Enhancing QoS Support for
Vertical Handoffs Using Implicit/Explicit
Handoff Notifications
Ling-Jyh Chen, Guang Yang, Tony Sun,
M. Y. Sanadidi and Mario Gerla
Computer Science Department
University of California, Los Angeles
August 24, 2005
QShine 2005, Orlando, Florida
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QoS Enhancement for Vertical Handoffs

Goal: Providing highly agile system for mobile applications.
August 24, 2005
QShine 2005, Orlando, Florida
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Related Work

Conventional adaptive method: AIMD


TCP, RAP, …
TEAR, TFRC, …

They DO NOT perform well in vertical handoffs
[Gurtov et al, MC2R ’04].

Our solution:


Handoff Notifications: Implicit Handoff Notification and
Explicit Handoff Notification
Enhanced Service Agility Schemes: Fast Rate Adaptation and
Early Rate Reduction
August 24, 2005
QShine 2005, Orlando, Florida
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Implicit Handoff Notification

Vertical handoffs usually result in drastic
changes in the link capacity.

By monitoring the end-to-end capacity, one can
identify the occurrence of a vertical handoff.

Passive capacity monitoring tools:
TFRC Probe & TCP Probe (CapProbe based)
August 24, 2005
QShine 2005, Orlando, Florida
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Background on CapProbe
20Mbps
10Mbps
5Mbps
10Mbps
T1
20Mbps
8Mbps
T3
Narrowest Link
T2
T3
T3
T3
Capacity = (Packet Size) / (Dispersion)
August 24, 2005
QShine 2005, Orlando, Florida
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TFRC: TCP Friendly Rate Control


TFRC is an equation based unicast protocol.
TFRC mimics the TCP long-term throughput by utilizing
s
the function: T 
R


 3p 
2p
2
 t RTO  3
 p 1  32 p
3
8 



The receiver is responsible for calculating the loss event
rate p and sending it back to the sender once per roundtrip time.
The sender is responsible for adjusting its sending rate
Tactual to be close to T.
August 24, 2005
QShine 2005, Orlando, Florida
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TFRC Probe

Embed CapProbe algorithm within TFRC by sending two
packets back-to-back every n packets

TFRC Probe was assessed in [E2EMON’04].
August 24, 2005
QShine 2005, Orlando, Florida
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TCP Probe
CapProbe:
Delayed Ack
TCP Probe:

TCP Probe was assessed in [GI’05].
August 24, 2005
QShine 2005, Orlando, Florida
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Implicit Handoff Notification

TCP/TFRC Probe continuously estimate the link
capacity, using 50 samples per estimate.

A vertical handoff can be identified when a dramatic
capacity change is observed.

An IHN is generated when
1. Cnew > αCpre or
2. Cnew < βCpre
where α =5 and β=0.2.
August 24, 2005
QShine 2005, Orlando, Florida
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Enhancing QoS using IHN
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Fast Rate Adaptation (FRA)



FRA monitors the capacity.
When vertical handoff from LOW to HIGH is observed, FRA
changes ssthresh of TCP/TFRC.
TCP/TFRC enter slow start, instead of staying in congestion
avoidance.

FRA+IHN is end-to-end.

However, IHN does not work with HIGH to LOW handoff.

Decreasing ssthresh does not change TCP/TFRC behavior.
August 24, 2005
QShine 2005, Orlando, Florida
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Explicit Handoff Notification

Need an “Intelligent Handoff Manager”.
EHN is generated prior to
the occurrence of handoff.

Smart Decision Model

August 24, 2005
QShine 2005, Orlando, Florida
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Enhancing QoS using EHN
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LOW-to-HIGH:


Fast Rate Adaptation (FRA)
HIGH-to-LOW:

Early Rate Reduction (ERR):
Reduce rate in advance to prevent bulk packet losses.
August 24, 2005
QShine 2005, Orlando, Florida
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Evaluation
Simulation Scenario (NS-2)
 Vertical handoff between 802.11b
(5.5Mbps) and 1xRTT (150kbps).

August 24, 2005
QShine 2005, Orlando, Florida
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Evaluation
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TFRC: vertical handoff from LOW to HIGH
August 24, 2005
QShine 2005, Orlando, Florida
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Evaluation

TCP: vertical handoff from LOW to HIGH
August 24, 2005
QShine 2005, Orlando, Florida
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Evaluation



TFRC: vertical handoff from HIGH to LOW
EHN (a): EHN when the handoff occurs
EHN (b): EHN one OWD before the handoff
August 24, 2005
QShine 2005, Orlando, Florida
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Evaluation

TFRC: vertical handoff from HIGH to LOW
August 24, 2005
QShine 2005, Orlando, Florida
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Summary

We have proposed IHN and EHN to notify the
sender about the occurrence of vertical handoff.

We have proposed FRA (for IHN) and ERR (for
IHN/EHN) for better QoS for vertical handoff.
August 24, 2005
QShine 2005, Orlando, Florida
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Thank you
Questions are welcome.
 For more info, please visit our web at
http://www.cs.ucla.edu/NRL/

August 24, 2005
QShine 2005, Orlando, Florida
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