIP 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)
April 10, 2023 14IIT Bombay
VOIP QoS : DiffservVOIP 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
April 10, 2023 15IIT Bombay
VOIP QoS : MPLSVOIP 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.
April 10, 2023 16IIT Bombay
VOIP QoS : SummaryVOIP 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
April 10, 2023 17IIT Bombay
VOIP in Wired NetworkVOIP in Wired Network
IP NetworkPSTN gateway
PBXgatekeeper PBX
PSTN gateway
RSVP/Diffserv/MPLS/Engineered Network
(Delay bounded VOIP system)
April 10, 2023 18IIT Bombay
OutlineOutline• 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
April 10, 2023 19IIT Bombay
Wireless NetworkWireless 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
April 10, 2023 20IIT Bombay
IEEE 802.11 networkIEEE 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)
April 10, 2023 21IIT Bombay
DCFDCF• 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)
April 10, 2023 22IIT Bombay
DCF (cont.)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
April 10, 2023 23IIT Bombay
DCF (cont.)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.
12 ikiCW
April 10, 2023 24IIT Bombay
DCF Timing diagramDCF Timing diagram
Ack
Data
Next MPDU
Src
Dest
Others
Contention Window
Defer Access Backoff after Defer
DIFS
SIFS
DIFS
April 10, 2023 25IIT Bombay
DCF ExampleDCF Example
data
waitB1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2
cw = 31
B2 = 10
April 10, 2023 26IIT Bombay
PCFPCF(Point Coordination Function)(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
April 10, 2023 27IIT Bombay
VOIP over Wireless (VoW)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
April 10, 2023 28IIT Bombay
VOIP over Wireless (cont.)VOIP over Wireless (cont.)
IP NetworkCPE router
SIP proxy
CPE router
(A VOIP over Wireless System) Mobile IP phone
Mobile IP phone
PSTN
PSTN Gateway
Soft phone
April 10, 2023 29IIT Bombay
OutlineOutline• 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
April 10, 2023 30IIT Bombay
VOIP over Wireless (cont.) 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
April 10, 2023 31IIT Bombay
Enhanced DCFEnhanced DCF• Provides service differentiation• Traffic can be classified into 8 different
classes• Each station has 4 access categories to
provide service differentiation
April 10, 2023 32IIT Bombay
Access Category (AC)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
Bac ko
ff A
IFS [0]
BO
[0]
Bac ko
ff A
IFS [1]
BO
[1]
Bac ko
ff A
IFS [2]
BO
[2]
Bac ko
ff A
IFS [3]
BO
[3]
Transmission Attempt
April 10, 2023 33IIT Bombay
Differentiated Channel Differentiated Channel AccessAccess
• Each AC contends with – AIFS[AC] (instead of DIFS) and CWmin[AC],
CWmax[AC] (instead of CWmin, CWmax)
BusyMedium
SIFS
PIFS
AIFS[AC]
BackoffWindow
SlotTime
Defer Access Select Slot and decrement backoffas long as medium stays idle
AIFS[AC]+SlotTime
Contention Windowfrom [1,1+CWmin[AC]]
Immediate access whenmedium is idle >=AIFS[AC]+SlotTime
Next Frame
April 10, 2023 34IIT Bombay
Priority to AC MappingPriority to AC MappingPriority 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
April 10, 2023 35IIT Bombay
Distributed Fair Scheduling Distributed Fair Scheduling (DFS)(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
April 10, 2023 36IIT Bombay
Distributed Fair Scheduling Distributed Fair Scheduling (cont.)(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
ki
i
LfactorscalingB
_
April 10, 2023 37IIT Bombay
Wireless Token Ring Wireless Token Ring ProtocolProtocol
• 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
April 10, 2023 38IIT Bombay
WTRP (cont.)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
April 10, 2023 39IIT Bombay
WTRP (cont.)WTRP (cont.)
A
BB
CC
B
DE
F
Transmission range of E
seq = 1 F
seq=2 A
Seq=3 unknown
seq=4 unknown
seq=5 D
Connectivity table of E
April 10, 2023 40IIT Bombay
WTRP (cont.)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
April 10, 2023 41IIT Bombay
BlackburstBlackburst• 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
April 10, 2023 42IIT Bombay
BlackburstBlackburst• 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
April 10, 2023 43IIT Bombay
BlackburstBlackburst• 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
April 10, 2023 44IIT Bombay
VoW VoW
IP NetworkCPE router
SIP proxy
CPE router
(Delay bounded VoW system)
Mobile IP phone
Mobile IP phonePSTN
PSTN Gateway
Soft phone
RSVP/Diffserv/MPLS/Engineered network
EDCF/DFS/WTRP
EDCF/DFS/WTRP
April 10, 2023 45IIT Bombay
VoW (cont.)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
April 10, 2023 46IIT Bombay
OutlineOutline• 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
April 10, 2023 47IIT Bombay
MobilityMobility• 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)
April 10, 2023 48IIT Bombay
MobilityMobility
Hot Spot A Hot Spot B
Micro mobility Micro mobilityMacro mobility
AP APAP AP
Internet
April 10, 2023 49IIT Bombay
MobilityMobility• Two approaches available:
– Mobile IP• handoff at network layer
– SIP• handoff at the application layer
April 10, 2023 50IIT Bombay
Handoff using Mobile IPHandoff using Mobile IP• 3 Parts of Mobile IP
– Advertising Care-of Addresses– Registration– Tunneling
April 10, 2023 51IIT Bombay
Mobile IPMobile 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
April 10, 2023 52IIT Bombay
RegistrationRegistration
April 10, 2023 53IIT Bombay
TunnelingTunneling
April 10, 2023 54IIT Bombay
Handoff using SIPHandoff using SIP• Two scenarios
– Pre-call mobility– Mid-call mobility
April 10, 2023 55IIT Bombay
Pre-call mobilityPre-call mobility
SIP server
Mobile
node
Correspondentnode
Visited network(1) Registration ofNew contact withregistrar
(2)INVITE
(3) 302 movedtemporarily
(4) INVITE (5) 200 OK
Home Network
April 10, 2023 56IIT Bombay
Mid-call mobilityMid-call mobility
SIP server
Mobile
node
Correspondentnode
Visited network
(1) re-INVITE (2) 200 OK
Home Network
April 10, 2023 57IIT Bombay
OutlineOutline• 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
April 10, 2023 58IIT Bombay
SummarySummary• 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
April 10, 2023 59IIT Bombay
Summary (cont.)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
April 10, 2023 60IIT Bombay
ReferencesReferences• 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.
April 10, 2023 61IIT Bombay
ReferencesReferences• 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.