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Ensuring QoS in Your VoIP Development

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  • 1. Ensuring QoS in Your VoIP Development Choon Shim CTO and Senior VP of Engineering [email_address] , http://www.qovia.com
  • 2. VoIP problems
    • Outage:
    • - Infrastructure: switch, router, bridge, UPS, etc
    • - VoIP element: call server, SIP server, GW, GK, MCU, handsets.
    • - Carrier: T1/E1, analog signal trunk lines.
    • Voice Quality:
    • - Delay: network bandwidth, processing power
    • - Echo: hybrid, acoustic
    • - Jitter: jitter buffer calculation, variable delay
    • - Packet loss: sender base, receiver base
    • - Out of order: complex topology
  • 3. Roots of the problems
    • IP is not designed for carrying real-time media stream.
    • Management was not considered by System/Elements Vendors.
    • Too many moving parts.
    • Too many protocol layers.
    • Too many API layers.
    • Multi vendor products.
  • 4. VoIP ready network
    • Fast network: low latency and jitter
    • Clean network: few packet loss and retransmit
    • QoS ready network: Voice packet has priority
    • Fault tolerant network: redundancy and backup
    • Manageable network: monitoring and management
  • 5. Bandwidth
    • Required bandwidth per call (bps):
    • BW = (V + I + L) * 8 * P
    • Where, V is size of voice sample,
    • I is IP/UDP/RTP overhead,
    • L is data link overhead and
    • P is packets generated per second.
    • Example) (160 + 40 + 18) * 8 * 50 = 87.2 kbps
    • Required bandwidth total:
    • Total BW = BW * N
    • Where, N is total number of simultaneous calls
    • Example) 87.2 * 50 = 4.36 Mbps
    • => Increase bandwidth
  • 6. To reduce bandwidth requirement
    • Bandwidth requirement by codec and duplex
    • cRTP reduces 2-5 bytes overhead
    • VAD reduces up to 50% payload
    29.4 87.2 110.4 Full duplex 78.4 174.4 220.8 Half duplex G.729 (20 ms) G.711 (20 ms) G.711 (10 ms) Link Type (Sample time)
  • 7. Clean network
    • Reduce hop counts
    • Reduce complexity of network topology
    • Remove duplex mismatch
    • Remove black hole and loop
    • Avoid half duplex link
    • Use common sense for cabling
  • 8. QoS ready network
    • Layer 3:
      • Type of Service (TOS)
      • RSVP signaling (RFC 2205)
      • DiffServ (RFC2474)
      • Multiprotocol Label Switching (MPLS)
    • Layer 2:
    • - 802.1p and 802.1q
    • - Ethernet Class of Service (COS)
  • 9. Fault tolerant network - Outage detection
    • Carrier failure: T1, E1, Analog
    • - No incoming or outgoing calls.
    • - Checking the module LED.
    • - Checking the event log, management console.
    • - Running a loop back test for T1/E1.
    • - Checking with T1 tester.
    • - Receiving an alarm from the call server.
  • 10. Fault tolerant network – Outage detection (cont)
    • Infrastructure failure:
    • - No dial tone or bad voice quality
    • - Checking NMS console
    • - Checking SNMP Traps
    • - Testing cables
    • - Testing switches, routers, bridges, etc
    • - Checking UPS power load, power level, connection
  • 11. Fault tolerant network – Outage detection (cont)
    • VoIP element failure:
    • - No dial tone.
    • - Checking SNMP trap.
    • - Checking NMS console.
    • - Checking with the vendor management console.
    • - Checking event log, trace, etc.
  • 12. Outage detection issues
    • Lack of alarm implementation.
    • Too many consoles to monitor: NMS, vendor supplied management, third party software, carrier OSS/EOSS.
    • Too many elements could go wrong.
    • Carriers are not monitoring the CSU or CPE.
  • 13. Alarm – Event driven Switch Router T1/E1 UPS GK GW TE Bridge Analog Environ Server Management Server SNMP Trap Email/Pager Carrier console VoIPm Console
  • 14. Checking vital signs
    • Blind polling: send a ping to every elements every x minutes. It triggers extra network traffics. Total number of packets per hour N = e * x / 60, where e = number of elements, x is minutes.
    • Severity base polling: send a ping to critical elements more often. For example) GW: every 5 mins, GK: every 6 mins, Switch: every 10 mins, Terminal element: 30 mins, etc.
    • Dynamic polling: recalculates number of pings based on the previous faults, traffic or volume. Number of packets N =  f(1)..f(e), where f is the function being used for calculating faults, traffic and volume.
  • 15. Manageable VoIP Network – Voice quality measurement
    • MOS (Mean Opinion Score):
    • - Subjective measurement of VoIP.
    • - Pre selected voice sample over different media, replayed to mixed group of men and woman, who rate them from 1 to 5.
    • 4 – 5: Toll Quality
    • 3 – 4: Communication quality
    • < 3 : Synthetic quality
  • 16. Voice quality measurement
    • PSQM (Perceptual Speech Quality Measurement, ITU-T P.861):
    • - A utomated scoring process using an algorithm that enables computer-derived scores to correlate to MOS scores.
    • - Designed for circuit-switched network and does not take into effect important parameters such as jitter and packet loss, which affect voice quality on a VOIP network adversely.
  • 17. Voice quality measurement
    • PAMS (Perceptual Analysis and Measurement System):
    • - Designed an intrusive listening speech quality assessment tool where speech quality is computed by injecting a speech like signal at one end and analysing the degraded signal at other end of the network.
    Parameter SCORE 1 2 3 4 5 Latency (ms) <50 50-75 75-100 100-200 >200 Packet/Loss (%) 0 0-1 1-2 2-3 >3 Jitter (ms) <5 5-10 10-50 50-100 >100
  • 18. Voice quality measurement
    • PESQ : PESQ (ITU–T P.862):
    • - The latest standard for assessing voice quality and is expected to eventually replace PSQM.
    • - It builds on the PSQM and PAMS algorithms by adding additional processing steps to account for signal-level differences and the identification of errors associated with packet loss.
  • 19. Voice quality measurement
    • Delay guideline: ITU-T G.114
    Acceptable Acceptable under conditions Unacceptable 0 150 400 0 – 150 ms: Good quality and no echo 151 – 400 ms: Acceptable under certain conditions and echo canceling is needed 401+: Unacceptable for real-time voice traffic and planning and testing purposes only
  • 20. Quality problem detection
    • Interpret RTCP and RTCP XR.
    • Packet monitoring by Layer2 switch taping or port mirroring.
    • Probing and active monitoring by injecting a test packet.
    • SNMP, RMON or sFlow gathering.
  • 21. Problem isolation procedure Central QoS server RTCP Packet monitoring SNMP ALARM Console VoIP Network 1 2 3 4 5 6
  • 22. Central QoS management server
    • Discover VoIP components/elements dynamically.
    • Create a topology and aggregate multiple call servers, GW, GK, MCU, SIP Servers, etc.
    • Collect performance/delay data from various sources.
    • Calculate variable polling period and injects an active packet.
    • Make a statistical model to use for assign QoS.
    • Organize elements/QoS data in the relational DBMS.
    • Detect voice quality problem and send an alarm to console.
    • Inject an active test packet to isolate the problem as per console.
  • 23. Console
    • Display overall call quality.
    • Display topology and status display.
    • Display drill down to detail elements with MOS/PESQ.
    • Display real time status and quality changes.
    • Trigger the problem isolation procedure.
  • 24. Q & A
    • Thank you!
    • Any questions?