1. Is Random Access Fundamentally Inefficient?
Rajmohan Rajaraman
Northeastern University

2. 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., DCF)
What is not random access?
Coordinated access to the channel (e.g., TDMA)
Using information from higher layers (e.g., flow rates, interference)

3. 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

4. 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

5. 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

6. Informed Random Access
A joint scheduling/rate control/routing optimization scheme computes local link rates [Tassiulas-Ephremides 92, Cruz-Santhaman 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

7. 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:
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

8. What was Missing Power control and energy efficiency
Security and resiliency to DoS attacks
Fairness
Specific QoS issues