IIT Bombay VOIP over Wireless Network

IIT Bombay

VOIP over Wireless VOIP over Wireless NetworkNetwork

Prof. Anirudha SahooProf. Anirudha SahooKReSITKReSIT

IIT BombayIIT Bombay

April 12, 2010 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


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


VOIP SystemVOIP System

IP NetworkPSTN gateway

PBXgatekeeper

PBX

PSTN gateway

(A typical VOIP system)

PSTN Network

(A typical PSTN system)


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





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


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)


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


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


Network QoSNetwork QoS• Can be provided by few approaches

– Engineering the network– IntServ– DiffServ– MPLS-based


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.


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)


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


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.


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


VOIP in Wired NetworkVOIP in Wired Network

IP NetworkPSTN gateway

PBXgatekeeper PBX

PSTN gateway

RSVP/Diffserv/MPLS/Engineered Network

(Delay bounded VOIP system)





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


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)


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)


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


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


DCF Timing diagramDCF Timing diagram

Ack

Data

Next MPDU

Src

Dest

Others

Contention Window

Defer Access Backoff after Defer

DIFS

SIFS

DIFS


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


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


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


VOIP over Wireless (cont.)VOIP over Wireless (cont.)


SIP proxy

CPE router

(A VOIP over Wireless System) Mobile IP phone

Mobile IP phone

PSTN

PSTN Gateway

Soft phone





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


Enhanced DCFEnhanced DCF• Provides service differentiation• Traffic can be classified into 8 different

classes• Each station has 4 access categories to

provide service differentiation


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

Backo

ff AIF

S[0]

B

O[0]

Backo

ff AIF

S[1]

B

O[1]

Backo

ff AIF

S[2]

B

O[2]

Backo

ff AIF

S[3]

B

O[3]

Transmission Attempt


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


Priority to AC MappingPriority to AC Mapping

Voice37

Voice36

Video25

Video24

Video Probe13

Best Effort02

Best Effort01

Best Effort00

Designation (Informative)

Access Category (AC)

Priority


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


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

φρ _


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


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


WTRP (cont.)WTRP (cont.)

A

BB

CC

B

DE

F

Transmission range of EDseq=5

unknownseq=4

unknownSeq=3

Aseq=2

Fseq = 1

Connectivity table of E


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


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


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


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


VoW VoW


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


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





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)


MobilityMobility

Hot Spot A Hot Spot B

Micro mobility Micro mobilityMacro mobility

AP APAP AP

Internet


MobilityMobility• Two approaches available:

– Mobile IP• handoff at network layer

– SIP• handoff at the application layer


Handoff using Mobile IPHandoff using Mobile IP• 3 Parts of Mobile IP

– Advertising Care-of Addresses– Registration– Tunneling


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


RegistrationRegistration


TunnelingTunneling


Handoff using SIPHandoff using SIP• Two scenarios

– Pre-call mobility– Mid-call mobility


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


Mid-call mobilityMid-call mobility

SIP server

Mobile

node

Correspondentnode

Visited network

(1) re-INVITE (2) 200 OK

Home Network





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


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


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.


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.

http://www.computer.org/internet/v2n1/perkins.htm

IIT Bombay VOIP over Wireless Network

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