TCP over Radio Links with Power Control and. Link Level Retransmissions. Niels Möller and Karl Henrik Johansson. Department of Signals, Sensors and ...
TCP
over Radio Links with Power Control and Link Level Retransmissions Niels Möller and Karl Henrik Johansson
Department of Signals, Sensors and Systems, KTH, Stockholm
TCP
over Radio Link
TCP
properties PTO for three power control step sizes
0.1
ACK (4)
f(r) 0.2dB 0.06dB 0.02dB
0.09
1.2
0.08
1.0
0.07
0.8 0.06
Trans. Recv. TCP Power PI SIR (1) Block − error (2) +
Network
SIR ref
PC RRQ (3)
ARQ
0.05
0.6
0.04
0.4 0.03
0.2
0.02
0.01
0
0.0 0
1
2
3
4
5
6
0
2 4 6 8 Radio blocks per IP packet
10
Left: Stationary distribution of the power control Markov chain. Right: The TCP PTO probability, for three values of the SIR ref step size (green is smallest, red is largest).
PTO = P (spurious timeout) = P (X > E(X) + 4σ(X))
Note the four cascaded feedback loops: (1) Inner loop of power control (2) Outer loop of power control (3) Retransmissions of damaged blocks (4) TCP congestion control
PTO(%)
TCP
• X uniformly distributed =⇒ PTO = 0 • X Gaussian =⇒ PTO ≈ 0.015% • σ(X) < ∞ =⇒ PTO < 1/16 ≈ 6% For our radio link, the above figures gives a PTO that is of the order 1%.
Power Control The inner loop is a fast controller tracking SIR ref . The outer loop is a Markov chain driven by block errors.
Conclusions • •
8
9
10 11 12 13 14 15 16 17 18 19 • SIR ref
SIR ref is decreased in small steps when no error occurs, and increased by a larger step when an error occurs.
The control loops below the IP layer interact with TCP congestion control. From a model of these control loops, we can derive IP-level properties. In particular, we get a probability of spurious timeouts on the order of 1%, two orders of magnitude larger than for links with Gaussian delay.
SIR ref
[1] A. Canton and T. Chahed, “End-to-end reliability in umts: Tcp over arq,” in Globecom 2001, 2001. [2] P. M. Garrosa, “Interactions between tcp and channel type switching in wcdma,” M.S. thesis, Chalmers University of Technology, 2002.
time Typical variation of
SIR ref
with time.
Link Layer The underlying Markov chain determines the block errors. Damaged blocks are scheduled for retransmission.
Retransmit
[3] Fredrik Gunnarsson and Fredrik Gustafsson, “Power control in wireless communications networks — from a control theory perspective,” survey paper in IFAC World Congress, Barcelona, 2002. [4] Ashwin Sampath, P. Sarath Kumar, and Jack M.Holtzman, “On setting reverse link target sir in a cdma system,” in Proc. IEEE Vehicular technology conference, May 1997. [5] F. Khan, S. Kumar, K. Medepalli, and S. Nanda, “Tcp performance over cdma2000 rlp,” in Proc. IEEE 51st VTC’2000Spring, 2000, pp. 41–45.
