Reflections on ad-hoc and
partially disconnected networks
Henning Schulzrinne
Suman Srinivasan
Arezu Moghadam
Andy Yuen
Columbia University
Introduction
• Are ad-hoc and sensor networks the
next active networks?
• What are the uses and users?
• What are missing pieces in the wireless
puzzle?
Ad-hoc/sensor networks
• More research interest than application interest:
– limited, mostly military applications
– always repeat the same handful of examples
• vineyards, glaciers, …
– number of papers >> number of users
• cf. active networks
• brittle for regular users:
– easily splits into disconnected sub-networks
– difficult to plan
• mesh networks: early experiences dubious
– business model? (Singapore)
– reliability and availability
– frequency management in dense deployments
What’s missing?
• Lots of practical problems
• 802.11 configuration and debugging
– IETF experience: 1500 engineers can’t keep networks up
and running
– manual channel assignment, no load balancing,
gratuitous channel dropping  application crash, long
association delays
– no location information (cf. Skyhook)
– security mechanisms
• something other than typing in 16 hex digits
• opportunistic security and association (e.g., get token)
• 3G (IMS)
– configuration
– system complexity
– new applications?
A set of predictions
• WiMax for rural areas (water tower)
• 3G/4G (= 3G without the PSTN legacy) in (sub)urban
areas and on major transportation corridors
– easier to deploy than mesh
– better power management
– but hard to deploy for non-carriers
• 2.5G in rural areas
• 802.11g/n indoors and as last-hop access
– cheap
– on every laptop
– reasonably fast
– easy to deploy
Motivation
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802.11 currently hard to deploy across
city or large area
Problem: How can mobile devices /
gadgets get information?
Peer-to-Peer data sharing Network
Solution: 7DS!
Wireless networks
access
cellular mesh
(802.11) (3G)
ad-hoc sensor
7DS
speed
10 Mb/s
1 Mb/s
500 kb/s
500 kb/s? 100 kb/s
10 Mb/s
ubiquity
islands
(100’)
urban
urban
islands
(500’)
islands
(500’)
dispersed
high
high
locally
high
low
density
dataflow
sink
sink
mesh
mesh
sink
sinks
power
high
medium
high
low -medium
low
low
Illustration
In the absence of the Internet, nodes
can exchange information amongst
themselves
Internet
7DS Overview

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
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Information Dissemination and Resource
Sharing
Disconnected
No Global Network Connection
Dynamically Changing Topology
Reactive Routing
Data-Centric
Unattended Network
Uses Multicast to propagate request
Network
Internet
7DS Network
Bob
Alice
Proxy
Multicast
Cache
SMTP Server
System Architecture & Proxy Server

Proxy Server listens to the incoming HTTP Requests

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Peer’s user client uses localhost proxy server by default
Query Multicast is sent through a Query Listener & Scheduler
SMTP Server listens to the incoming messages and dumps them up
to the MTA
Multicast
Server
Proxy Server
Search
Engine
Web Server
(Mini HTTP)
Cache
Cache
Manager
Relay Email
MTA
SMTP Server
To Next
Client
Search Engine




Provides ability to
query self for results
Searches the cache
index using Swish-e
library
Presents results in
any of three formats:
HTML, XML and
plain text
Similar in concept to
Google Desktop
Query Multicast Engine


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Used to actually
exchange information
among peers
Requesting peer
broadcasts a query to
the network
Responding peers reply
if they have information
 Send encoded string
with list of matching
items
Requesting peer
retrieves suitable
information
Email Delivery
7DS enables mobile nodes to discover each other
and relay messages behaving as MTA.
 Each node calculates statistics and keeps track of
each outgoing message using a database.

Node Discovery
Zero-Configuration Network
 On-Demand Publishing and Discovering of Services
 Connection set up on-demand using zeroconf protocol
 Similar to AppleTalk, Microsoft NETBIOS, Novell IPX

Wireless Coverage
Wireless Coverage
Zero Configuration
Zero Configuration
AP
AP
Community Extensions (Proposal)
Users can generate and share content
in the spirit of Web 2.0
1. Users can
contribute
community
information
7DS Access Box
at 116th & Broadway
2. Users
can search
for and
read
community
information
7DS in Cluster Networks
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Sparse scenario
 Heavily partitioned network; opportunistic p2p data
sharing
Dense scenario
 full network connectivity; multihop routing for
communication
Cluster network
 A cluster is an isolated island disconnected from the
world
 Nodes within a cluster connected by multihop routes
 Network consists of multiple clusters
 Likely scenario since nodes are heterogeneous
distributed

Context: Email delivery application

Should we incorporate multihop forwarding to 7DS?
A Snapshot of a Cluster Network
AP
Route exists
to connect
to AP
No route to
connect to
AP
Mean Cluster Size E[Cu(n)]
• E[Cu(n)]=A exp (B )
•  denotes # neighbors
• Least square fitting 
A=0.9694; B=0.9992
• Mean cluster size
exponentially related
to mean # neighbors
2000 simulations
n=200 nodes
uniformly distributed
• Percolation theory
shows that many metrics
are bounded by
exponential function of
node density
Small
Small variance
variance of
of sample
sample mean
mean
of
of cluster
cluster size
size
– We have identified
bound is (almost) exact
for E[Cu(n)]
Email Delivery Application
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If multihop route discovery fails
to find AP, i.e. Pc <1,
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If route discovery fails to
find AP, it is likely cluster
size is small
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it is likely <4
mean cluster size < e=55
Flooding cluster with replicas
is justified
Overhead for finding cluster
boundary using MST is
also small
Always perform route
discovery to find route to AP
for immediate email delivery
If no route is found, SRC node
creates replicas according to
message replication schemes
Pc: Prob. of connecting to AP
nAP: #APs (nAP« n)
n: # nodes (n=200)
3 Message Replication Schemes
Boundary
Gossiping
Random Walk
Nodes at cluster
boundary are more
likely to meet an AP
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Each node forwards a
message with some prob.

No boundary discovery
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Discover cluster
boundary using MST
or Dijkstra shortest
path algorithm

Most replicas are close
to SRC, not boundary
 inefficient

Source node creates m
replicas
Tx node deletes the
replica after successful
transmission
# replicas independent
of cluster size
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Conclusions
• 7DS makes transparent data exchange, even
in absence of Internet, possible
• Data Propagation through and out side of the
local network
– By new nodes joining and others leaving 7DS
Network.
• No user intervention unless absolutely
necessary
• New step in practical, large-scale wireless
networking with gadgets?
– Remains to be seen