Hierarchical Cellular-Based Management for Mobile Hosts in Ad-Hoc Wireless Networks Chih-Yung Chang*, Chao-Tsun Chang+ Department of Computer and Information Science, Aletheia University
[email protected] + Department of Computer Science and Information Engineering, National Central University
[email protected] *
Corresponding author: Prof. Chih-Yung Chang Department of Computer and Information Science, Aletheia University 32 Chen-Li St., Tamsui, Taipei, Taiwan Tel: +886-2-29903836 Fax:+886-2-89944322 Email:
[email protected]
Abstract This study proposes a hierarchical Cellular-Based management model for Mobile Ad-Hoc Networks. The proposed management model allows the mobile host to establish a stable communication path with fewer flooding messages and smaller hop count. In the single-level management, a host is selected to act as manager, constructing a communication path and managing the mobile hosts in a specific cellular region to significantly reduce the number of flooding packets. Two-level management is also proposed, to reduce the hop count and enhance the efficiency of constructing the QoS routing path. The Cellular-Based management model is compared with Triangular and Zone based management models. Performance evaluation shows that Hierarchical Cellular-Based management more efficiently reduces the number of flooding messages and the hop count of the routing paths.
Keywords: Mobile Ad-Hoc Networks (MANET), mobile host, routing path, flooding, hop count, QoS.
1. Introduction Scientific and technological progress is increasing the demand for communication quality and the ability to communicate with other persons at any time and any place. A PCS (Personal Communication System) provides one-hop communication in which the base station (BS) facilitates communication with mobile hosts. However, certain regions are infeasible for establishing base stations or access points due to high cost, low utilization, or poor performance. Additionally, wireless infrastructure is unavailable in situations such as battlefield communications and 1
search and rescue operations. An Ad-Hoc network consisting of mobile hosts provides low cost and high mobility communications without requiring a supporting base station or access point. Unlike a static network, Ad-Hoc networks have no infrastructure. Each mobile host acts as a router, relaying information from one neighbor to others. Packet flooding is a general technique for establishing a routing path from source host to the destination. Recently, many researchers have investigated the routing problem, proposing many protocols to enhance performance. Most of these protocols can be classified into two categories: Table-Driven protocols or On-Demand protocols. In Table Driven [4] protocols, each host maintains a routing table according to the link states of neighbors. Consequently, although the optimal path can be found whenever a source host issues a request for communication with another host, the Table Driven routing protocol has considerable overheads in maintaining routing table. On the other hand, the On-Demand routing protocols [1][2][3][5][7][9] establish routing path in an on-demand manner. The source host issues a search packet and spreads the packet through flooding to look for the destination host. When executing the flooding operations, hosts record the visiting path in the search packet. The destination host chooses the shortest path from the received packets and then sends a reply to the source host. Consequently, the On-Demand routing protocol takes longer to establish a routing path than the Table-Driven protocol does in MANET. The search packet is spread from the source host to the destination host by a flooding operation which is very costly and results in serious redundancy, contention, and collision[10]. A set of protocols belonging to the On-Demand class can be further divided into two subclasses, namely either flat or cluster-based management. To reduce the amount of flooding packets, many protocols [8][12] partition the MANET into several clusters or grids. Hosts in each partitioned cluster will vote for a header to manage the cluster, with the manager being responsible for controlling message exchange with the managers of neighboring clusters. Hosts wishing to establish a communication path should first send a request packet to their cluster manager, and the manager will then relay the packet to neighboring managers through flooding until the manager of the destination host is found. These two proposed management protocols alleviate flooding, but they increase management overheads. Some other location-aware protocols [7][11] use GPS(Global Positioning System) to provide the location information for establishing a routing path. In [7], the MANET is geographically partitioned into several grids (or called zones). Equipped with GPS, the host can identify which grid it is located in. In each grid, a host that is geographically near the center of this grid will be considered to be 2
a manager for executing the information exchange. However, the grid size is not determined. Furthermore, in a large grid the movement of the manager will incur a situation in which the signal transmitted by the manager is too weak to communicate directly with neighboring managers. In this case, a gateway host is needed to relay the manager’s message to the neighboring managers. To efficiently establish a communication path, alleviate the flooding phenomenon, minimize the data of routing table and increase the stability, a two-level Cellular-Based management protocol is offered herein. We geographically partition the MANET in a cellular way and develop a QoS routing protocol to extract a communication path with a smaller hop count by fewer flooding operations. The rest of this study is organized as follow. Section 2 illustrates the background and basic concepts of single-level Cellular-Based management. To reduce the hop count and resolve the congestion problem, a two-level management protocol is proposed in Section 3. Meanwhile, Section 4 proposes the performance evaluation of Cellular-Based management. Conclusions are finally made in Section 5.
2. Single-Level Cellular-Based Management This section first reviews the conventional routing protocol, then presents a single-level cellular-based management protocol.
2.1 Previous Works The Ad-Hoc wireless network contains no base station to support communication among mobile hosts. The source host constructs the routing path by first sending the request to all its neighbors. As soon as the neighboring hosts receive the request message they check the destination ID of the request packet. Hosts that are not the destination host should then relay the request packets to all of their neighbors. By considering all possible paths from the source to destination, the source host can establish the shortest communication path (with the minimal hop count). For those protocols developed in a flat model, all hosts act as routers relaying messages among their neighbors. However, applying these protocols usually causes packet flooding. Ni[10] presented the problem of broadcasting storm and demonstrated the negative effects of the flooding operation. The cluster-based model is proposed to alleviate flooding. The developed cluster-based routing protocol can be categorized into two classes, according to whether or not the GPS is adopted. Without GPS, mobile hosts should execute a distributed algorithm to define a cluster group and select a host as the cluster head (or manager). Whenever a member host desires to establish a communication path, it sends a request to the local manager and the manager and gateway of each cluster then take responsibility for setting up the communication path. The extent of packet flooding is thus significantly reduced since only managers or gateways relay the 3
request packets between clusters. In this scheme the manager must maintain the information of the mobile hosts that are located in its cluster. Applying management rules to reduce the number of flooding packets may increase the number of hops on a routing path. As Fig. 1 shows, applying cluster-based management allows a 4-hop communication path to be constructed for hosts A and B. Communication can run through a one-hop link since hosts A and B can communicate with each other directly, rather than relaying message via their managers. A
Member
B
Manager
Gateway
Figure 1: A routing path is constructed by applying cluster-based management protocol.
Recently, some researchers [7][11] have designed grid(or zone)-based management for mobile hosts, under the assumption that all mobile hosts are equipped with GPS and that the location at any time is known. In [11], the authors propose a peer-to-peer technique for managing mobile hosts. The proposal involves geographically dividing MANET into several zones. Each host can then evaluate the zone number in which it is located. Many flooding messages are incurred within the zone, since each host periodically sends its link state to all the other hosts in the same zone. Whenever a host moves between zones, all hosts in the old and new zones should update their routing table to ensure that the link states are fresh. This procedure creates significant maintenance overheads for each host. In [7], the MANET area is partitioned into many grids. Equipping each mobile host with GPS allows it to determine which grid it belongs to. Within each grid, the host located closest to the center of the grid will be selected as a manager, handling the information of all the other hosts located in that grid. The manager is responsible for exchanging information or communications with managers of the neighboring grids. When a source host wishes to establish a routing path to a destination host located in a different grid, the source host first issues a request to its manager. The routing path is then constructed by executing the manager-level flooding operation. Grid size determines whether a manager can communicate directly with other managers of the neighboring grids. All managers and hosts are assumed to have equivalent signal strength. In the worst case, two managers may move to the boundary of the grid in opposite directions. If the grid is too large, two neighboring 4
managers may be unable to communicate with each other since their signal strength will be too weak to cover the distance between them. In this case, gateways should be available to relay the message between the two managers, causing maintenance overheads.
Y (x,y+2) (x-1,y+1) (x+1,y+1) (x,y)
(3,5) (1,3) (0,2) (1,1) (0,0)
Member
(x-1,y-1) (x+1,y-1) (x,y-2)
(3,1) (2,0)
X
Manager
(a) Cellular-based partition.
(b) Cellular-id.
(c)The Relation of cellular-id of two neighboring cellulars.
Figure 2: The cellular-based partition.
2.2 Single-Level Cellular-Based Management Cellular-Based Management partitions the MANET into several disjoint and equally sized cellular regions. Figure 2(a) illustrates the partitioned MANET. Each cellular is assigned a unique cellular-id, as shown in Fig. 2(b). Figure 2(c) presents the relationship between the cellular-id of two neighboring cellulars. The manager of one region is assumed to be able to communicate directly with the managers of the neighboring regions. This assumption is made for two reasons. First, no gateway is needed to relay the message between managers, saving on overheads for maintaining the mobility of the gateway. Second, the routing, geocasting, multicasting, or broadcasting protocols are simple to develop using this management model. The model presented herein allows the manager to communicate directly with neighboring managers. In this paper, the signal radius of each host is assumed to be S. Here, we discuss and compare the partitioning of MANET into several basic units, namely Triangle, Zone, and Cellular. To meet the requirement that each manager can directly communicate with neighboring managers, the Zone, Triangle, or Cellular areas are bounded by a specific value. If a partitioned area becomes too large, the worst outcome is that the distance between two neighboring managers may exceed their transmission signal radius. Meanwhile, if the cell size is too small, a MANET may contain too many cells. Since each cell uses a manager to perform path construction work, too many cells will means too many managers and hence too many flooding packets during the establishment of a routing path. The following 5
illustrates the cellular-based partition in detail.
Selecting the Manager The manager selection process is similar to [7]; that is, the host closest to the center of a cellular region will be selected as a manager. The manager should broadcast its ID to other members, receive all member IDs, and keep these IDs in its cache table, and should also update member information according to the mobility of all hosts in the cellular. Before the manager moves to another cellular, it should select the host nearest the center of the old cellular as the new manager, and should transfer the contents of its cache table to the new candidate manager.
The Size of a Cellular Two factors should be considered in partitioning the MANET. First, to reduce flooding packets a cellular region should be as large as possible. However, an excessively large cellular region will prevent managers from communicating directly with neighboring managers, causing additional overheads such as gateway maintenance. Consequently, an upper bound of the size of a cellular region exists for a given signal strength S. 3 3 2 S ≅ 0.199852 S 2 . 26 According to this cellular size, MANET can be partitioned into a minimal number of Size of Cellular =
cellular such that each manager can directly communicate with other neighboring managers. To illustrate the effectiveness of the Cellular-Based partition in reducing flooding messages, the cellular-based partition is compared below to the triangular-based and zone-based [7] partitions. The signal radius of a mobile host is assumed to be S. To satisfy the two factors mentioned before, the size of a basic triangular region and basic zone region are, respectively Tri =
3 2 1 S ≅ 0.144337 S 2 and Zone = S 2 = 0.125 S 2 . 12 8
To ensure that each manager can communicate directly with neighboring managers, the maximum distance between two neighboring cells is set as S, as displayed in Fig. 3. Since the size of each cell, partitioned by a specific shape, is known, the number of cells in MANET can be evaluated. The measured MANET is assumed to have a size of K*K, and is partitioned into different shaped cells. The number of partitioned cells can be used to measure the number of flooding packets, since each cell contains a manager and only the manager has access to the flooding packets. Let CostT, CostZ, 6
and CostC respectively represent the number of flooding packets in the environment in which the MANET is partitioned by shapes: Triangular, Zone, and Cellular. The ratio of the flooding cost is listed as below. Cost T : Cost z = 0.144337 : 0.125 ≅ 0.875 Cost c : Cost z = 0.125 : 0.199852 ≅ 0.624 The Triangular-Based partition saves almost 12.5% more flooding packets than Zone-Based partition, but the Cellular-Based partition saves almost 37% more. Consequently, the Cellular-Based partition is the most effective in reducing flooding packets. Z
Signal radius S
Signal radius S
Z
ĉ
ĉ
(a) MANET is partitioned by shape of triangular. The signal radius is 3 Z.
(b) MANET is partitioned by shape of grid. The signal radius is 2 2 Z.
Signal radius S
Z
ĉ
(c) MANET is partitioned by shape of cellular. The signal radius is 13Z.
Figure3: The MANET is partitioned by different shape. Table 1. Measurement of hops count of routing path established by different shape partitioning for MANET Angle made by source and destination hosts Shape of Cell 0 30 45 60 90 Optimal 100% 100% 100% 100% 100% Cellular 240% 208% 328% 240% 208% Zone 283% 283% 283% 283% 283%
Besides number of flooding packets, the number of hop counts of a routing path is another factor that determines the communication performance in MANET. The improvement in reducing the number of hops on a routing path is measured by setting 7
some specific angles created by the source and destination hosts. Table 1 lists these measurements. In Table 1, value in the “Optimal” row denotes the minimal hop count established for the routing path via unmanaged flooding. In this condition, hosts can establish a shortest routing path without the constraint that path can only be established by managers, as defined in the Cellular and Grid management models. As described previously, establishing the shortest routing path by flooding causes a large number of flooding packets in MANET during path construction. The number of hops in two different management models, Cellular-Based partitioning and Grid(or Zone)-based partitioning [7][11], are compared herein. When the angle formed by the source and destination hosts is 45, Grid-based management more effectively constructs a routing path with fewer hops than Cellular-Based management. Figure 4 indicates that this is because Grid-Based management can construct a routing path along the left-up direction, thus needing just one hop where Cellular-Based management needs two to establish the routing path from source host to destination host. However, in most cases, Cellular-Based management behaves better than Grid-Based management.
Figure 4: When the angle made by source and destination hosts is 45, Grid-Based management has a better behavior than Cellular-Based management.
Cellular-Based Management Protocol Owing to mobility of mobile hosts, the topology of the Ad-Hoc network changes dynamically. The following proposes management protocols for mobile hosts such that manager can keep track of the newest member information.
Host-id C B D A E
Manager
B
Member
A C
member manager
D E
Figure 5Member table (MT) stored in manager B.
8
As mentioned before, the host located closest to the center of the cellular region will be selected as a manager. A manager of a cellular zone will be changed if it moves to a new cellular. Each host in a cellular should record the ID of the cellular region’s manager so that they can relay requests to the manager whenever they need to construct a routing path. The manager will maintain a member table(MT) containing the ID of all the hosts located in a cellular. Figure 5 displays the member table kept in the cache of the manager. The manager should periodically broadcast a packet MANAGER(cellular-id, manager-id) to notify members of its region and managers of neighboring regions of its presence. A new manager must be sought if the current manager suffers a fault or suddenly loses power. If no MANAGER packet is received for a long time, member hosts will repeat the manager selection process. In such cases, host members will issue a CHOICE(cellular-id, host-id, location) packet. Host that receives the CHOICE packet will calculate the distance between the cellular center and the sender. If the member host receives a CHOICE packet and knows that the sender’s location is nearer to the cellular center than its current location, the member host will stop issuing its own CHOICE packet, thus guaranteeing that only one member host can be selected as the new manager. The new manager then broadcasts a SELECT(cellular-id, host-id) packet to notify all other members of its host-id. When a host moves to a new cellular, it should issue a MOVE(host-id, old-cellular-id, new-cellular-id) packet. Managers of the old and new cellulars then update their member tables to ensure freshness. In the case that manager moves to a new cellular, it should issue a LOCATE(old-cellular-id) packet to their members. All members located in the old cellular will then reply by sending a LOCATION(cellular-id, host-id, location) to the current manager, thus allowing the old manager to select a replacement. The old manager issues an ASSIGN(cellular-id, host-id) packet to the selected candidate, and then transfers its member table to this candidate manager. The candidate then broadcasts a MANAER packet to inform all member hosts located in the old cellular zone.
Cellular-Based Routing Protocol Hosts hoping to establish communication paths with another hosts issue a route request packet RREQ(source-id, broadcast-id, destination-id, hop-count, timeout) to their manager. On receiving the RREQ packet, the manager first checks whether or not the destination host appears in its table. If the destination host appeared in the MT, the manager sends a reply packet RREP(source-id, destination-id, my-id, previous-id) back to the manager of the source host to construct the routing path. Meanwhile, if the destination host is not found in MT, the manager determines whether or not it 9
should relay the routing request packet to all its neighboring managers. The manager first checks whether or not the packet has been received, based on information such as source-id, destination-id, and broadcast-id. If the manager receives a RREQ packet containing a new broadcast-id, it forwards the received packet to all its neighboring managers. On the contrary, if the manager receives a RREQ packet containing a broadcast-id that it has already received, it uses hop-count to decide whether or not it should forward the RREQ packet. If the RREQ packet received by manager has a smaller hop count than those packets it has already been received, the manager will relay the RREQ packet to neighboring managers. The flooding operation is then performed at the manager level until the destination host is found. Figure 6 depicts the construction of the routing path from source host S to destination host D, while Table 2 lists the information collected by managers A, B, C, E, F, G, H, I, J when they receive the RREP packet. Table 2: information collected by mangers when they receive the RREQ packet. Manager Source
A S
B S
C S
E S
F S
G S
H S
I S
J S
Broadcast-id
1
1
1
1
1
1
1
1
1
Destination
D
D
D
D
D
D
D
D
D
Upstream Hop-count
(1,3) 2
S 1
(1,3) (1,3) (1,1) (2,4) (2,4) (2,0) 2 2 3 3 3 4
(3,1) 4
Timeout
Host S first issues a RREQ packet to its manager B. Manager B then verifies that destination host D is not listed in its member table, and determines to relay the RREQ packet to all its neighboring managers, as displayed in Fig. 6(a). The RREQ packet is thus flooded from manager to manager. As soon as manager J, the manager of the destination host, receives the RREQ packet, it determines the routing path with the smallest number of hop counts, and sends a RREP packet to host H. Managers will modify information based on this RREP packet, as listed in Table 2. Table 3 lists the information collected by managers B, E, H and J. Whenever the RREP packet arrives at host B, a routing path S-B-E-H-J-D is constructed, as shown in Figure 6(b). During route maintenance, whenever a manager that lies on the routing path moves to another new cellular, that manager will transfer its routing information, as listed in Table 3, to the new candidate manager. If the old cellular contains no host, the routing path will be broken. Under this condition, the original manager will issue a REBUILD(source-id, destination-id, my-cellular-id, new-router-id) packet to partially reconstruct another routing path in place of the original path. As Fig. 7 illustrates, manager E (assumed in cellular (2,4)) determines that manager F is the most suitable host to join the routing path as its own replacement. Manager E 10
therefore issues a REBUILD(S, D, (2,4), (2, 2)) packet to managers B, H , located in the upstream and downstream of the original routing path, respectively. Simultaneously, manager F receives the REBUILD packet and joins the routing path as a new router. When managers B and H, which are respectively located in cellulars (1, 3) and (3, 3), receive the REBUILD packet, they substitute manager F for manager E in their original routing table, as listed in Table 3. Meanwhile, manager E also transfers its routing table to manager F. G
G A
E
A S B
E H
S B
H J
C
J D
I
C
F
member manager
I
D
F
member manager RREP routing path
RREQ routing path duplicate RREQ
(a) RREQ packet is flooded by managers.
(b) RREP packet is replied by manager J.
Figure 6: An example of route construction from source host S to destination host D.
Table 3
ÍRecords collected by managers B,E,H,J when they receive RREP packet
Manager Source Destination Upstream Downstream
B S D S (2,4)
E S D (1,3) (3,3)
H S D (2,4) (4,2)
J S D (3,1) D
G A
E H
S B
J C
F
Cell’s member New route Move direction Join packet
D I
Cell’s manager Old route REBUILD packet
Figure 7:Manager F will replace manager E to partially reconstruct a new route. 11
As mentioned previously, flooding produces the shortest routing path. Although the use of managers significantly reduces the extent of flooding during path construction, the routing path constructed by managers may have a larger hop count. The next section introduces two-level cellular-based management, which further reduces the hop count during construction of a QoS routing path.
3. Two-Level Cellular-Based Management The previous section described single-level cellular-based management protocols. These protocols include the manager management protocol, routing protocol, and route maintenance protocol. The cellular-based partition was also demonstrated to be more effective than Zone(Grid)-Based partition in reducing the number of flooding packets and hops when constructing a routing path. However, single-level management suffers from two shortcomings. First, two routing paths may pass through the same cellular. This situation makes constructing a QoS routing request that requires a minimal path bandwidth difficult, because a manager may not be able to afford the bandwidth requests of two routing paths. This challenge is called the bandwidth-congestion problem. Fig. 8 shows the bandwidth-congestion problem as it occurs with manager A. Two source hosts, S1 and S2, hope to create a routing path to destination hosts D1 and D2, respectively. These two paths pass through manager A, but the bandwidth of A can not simultaneously support communication for both these paths. Another problem of single-level management is that the hop count may increase, compared to a routing path constructed via host flooding. As with the single-level management protocol, flooding performed by managers only, rather than by all hosts in a cellular region, can significantly reduce the number of flooding messages. However, only allowing managers to participate in the flooding operations may extend the routing path. Fig. 9 illustrates this phenomenon. In Fig. 9(a), source host S1 can communicate with destination host D1 in a single hop. However, path construction by the manager will establish a 4-hop communication path, as shown in Fig. 9(b). This section presents a two-level cellular-based management approach to solve the above two problems. Two-level cellular-based management will reduce not only the number of flooding packets but also the number of hops. Besides these advantages, the bandwidth-congestion problem also can be solved. S1 D2
Source/Destination manager Packet forwarding
A D1
S2
Figure 8: The congestion problem in one-level cellular-based management. 12
S1 D1
(a) A one-hop communication path is constructed by host level flooding. S1 D1 A
(b) A 4-hop communication path is constructed by manager-level flooding. Figure 9: Manager-level flooding will possibly increase the number of hops of a communication path.
Two-Level Cellular-Based Partition The central idea of two-level cellular-based management is to group seven small cellular regions into a super cellular region. The manager closest to the center of the super cellular region will be considered the header of the super cellular. As Fig. 10 shows, cellulars that share the same symbol are considered a super cellular. For example, cellulars marked by A0, A1,…, A6 are grouped into a super cellular, and the manager of cellular A0 is the header of the super cellular. The header of a super cellular records the seven managers in the super cellular region. The management can be divided into two levels, header-level and manager-level. Only headers participate in flooding when constructing a routing path. When the path is constructed the header can assign one of its managers to serve as a router in the constructed path. This approach avoid the bandwidth-congestion problem, since the header can assign different managers to serve as the routers of different paths that simultaneously pass through the same super cellular region. A1 A2
A6 A0
A3
A5 A4
Figure 10: Two-level cellular-based partition.
To guarantee that header can communicate directly with neighboring headers, 13
the small cellular should be made smaller, compared with the single-level cellular-based management. Let S denote the signal distance that a mobile host can communicate within one hop. The size of the small cellular and super cellular are respectively 3 2 S ≅ 0.054126 S 2 and 32 3 2 S ≅ 0.378861S 2 . size of super cellular = 32 Fig. 11 displays the size of small cellular and super cellular in relation to S. Since the size of small cellular =
header is mobile in the central cellular of the super cellular, the longest distance between two neighboring headers is set at S, as shown in Fig. 11.
S
Figure 11: Relation of S and size of cellular in two-level cellular-based partition.
Two-Level Management of Cellular-Based Partition This subsection presents the management protocols and tables for the manager and header. A super cellular comprises seven basic cellular regions. Within a basic cellular, the host located closest to the center of the basic cellular is selected as the manager. Meanwhile, the manager of the central cellular of the super cellular is selected as the header of the super cellular. This guarantees that the header will not be changed frequently owing to mobility. In first-level management, the header is responsible for managing the managers within a super cellular. Second-level management is similar, but instead involves the manager keeping track of all the hosts within the same basic cellular. Within the super cellular region, only header can communicate with neighboring headers. Whenever a header moves to a new cellular, a new manager will be selected as the replacement header. New manager is created similarly to the single-level cellular-based management described previously, and the old header transfers all of its information to the newly chosen header. Within a basic cellular, the management rules resemble those in single-level cellular-based management, including the rules for dealing with situations where a manager is powered off or suffers a fault and manager, or a host moves to a new cellular. The manager records the header-id so that it can relay the route search request packet from 14
its member to header. Within a super cellular, a header has two tables, a neighboring header table that records the ID of neighboring headers, and a member table that records the ID of managers located in the same super cellular. Fig. 12 presents an example of tables stored in header A0. In Fig. 12(a), header A0 records its neighboring headers B0, C0, D0, E0, F0, and G0 in its neighboring header table, while in Fig. 12(b), managers with IDs A1, …, A6 are recorded in header A0.
Header
B0 G0 A6
A1
C0
A2
A0 F0
A0
A3
A5 A4
Neighboring headers B0 C0 D0 E0 F0 G0
Header managers A1 A2 A0 A3 A4 A5 A6
D0 E0
(a) Neighboring header table stored in Header A0.
(b) Id of managers stored in Header A0.
Figure 12: Tables stored in header A0.
QoS Routing Protocol for Two-Level Cellular-Based Management This subsection describes the routing protocol and presents the routing tables stored in the manager and header. Under hierarchical cellular-based management, the header can assign one of seven managers to be a router and relay a message along a routing path. To reduce the hop count of a routing path and efficiently construct a QoS routing path, each header should create a bandwidth Table. A bandwidth table comprises 6 rows and 42 columns storing the bandwidth (or the number of free channels) between the manager of the same super cellular and that of the neighboring super cellular. Fig. 13 displays the bandwidth table stored in header B0. The first column lists the ID of managers B0, B1,…, B6, managed by header B0, while the first row presents the ID of all the managers managed by neighboring headers. The values in the table itself indicate the bandwidth of the two managers appearing in specific rows and columns. For example, the bandwidth of managers B4 and C4 is 5. Notably, two managers separated by a distance larger than the signal distance S will have a zero value in the bandwidth table. An example of this situation can be found in the bandwidth value of managers B1 and C3, as shown in Figs. 13 and 14. If a QoS 15
communication request with a minimal bandwidth of 4 is received by header B0, header B0 will consult its bandwidth table, presented in Fig. 13, and determine that manager B4 can serve as a router for the path from B4 to C4, as shown in Fig. 14.
B0 B1 B2 B3 B4 B5 B6
C0 2 2 2 1 2 0 0
C1 3 3 3 2 3 0 0
C2 0 0 2 3 0 0 0
C3 0 0 4 1 0 0 0
C4 1 3 3 2 5 0 0
C5 4 2 1 1 3 2 2
C6 3 2 4 2 2 3 1
Figure 13: Bandwidth table stored in header B0 A6 A0 B6
B1 B0
B5
B2
C0
B3
C4
B4
Figure 14: Two-level management to avoid the bandwidth congestion problem.
Besides resolving the bandwidth congestion problem and efficiently constructing a QoS routing path, the two-level cellular-based management can also reduce the hop count required to construct a routing path. To reduce the hop count, the bandwidth table stored in each header should be modified to create another table consisting of 49 rows and 49 columns. The first row and first column store the IDs of all managers located in the header’s super cellular, plus the IDs of all managers located in the neighboring super cellulars. Meanwhile, the value in the bandwidth table reveals the bandwidth of the corresponding two managers. Notably, two managers separated by a distance greater than the signal distance S will have a zero value in the bandwidth table. During path construction, the RREQ packet is flooded by headers. Whenever the header of the destination host receives the RREQ packet, it selects the path with minimal hop count and sends the RREP packet back to the header of source host. Meanwhile, when the header receives the RREP packet, it refers to the bandwidth table and determines whether the route path can jump itself and directly relay the message between the managers of the two neighboring super cellulars. Fig. 15 presents the bandwidth table stored in header C0. As the header C0 receives the RREP packet issued by header B0, it checks whether the downstream manager Bi can directly communicate with upstream manager Ej for some 0 ≤ i, j ≤ 6. In Fig. 16, 16
since the downstream manager B6 can directly communicate with upstream manager E6 and since the bandwidth meets the QoS minimal requirement, header C0 determines to jump over itself, thus saving one hop in routing path. Consequently, the number of hops of the constructed routing path can be reduced by headers when they receive the RREP packet. As mentioned before, the two-level cellular-based management can reduce the number of flooding packets and the hop count and avoid the bandwidth-congestion problem. The following section proposes a performance study that compares the hop count and number of flooding packets of two-level cellular-based management with Zone (Grid)-based management.
B0 B1 B2 B3 B4 B5 B6
E0 2 0 0 1 2 0 0
E1 3 3 3 2 3 0 0
E2 3 2 2 3 1 0 0
E3 2 0 0 1 3 0 0
E4 1 0 0 2 2 0 0
E5 0 0 0 0 0 2 2
E6 0 0 0 0 0 3 5
Figure 15:The bandwidth table stored in header C0.
D0
B6
C0
BB
0
E2
E6 E0
A0
A RREQ forwarding path RREP forwarding path Route path
Figure 16:One hop count is reduced by header C0.
4 Performance Study This section uses simulation to evaluate performance, in terms of the hop count of a routing path and the number of flooding packets. The simulation environment is 17
as follows. The size of MANET is 100*100 basic units, and signal strength is set at either 10 or 20 basic units. Fig. 17 compares single-level zone(grid)-based and cellular-based management regarding the number of flooding packets. The signal strength in Figs. 17(a) and 17(b) is set at 10 and 20 units, respectively. Meanwhile, the location of the mobile host is randomly generated in MANET. To guarantee that two neighboring managers can communicate directly, the size of zones and cellulars is set according to the calculation discussed in Section 2.
Amount of flooding packets
Zone-based
Cellular-based
700 600 500 400 300 200 100 0 400
500
600
700
800
900 1000 1100 1200 1300
T he number of hosts
(a) Signal strength is set at 10 units.
Amount of flooding packets
Zone-based
Cellular-based
150 100 50 0 80
100
120
140
160
180
200
220
240
260
T he number of hosts
(b) Signal strength is set at 20 units. Figure 17: The comparison of single-level zone-based management and cellular-based management in the number of flooding packets.
When few hosts exist, many zones have no host. This phenomenon occurs because the number of basic zone exceeds the number of basic cellulars. Consequently, the zone-based partition is generally unable to find any routing path for a pair of randomly generated source and destination hosts. The flooding packet will not be spread in zones without a host. Because of the above, zone-based management has fewer flooding packets than cellular-based management. The phenomenon can be 18
observed in Fig. 17(a) when the number of hosts is below 600, and in Fig. 17(b) when the number of hosts is below 140. Fig. 18 displays the success rate. Consider Fig. 18. As the number of hosts is more than 600, almost all zones and cellulars have hosts. Hence, zone-based management has more flooding packets than cellular-based management.
Cellular-based
10 00 11 00 12 00 13 00
90 0
80 0
70 0
60 0
50 0
120.00% 100.00% 80.00% 60.00% 40.00% 20.00% 0.00% 40 0
Successful rate
Zone-based
T he number of hosts
(a) Signal strength is set at 10 units.
Zone-based
Cellular-based
Successful rate
120.00% 100.00% 80.00% 60.00% 40.00% 20.00% 0.00% 80
100 120 140 160 180 200 220 240 260 T he number of hosts
(b) Signal strength is set at 20 units. Figure 18: The comparison of single-level zone-based management and cellular-based management in the successful rate for finding a routing path.
Fig. 19 compares hop count in zone-based and cellular-based partitioning schemes. The signal strength of a mobile host is set at 10 and 20 units, respectively, in Figs. 19(a) and 19(b). When the number of hosts is below 500, as in Fig. 19(a), many zones have no host. The successful path is generally short. In this unstable case, the zone-based partition has a smaller hop count than the cellular-based partition. However, as the number of hosts increases, the length of the constructed paths also grows. In these cases, cellular-based partition has a lower hop count than the zone-based partitioning scheme. 19
In comparison with single-level management, two-level cellular-based management requires a larger cache size for storing the bandwidth table. The super-cellular management procedure also creates the maintenance overhead for each header. However, the two-level cellular-based management reduces the hop count of a communication path and resolves the congestion problem. Fig. 20 displays the effect of two-level cellular management. The signal strength is set at 10 and 20, respectively, in Figs. 20(a) and 20(b). Compared with the single-level cellular management, the two-level cellular management has a lower hop count. Fig. 21 compares the congestion probabilities when the number of communication paths simultaneously being constructed is varied. As Fig. 21 illustrates, no congestion occurs with two-level cellular-based management when the number of paths is under 8, because that the super manager can assign different manager as the routers of a path. Generally, two-level cellular management has less congestion and a smaller hop count than single-level cellular management.
The number of hop count
Zone-based
Cellular-based
20 15 10 5 0 400
500
600
700
800
900 1000 1100 1200 1300
T he number of hosts
(a) Signal strength is set at 10 units.
The number of hop count
Zone-based
Cellular-based
10 8 6 4 2 0 80
100
120
140
160
180
200
220
240
260
T he number of hosts
(b) Signal strength is set at 20 units. Figure 19: The comparison of single-level zone-based management and cellular-based management in the number of hop count.
20
The number of hop count
Single-level management
T wo-level management
20 15 10 5 0 400
500
600
700
800
900 1000 1100 1200 1300
T he number of hosts
(a) Signal strength is set at 10 units.
Single-level management
T wo-level management
The number of hop count
10 8 6 4 2 0 80
100
120
140
160
180
200
220
240
260
T he number of hosts
(b) Signal strength is set at 20 units. Figure 20: The comparison of single-level and two-level cellular-based management in the number of hop count.
Probability of congestion
Singal-level management
T wo-level management
120.00% 100.00% 80.00% 60.00% 40.00% 20.00% 0.00% 2
4
6
8
10
12
14
16
18
20
T he number of communication paths
Figure 21: The comparison of single-level and two-level cellular-based management in the probability of congestion.
4 Conclusions and Future works This study proposed two-level cellular-based management for Mobile Ad-Hoc 21
Networks. In comparison with the Zone (or Grid)-Based management model, single-level cellular-based management generates fewer flooding packets during path constructing. However, to resolve bandwidth congestion and reduce the number of hops on the established routing path, two-level cellular-based management was proposed herein. Simulation results show that Cellular-Based management works better in reducing the number of flooding packets and number of hops in a routing path. Future investigations are necessary to develop multicasting, geocasting, and broadcasting protocols based on the Cellular-Based management model.
Acknowledgements: The authors would like to thank the Ministry of Education, ROC, for partially supporting this research under Contract No. 89-H-FA07-1-4 (Learning Technology).
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