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Over-the-Air VoIP Issues and Recommended Phases

Over-the-Air VoIP Issues and Recommended Phases. Raymond Hsu Tao Chen Joe Odenwalder Ed Tiedemann. Outline. Identify issues concerning VoIP over the air IP/UDP/RTP header compression Link layer overhead Capacity loss QoS Recommendation Phasing approach to VoIP over the air

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Over-the-Air VoIP Issues and Recommended Phases

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  1. Over-the-Air VoIP IssuesandRecommended Phases Raymond Hsu Tao Chen Joe Odenwalder Ed Tiedemann

  2. Outline • Identify issues concerning VoIP over the air • IP/UDP/RTP header compression • Link layer overhead • Capacity loss • QoS • Recommendation • Phasing approach to VoIP over the air • Requirements for VoIP over the air

  3. Header Compression Issues • A voice frame is transported over IP/UDP/RTP that has 40-byte of header. • RFC 2508 is capable of compressing the header to a minimum of 2 bytes. • RFC 2508 does not perform well on cellular links that lossy and slow. • As BER increases, the average size of the compressed header increases. For example, the average size is more than 5 bytes when BER is 10-2 [draft-jonsson-robust-hc-02.txt. • IETF formed the Robust Header Compression working group to develop header compression schemes for cellular links.

  4. Link Layer Issues • A VoIP packet is sent over the air in a link-layer frame. • The frame overhead should be minimized • The link-layer protocol provides a mechanism for the link-layer end-points to negotiate the capability and parameters of IP/UDP/RTP header compression. • Using PPP as an example: • Normally 8-byte overhead per frame. • Address and Control field can be null (RFC 1661) • Protocol field can be compressed to 1 byte (RFC 1661) • FCS field can be null (RFC 1570) • Trail Flag is not required (RFC 1662) • Achieve a minimum of 2-byte overhead per frame. • Negotiate IP/UDP/RTP header compression (RFC 2509)

  5. Capacity Issues: Rate Switching • 8-kbps vocoders: EVRS, SMV (Mode 1) • In IS-95, each voice frame is carried by a physical frame at a rate (full, 1/2, 1/4, 1/8) in Rate Set 1 (RS1). • An example of voice activity: 20% at full rate, 20% at 1/2 rate, 10% at 1/4 rate, and 50% at 1/8 rate.

  6. Rate Switching • The effect of using VoIP to carry EVRC or SMV: • At full rate, just 1 byte of VoIP overhead will require switching from 9600 kbps in RS1 to 14400 kbps in RS2. • At 1/8 rate, just 1 byte of VoIP overhead will require switching from 1200 kbps in RS1 to 3600 kbps in RS2. Note: 1800 kbps in RS2 cannot accommodate the additional VoIP overhead.

  7. Rate Switching • The table summarizes the rate switching as a result of using VoIP to transport EVRC or SMV. • This quantization effect causes capacity loss.

  8. Capacity Loss on IS-95 Forward Link • If the VoIP overhead is just 1 byte per frame, systems using EVRC or SMV would lose about 40% of respective forward link capacity.

  9. Capacity Loss on IS-2000 Forward Link • If the VoIP overhead is 4 bytes per frame, systems using EVRC or SMV would lose about 25% of respective forward link capacity if flexible data rates are supported. • If flexible data rates are not supported, the IS-2000 capacity loss is the same as IS-95.

  10. Capacity Loss on IS-2000 Reverse Link • If the VoIP overhead is 4 bytes per frame, systems using EVRC or SMV would lose about 23% of respective reverse link capacity.

  11. Capacity Issues: IS-95 Signaling • On the forward link, dim-and-burst cannot be used, because the base station does not have local control over the vocoder. Thus, blank-and-burst is used to transport IS-95 signaling messages. • The disadvantage of using blank-and-burst is that If the IS-95 signaling messages are sufficiently long (spanning few frames), there can be degradation to VoIP quality. • On the reverse link, dim-and-burst can be used to alleviate the problem, because the mobile station has local control over the vocoder.

  12. Capacity Issues: IS-2000 Signaling • On the forward link, IS-2000 signaling can be sent on a Forward Dedicated Control Channel (F-DCCH), while VoIP is sent on a Forward Fundamental Channel (F-FCH). • The disadvantage of this method is that it requires two Walsh codes. If the base station doesn’t have a sufficient number of Walsh codes, this can result in a loss of base station capacity.

  13. QoS Issues • VoIP packets generally have more stringent latency requirements than data packets. • VoIP packets should have higher transport priority than data packets over the cellular link. • Coordination is required between IP-layer QoS (e.g. diff-serv) and cellular QoS (e.g. assured packet data service QoS parameters). • It is not desirable to re-transmit erroneous VoIP packets over the cellular link. • Different service options may be needed for VoIP packets and data packets over the air.

  14. Recommendation - Phasing Approach • The Phasing approach is from the air interface perspective. • Phase 1: The mobile station uses IS-95 or IS-2000 voice services over the air. VoIP is in the networks. • Phase 2: In addition to using IS-95 or IS-2000 voice services over the air, the mobile station may use VoIP over the air. • The advantages of this phasing approach: • Support legacy mobile station using IS-95 voice service. • Phase 1 safeguards the over-the-air capacity of IS-95 and IS-2000, and leverages the IP advantages in the networks. • Given the difficult issues of VoIP over the air, it may not be wise to rush it into Phase 1 without solving the key issues and understanding the capacity impact.

  15. Recommendation - Phase 2 Requirements • The link-layer end-points shall perform IP/UDP/RTP header compression and decompression. • The compression performance shall be optimized for over-the-air link. • The compression method(s) shall be based on IETF standards. • The link-layer protocol shall support a mechanism for the link-layer end-points to negotiate profiles for IP/UDP/RTP header compression. • The link-layer frame overhead shall be minimized. • The link-layer protocol and framing shall be based on IETF standards. • The latency to deliver VoIP on over-the-air link shall be minimized. • The IP-layer QoS and over-the-air QoS shall be coordinated to ensure the VoIP performance. • The mobile station shall use IETF-based signaling method(s) for VoIP calls.

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