Quick Answer: Voice quality is measured by MOS (Mean Opinion Score): 4.0+ is excellent, 3.5-4.0 is good, below 3.0 is poor. Quality depends on ITU G.114 latency (<150ms one-way), jitter (<20ms), and packet loss (<1%). Broadband under 20Mb or contended connectivity will fail. Resellers must conduct pre-deployment QoS assessments and recommend upgrades to leased lines or SD-WAN policies when broadband is insufficient. Selling VoIP over poor connectivity damages customer satisfaction and your reputation.
QoS Thresholds and Service Level Targets
These thresholds define acceptable voice quality. Use them to assess customer readiness and set SLA commitments.
| QoS Parameter | Excellent (MOS 4.0+) | Good (MOS 3.5-3.99) | Acceptable (MOS 3.0-3.49) | Poor (MOS <3.0) | Measurement Method | Reseller Action |
|---|---|---|---|---|---|---|
| One-Way Latency (ITU G.114) | <150ms | 150-200ms | 200-250ms | >250ms | SIP packet round-trip time (RTT) / 2. Use ping or SIP ALG monitoring tools. | <150ms: OK to deploy. 150-250ms: Monitor and optimize. >250ms: Recommend VPN/SD-WAN or upgrade connectivity. |
| Jitter (Packet Delay Variation) | <20ms | 20-30ms | 30-50ms | >50ms | Measure variance in packet arrival times. Use VoIP monitoring tools (Cisco, SolarWinds, Avaya). | <20ms: OK. 20-50ms: QoS policies needed. >50ms: Significant issue; recommend network assessment. |
| Packet Loss (One-Way) | <0.5% | 0.5-1.0% | 1.0-3.0% | >3.0% | RTCP (RTP Control Protocol) feedback. Use VoIP monitoring or provider reports. | <1%: OK. 1-3%: Acceptable but edge case; monitor. >3%: Unacceptable; investigate and remediate. |
| Bandwidth Utilisation (Peak) | <50% of available capacity | 50-75% of available capacity | 75-85% of available capacity | >85% of available capacity | Monitor total outbound/inbound traffic during peak calling periods. Use NetFlow or provider dashboard. | <50%: Good headroom. 50-75%: Acceptable. 75-85%: Warn customer of congestion risk. >85%: Recommend upgrade or QoS policies. |
| MOS (Mean Opinion Score, calculated) | 4.0-4.5 (excellent perception) | 3.5-3.99 (good perception) | 3.0-3.49 (acceptable but noticeable quality loss) | <3.0 (poor; callers notice strain, delay, breaks) | Calculated from latency, jitter, packet loss, codec. Formula: ITU G.107. Tools: SolarWinds, NETSCOUT, Avaya Aura. | Target MOS 3.5+. Set SLA goal: average MOS >3.7. Alert on MOS <3.5. Customer satisfaction drops sharply below MOS 3.0. |
Codec Bandwidth and MOS Comparison
Codec choice affects bandwidth consumption and call quality. This table helps resellers right-size capacity and manage QoS trade-offs.
| Codec | Bandwidth per Call (kbps) | Bandwidth with IP/UDP Overhead (kbps) | MOS at Optimal Conditions | Sensitivity to Packet Loss | Sensitivity to Latency | Use Case and Reseller Recommendation |
|---|---|---|---|---|---|---|
| G.711 (μ-law, A-law) | 64 kbps | 87.2 kbps (with RTP, IP, UDP headers) | 4.13 (excellent) | Moderate. Can tolerate up to 1% loss without noticeable degradation. | Moderate. Latency visible over 200ms but acceptable to 300ms. | Default codec. Use for all customers with >20Mb broadband. Best quality if bandwidth available. PSTN interoperability. |
| G.729 (8 kbps) | 8 kbps | 31.2 kbps (with headers) | 3.92 (good) | High. Sensitive to packet loss. 1% loss = noticeable quality drop. Requires FEC or packet redundancy. | Moderate-High. Notices latency over 150ms. Use with low-latency networks. | Bandwidth-constrained networks. Satellite links, slow broadband (<10Mb). International calling to reduce PSTN costs. Not recommended for domestic SMEs with decent broadband. |
| GSM-AMR (Adaptive Multi-Rate) | 4.75-12.2 kbps variable | 25-35 kbps (with headers) | 3.70 (good to acceptable) | High (similar to G.729). Variable bitrate adapts to network congestion. | Moderate. Adaptive to network conditions. Good for poor networks. | Mobile VoIP over cellular data (4G, 5G). Desktop VoIP over poor home broadband. Niche use case. Resellers rarely deploy. |
| Opus (variable) | 6-128 kbps (adaptive bitrate) | 25-150 kbps (with headers and bitrate-dependent) | 4.0+ at 24 kbps; 4.5+ at 48+ kbps (excellent to superb) | Low. Very robust to packet loss due to FEC and redundancy built-in. | Low. Handles latency well up to 300ms+. | Future standard for cloud voice (Teams Direct Routing, WebRTC). Best-in-class quality over poor networks. Recommend for new deployments. Requires codec support in PBX/client. |
| iLBC (Internet Low Bitrate Codec) | 13-15 kbps | 33-35 kbps (with headers) | 3.7 (good to acceptable) | Low. Built-in error concealment. Good for packet loss resilience. | Low. Handles latency well. | Satellite and harsh networks. Open-source (royalty-free). Rarely used commercially. Not recommended unless specific requirement. |
SD-WAN QoS Policies for VoIP Prioritisation
SD-WAN (Software-Defined WAN) allows granular traffic prioritisation. This table maps QoS policies to VoIP performance improvements.
| SD-WAN QoS Policy | Configuration | Benefit to VoIP | Typical Implementation Cost | Reseller Positioning | Customer Use Case |
|---|---|---|---|---|---|
| Voice Traffic Prioritisation (DSCP EF) | Mark all SIP and RTP packets with DSCP Expedited Forwarding (EF, value 46). Prioritise in WAN queues. | VoIP gets priority over data. Jitter reduced. Latency more consistent. MOS improves 0.2-0.5 points in congested networks. | £0 (policy configuration only, if SD-WAN already deployed). £50-£150/month SaaS SD-WAN service if new deployment. | Bundle SD-WAN with VoIP service. Upsell value: “Guaranteed voice quality even during peak data usage.” | Organisations with heavy data usage (large file transfers, video conferencing, cloud apps) competing with voice for bandwidth. |
| Link Bonding and Failover (Dual WAN) | Configure primary broadband link and secondary 4G LTE link. SD-WAN automatically failover if primary fails. | Voice calls do not drop during primary link failure. Automatic switchover within 100-500ms (transparent to user). Resilience SLA: 99.9%+. | £100-£300/month (secondary 4G link cost) + £50-£100/month SD-WAN SaaS fee. | Premium offering. Position as “Resilience-as-a-Service” for critical sites (HQ, contact centre, healthcare). | Sites where VoIP downtime has high business cost (contact centre, hospital, financial services). Multiple calling dependencies. |
| Application-Based Routing (ABR) | Route VoIP traffic (SIP/RTP) over lowest-latency path. Route data (HTTP, email) over lowest-cost path. | VoIP always takes optimal path (lowest latency). Data takes cheaper path. Reduces carrier costs. MOS improvement: 0.3-0.8 points on multi-WAN setups. | £50-£150/month SD-WAN service (includes ABR feature). | Upsell to multi-WAN customers (office + backup MPLS). Highlight cost savings + quality improvement. | Organisations with dual-WAN (broadband + leased line). Want to optimise both for cost and quality. |
| Bandwidth Reservation (CAC, Call Admission Control) | Reserve fixed bandwidth for VoIP (e.g., 2Mb out of 20Mb broadband). If voice reaches limit, reject new calls rather than degrade existing calls. | Existing calls maintain quality. New call is rejected with SIP error; customer is notified. Better than all calls degrading. | £0 (policy configuration if SD-WAN exists). Requires careful sizing. | Positioning: “Guaranteed quality for all calls in progress. New calls fail gracefully.” Important for small-medium office. | SME offices with limited broadband (<20Mb) and many concurrent calls. Prioritise existing calls over new ones during congestion. |
| Packet Redundancy and FEC (Forward Error Correction) | Duplicate RTP packets or add FEC packets. If 1 packet is lost, can reconstruct from redundancy without retransmission. | Packet loss becomes invisible to codec. Tolerates 1-2% loss without quality impact. MOS improvement: 0.3-0.5 at high loss networks. | £0-£50/month (feature in advanced SD-WAN products). Some carriers offer FEC SIP trunk add-on (£20-£50/month). | Position for lossy networks (satellite, 4G backup links). Resellers in rural areas with poor broadband should consider this. | Sites with satellite backup, high packet loss links (>1%), or high-jitter networks (WiFi in rural areas). |
Frequently Asked Questions
What QoS assessment should I do before deploying VoIP?
Run a pre-deployment assessment: (1) Measure broadband speed and stability (ping, continuous speed test for 1 hour). (2) Identify concurrent call requirement (number of users x percentage on call simultaneously). (3) Calculate bandwidth needed: concurrent calls x 87 kbps (G.711) + 30% buffer for other traffic = minimum uplink and downlink required. (4) If measured bandwidth < required, recommend upgrade. (5) If jitter or latency high, recommend SD-WAN or leased line.
Why is my broadband 50Mb but VoIP quality poor?
Broadband speed is headline rate, not usable capacity. Actual throughput may be 30-40Mb. Quality also depends on jitter and packet loss. Cable broadband can have high jitter during peak hours. WiFi (especially shared home networks) can have poor latency. Test actual performance, not headline speed. Consider wired Ethernet, not WiFi.
Should I offer G.729 to save bandwidth?
Only if customer requests or bandwidth is severely constrained. G.729 requires licensing and quality is noticeable lower (MOS 3.92 vs 4.13 for G.711). If broadband is >20Mb, use G.711. G.729 is niche for satellite, expensive international calling, or very remote sites.
What SLA should I commit to for VoIP quality?
Typical SLA: “Average monthly MOS > 3.7 and 99% of calls > MOS 3.5. Availability: 99.5%.” If you can’t measure MOS, fall back to: “Latency < 150ms, jitter < 20ms, packet loss < 1%, measured monthly average.”
Do I need SD-WAN to deliver quality VoIP?
Not always. If customer has dedicated broadband line (not shared), low concurrent calling, and quiet network, standard VoIP works. SD-WAN adds value if: (1) broadband is shared (office doing heavy downloads), (2) multi-WAN failover needed, (3) latency or jitter is high. Position as optional premium add-on for quality assurance.
How do I explain MOS to customers?
Simple analogy: MOS 4.0+ = sounds like landline. MOS 3.5-3.9 = sounds like mobile phone, acceptable. MOS 3.0 = noticeable quality loss, strain to listen. MOS <3.0 = poor, frustrating. Most business customers are satisfied with MOS 3.5+.
Sources
Cisco QoS and VoIP Best Practices
ITU G.114 Latency and G.107 MOS Calculation Standards
IETF RTP and QoS Standards (RFC 3550, RFC 3551)
Ofcom QoS and Network Performance Standards
Microsoft Teams Phone QoS Guidelines
Partner Playbook: Pre-Deployment QoS Assessment Template
| Assessment Item | Measurement / Question | Target / Pass Criteria | Remediation If Fail | Status |
|---|---|---|---|---|
| Broadband Speed (Download) | Run speed test (speedtest.net or customer ISP dashboard). Record average over 3 tests. | > 20Mb for office of 10-50 users. Scale up 5Mb per 20 additional users. | Recommend upgrade to FTTP, cable, or leased line. Assess lead time. May delay VoIP deployment 4-6 weeks. | [ ] |
| Broadband Speed (Upload) | Run speed test (upload component). | > 5Mb for office of 10-50 users. Upload is voice-critical direction. | Recommend upgrade. Upload typically the constraint. Alternative: leased line with symmetric speeds. | [ ] |
| Latency (Ping) | Ping customer’s broadband gateway and ISP gateway. Average over 10 pings. | < 50ms to gateway. < 100ms to ISP. < 150ms to SIP trunk provider. | <150ms acceptable. 150-250ms: marginal; monitor. >250ms: investigate routing, consider VPN/SD-WAN. | [ ] |
| Jitter (Packet Delay Variation) | Run extended ping or use VoIP monitoring tool. Measure variance over 60 seconds. | < 20ms jitter. | 20-50ms: acceptable but watch. >50ms: network issue; recommend wired Ethernet (not WiFi), QoS policies, or upgrade connectivity. | [ ] |
| Packet Loss | Use VoIP monitoring tool or request from ISP. Measure during business hours (peak load). | < 0.5% packet loss. | 0.5-1%: acceptable. >1%: investigate and remediate. May indicate network congestion or QoS needed. | [ ] |
| Concurrent Call Requirement | Interview customer. Estimate peak concurrent calls (not total users). Assume 10-20% of staff in call simultaneously. | Right-size SIP trunk count and bandwidth. Example: 50 users x 15% = 7-8 concurrent calls x 87kbps = 610kbps + buffer. | If actual concurrent load exceeds available bandwidth, recommend SD-WAN QoS or upgrade broadband. | [ ] |
| Expected MOS Score (Calculated) | Use ITU G.107 calculator or vendor tool. Input: codec (G.711), latency, jitter, packet loss from above tests. | Expected MOS > 3.5 (good quality). Target MOS > 3.7 (excellent). | If MOS < 3.5, recommend: broadband upgrade, SD-WAN, codec change, or connectivity upgrade (leased line). | [ ] |
| Deployment Approval | Obtain customer sign-off on QoS assessment and any required remediation (broadband upgrade, SD-WAN, etc.). | Customer acknowledges QoS findings and approves deployment plan (as-is or with remediation). | If QoS insufficient, document delay and upgrade timeline in project plan. | [ ] |
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