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MCBC: Highly Scalable MAC Protocol
Computer Laboratory
Approaching theoretical throughput limits in Digital Technology Group
contention-based wireless networks
Bogdan Roman† and Frank Stajano
Overview subcarrier bursts (i.e. non-modulated 200
B: 24Mbps
Number of nodes
Runtime: 5min
subcarriers). 150
Avg. frames: 580.04
One can imagine a future where most Min. frames: 505
100
Std. deviation: 18.79
people own wireless devices potentially Subc. idle
50
running network hungry applications in Contender
Detects subc.
Contender
j ≠ null
large conference halls, congested high- Referee
0
0 0 0 0 0 0 0 0 0 0 0 0
Activates Detects 44 46 48 50 52 54 56 58 60 62 64 66
ways etc. Such networks may easily subcarrier i
Referee
subc. i or idle
Successful frames per node
Detects subc.
reach hundreds and even thousands of j ≠ i ≠ null
Nominee Nominee A network of 1000 nodes under asymptotic load
active nodes. Vehicular networks in
particular have very high mobility levels Contention slot Feedback slot
• Topology independent. The col-
and rapid topology changes imposing Simplified state diagram of the contention alg. lision resolution algorithm is non-
very harsh conditions for wireless com- adaptive and does not rely on outcomes
munication. Characteristics and Advantages of past transmission attempts.
Contention-based MAC protocols are • Virtually constant throughput.
suited for ad hoc networking but, in or- • Reduced contention overhead. The high probability of non-collision
der to support such extreme demands, With the modified PHY, no data en- and the efficiency of the RTS/CTS
the collision resolution algorithm needs capsulation is needed at the MAC and scheme allows the protocol to maintain
to be highly efficient to minimize the no symbol modulation is needed at network throughputs close to theoreti-
collision probability for any network the PHY, thus drastically reducing the cal limits, even at extreme loads.
size, impose a low overhead to main- global turnaround time.
MCBC, 6M 802.11a, 6M
tain high throughput and, ideally, net- MCBC, Ps
802.11a, Ps
MCBC, 24M
MCBC, 54M
802.11a, 24M
802.11a, 54M
work topology independent. CW RTS 1.0
0.9
c1 b 1 c 2 b 2 c 3 b 3 cns bns
Multi-Carrier Burst Contention 2nsts
0.8
0.7
We propose a high performance solu- Contention (c) and feedback (b) slots 0.6
S, Ps
tion based on active contention through 0.5
node elimination, with the following • Low collision probability. The con-
0.4
0.3
main components: tention scheme using both the time and
0.2
frequency domains is able to finish with 0.1
• Tweaked OFDM PHY. With minor a unique contention winner with a very
0.0
modifications to the standard 802.11a high probability, as the number of con- 50
0
10
00
15
00
20
00
25
00
30
00
35
00
40
00
45
00
50
00
nodes nn
PHY, a subset of the 52 OFDM sub- tenders decrease exponentially.
carriers set can be used by the MAC to Normalized network throughput
send and sense very short bursts, form- n = 500, n = 5, n = 4, p = 0.5 n s f t
n=264 c=70 n=38 c=11 n=6 c=2 n=2 c=1 n=0 c=1
ing the basis of the active contention f r=236 r=430 r=462 r=489 r=494 r=498 r=498 r=499 r=499 r=499
Other Applications
algorithm. f
f
70
62
11
8
4 2
2 1
3
f2 68 10 - Because of the ability to deal with
1
MAC
CSS
(bitmask)
f1
c
64
1b c b 1 c
9
b
2 c b c
2 b
hidden nodes before any MAC data is
1
3 3
1
4 4 5 5
t
Down-mix
FFT 64
nf
Symbol det.
MAC
transmitted, the throughput of the pro-
+ ADC + P2S Typical contention session example with 500 nodes
tocol can be increased by as much as
• Hidden node handling. The proto- 25% by dropping the RTS/CTS scheme
Tweaked OFDM PHY for fast active contention
• Microsecond synchronization. In col inherently deals with hidden nodes and employing a fast-adaptive algo-
order to further reduce the contention as early as the contention session due rithm to maintain a low collision prob-
to its feedback nature. ability.
overhead, an external synchronization
- Strict and relaxed QoS can be
source should be used that provides mi-
achieved by mapping the traffic cat-
crosecond accuracy. A GPS clock offers (a) (b)
egories and statistical round-robin
up to 15 ns accuracy. 1 2 3 4 5 schemes onto the subset of OFDM sub-
• Active contention algorithm. A Hidden nodes scenarios carriers used for contention.
node elimination algorithm lies at the - The fast convergence and lack of mes-
base of the contention scheme where • Fairness. All nodes use the same sage exchange allow the proposed con-
contender nodes are eliminated by ref- contention parameters so fairness and tention algorithm to be used in fast
eree nodes by activating and sensing no-starvation are statistically ensured. leader election protocols.
† Supported in part by the Ratiu Foundation UK and Felix Telecom Romania.
Based on
[1] Bogdan Roman and Frank Stajano. An Arbitrarily Scalable MAC Protocol for Wireless Networks. (in submission)
http://www.cl.cam.ac.uk/research/dtg
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