Why is TCP not good enough for Mobile Operators?

1. Why is TCP not good enough for Mobile Operators? Ulas C. Kozat kozat@docomolabs-usa.com

2. Punch Lines • Cellular networks are carefully engineered: • Starvation observed in ad hoc/mesh deployments or with 802.11 MAC are unlikely to occur in cellular. • But, TCP can lead to substantially sub-optimal operating points for highly optimized/expensive cellular radio. • An operator like NTT DoCoMo do not use standard TCP. • Split with proxies, use a modified proprietary TCP version.

3. Features of state-of-the-art Wireless Transmission • An explicit fairness consideration per user/connection. • Constant-size radio link layer PDUs. • E.g. 1024 bits in HDR, 320 bits in HSDPA. • Time-slotted (1.67ms for HDR, 2ms for HSDPA). • Separate (i.e., out-of-band) control channels. • Fast rate adaptation per connection. • Hybrid ARQ at PHY layer and aggressive re-transmission at link layer. • Less Radio Network delays (e.g., < 50 msec) • Opportunistic scheduling across users for high capacity. • Currently limited to downlink. • In few years, it will be common for the uplink as well.

4. Multi-user Diversity and Proportional Fair Sharing (PFS) • Each user has a separate queue. • Users are sorted w.r.t.: • Ci[n]: Achievable rate at slot n. • Ti[n]: Weighted average throughput until slot n. Rate (Kbps) Time (slot number)

5. Simulated RLL, MAC layer & Scheduler • RLL fragments/defragments IP packets • PDU length = 1024 bits. • No concatenation, shorter fragments are padded. • Scheduler has perfect CSI information before each slot. • Slot length = 1.67ms. • No packet losses over the wireless channel. • Scheduler uses α=0.002 to compute weighted throughput. • Per user scheduling queue • Limited to 160 PDUs.

6. Simulated PHY Layer *These modes are used for downlink only in 1x-EVDO, but I assume symmetric UL/DL

7. Channel Model • ITU IMT-2000 channel models are used • COST231-Walfish-Ikegami model • Path Loss model takes into account • BS-MS separation distance, Carrier frequency, BS antenna height, Difference of the BS height from the mean building height, Average separation between rows of buildings. • Shadow Fading model • Log-normal distribution (zero mean and Std.Dev=10 dB) • Correlation with distance follows Gudmundson’s model • Multipath Fading model • Pedestian (3km/hr) model (4 tap channel model) • Vehicular (100 km/hr) model (6 tap channel model) • Doppler correlation follows Clark’s model • Receiver is assumed to have 3 fingers tuned to the three strongest paths. Remaining paths only provide interference. • Mobiles are assumed to detect signals at a maximum C/I of 13 dB due to hardware imperfections

8. TCP model • Ns-2 implementation is used: • set tcp [new Agent/TCP/Fack] • set sink [new Agent/TCPSink/Sack1/DelAck] • Payload+TCP/IP header = 1024 bytes • Perfectly aligned with RLL PDU and slot duration.

9. Average rate (Kbps) MH2 MH1 Instantaneous rate (Kbps) MH3 MH2 MH3 MH1 Time (sec) Demo-1 Scenario • 1 down-stream video (1.2Mbps) & 3 uploads in the same cell. • Wireless capacity is the bottleneck. • Each user sees symmetric channel rates. • We compare TCP vs. backlogged UDP. S2 S1 50Mbps, 25ms 50Mbps, 25ms Wireless Channel Quality

10. MH3 MH1 MH2 Demo-2 Scenario • 2 Downloads and 1 P2P streaming (600Kbps). • Wireless capacity is the bottleneck. • Each user sees symmetric channel rates. • We compare TCP vs. backlogged UDP. S1 Wireless Channel Quality 50Mbps, 25ms MH1 Averate rate (Kbps) MH2 MH3 Time (sec)

11. Summary of Problems • ACK traffic substantially interferes with the payload traffic. • Load asymmetries substantially impact the performance. • TCP fairness and scheduler fairness are not necessarily the same. • Large RTT misses transmission opportunities. • Mobile P2P with TCP looks problematic. • Unmatched channel states increases RTT.

12. Acknowledgements • Sandeep Kanumuri@DoCoMo USA Labs • Reha Civanlar@DoCoMo USA Labs ANY QUESTIONS?