Studies of TCP Performance in SUMTS Using Radio Link Control with Fragmentation and Retransmission

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Studies of TCP Performance in S-UMTS Using Radio
Link Control with Fragmentation and
Retransmission
Zhili Sun, Zongyang Luo Centre for Communication
Systems Research University of Surrey Guildford U.
K.
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Outline

• Introduction
• Objectives
• Modeling
• Simulation and result analysis
• Conclusion and future work

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Introduction

• Satellite-based Universal Mobile
  Telecommunications System (S-UMTS) provides a
  global seamless service for mobile users.
• Satellite channels characters, such as high
  error bit rate combined with fading, long
  propagation delay, affect the service quality.
  TCP as one of the important transport layer
  protocol is also affected by this.
• In order to enhance TCP performance in satellite
  networks, it is necessary to do research on the
  interaction between the transport layer and link
  layer.
• Based on the research result, it is possible to
  provide some effective cross layer design
  mechanism to enhance TCP performance in satellite
  networks.

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Objectives

• Modeling the interaction between TCP and Radio
  Link Control (RLC) in the S-UMTS.
• Adapt simulation methodology to do the research
  on the interaction between TCP and RLC.
• Provide a framework to enhance TCP performance
  via cross layer mechanism for satellite networks.

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Background (1/2)

• S-UMTS radio interface is divided into three
  layers, i.e., network layer, data link layer, and
  physical layer. Layer 2 comprises RLC and MAC.
• RLC provides segmentation/reassembly and
  retransmission services between the User
  Equipment (UE) and UMTS Satellite Radio Access
  Network (USRAN).
• RLC can be configured in three modes
• Transparent Mode (TM)
• Unacknowledged Mode (UM)
• Acknowledged Mode (AM)
• Acknowledged Mode provides Automatic Repeat
  Request (ARQ) mechanism to enhance guaranteed
  delivery, which may work combined with Forward
  Error Correction (FEC).

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Background (2/2)
Higher Layer PDU
Higher Layer PDU (IP,etc.)
RLC SDU
RLC SDU
Segmentation and reassembly

RLC header
RLC PDU
RLC PDU
RLC PDU
Payload
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Modeling (1/4)

• For a Point to Point (P2P) connection, we make
  following assumptions and definitions
• P P2P connection packet probability
• Pl Unsuccessfully packet transmission
  probability of one attempt
• M Maximum number of retransmission
• N Fragmentation number of one packet
• PT Loss probability of transmitting a TCP packet
• P1 The up-link packet loss probability
• P2 The down-link packet loss probability
• P1 and P2 are independent

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Modeling (2/4)

• Based on the model in 1. We can have
• For a P2P connection,
• For satellite networks, we have two hops, up-link
  and down-link, then

1 Fethi Filali, Link-Layer Fragmentation and
Retransmission Impact on TCP Performance on TCP
Performance in 802.11-based Networks, the 7th
IFIP MWCN, 2005.
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Modeling (3/4)
Fig.1 TCP packet loss rate when fragmentation
number changes
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Modeling (4/4)
Fig 2. TCP packet loss rate when retransmission
times change
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Simulation and Analysis (1/5)

• We use NS2 as our simulation tool.
• In simulation, fragmentation number and
  retransmission times change.
• TCP throughput and congestion window are used as
  important metrics to compare different scenarios.
• The result presented here is from TCP NewReno.
  Other versions have also been simulated.

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Simulation and Analysis (2/5)
Fig 3. TCP Throughput with retransmission times
change
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Simulation and Analysis (3/5)
Fig 4. TCP throughput with fragment number changes
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Simulation and Analysis (4/5)
Fig 5. TCP congestion window comparison with RCL
and without RLC When BER is 10-4
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Simulation and Analysis (5/5)

• From the result, we can see
• When increasing fragmentation number, the TCP
  throughput increases when the channel is in high
  bit error rate.
• When increasing retransmission times, the TCP
  throughput increases when the channel is in high
  bit error rate.
• When the error bit rate decreases to 10-5, the
  difference between different configuration
  becomes small.
• The TCP congestion window size also can show the
  TCP performance is better when apply RLC
  fragmentation and retransmission mechanism in
  high error bit rate channel.

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Conclusion and future work

• RLC fragmentation and retransmission mechanism
  can improve the TCP performance in high bit error
  rate satellite channel.
• Optimized configuration needs to be considered to
  acquire best TCP performance.
• In this work, we only consider satellite working
  in bent-pipe mode. When apply onboard
  processing, TCP performance can be improved more.

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