RAIN: A Reliable Wireless Network Architecture
Chaegwon Lim
†‡
, Haiyun Luo
†
, and Chong-Ho Choi
‡
†
Department of Computer Science, University of Illinois at Urbana-Champaign, USA
‡
School of Electrical Engineering and Computer Science, Seoul National University, Korea
chaegwon@csl.snu.ac.kr, haiyun@cs.uiuc.edu, chchoi@csl.snu.ac.kr
Abstract— Despite years of research and development, pioneer-
ing deployments of multihop wireless networks have not proven
successful. The performance of routing and transport is often
unstable due to contention-induced packet losses, especially when
the network is large and the offered load is high. In this paper
we propose RAIN, a reliable wireless network architecture for
large-scale multihop wireless networks. A RAIN network enforces
contention control by limiting the queue length at intermediate
wireless routers to the minimum. To keep the queue short a RAIN
network enforces congestion control through in-network implicit
back-pressure. RAIN congestion control is built on wireless
datalink layer mechanisms, e.g., mandatory per-frame acknowl-
edgement and inter-frame backoff in popular CSMA/CA wireless
transceivers, therefore very efficient and effective compared with
those defined at the network or transport layer for the wired
Internet. As a result of the built-in contention and congestion
control, RAIN presents the end hosts a highly reliable network
service model, even more reliable than that of the wired Internet.
The end hosts only need to deal with packet losses due to router
or routing failures. Therefore, the transport protocol can be
significantly simplified. This is in stark contrast to the existing
approach of adding more and more complexity to adapt TCP
for multihop wireless networks. We propose the details of RAIN
datalink layer protocol, and a simple transport protocol at the
end hosts. Performance evaluation through intensive simulations
shows that RAIN improves the throughput by up to 92% and
fairness by up to 48%, with packet losses due to contention and
congestion significantly reduced.
I. INTRODUCTION
Multihop wireless networks first emerged in response to
the demand for instant networking in the forms of mobile ad
hoc networks (MANETs). It recently developed into wireless
mesh networks to offer high-speed broadband Internet connec-
tivity. However, despite years’ research and development in
MANETs and wireless mesh, pioneering deployments of mul-
tihop wireless testbeds [1], [2], [3], [12], [38] have encountered
serious stability problem [4]. Routing and transport protocols,
designed for the wired, first-/last-hop wireless Internet, are
often extremely unstable and unpredictable in a multihop
wireless network as the network grows, especially when the
offered load is high. Reports of excessive packet drops [23],
[46], unfair channel bandwidth sharing and starvation [25],
[26], and extremely volatile path properties [9], [40], [46] have
been frequently cited to question the feasibility of multihop
wireless networks.
One major cause of these problems is that wireless trans-
missions are broadcast
1
in nature. They contend with each
other even between packet transmissions of the same flow.
Different from the Internet that are usually connected by
point-to-point well-insulated wires, in wireless networks the
transmission between a sender and a receiver relies on the
medium access control (MAC) to resolve the contention for
the shared wireless channel in a neighborhood with variable
traffic demands. However, wireless transmissions interfere
with each other in a range that is usually longer than the
transmission/receiving range (or “one hop” direct communica-
tion range) and unknown a priori. A wireless MAC does not
have the explicit information regarding those contending nodes
that are interfering from beyond the direct communication
range. It is therefore very challenging, if not impossible, for
a wireless MAC to coordinate interfering nodes with which it
cannot directly communicate [14], [13]. Furthermore, wireless
interference comes and goes as interfering nodes start and
finish transmitting a packet. This fine time granularity renders
end-to-end mechanisms, e.g., TCP wireless variants [22], [27],
[33], [41], [43], ineffective in contention control since end-
to-end mechanisms operate at a significantly coarser time
granularity of a round trip time. As a result, the contention
becomes the primary cause to packet losses in a multihop
wireless network [9], [23], [44], [46], as compared with the
wired Internet where packet losses are mostly caused by
network congestion and router buffer overflow.
The above analysis suggests that effective contention control
must be in-network for timeliness, and go beyond the direct
one-hop neighborhood of individual wireless routers. To this
end, we start our design with a novel approach that bounds
the contention level in a multihop wireless network through
buffer management. Since all wireless nodes with non-empty
packet queues will contend for the channel, the contention
level is a function of the number of nodes with backlogged
queues. Therefore, if we can reduce the number of backlogged
queues in the network we can keep the contention level
low. In specific, we limit the maximum length of the transit
traffic queue
2
to the minimum possible, i.e., close to one
packet. Because each intermediate wireless router only holds
a very small number of packets, typically one packet only,
the contention between upstream and downstream wireless
1
Directional antenna may reduce the broadcast area, but cannot completely
eliminate interference due to side lobes. It also introduces new sources of
channel contentions such as carrier sense deafness [15].
2
Note that the source or destination node will maintain extra queue for
packets generated by or destined for the node itself.
1-4244-0593-9/06/$20.00 ©2006 IEEE