Is Random Access
Fundamentally Inefficient?
Rajmohan Rajaraman
Northeastern University
What is Random Access?
• Each node accesses the channel independently
with a certain probability
– Fundamentally a symmetry-breaking technique
• Different protocols vary according to how this
probability is chosen:
– Fixed (e.g., Aloha)
– Fixed with carrier sense (e.g., basic CSMA protocols)
– Varies dynamically including collision avoidance and
backoff (e.g., 802.11 DCF)
• What is not random access?
– Coordinated access to the channel (e.g., TDMA)
– Using information from higher layers (e.g., flow rates,
interference)
Single-Hop Scenario
• All nodes can hear one another
• Uniform rate r for each of n links -- nr < 1:
– Random access with fixed probability can achieve throughput
within 1/e of optimal (Aloha)
– For nr close to 1, TDMA clearly more efficient
• Arbitrary rates ri with  ri < 1:
– Random access with uniform probability clearly inefficient
– Random access with rate-dependent probabilities can achieve
throughput within 1/e of optimal (Chafekar et al 2008)
– Again, as the aggregate rates come close to 1, scheduled
schemes outperform
Dynamic Link Rates
• Random access determines access probabilities based
on channel contention
– Backoff schemes
• For low aggregate rates, backoff methods efficient
• In theory, exponential backoff schemes converge much
slowly and are unstable for even small aggregate rates
bounded away from 1 (Leighton et al 88)
– Polynomial backoff schemes -- contention window polynomial in
no. of collisions -- more effective (Goldberg-Mackenzie 97)
– Poor average delay
• In the absence of information about dynamics, random
access with backoff probably competitive
Multihop Networks
• Random access (in its basic form) susceptible to
interference problems
– Information available from channel different for different users
sharing the same channel
– Fundamentally inefficient (cannot address hidden/exposed
terminals)
• Random access also oblivious to end-to-end flow control
– Access probabilities based on channel contention may be
inefficient and inconsistent with flow control
• The above problem is not with the “random” part of
“random access” -- it is due to the lack of information
Informed Random Access
• A joint scheduling/rate control/routing optimization scheme
computes local link rates [Tassiulas-Ephremides 92, CruzSanthaman 03, Jain et al 03]
– Takes interference into account
– End-to-end flow control and packet routing
– A very complex problem, but orthogonal to whether random access is
used at the MAC level
• Each node made aware of rates at which adjacent links would be
used
• Random access based on these link rates [Yi et al 07]
• In some sense reduces multihop case to single-hop case:
– Random access efficient if aggregate channel rates low [Joo-Shroff 07]
– Possibly competitive when link rates vary highly dynamically
In Closing
• The answer depends on:
– The information available to the MAC layer and whether random access
uses this information
– Whether joint rate control/routing optimization being done at higher
layers
– Saturated vs unsaturated conditions
– Kind of traffic (bursty vs constant-rate)
• Scenarios where random access could be competitive:
– Unsaturated (load in all parts of the multi-hop network should be well
below saturation)
– Highly dynamic environment where joint optimization is impractical
• Random access ineffective when:
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–
–
–
Saturated conditions
Access does not take into account interference/routing/flow info
Static non-uniform traffic patterns much better handled by scheduling
Need delay guarantees
What was Missing
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Power control and energy efficiency
Security and resiliency to DoS attacks
Fairness
Specific QoS issues