The Impact of Multihop
Wireless Channel on TCP
Throughput and Loss
Zhenghua Fu, Petros Zerfos, Haiyun Luo, Songwu Lu, Lixia Zhang,
Mario Gerla (UCLA), INFOCOM 2003, San Francisco, Mar. 2003.
Presented by
Scott McLaren
Overview
Introduction
 Background
 Throughput in Multihop Wireless Networks
 Loss Behavior
 Improving Performance
 Conclusions

Introduction
Improve channel utilization by spatial
channel reuse
 A TCP window size W* exists at which
throughput is maximized by achieving best
spatial reuse
 Increasing the window size past W* will
reduce throughput
 Standard TCP typically grows its average
window much larger than W*

Techniques to improve efficiency

Link-RED
 Tune

the wireless link’s drop probability
Adaptive link-layer pacing scheme
 Increase

the spatial reuse of the channel
Allow TCP to operate in the contention
avoidance region
802.11

RTS/CTS messages
 Nodes
hearing this handshake defer
transmission until current transmission is
finished
 Data is dropped if no CTS is received after 7
RTS retries
 Data is also dropped if 4 transmissions are
sent without an receiving an ACK
Hidden Terminals

A hidden terminal is a node in the
receiver’s neighborhood, that can’t detect
sender and may disrupt transmissions




Nodes are 200m apart
Transmission range is 250m
Carrier sensing and interference range is 550m
D is a hidden terminal of A  B
 Cannot
hear CTS ( > 250m )
 Cannot hear data from A, A is outside of D’s carrier
sensing range
 D can transmit to E


Causes collision at B, since D is within 550m
interference range for B
Contention loss at B
Chain Topology

Best throughput when window size is h/4
 Assuming
ideal MAC protocol and equal
packet sizes
Max concurrent senders is h/4, where max
spatial reuse is achieved
 TCP window size < h/4  under utilization
 TCP window size > h/4  reduced
throughput

Cross Topology



2 TCP flows
Best window W* = 2,
measured window = 12
20% throughput
reduction
Grid Topology



4, 8, and 12 TCP
flows
½ of flows in each
direction
Measured TCP
windows are larger
than max achievable
throughput
Results
TCP Loss Behavior

Using 8-hop chain, all 165 TCP drops out
of 12349 transmissions were due to link
drops
TCP Loss Behavior
Corollaries




m – number of backlogged nodes
B* – the max number of nodes that can transmit their DATA packets
concurrently without collision
C* – denotes the max number of nodes that can initiate RTS
messages
Corollary 4.1



Corollary 4.2



m > B*
Pl increases as m increases
Corollary 4.3



m < B*
Pl ≈ 0
m > C*
Pl remains constant
Throughput reduction due to Wavg >> W*, Pl > 0, Link contention > 0
reducing spatial reuse
Improving TCP Performance
Distributed Link RED (LRED)
 Adaptive Pacing

LRED




Easy way is to improve performance by reducing
buffer size, but problems with bursty traffic
LRED exploits dropping in 802.11 MAC
RED provides a linearly increasing drop curve
as queue exceeds a min size
LRED provides a linearly increasing drop curve
as link drop probability exceeds a min size
LRED




Link layer maintains
average number of retries
Next packet is
dropped/marked with
probability based on
average number
If average number of
retries is small, packets
are not dropped/marked
When retries increase,
the dropped/marked
probability is calculated
Adaptive Pacing
Improve spatial channel reuse by
balancing traffic among nodes
 Exposed receiver problem
 Let a node backoff an additional packet
transmission time when necessary

Adaptive Pacing



Enabled from LRED
If average retries <
min_th calculate
backoff time as usual
If pacing, backoff time
increases by a time
equal to the
transmission time of
the previous packet
Performance

Chain Topology
 In
all cases LRED & Pacing increased TCP
throughput by up to 30%
 TCP stabilizes at a window size close to the
optimal value
 The longer the chain, the better the
improvement, due to pacing optimizing spatial
channel reuse
Chain Topology
Performance

Cross Topology
Increased throughput and improves fairness (Jain’s) for
both flows
 TCP NewReno has large unfairness, due to 802.11
capture characteristic (collision of 2 packets, one weaker
than the other. The stronger packet is received)

Performance

Grid Topology
 Also
increases throughput and fairness
Conclusions
Only when buffer is small do buffer
overflow drops dominate
 As buffer increases, link-layer drops
dominate
 Link drop acts as a RED gateway, but is
insufficient

Questions