CMPE 257: Wireless and
Mobile Networking
SET 3b:
Medium Access Control
Protocols
Spring 2005
CMPE257
1
Channel Access Schemes

Contention based schemes
ALOHA, CSMA/CA (FAMA, MACA, MACAW,
IEEE 802.11) : with/without RTS/CTS
handshakes.
 Difficulties: not scalable, fairness, QoS.


Scheduled schemes
FDMA/TDMA/CDMA in multi-hop networks:
graph coloring problem — UxDMA.
 Node/link activation based on NCR
(Neighbor-aware Contention Resolution)

Spring 2005
UCSC CMPE257
2
UxDMA [R01]


Channel assignments (code in CDMA, timeslot in TDMA and frequency in FDMA) are
abstracted as graph coloring problems.
Several atomic constraints are identified.
Node-based constraint
Edge-based constraint
B
B
A
A
C
Vtr0
Spring 2005
E.g.: Two adjacent cells cannot
use the same freq. set.
E tr0
UCSC CMPE257
E.g.: A node (A) cannot
transmit and receive at the
same time.
3
UxDMA (Cont’d)

Channel assignments can be classified
based on certain sets of constraints.
(T/F)DMA broadcast schedule/assignment
Vtr0 , Vtt1
 RTS/CTS protocols

0
rr
0
tt
0
tr
1
tt
1
tr
E ,E ,E ,E ,E

Then a unified algorithm for efficient
(T/F/C)DMA channel assignments is
proposed using global topology.
Spring 2005
UCSC CMPE257
4
Scheduled Access

Problem description:


Given a set of contenders Mi of an entity i
in contention context t, how does i
determine whether itself is the winner
during t ?
Topology dependence:
Exactly two-hop neighbor information
required to resolve contentions.
 In ad hoc networks, two-hop neighbors are
acquired by each node broadcasting its
one-hop neighbor set.

Spring 2005
UCSC CMPE257
5
Goals to Achieve



Collision-free — avoid hidden terminal
problem, no waste on transmissions;
Fair — the probability of accessing the
channel is proportional to contention;
Live — capable of yielding at least one
transmission each time slot.
Spring 2005
UCSC CMPE257
6
Neighbor-Aware Contention
Resolution (NCR)

In each contention context (time slot t ):

Compute priorities
p  Rand (k  t )  k , k  M i  {i}
t
k

i is the winner for channel access if:
j  M i , p  p
t
i
6
Spring 2005
t
j
4
9
a
UCSC CMPE257
5
c
e
b
2
Contention Floor d
7
Channel Access Probability:


Dependent on the number of contenders
in the neighborhood.
Channel access probability:

Bandwidth allocation general formula to i
qi 
Spring 2005

Ii
I
kM i {i} k
UCSC CMPE257
8
NAMA: Node Activation
Multiple Access (Broadcast)



Channel is time-slotted.
Transmissions are broadcasts via omnidirectional antenna: all one-hop neighbors
can receive the packet from a node.
The contenders of a node for channel
access are neighbors within two hops
because of direct and hidden terminal
contentions.
Spring 2005
UCSC CMPE257
9
Algorithm
Spring 2005
UCSC CMPE257
10
Illustration of NAMA
8
A
5
B
1
F
6
C
3
D
9
G
Spring 2005
UCSC CMPE257
4
E
2
H
11
NAMA Improvements

Inefficient activation in certain scenarios.

For example, only one node, a, can be
activated according NAMA, although several
other opportunities exist.
10
a
8
b
1
f
7
g
6
c
5
d
4
e
3
h
—— We want to activate g and d as well.
Spring 2005
UCSC CMPE257
12
Node + Link (Hybrid) Activation

Additional assumption



Radio transceiver is capable of code division
channelization (DSSS —— direct sequence
spread spectrum)
Code set is C .
Code assignment for each node is per
time slot:
i .code = i .prio mod |C |
Spring 2005
UCSC CMPE257
13
Hybrid Activation Multiple
Access (HAMA)

Node state classification per time slot
according to their priorities.





Receiver (Rx): intermediate prio among onehop neighbors.
Drain (DRx): lowest prio amongst one-hop.
BTx: highest prio among two-hop.
UTx: highest prio among one-hop.
DTx: highest prio among the one-hop of a
drain.
Spring 2005
UCSC CMPE257
14
HAMA (cont.)

Transmission schedules:



BTx —> all one-hop neighbors.
UTx —> selected one-hops, which are in Rx
state, and the UTx has the highest prio
among the one-hop neighbors of the receiver.
DTx —> Drains (DRx), and the DTx has the
highest prio among the one-hops of the DRx.
Spring 2005
UCSC CMPE257
15
HAMA Operations


Suppose no conflict in code assignment.
Nodal states are denoted beside each node:


Node D converted from Rx to DTx.
Benefit: one-activation in NAMA to four possible
activations in HAMA.
Spring 2005
10-BTx
a
8-Rx
b
1-DRx
f
7-UTx
g
6-Rx
c
5-DTx
d
UCSC CMPE257
4-DRx
e
3-DRx
h
16
Other Channel Access Protocols

Other protocols using omni-directional
antennas:



Protocols that work when uni-directional
links exist.


LAMA: Link Activation Multiple Access
PAMA: Pair-wise Activation Multiple Access
Node A can receive node B’ s transmission
but B cannot receive A’ s.
Protocols using direct antenna systems.
Spring 2005
UCSC CMPE257
17
Channel Access Probability
Analysis of NAMA

The channel access probability for a single
node i is given by
Ii
qi 
kM {i} I k
i

We are interested in average probability of
channel access in multi-hop ad hoc
networks.
Spring 2005
UCSC CMPE257
18
Ad Hoc Network Settings



Equal transmission range;
Each node knows its one- and two-hop
neighbors — Mi .
Nodes are uniformly distributed on an
infinite plane with density

.
A node may have different numbers of
neighbors in one-hop and two-hop.
Spring 2005
UCSC CMPE257
19
Counting One-Hop Neighbors


The prob of having k nodes in an area of
size S is a Poisson distribution:
Average one-hop neighbors is:

Note: the mean of r.v. with Poisson dist is
S  r 2
Spring 2005
UCSC CMPE257
20
Counting Two-hop Neighbors

Two nodes become two-hop nbrs if they
share at least one one-hop neighbor.

Average number in B(t):
Spring 2005
UCSC CMPE257
21
Counting Two-hop Neighbors



Probability of becoming two-hop:
Prob of a node staying at tr is 2t.
Summation of nodes in ring (r,2r) times
the corresponding prob of becoming twohop --- number of two-hop neighbors:
Spring 2005
UCSC CMPE257
22
Total One- and Two-hop
Neighbors


Sum:
This is average number of one-hop and
two-hop neighbors.
Spring 2005
UCSC CMPE257
23
Average Probability of
Channel Access

Apply Poisson distribution with the mean
(number of one- and two-hop neighbors)
Spring 2005
UCSC CMPE257
24
Plotting Channel Access
Probability
Spring 2005
UCSC CMPE257
25
Comparison of Channel Access
Probability
Spring 2005
UCSC CMPE257
26
Delay per Node



Delay is related with the probability of
channel access and the load at each node.
Channel access probability can be
different at each node.
Delay is considered per node.
Spring 2005
UCSC CMPE257
27
Packet Arrival and Serving:


M/G/1 with server vacation: Poisson arrival
(exponential arrival interval), service time
distribution (any), single server.
FIFO service strategy: head-of-line packet waits
for geometric distributed period Yi with
parameter 1-qi ̶ qi is the channel access
probability of node i.
Spring 2005
UCSC CMPE257
28
Service Time:



Service time: Xi = Yi + 1.
The mean and second moment of service
time:
Server vacation: V=1,
Spring 2005
UCSC CMPE257
29
Delay in The System

Pollaczek-Kinchin formula:

Take in Xi and Vi :

Delay in the system: (q>)
Spring 2005
UCSC CMPE257
30
Plotting System Delay
Spring 2005
UCSC CMPE257
31
System Throughput


Multi-hop networks have concurrent
transmissions >1.
The system can carry as many packets at
a time as all nodes can be activate.

Simple!
Spring 2005
UCSC CMPE257
32
Comparisons with CSMA
CSMA/CA by Analysis

Different slotting:
NAMA long slots
 CSMA CSMA/CA short slots


CSMA(CA) assumptions:
Heavy load (always have packets waiting)
 Channel access regulated by back-off
probability p’ in each slot.


Convert the load to comparable one in
NAMA.
Spring 2005
UCSC CMPE257
33
Convert Load in CSMA(CA) to
the Load in NAMA



Each attempt to access channel is a
packet arrival p’.
Packet duration is geometric with average
1/q.
Two state Markov chain to compute the
load.
Spring 2005
UCSC CMPE257
34
NAMA Load



Relation:
i=b is the load for each node.
qm is the channel access probability of
each node.
Spring 2005
UCSC CMPE257
35
Protocol Throughput Comparison
Spring 2005
UCSC CMPE257
36
Simulations

Two scenarios:
Fully connected: 2, 5, 10, 20 nodes.
 Multi-hop network:

100 nodes randomly placed in 1000x1000 area.
 Transmission range: 100, 200, 300, 400.


Compare with UxDMA:
Spring 2005
UCSC CMPE257
37
Fully Connected Network
(Throughput)
Spring 2005
UCSC CMPE257
38
Fully Connected Network (Delay)
Spring 2005
UCSC CMPE257
39
Multi-hop Network (Throughput)
Spring 2005
UCSC CMPE257
40
Multi-hop Network (Delay)
Spring 2005
UCSC CMPE257
41
Conclusions





NCR ensures collision-free transmissions.
Only two-hop topology information is
needed.
HAMA performs better than static
scheduling algorithms (UxDMA).
HAMA performs better than contentionbased protocols.
The use of directional antennas can
improve performance further. (Next topic)
Spring 2005
UCSC CMPE257
42
Comments



Scheduled-access protocols are evaluated
in static environments and what about
their performance in mobile networks?
Neighbor protocol will also have impact on
the performance of these protocols
Need comprehensive comparison of
contention-based and scheduled access
protocols.
Spring 2005
UCSC CMPE257
43
References



[R01] S. Ramanathan, A unified framework and algorithm for
channel assignment in wireless networks, ACM Wireless
Networks, Vol. 5, No. 2, March 1999.
[BG01] Lichun Bao and JJ, A New Approach to Channel Access
Scheduling for Ad Hoc Networks, Proc. of The Seventh ACM
Annual International Conference on Mobile Computing and
networking (MOBICOM), July 16-21, 2001, Rome, Italy.
[BG02] Lichun Bao and JJ, Hybrid Channel Access Scheduling in
Ad Hoc Networks, IEEE Tenth International Conference on
Network Protocols (ICNP), Paris, France, November 12-15,
2002.
Spring 2005
UCSC CMPE257
44