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VOIP over Wireless Network. Prof. Anirudha Sahoo KReSIT 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. Voice Over IP (VOIP).

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voip over wireless network

VOIP over Wireless Network

Prof. Anirudha Sahoo

KReSIT

IIT Bombay

outline
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
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 system

PSTN Network

IP Network

PSTN gateway

PSTN gateway

gatekeeper

VOIP System

PBX

PBX

(A typical PSTN system)

(A typical VOIP system)

voip system cont
VOIP System (cont.)

IP Network

CPE router

CPE router

LAN

LAN

PSTN Gateway

SIP proxy

PSTN

Soft phone

IP phone

IP phone

(Another VOIP system)

outline1
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
qos in voip
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
delay
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)
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 cont1
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 qos
Network QoS
  • Can be provided by few approaches
    • Engineering the network
    • IntServ
    • DiffServ
    • MPLS-based
network qos engineering the network
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.
voip qos intserv
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)
voip qos diffserv
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
voip qos mpls
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.
voip qos summary
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
voip in wired network
VOIP in Wired Network

RSVP/Diffserv/MPLS/

Engineered Network

IP Network

PSTN gateway

PSTN gateway

gatekeeper

PBX

PBX

(Delay bounded VOIP system)

outline2
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
wireless network
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
ieee 802 11 network
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)
slide21
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)
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 cont1
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.
dcf timing diagram
DCF Timing diagram

DIFS

Src

Data

SIFS

Dest

Ack

DIFS

Contention Window

Others

Next MPDU

Backoff after Defer

Defer Access

dcf example

B1 = 25

B1 = 5

wait

data

data

wait

B2 = 10

B2 = 20

B2 = 15

B1 and B2 are backoff intervals

at nodes 1 and 2

cw = 31

DCF Example
pcf point coordination function
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
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.)

IP Network

CPE router

CPE router

PSTN Gateway

SIP proxy

PSTN

Soft phone

Mobile IP phone

Mobile IP phone

(A VOIP over Wireless System)

outline3
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
voip over wireless cont1
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 dcf
Enhanced 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

A

A

A

A

B

B

B

B

I

I

I

I

B

B

B

B

a

a

a

a

F

F

F

F

c

c

c

c

O

O

O

O

S

S

S

S

[

[

[

[

k

k

k

k

[

[

[

[

o

o

o

o

0

1

2

3

0

1

2

3

f

f

f

f

]

]

]

]

]

]

]

]

f

f

f

f

Virtual Collision Handler

Transmission

Attempt

differentiated channel access
Differentiated Channel Access
  • Each AC contends with
    • AIFS[AC] (instead of DIFS) and CWmin[AC], CWmax[AC] (instead of CWmin, CWmax)
distributed fair scheduling dfs
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
distributed fair scheduling cont
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
wireless token ring protocol
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
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 cont1

B

B

B

A

C

C

F

E

D

Transmission range of E

WTRP (cont.)

Connectivity table of E

wtrp cont2
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
blackburst
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
blackburst1
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
blackburst2
Blackburst
  • After successful transmission of a frame, the station schedules the next access instant tschseconds 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
slide44
VoW

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)

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
outline4
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
mobility
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)
mobility1
Mobility

Internet

Hot Spot B

Hot Spot A

AP

AP

AP

AP

Micro mobility

Macro mobility

Micro mobility

mobility2
Mobility
  • Two approaches available:
    • Mobile IP
      • handoff at network layer
    • SIP
      • handoff at the application layer
handoff using mobile ip
Handoff using Mobile IP
  • 3 Parts of Mobile IP
    • Advertising Care-of Addresses
    • Registration
    • Tunneling
mobile ip
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
handoff using sip
Handoff using SIP
  • Two scenarios
    • Pre-call mobility
    • Mid-call mobility
pre call mobility
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

mid call mobility
Mid-call mobility

Correspondent

node

Home Network

SIP

server

(2) 200 OK

(1) re-INVITE

Visited network

Mobile

node

outline5
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
summary
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
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
references
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.
references1
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.