Download Mesh vs. point-to-multipoint topology

MESH VS. POINT-TO-MULTIPOINT TOPOLOGY: A COVERAGE AND SPECTRUM
EFFICIENCY COMPARISON
Siamiik Naghian
P.O. Box 301, FIN-00045 NOMA GROUP
E-mail: [email protected]
Abstruct - AdHoc and Mesh networks are currently
under heavy research around the world. It is also
becoming an area of great interest in the future
network architecture such as 4G. It is expected that
these networks will be dominantly present both in the
proximity and wide area networks. In this paper we
present performance perspectives of wireless mesh network
topology in compared with Point-to-Multi Point (PMP) in
terms of coverage and spectrum effciency. Simulation
results shows that thought the coverage effciency of mesh
networking is overwhelming, advanced methods should be
used tu improve its spectrum efficiency. The key results o f the
performance evaluation based on the simulations are
presented.
Keywords - MANET, Ad-Hoc, mesh, PMP, networks.
1. INTRODUCTION
Wireless networking technology is entering a new era in
terms of the networking paradigms and architecture.
Relying on the principles of Ad-Hoc networking, Wireless
Mesh Network (WMN) is an attractive alternative solution
for broadband Intemet access and the future networks such
as 4G that are expected to be more self-organized and
distributed than legacy networks. The network is formed
by a number of nodes that are capable of packet routing
and mesh link formation. The primary characteristic of
WMN networks is that they are self-organizing and selfhealing networks that can be deployed easily and
incrementally, making them cost efficient and flexible.
Thought, WMNs are mainly fixed or the degree of
mobility they provide is very limited e.g. nomadic
mobility, yet the network topology remains highly
dynamic.
WMN and Ad-Hoc networks have been an area of great
interest during last decades within the academia. Recently,
it has attracted the attention of the industry, as well. The
inception of infrastructureless Ad-Hoc networks backs to
the 1970s [ I ] when it was referred to as packet radio
networks. Since then, wireless Ad-Hoc networks have
been investigated specifically within the scope of wireless
routing. The results of the earlier work have been
materialized by developing a number of routing methods
[2, 31. A number of them constitute the main IntemetDrafts in Mobile Ad-hoc Network (MANET). Out of them
Ad Hoc On-demand Distance Vector (AODV) has very
recently been requested for comments as the first
Experimental method [4]. Rapid growth in the number of
0-7803-8523-3/04/$20.00 02004 IEEE.
mobile users coupled with the convergence of mobile
systems, the Internet, and short-range networking
technologies are creating unprecedented possibilities and
needs for self-organized architecture. This require much
research on the performance aspects and the capabilities of
WMNs in terms of capacity, coverage, latency and its
relation with multihop, throughput, etc.
The shadowed side of WMN is related to the overall
performance of such networks. Network modeling and
dimensioning has always been a challenge for traffic
engineers. However, with the emerge of Ad Hoc and mesh
networks, network modeling is getting more complicated
and ambiguous; modeling WMN mesh networks with
highly unstable topologies, unpredictable network
partitioning and radio resource sharing patterns, coupled
with the tight relation between routing, mobility and
multihop architecture bring demanding challenges for the
network and traffic engineering. So far, there have been
numerous studies on evaluating the performance of Ad Hoc
networks mainly from the routing and Medium Access
Control (MAC) standpoints and basically within the scope
of IEEE 802.11 MAC and MANET [5, 61. More overall
performance of Ad Hoc networks have been analyzed and
evaluated in [7] by showing that the overall per-node
throughput decreases as the number of nodes increases in a
mobile Ad Hoc network. Very recently, an interesting
approach has been presented [8] to model the nominal
capacity of Wireless Mesh Networks (WMN), tying the
network topology to the aggregate throughput. The paper
goes one step further by showing that in the presence of the
bottleneck collision domains the per-node capacity
decreases even more rapidly than what was claimed in [7],
that is O(l/n), where n represents the number of nodes in the
mesh network.
The organization of this paper is as follows. In the next
section, we present the key architectural principles and
elements of semi-infrastructured Ad-Hoc networks. The
section shows the placement of Ad-Hoc networks in the
future network environments. After that we describe the AdHoc enabling technologies and the ways they form local
Ad-Hoc networks. Concluding remarks and some thoughts
on the future research are introduced in the last section.
11. NETWORK ARCHITETCURE
The reference network topologies of this paper are mesh and
PMP topology based on IEEE 802.16. The basic building
block of mesh networks is a Wireless Router (WR). Being
mainly an emerging proprietary technologies, WRs are
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becoming very attractive for forming Wireless Mesh
Networks (WMN) based on the principles of Ad Hoc
networking. As shown in the mesh clusters in Fig I, WR is
the fundamental building block of mesh network
architecture. In this respect, WR-based mesh networks
resemble the structure of the wired Internet. A master
element of mesh access network connects the Ad-Hoc
network to the IP network, acting as a wired or wireless
backhaul. Subscriber routers are deployed throughout the
coverage area of the master element. Each subscriber
router
. ...
j
i .
-
Figure 1. WR-based mesh topology
not only provides access for attached users, hut also
becomes part of the network infrastructure by routing the
data through the network over multiple hops. This allows
wireless routers to join and detach the network even out of
the coverage of the master element. Different resource
sharing mechanisms can he utilized to control and routes
the traffic through the Ad-Hoc network.
In addition to a few proprietary WR-based Ad-Hoc
solutions, IEEE 802.16 for Point-to-Multi Point (PMP) and
IEEE 802.15 are developing standards for wireless access
systems with mesh configuration options. The former
encompasses the point-to-multipoint fixed access network
while the later handle low mobility, as well.
In addition to WR, mesh topology can be create by
utilizing other technologies such as Wireless LAN [9] and
Bluetooth [IO], as well.
Up to a certain extend, and regarding the link layer, also
IEEE 802.11 standard supports mesh configuration. In its
Ad Hoc mode, fixed Access Points (AP)are not required.
Instead, one promiscuous node is "elected as the master
of the network, forming a local network with other nodes
as slave nodes. Any node can handle the logical function
of the master. The nodes communicate directly with each
other on a peer-to-peer basis while sharing a given cell
coverage area of the master. Single shared channel, lack of
multihop routing, hidden and exposed effects coupled with
shortage in conveying secure and efficient Transmission
Control Protocol (TCP) traffics over multi-hops [I I ] are the
most severe limitations of the Wireless LAN when forming
a pure Ad-Hoc network.
Yet another mesh enabler is Bluetooth (BT) [IO, 121. It
supports point-to-point and point-to-multipoint connections
using star link topology, in which all the traffic goes via
master node. A group of BT nodes can form a piconet,
sharing the same radio channel. The BT technology defines
a scatternet structure to interconnect overlaying piconets,
forming a multihop Ad-Hoc network. The Ad-Hoc can be
formed when two or more piconets are interconnected via a
node that is a common member of neighbouring piconets,
result in a scatternet structure. In this context, the BT node
can act as a master in only one piconet but it can be
simultaneously a slave member of multiple piconets. In
addition, a BT node can only transmit and receive data in a
single piconet at a specific time span. Time-division
transmission helps when building inter-cell scheduling in
scatternets. Despite of it, he established Ad-Hoc network is
tightly limited by the BT star topology that makes the
technology inefficient for the pure Ad-Hoc networking. The
scatternet maintenance has been kept separated from the real
traffic conditions and traffic requests, leading to
unnecessary link maintenance that waste the scarce
resources in terms of power and bandwidth.
111. SIMULATION RESULTS AND DISCUSSION
The simulations were extracted based on the network
topologies illustrated in Fig. 1 in which mesh clusters and
PMP hackhaul are illustrated. The performance of the mesh
was compared with a PMP topology, as employed in the
underlying topology used in legacy networks such as radio
access part of cellular networks. It is assumed that the
wireless routers involved in the mesh are stationary nodes.
The mesh-network consists of a cluster of stations each of
which can have radio connection to one or more other
stations. All nodes, may act as repeaters with local access
for packet data traffic. Traffic is routed via one or more
intermediate nodes in its way to the destination node. The
key parameters used in the simulation are included in Table
1 and Fig. 1.
At the initial phase mesh-network needs a small number of
so called seed nodes to generate an acceptable level of
coverage. At the same time that every new node increases
the coverage for potential newcomers, the existing can he
optimized. When the network becomes denser the hop
lengths will, on average, become shorter because more
multihop connections can be applied. As a consequence,
transmit power levels can be decreased accordingly or the
modulation method can he changed to increase hopcapacity.
When the network capacity becomes more and more loaded
especially on the AP neighbouring hops, the situation can he
relieved by providing another node with a new backhaul
link to tum it into a new core network AP.This will divide
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the mesh-network into more mesh clusters with more
capacity available near the AP and possibly results in
shorter individual hops. For further capacity increase at the
access node, sectoral antennas instead of omnidirectional
antennas may be applied at these nodes.
Therefore, by utilizing a mesh topology, a good coverage
within a geographical area can be reached fast. It's enough
to have a connection to any neighbours to have access to
the others and also to the network AP.
Theoretically, in a mesh topology, the coverage probability
increases rapidly as the number of nodes in the meshcluster increases and makes the turning up of new low loss
paths more and more probable for new nodes.
In reality, the probability of reliable link between any two
points is not typically equal but depends on used system
parameters; transmit power, antenna gains and receiver
sensitivity. It is also affected by the attenuation of a link
depending on distance between the two points and
antennas heights.
Table 1. The modulation. codine rate. receiver sensitivitv
314
QPSK
16QAM
64QAM
112
314
112
314
213
3 14
-8 1
-79
9
12
-77
18
-74
24
36
48
54
-70
-66
-65
Fig. 2 shows the results of PMP vs. MESH comparison at
3.5 GHz using extension of Okumura Hata model for CS
antennas heights at 20m, 30m, and 50m, CS Gain of
IOdBi, TS Gain of IBdBi, mesh AP gain of IOdBi, node
Gain of lOdBi, and transmit power of 30dB and ZOdB, for
PMP and mesh, respectively.
In this calculation, the mesh antennas height was 9m, the
PMP Terminal Station (TS) antennas height was IOm,
PMP Central Station (CS) heights were at 20m and 50m
for different bit rates. The transmit power was set at
20dBm and 30dBm for mesh and PMP, respectively. Table
1 describes more values used for the modulation, receiver
sensitivity, and the bit rate used in the simulation.
The coverage and achievable capacities of PMP cell and
equal-size mesh cluster was simulated in suburban
environment using extension of the Okumura empirical
mode1 1131.
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Figure 2. A coverage comparison ofPMP vs. MESH at 3.5
GHz using extension of Okumura Hata model
Fig. 3 on the other hand depicts a comparison of total
capacity of mesh and PMP systems using a single channel
for each. For PMP system the total capacity in each
individual simulation is the mean of all achievable link bit
rates. For mesh topology the throughput capacity is
calculated as a weighted sum of achievable bit rates for hops
directly connected to mesh access point and other hops
nearer the mesh cluster edge divided by average number of
hops to subscriber. With PMP cell of 0.5 km radius
practically all hops can use highest bit rate of 54 Mhit/s. As
cell size increases increasing number of hops have to use
lower bit rates and total capacity decreases. Note that the
total capacity is calculated as an average of achievable links
meaning that the decrease of coverage has no effect on this
figure. The total capacity of mesh topology is always lower
than that of PMP systems since all of the mesh nodes cannot
have direct one hop-connection to mesh access point. With
increasing service area the total capacity of mesh topology
decreases faster than in PMP case due to the increase in the
number of hops.
We observed that the achievable aggregate capacity of mesh
network is about 114 to 112 of PMP system due to multihop
nature of mesh network.
This result applies only for noise limited case where the
interference of possible other PMP cells or mesh clusters is
not analyzed hut it is expected that PMP will suffer more
from interferences than mesh due to higher CS-antennas.
Also mesh nodes use on average lower transmit powers and
mesh-network therefore benefit from lower overall
interference levels.
Simulations were performed for the case without possible
surrounding PMP cells or mesh clusters causing
interference, meaning that these comparisons are valid only
for early deployment of these systems.
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networking approach isn’t so obvious. Sharing the radio
channel between neighbouring nodes, huge amount of
control signaling, multihop, etc. are the key factors that
make the efficient use of spectrum complicated.
More research is required to investigate the overall
operation of mesh topology and its behaviour in mobile
AdHoc networks. The impact of multihop on the overall
capacity of the system, end-to-end delay, and jitter are
becoming the bottleneck of such networks especially in a
mobile environment.
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REFERENCES
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Figure 3. A total capacity comparison of mesh and PMP
systems as a function of service area radius
These simulations included few practical factors which are
favourable for PMP systems; PMP terminal stations used
directional antennas, higher transmitted powers, and the
path attenuation levels of PMP systems are lower due to
higher antenna heights especially at central station. Even
with these assumptions Ad-Hoc mesh topology has
comparable or better coverage than PMP systems.
Furthermore, being very scalable, the coverage vs.
capacity of mesh network can be optimized based on the
use of the power control and routing algorithms. The total
capacity per unit area can increase at nearly linear rate
with increasing density. On the other hand, the scaling
down of the microcells around the nodes permits efficient
reuse of the spectrum throughout the region.
IV. CONCLUSIONS
This paper presents the performance of Mesh vs. PMP
topology in terms of coverage and spectrum efficiency.
AdHoc networking is becoming an attractive technology
when shifting to the radical development of the future
wireless networks. The networking technology can be
utilized to extend the coverage of the future networks
efficiently, with high scalability and cost-effectively. Also,
Ad-Hoc networking is presented as a potential means for
creating local networks efficiently and on a demand basis.
The Ad-Hoc Domain is foreseen to he the most likely
underlying domain for providing group and local
networking. Furthermore, it helps alleviate the
interoperation between different radio accesses involved in
the future networks.
In spite of long-term research on AdHoc networks, there
are still some essential aspects of the technology that need
to be brightened. Thought, the coverage efficiency of mesh
topology is overwhelming the spectrum efficiency of the
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