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IIT Bombay VOIP over Wireless Network
 

IIT Bombay VOIP over Wireless Network

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    IIT Bombay VOIP over Wireless Network IIT Bombay VOIP over Wireless Network Presentation Transcript

    • 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)
      • 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 PBX PBX (A typical VOIP system) (A typical PSTN system) IP Network PSTN gateway gatekeeper PSTN gateway PSTN Network
    • 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
    • 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
      • 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
      • 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.)
      • 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.)
      • 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
      • Can be provided by few approaches
        • Engineering the network
        • IntServ
        • DiffServ
        • MPLS-based
    • 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
      • 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
      • 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
      • 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
      • 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 IP Network PSTN gateway PBX gatekeeper PBX PSTN gateway RSVP/Diffserv/MPLS/ Engineered Network (Delay bounded VOIP system)
    • 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 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
      • 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)
    • 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.)
      • 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.)
      • 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 Ack Data Next MPDU Src Dest Others Contention Window Defer Access Backoff after Defer DIFS SIFS DIFS
    • 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
    • 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)
      • 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.) IP Network CPE router SIP proxy CPE router (A VOIP over Wireless System) Mobile IP phone Mobile IP phone PSTN PSTN Gateway Soft phone
    • 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 (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
      • 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) 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 B a c k o f f A I F S [ 0 ] B O [ 0 ] B a c k o f f A I F S [ 1 ] B O [ 1 ] B a c k o f f A I F S [ 2 ] B O [ 2 ] B a c k o f f A I F S [ 3 ] B O [ 3 ] Transmission Attempt
    • Differentiated Channel Access
      • Each AC contends with
        • AIFS[AC] (instead of DIFS) and CWmin[AC], CWmax[AC] (instead of CWmin, CWmax)
    • Priority to AC Mapping Voice 3 7 Voice 3 6 Video 2 5 Video 2 4 Video Probe 1 3 Best Effort 0 2 Best Effort 0 1 Best Effort 0 0 Designation (Informative) Access Category (AC) Priority
    • 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.)
      • 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 (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.)
      • 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.) Connectivity table of E A B B C C B D E F Transmission range of E D seq=5 unknown seq=4 unknown Seq=3 A seq=2 F seq = 1
    • 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
      • 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
    • 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
    • Blackburst
      • After successful transmission of a frame, the station schedules the next access instant t sch 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 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
    • 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
    • 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 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)
    • Mobility Hot Spot A Hot Spot B Micro mobility Micro mobility Macro mobility AP AP AP AP Internet
    • Mobility
      • Two approaches available:
        • Mobile IP
          • handoff at network layer
        • SIP
          • handoff at the application layer
    • Handoff using Mobile IP
      • 3 Parts of Mobile IP
        • Advertising Care-of Addresses
        • Registration
        • Tunneling
    • 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
    • Registration
    • Tunneling
    • Handoff using SIP
      • Two scenarios
        • Pre-call mobility
        • Mid-call mobility
    • 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
    • Mid-call mobility SIP server Mobile node Correspondent node Visited network (1) re-INVITE (2) 200 OK Home Network
    • 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
      • 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.)
      • 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
      • 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 8 th 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.
    • 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.