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Wireless IP Multimedia. Henning Schulzrinne Columbia University MOBICOM Tutorial, September 2002. Types of wireless multimedia applications streaming interactive object delivery Properties of multimedia content loss resiliency delay reordering 3G and WLAN MM-related channel properties

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Wireless IP Multimedia


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    1. Wireless IP Multimedia Henning Schulzrinne Columbia University MOBICOM Tutorial, September 2002

    2. Types of wireless multimedia applications streaming interactive object delivery Properties of multimedia content loss resiliency delay reordering 3G and WLAN MM-related channel properties effective bandwidth packet loss delay Header and signaling compression cRTP ROHC signaling compression Packet FEC UMTS multimedia subsystem (IMS) QoS Session setup Fast handoff mechanisms Multimodal networking Overview

    3. Types of wireless multimedia applications • Interactive • VoIP • multimedia conferences • multiplayer games • Streaming • video/audio on demand • broadcast TV/radio • may be cached at various places, including end system • Object retrieval • peer-to-peer • user may be waiting for result • Messaging • store-and-forward (e.g., MMS) • can be batched

    4. IETF (multimedia) protocols media encap (H.261. MPEG) Media Transport Signaling SAP SDP MGCP DHCPP SIP H.323 RTSP RSVP RTCP RTP DNS Application LDAP TCP UDP CIP MIPv6 IDMP Network MIP ICMP IGMP MIP-LR IPv4, IPv6, IP Multicast Kernel PPP AAL3/4 AAL5 PPP Physical CDMA 1XRTT /GPRS SONET ATM 802.11b Ethernet Heterogeneous Access

    5. Common wired & wireless audio codecs

    6. Audio codecs, cont'd.

    7. Audio codecs • MP3 and AAC: delay > 300 ms  unsuitable for interactive applications • GSM and AMR are speech (voiceband) codecs  3.4 kHz analog designed for circuit networks with non-zero BER • Wideband = split into two bands, code separately  conferencing • AMR is not variable-rate (dependent on speech content) • receiver sends Codec Mode Request (CMR) to request different codec, piggy-backed on reverse direction • trade-off codec vs. error correction

    8. Audio codecs • Typically, have algorithmic look-ahead of about 5 ms  additional delay • G.728 has 0.625 ms look-ahead • AMR complexity: 15-25 MIPS, 5.3 KB RAM original 4 6 8 10 12 14 18 20 22 24 16 G.723.1 G.729 G.729A AMR-NB AMR-WB www.voiceage.com

    9. Audio codecs - silence • Almost all audio codecs support Voice Activity Detection (VAD) + comfort noise (CN) • comfort noise: rough approximation in energy and spectrum  avoid "dead line" effect • G.729B • AMR built-in: CN periodically in Silence Indicator (SID) frames = discontinuous transmission (DTX)  saves battery power • or source controlled rate (SCR)

    10. Audio codecs - silence • silence periods depend on • background noise • word vs. sentence vs. alternate speaker • particularly useful for conferences • small ratio of speakers to participants • avoid additive background noise

    11. Video codecs JPEG common code words  shorter symbols Huffman, arithmetic coding e.g., DCT: spatial  frequency Transform, Quantization, Zig- Zag Scan & Run- Length Encoding Motion Estimation & Compensation Symbol Encoder Frames of Digital Video Bit Stream predict current frame from previous Quantization changes representation size for each symbol adjust rate/quality trade-off Run-length encoding: long runs of zeros  run-length symbol courtesy M. Khansari MPEG, H.26x

    12. History of video codecs H.261 H.263++ H.263 H.263+ H.263L ITU-T MPEG 1 MPEG 4 ISO MPEG 2 MPEG 7 1990 1992 1994 1996 1998 2000 2002 courtesy M. Khansari

    13. H.263L example 64 kb/s, 15 fps courtesy of Siemens CT

    14. Delay requirements • In many cases, channel is delay constrained: • ARQ mechanisms • FEC • low bandwidths • ITU G.114 Recommendation: • 0..150 ms one way delay: acceptable to most users • 150..400 ms: acceptable with impairments • Other limits: • telnet/ssh limit ~ 100-200 ms [Shneiderman 1984, Long 1976]? • reaction time 1-2 s for human in loop [Miller 1968]: • web browser response • VCR control for streaming media • ringback delay for call setup • can often be bridged by application design

    15. 802.11 architecture ESS Existing Wired LAN AP AP STA STA STA STA BSS BSS Infrastructure Network STA STA Ad Hoc Network Ad Hoc Network BSS BSS STA STA Mustafa Ergen

    16. 802.11b hand-off Kanter, Maguire, Escudero-Pascual, 2001

    17. 802.11 delay channel idle is busy Data ACK idle slots slots time DIFS SIFS DIFS (DCF interframe space) (short IFS) idle idle RTS CTS Data ACK slots slots time DIFS SIFS SIFS SIFS DIFS M. Zukerman

    18. 802.11 delay • 802.11b: 192 bit PHY headers  192 µs (sent at 1 Mb/s) • 802.11a: 60 µs • three MAC modes: • DCF • DCF + RTS • PCF: AP-mode only

    19. 802.11 delay

    20. 802.11 delay

    21. 802.11a delay for VoIP

    22. 802.11b channel access delay Köpsel/Wolisz • 12 mobile data nodes, 4 mobile with on/off audio • 6 Mb/s load

    23. 802.11b VoIP delay • Köpsel/Wolisz WoWMoM 2001: add priority and PCF enhancement to improve voice delay DCF Köpsel/Wolisz

    24. 802.11b – PCF+priority • poll only stations with audio data • move audio flows from PCF to DCF and back after talkspurts Köpsel/Wolisz • IEEE 802.11 TGe working on enhancements for MAC (PCF and DCF) • multiple priority queues

    25. 802.11e = enhanced DCF Mustafa Ergen

    26. 802.11e back-off

    27. Metric of VoIP quality • Mean Opinion Score (MOS) [ITU P.830] • Obtained via human-based listening tests • Listening (MOS) vs. conversational (MOSc)

    28. FEC and IP header overhead • An (n,k) FEC code has (n-k)/k overhead • Typical IP/UDP/RTP header is 40 bytes

    29. Predicting MOS in VoIP • The E-model: an alternative to human-based MOS estimation • Do need a first-time calibration from an existing human MOS-loss curve • In VoIP, the E-model simplifies to two main factors: loss (Ie) and delay (Id) • A gross score R is computed and translated to MOS. • Loss-to-Ie mapping is codec-dependent and calibrated

    30. Predicting MOS in VoIP, contd • Example mappings • From loss and delay to their impairment scores and to MOS

    31. Predicting MOS under FEC • Compute final loss probability pf after FEC [Frossard 2001] • Bursty loss reduces FEC performance • Increasing the packet interval T makes FEC more efficient under bursty loss [Jiang 2002] • Plug pf into the calibrated loss-to-Ie mapping • FEC delay is n*T for an (n,k) code • Compute R value and translate to MOS

    32. Quality Evaluation of FEC vs. Codec Robustness • Codecs under evaluation • iLBC: a recent loss-robust codec proposed in IETF; frame-independent coding • G.729: a near toll quality ITU codec • G.723.1: an ITU codec with even lower bit-rate, but also slightly lower quality. • Utilize MOS curves from IETF presentations for FEC MOS estimation • Assume some loss burstiness (conditional loss probability of 30%) • Default packet interval T = 30ms

    33. G.729+(5,3) FEC vs. iLBC • Ignoring delay effect, a larger T improves FEC efficiency and its quality • When considering delay, however, using a 60ms interval is overkill, due to higher FEC delay (5*60 = 300ms)

    34. G.729+(5,2) vs. iLBC+(3,2) • When iLBC also uses FEC, and still keeping similar gross bit-rate • G.729 still better, except for low loss conditions when considering delay

    35. G.729+(7,2) vs. iLBC+(4,2) • Too much FEC redundancy (e.g., for G.729)  very long FEC block and delay  not always a good idea • iLBC wins in this case, when considering delay

    36. G.729+(3,1) vs. iLBC+(4,2) • Using less FEC redundancy may actually help, if the FEC block is shorter • Now G.729 performs similar to iLBC

    37. Comparison with G.723.1 • MOS(G.723.1) < MOS(iLBC) at zero loss  iLBC dominates more low loss areas compared with G.729, whether delay is considered or not

    38. G.723.1+(3,1) vs. iLBC+(3,2) • iLBC is still better for low loss • G.723.1 wins for higher loss

    39. G.723.1+(4,1) vs. iLBC+(4,2) • iLBC dominates in this case whether delay is considered or not, • (4,2) code already suffices for iLBC • (4,1) code’s performance essentially “saturates”

    40. The best of both worlds • Observations, when considering delay: • iLBC is usually preferred in low loss conditions • G.729 or G.723.1 + FEC better for high loss • Example: max bandwidth 14 kb/s • Consider delay impairment (use MOSc)

    41. Max Bandwidth: 21-28 kb/s

    42. Effect of max bandwidth on achievable quality • 14 to 21 kb/s: significant improvement in MOSc • From 21 to 28 kb/s: marginal change due to increasing delay impairment by FEC

    43. UMTS and 3G wireless • Staged roll-out with "vintages"  releases: • Release 3 ("1999")  GPRS data services • Multimedia messaging service (MMS) = SMS successor ~ MIME email • RAN via evolved CDMA • Release 4: March 2001 • Release 5: March-June 2002 • Release 6: June 2003  all-IP network • Main future new features (affecting packet services): • All-IP transport in the Radio Access and Core Networks • Enhancements of services and service management • High-speed Downlink Packet Access (HSDPA) • Introduces additional downlink channels: • High-Speed Downlink Shared Channel (HS-DSCH) • Shared Control Channels for HS-DSCH

    44. UMTS • Follow-on to GSM, but WCDMA physical layer • new ($$$) spectrum around 2 GHz • radio transmission modes: • frequency division duplex (FDD): 2 x 60 MHz • time division duplex (TDD): 15 + 20 MHz • Chip rate 3.84 Mcps  channel bandwidth 4.4 – 5 MHz

    45. cdma2000 1X (1.25 MHz) cdma2000 3X (5 MHz) 1G-3G air interface 1G 2G “2.5G” 3G/ IMT-2000 Capable Existing Spectrum New Spectrum Analog AMPS IS-95-A/ cdmaOne IS-95-B/ cdmaOne 1XEV DO: HDR (1.25 MHz) IS-136 TDMA 136 HS EDGE TACS GSM GPRS EDGE GSM WCDMA HSCSD Ramjee

    46. The mysterious 4G • Fixes everything that's wrong with 3G  • Convergence to IP model: treat radio access as link layer that carries IP(v6) packets • not necessarily new radio channel • no new spectrum available • all-IP radio access network (RAN) • common mobility management • AAA and roaming • user identifiers • roaming across wired networks

    47. UMTS – 3GPP and 3GGP2 • Divided regionally/historically: • both from ITU IMT-2000 initiative • GSM  3GPP (ETSI) = WCDMA • US (CDMA)  3gpp2 (TIA) = CDMA2000 • 3GPP2: different PHY, but similar applications (not completely specified) • cdma2000

    48. UMTS W. Granzow

    49. 3GPP network architecture AS Jalava

    50. 3GPP network architecture - gateways Legacy Mobile Signaling Networks Multimedia IP Networks Roaming Signaling Gateway (R-SGW) Mm Mh Ms HSS CSCF Gi Cx Mg Mr Gi MRF Media Gateway Control Function (MGCF) Transport Switching Gateway (T-SGW) SGSN GGSN Gi Mc (= H.248) PSTN/Legacy/External Media Gateway (MGW) Media Gateway (MGW) Gi Alves