Prepared By: Amit Patel October 14, 2004

Performance, Optimization, and

Cross-Layer Design of Media Access

Protocols for Wireless Ad Hoc

Network

Stavros Toumpis and Andrea J Goldsmith

Department of Electrical Engineering, Stanford University, Stanford, CA, 94305

Prepared By: Amit Patel October 14, 2004

Prepared For:

EE653 - Cross Layer Design for Wireless Networks Stevens Institute of Technology, Hoboken, NJ, 07030

Agenda

„

Background and Overview.

Purpose of this paper.

„

Survey of wireless network features and how they

perform.

„

New MAC protocols.

Conclusions.

„

Shortcomings of the paper, future work.

References.

Purpose and Overview (1)

„

Want to study the effects of cross-layer design

choices of MAC protocols on a wireless ad-hoc

network.

„

Design choices that are associated with different

aspects of MAC protocols:

„ Power control.

„ Queueing discipline (ex. FIFO, priority…)

„ Routing decisions (single vs. multiple next hops…) „ Channel Access (CSMA/CA, TDMA, contention vs.

Purpose and Overview (2)

„

How to compare design choices ?

„

How will you know one choice performs better than

another ?

„

Metrics must be collected:

„ Throughput of data (overall capacity). „ Power consumed.

„ Latency and jitter. „ Congestion.

How to study these problems?

„

Method 1:

„ Build all different types of testbeds and individually change

design choices.

„ Run a series of experiments (or simulations). „ Collect data and compare final choices.

„

Method 2:

„ Build a mathematical model that encompasses a way to

implement the different aspects of design elements of the MAC protocols.

„ Insure the model can output desired metrics as design

elements are changed so results can be compared.

The Model (1)

„

Wireless link aspects of the model.

„ Essentially free space path loss model that ignores

shadowing and fast fading details.

„ Makes use of the free space formula for SINR and adds in an

additional noise component ( for thermal, background…)

„ Assumes if SINR is greater than some Threshold SINR, then

the packet is successfully received and decoded.

„

Queueing assumptions.

„ Traditionally, networks use FIFO ordering, but this model

The Model (2)

„ 7’s queue has a packet for Q[2], 8 has Q[7, 10]

„ Out of order transmissions permits 7, 8 to TX at the same time

to get to their respective destinations without causing delays and better utilizing the time available.

„ Non-FIFO allows for optimizing the transmission schedules.

The Model (3)

„ Model must capture the transport capability of an ad-hoc

network. Many issues to describe the network.

„ Different TX/RX power.

„ Different routing paths taken. „ Multiple hops vs. single hop.

„ Simultaneous TX (each of n src nodes tx to n-1 dst)

„ Results in n(n-1) dimensional convex polyhedron called a

capacity region.

„ All points inside the capacity region are described by a rate matrix. „ Rate matrices contain n(n-1) rates for all the src-dst pairs that

The Model (4)

„ Time division and multihop routing schedules that achieve an

optimal result are points on the boundary of the capacity region.

(Caveat: requires global knowledge).

„ Metric of comparison is Uniform Capacity:

„ Def: Max possible aggregate rate if all nodes tx to all other nodes

and comm. rates are equal for all n(n-1).

„ 12 node figure results in 12(11) = 132 dimensions.

„ So compare different pairs as slices of region (see later figs).

„ Now have a scheme to make different design elements and see

the effect on overall transport capability by examining the capacity region and uniform capacity.

Design Elements (2)

„

Power Control:

„ TX with max power vs. no TX at all (line b).

„ Not a critical improvement due to the multihop nature of the

network.

„ Good for reducing interferences with nearby neighbors. „ Good for overall energy consumption, if that is a concern.

„

Routing Effects:

„ Assuming single min-hop count routes used between 2

nodes where a link is viable only if its SINR > Threshold(SINR).

„ 3 threshold values were chosen for different link qualities

CSMA/CA

„

Classic channel access with known problems.

„

Shortcomings of 802.11 (RTS-CTS-MSG-ACK).

neighboring nodes.

„ High overhead due to collision avoidance mechanisms. „ No integrated power control.

„

MAC Protocols are described using simulation

and not analytically.

„ Will have similar slices of capacity regions and uniform

Additional Metrics (2)

„

CSMA/CA performs badly over weak links (as can be

seen from RP-10, RP-30, RP-120 data).

„ Current routing uses min-hop count which would utilize links

without any such consideration.

„ Weak links give rise to hidden terminals and hence extra

collisions.

„

Dichotomy Noticed:

„ Extra hops increases capacity in general because relying on

other nodes.

„ Extra hops increases usage of channel access, hence

reducing spatial reuse and increasing the probability of collisions (hence reducing bandwidth).

New Directions (1)

„

This problem coupled with the low efficiency of the

RTS-CTS handshake is a big problem for CSMA/CA

algorithms.

„

Slotting can help alleviate this by reducing contention

periods and hence lowering the probability of

collision. Slotting is the

KEY

idea behind the two

alternate protocols.

(Caveat: now requires global

time synchronization).

„

Depends on novel frame format based loosely on the

New Directions (2)

Contention slot is split into m mini-slot pairs (slotting is done at a much finer level). These contention intervals finally result in a src-dst pair agreeing to exchange data during the data slot portion of the frame. The duration of a mini-slot pair is

Progressive Back Off

Algorithm (PBOA) (1)

„ Protocol Outline:

„ Nodes contend during contention period.

„ Unsuccessful nodes progressively backoff during progression of contention period.

„ Successful nodes use remaining contention interval to discover minimum power needed to TX their data.

„ Benefits:

„ Energy conservation.

„ Backoff reduces interference and collisions.

„ PBOA Performance:

„ Provides power control, m, p design variables to be adjusted per network topology, minislot overhead, and traffic patterns.

„ Figures show PBOA is much better than CSMA/CA in throughput, power.

„ But PBOA nodes can still waste contention time by TX an RTS to a node who has already sent a CTS to another originator.

Progressive Back Off

Algorithm (PBOA) (2)

Basic state transition diagram for nodes participating in the PBOA algorithm.

Progressive Back Off

Algorithm (PBOA) (3)

Progressive Back Off

Algorithm (PBOA) (4)

Progressive Ramp Up

Algorithm (PRUA) (1)

„ Protocol Outline:

„ Nodes monitor the channel for favorable conditions and sends an RTS

with some probability p at the beginning of the next RTS minislot.

„ If successful, it continues to send RTS packets to notify others to

backoff. If unsuccessful, it backs off and tries again at a later RTS minislot.

„ No transmitter power control. „ Benefits:

„ Does not obey FIFO queueing, so best reachable destination is

transmitted to.

„ PRUA employees carrier sense but it is tuned to detecting CTS and

hence nodes will avoid extra RTS transmissions and unnecessary interference and collisions experienced by PBOA.

„ PRUA Performance:

Progressive Ramp Up

Algorithm (PRUA) (2)

„ All items combined below represent a set of single state

transition conditions for A to TX an RTS in ith minislot.

„ A has not TX a CTS in the last CTS minislot.

„ A has received the sum total power of all other transmitters and it

is less then threshold power Pt.

„ If no RTS was decoded in the last RTS timeout, and A has a

non-empty queue.

„ If A decoded an RTS from C and has a Q[B] where B!=C and B can

decode in presence of interference from C, A will contend.

„ A performs a biased coin toss with P(success) = p and succeeds.

„ A successful reception of CTS in last CTS minislot is required to

qualify to send data.

Conclusions (1)

„

Key observation of protocol performance is that they

look worse than some optimal choices because these

two protocols are distributed and hence require

global knowledge to schedule their transmissions.

„

Cross-layer design aspects of this paper.

„ Adaptive MAC protocol sensitive to contention.

„ Influence of network layer FIFO queueing on better BW

utilization.

„ Importance of transmission scheduling. „ Routing and power control interactions.

Conclusions (2)

„

Power control helps reduce energy consumption but

does not have a big impact on throughput efficiency.

„

FIFO queueing is not very useful.

„

Increased route diversity improves performance.

CSMA/CA is not good over weak links.

Shortcomings of this paper (or

future work)

„

Depends on global time synchronization.

„

Non FIFO queueing breaks some priority ordering of

packets provisioned for QoS by another layer.

„

Ignores multicast traffic (could be a large amounts).

Model assumes centralized time division and routing

scheme.

„

Assumes no routing overhead (discovery,

maintenance).

References

„

Toumpis, Stavros, Andrea Goldsmith. “Performance,

Optimization, and Cross-Layer Design of Media

Access Protocols for Wireless Ad Hoc Networks”. ICC

2003, May 2003, pp 2234-2240.

„

Toumpis, Stavros, Andrea Goldsmith. “Capacity

Regions for Wireless Ad Hoc Networks”. IEEE

Transactions on Wireless Communications, Vol 2.

NO. 4, July 2003.

Extra materials – Capacity

region model details (1)

„ Transmission scheme S is a

description of information flow between nodes in a network at some given time.

„ Linearity for summation of

weighted matrices holds.

„ A given transmission protocol

is described by a set of basic rate matrices.

Extra materials – Capacity

region model details (2)

„

A transmission protocol can be described by a rate

matrix representing some scheme and/or schedule.

Extra materials – Capacity

region model details (3)

„

Capacity regions are defined to be the convex hull

(CH) of the basic rate matrices. The full set of rate

matrices describes a particular protocol. (eqn. 4)

„

By solving the LP problem iteratively, the convex hull

representing the boundary points of the capacity

region can be found. (eqn. 5)

„

These points are the optimal operation mode for the

Extra materials – Capacity

region model details (4)

„

For different design choices, get different numbers of

schemes represented by different sets of matrices.

„ Single-hop routing, no spatial reuse. „ Multihop routing, no spatial reuse. „ Multihop routing, with spatial reuse. „ Power control.

„ Successive interference cancellation.

„

Eventually expanded model to cover:

„ Time-varying flat fading channels. „ Node mobility.