Project progress reports due today.
Homework 2 ready later today –
due 6/2 (next Friday)
Graded HW 1 and solutions ready
Third paper summary on ad-hoc
networks due next Wednesday.
Each node generates independent data
Any node can communicate with any other.
No centralized controller (self-configuring)
Data transmitted in (short) packets
Links typically symmetric.
Nodes may be mobile and/or power constrained.
Typically a large number of nodes
Short-term networks (e.g. convention)
Medical applications (on-body)
Cellular phone evolution
Communication infrastructure for
Widely different channel characteristics,
distances, mobility, and rate requirements.
Link Layer design
Channel sharing (MAC/reuse)
Must exploit synergies
between design layers
Link Layer Issues
Modulation and Coding
Rate, power, BER, code, framing.
Control and communication requirements
Binary or adaptive.
DS Spread Spectrum
FH Spread Spectrum
Dynamic channel allocation
hard for packet data
Fixed allocation inefficient
Hard to implement when node
locations dynamically change
Distributed dynamic channel
allocation hard to do
FD typically only used to
create hierarchical networks
Fixed allocation inefficient and
impractical (as in FD)
Hidden nodes degrade performance
Busy tone may interfere with
transmission to other nodes.
Common spreading code for all nodes
Collisions occur whenever receiver
can “hear” two or more transmissions.
Near-far effect improves capture.
Each receiver assigned a spreading
All transmissions to that receiver use
Collisions occur if 2 signals destined
for same receiver arrive at same time.
Can randomize transmission time.
Little time needed to synchronize.
Transmitters must know code of
Complicates route discovery.
Multiple transmissions for broadcasting.
Each transmitter uses a unique
Receiver must determine sequence of
Complicates route discovery.
Good broadcasting properties
Poor acquisition performance
Preamble vs. Data assignment
Preamble may use common code that
contains information about data code
Data may use specific code
Advantages of common and specific
Easy acquisition of preamble
Few collisions on short preamble
New transmissions don’t interfere with
the data block
Data link control
Packet acknowledgements needed
May be lost on reverse link
Should negative ACKs be used.
Combined ARQ and coding
Retransmissions cause delay
Coding may reduce data rate
Balance may be adaptive
Transmitter listens for forwarded packet
Not possible with directive antennas.
Large delays in FIFO queues.
More likely to experience collisions than a
Hop-by-hop or end-to-end or both.
Bit/Packet error rate
Linkcan adapt to maintain
connectivity (adapt rate, power,…)
Interaction with routing protocol.
Power increase may affect other
nodes (Bambos technique).
How many connected nodes
constitute a network
Or, take what you can get.
Broadcast packet to all neighbors
Robust for fast changing topologies.
Little explicit overhead
Routes follow a sequence of links
Explicit end-to-end connection
Less overhead/less randomness
Hard to maintain under rapid dynamics.
Packets forwarded towards destination
Route computed at centralized node
Most efficient route computation.
Can’t adapt to fast topology changes.
Distributed route computation
Each node transmits connectivity
information to other nodes.
Nodes determine end-to-end route
based on this local information.
Adapts locally but not globally.
Nodes exchange local routing tables
Node determines next hop based on
Deals well with connectivity dynamics.
Routing loops common.
Clusterhead gateway switch routing
Wireless routing protocol
On-demand distance vector routing
Dynamic source routing
Temporally ordered routing
Signal stability routing
*”A review of current routing protocols for ad hoc mobile
wireless networks,” Royer and Toh, IEEE Personal
Communications Magzine, April 1999.
Tradeoffs in overhead size
Synergies of routing and
packet forwarding with link
Interface with wired networks
Capacity limits of ad-hoc 3D
Data rates per node
Number of nodes
N users uniformly distributed over the interior
of a sphere.
Each user communicates with another user
randomly chosen among all users.
Signal power decays based on free space path
All users transmit at the same power.
No channel separation or diversity.
Interference acts as additive white Gaussian
The total number of bits that may be
transmitted by all users, per second,
C K N 3
Proportional to the cube root of N
Based on deterministic routing scheme.
Lower Bound Proof
Estimate the effects of interference in the
limit of large N.
Construct a series of cell tessellations with
Use the weak law of large numbers to
prove the existence of one user in each
Specify a routing and transmitting scheme
using time sharing.
Determine the capacity of this scheme,
which lower bounds the capacity of the
What has changed
Signal processing is better,
cheaper, and lower power.
More powerful channel codes.
Multiuser detection and smart
Signal strength measuring
techniques available in radios.
How would we leverage these
developments to make better
Data highly correlated in time and space.
Low homogeneous rates.
Links typically asymmetric.
Data flows to centralized location.
Energy is the driving constraint.
Have a common mission.
Very different from typical ad-hoc networks