Future wireless systems will be characterized by their heterogeneity - availability of multiple access systems in the same physical space.

1
Introduction

• Future wireless systems will be characterized by
  their heterogeneity - availability of multiple
  access systems in the same physical space.
• Each system differs from others in terms of its
  capabilities - data rates, latencies, cost per
  byte etc.
• Given such choice of systems, end users need not
  restrict themselves to any single system but
  rather at every instant, depending on their
  requirements choose the system or systems that
  best cater to his needs.

2
Example Scenario

• A laptop is equipped with multiple interfaces.
  The applications running on it have bandwidth
  requirements higher than can be offered by any
  single system.
• A solution is to aggregate the bandwidth offered
  by each of these different systems.
• For example, if the different interfaces provide
  a bandwidth of 56kbps, 128kbps and 144kbps, the
  laptop has at its disposal an aggregated
  bandwidth of (56128144) 328kbps.
• The goal is to minimize the cost while satisfying
  the QOS requirements of the applications.

3
Other Scenarios

• A corporate car fleet or rental cars equipped
  with multiple wireless interfaces.
• The multiple passengers have different profiles
  and requirements and have to share the combined
  bandwidth of the interfaces among themselves.
• An ad-hoc network formed by devices some of which
  are equipped with WWAN interfaces.
• The devices form the ad-hoc network with their
  WLAN interface (bluetooth,802.11).
• The WWAN interfaces provide access to the
  internet and the aggregated bandwidth is shared
  among themselves.

4
Laptop Scenario
5
Issues Involved

• The multiple access systems display variable data
  rate, latency, packet loss.
• These variations cause packet reordering.
• Buffering helps but it means increased delay.
• Situation only worsens when a reliable protocol
  like TCP is used.

6
Approach

• Problem can be addresses at different layers of
  the protocol stack- Network, Transport.
• Network layer approach
• A single TCP/UDP connection end to end.
• Mobile IP like infrastructure
• Scheduler distributes the traffic to the multiple
  systems at the network layer.
• Transport layer solution
• Multiple TCP connections one for each access
  system being used.
• A scheduler distributes application data to the
  multiple TCP connections.
• The connections can terminate at the home agent
  or correspondent node.

7
Network Layer Solution - Scheduling Algorithm

• Packets of different applications need to be
  scheduled onto the different links so that each
  application gets its share of bandwidth.
• Care should be taken so that delay, reordering
  and jitter experienced by the packets is
  minimized.
• Our approach is to use a Weighted Fair
  Queuing(WFQ) algorithm (that ensures that each
  application gets its share of bandwidth) and
  follow it with a channel striping algorithm.

App1
Link1
WFQ
Channel Striping Alg
App2
Link2
App3
Link3
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Channel Striping Algorithm
Channel Striping Algorithm
9
Channel Striping Algorithm EDF

• This algorithm schedules packets from a single
  input queue onto multiple links.
• In our algorithm (which we call EDF), we schedule
  packets on the link which delivers it the
  earliest.
• Each link l is associated with three quantities
• A variable S_l, which is the time the link
  becomes available for the next transmission.
• D_l, the delay (estimated) associated with the
  link
• BW_l, the bandwidth (estimated) of the link

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EDF cont..

• If we denote by a_i, the arrival instance of the
  ith packet and L_i, the size of the packet, we
  know that this packet when scheduled on link l
  would arrive at the receiver at R_l, where
• R_l MAX(a_iD_l,S_l) L_i/BW_l
• EDF schedules the packet on the link for which
  R_l is the minimum.
• Since the criteria is earliest delivery, it
  minimizes the delay, buffering required and
  jitter experienced by the packets.

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UDP Flows Simulation Setup
(350kbps,100ms)
WAN1
(10Mbps,20ms)
(100kbps,120ms)
Mobile Receiver
Higher Layers
Home Agent
WAN2
Sender
(Buffering)
(50kbps,150ms)
WAN3
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Details of Setup

• The sender generates packets according to an
  arrival and packet size distribution and forwards
  them to the home agent.
• The scheduling algorithm in the home agent
  distributes the packets onto the multiple
  interfaces.
• The WAN clouds induce delay according to a delay
  distribution.
• The hop to the mobile receiver is wireless and
  has a limited bandwidth (bottleneck). Packets
  would be dropped according to a packet loss
  model.
• The mobile receiver collects the packets and
  sends them in order to higher layers.

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UDP Flows Cont

• We compare the performance of our algorithm with
  two other algorithms.
• A Single Link Algorithm (SL), where the multiple
  links between the home agent and the mobile are
  replaced with a single link.
• Surplus Round Robin which distributes traffic in
  proportion to the bandwidth.

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Trace Driven Simulation

• Source Details
• An MPEG1 video trace ( a cable TV show)
• 5000 frames
• Capture rate 25fps
• Mean bit rate 440kbps, peak bit rate 1760kbps
• 4 MTU sizes(bits) 4000,8000,12000 and no
  restriction
• Network details
• Constant and truncated Gaussian delay
  distribution with standard deviation 50ms.
• No packet loss

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Buffering Required constant delay
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Buffering Required variable delay
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Jitter constant delay
18
Jitter variable delay
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TCP Flows Details

• Same setup as before except the protocol used is
  TCP.
• Application - File transfer of 2 Mbytes
• Download time is measured
• Two scenarios considered
• The packets are passed to TCP layer as they
  arrive.
• The packets are buffered and passed in order to
  TCP layer

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Download time (No buffering)
Algorithm Constant Delay Variable Delay
SL (500kbps) 33.4 69.9
EDF 33.6 140.9
SRR 111.4 177.0
SL (350kbps) 48.6 71.9
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Future Work

• UDP flows evaluate the performance of the
  system with packet loss and rate control.
• TCP flows
• Buffering at the receiver
• Opening multiple TCP connections
• Multiple Applications using different protocols
  (UDP,TCP) , having different QoS requirements
  sharing the aggregated bandwidth.
• Multiple users sharing the aggregated bandwidth.
• Effect of incorrect estimation of the delay and
  bandwidth on EDF.