PERFORMANCE ANALYSIS OF MICROMOBILITY PROTOCOLS IN IP - BASED CELLULAR NETWORKS

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1 PERFORMANCE ANALYSIS OF MICROMOILITY PROTOCOLS IN IP - ASED CELLULAR NETWORKS D.SARASWADY AND S.SHANMUGAVEL, Telematics Lab, Department of Electronics and Communication Engineering Anna University, Chennai INDIA Abstract: - The Internet Engineering Taskforce (IETE) Mobile IP protocol is considered to have limitation in its capability to handle large number of Mobile stations moving fast between different radio cells. However, Mobile IP is well suited for interconnecting disparate cellular networks effectively providing global mobility. The IETF Mobile IPV6 protocols have been developed for intra domain micromobility. Micromobility protocols like cellular IP, Hawaii and Hierarchical Mobile IP are developed to solve the problems of handoff latency and control overhead of Mobile IP. In these protocols, a tree access network topology is assumed. An actual network topology would be a mixture of a tree and mesh topology. For reasons of robustness against link failures and load balancing, extra uplinks and mesh links in the topology are desired. In this paper, we present the performance of micromobility protocols in the proposed partial mesh topology and hierarchical topology. The simulation results presented in this paper are based on the network simulator ns2.1b7a. From the results, it is observed that the cellular IP and Hierarchal Mobile IP do not take advantage of extra links present in the partial mesh network whereas Hawaii uses the presence of mesh to reduce handoff latency, UDP packet loss or duplication. Keywords: - Mobile IP, Micromobility protocols, Mesh Network. 1. Introduction Recent technological developments in the Internet and Mobile communication have changed people s attitude to communication in the last decade. As for today we witness the development of these two technologies still somehow are separate approaches. On one hand, we have observed continuous development of cellular systems toward support of Internet-style data services, in addition to classical mobile telephony. Already in GPRS and EDGE systems, which extend the low-level data services of GSM, the concept of Internet connectivity is explicitly stressed; in each consecutive release of the 3G system draft standards the move toward Internet based protocols has become more visible. Nevertheless, obviously enough, the recent 3G-protocol architecture is very far from the elegant simplicity of the classical Internet approach. On the other hand, increasing interest in further evolution of the classical Internet architecture towards wireless and mobile Internet is visible. The essential goal of these activities might be defined as adding to the classical Internet architecture as necessary in order to ensure efficient support of Internet over wireless access links and support mobility. Interest in such an approach has been essentially boosted by the development of wireless Ethernet. After IEEE82.11 [1] products really became interoperable, we could observe their explosive deployment and this has been paralleled by the definition of the ETSI Hiper LAN as well as an impressive amount of new activity within IEEE82, especially a noticeable activity aimed at developing transmission schemata with bit rates up to 54Mb/s and support of different quality of service requirements. In addition, we have also been witnessing a lot of effort in the area of short-range wireless, initiated by the luetooth development and followed by a plethora of personal area network solutions. All of these solutions can be utilized to provide fast IP access at a lower price per bit than the 2G or 3G systems, but on the other hand, limited in the geographical scalability. This paper utilizes the Mobile IPv6 protocol as the mechanism for providing mobility in a multi-access environment. Mobility of terminals has recently been one of the most important topics in the research community. Hosts normally belong to their home network, i.e. their home address can be accessed through standard routing. When a host moves to a new network and continue its movement, its address is no longer routable in the new location. Thus, mobility of hosts presents the network with the problem of routing the packets to the terminal s 1

2 new point of attachment to the network. The main goals of the mechanism supporting mobility are, to ensure that the break in communications during the handover is as short as possible and that no (or only a few) packets are lost. Hence, all applications including real time can be supported. Minimize signaling overhead to achieve the re-routing. This includes the load and delay, and also the storage and processing requirements at each router. Solutions to the basic mobility problem involve establishing some sort of dynamic mapping between the Mobile Host s (MH) permanent identifier and its current location. In order to fulfill the above-mentioned attributes, it is preferable to decouple the mobility management inside the access network (AN) and through the core network. Hence, by managing mobility locally, micro mobility protocols exploit the significant localization of a MH s movements. In this way, routes are updated through access network routers avoiding signaling message to network components far from the current location of the MH, thus reducing the signaling load in the core network and improving the re-routing latency. Macro mobility, thus deals with the movements of MH to a new AN, whereas micro mobility deals with movements of MH inside the same AN. Mobile IP and SIP are examples of protocols used for macro mobility management [2,3], whereas there are numerous protocols for local mobility management such as Cellular IP, Hawaii, Hierarchical MIP and many others. Although all the micromobility protocols are designed to work correctly irrespective of the topology of the IP domains [4,5], there is always a growing need to understand the performance of micromobility protocols in different network topologies. The topology has an important influence on the performance of the micromobility protocols. In this paper, we present a performance comparison of micromobility protocols in partial mesh topology and hierarchical topology. The redundant link present in the partial mesh topology helps in load balancing and delivering of datagrams to the mobile host even in the case of link failure, which increases the robustness of the networks. We use UDP probing traffic between the corresponding host and Mobile Hosts, and count the number of packets lost during handoff using Cellular IP, Hawaii and Hierarchical Mobile IP micromobility protocols, the number of packet duplication or loss as a function of buffering delay for Hawaii MSF scheme and end-enddelay of UDP packets for partial mesh and hierarchical topologies. The network simulator (ns2.1b7a) is used to evaluate the performance of the proposed partial mesh topology and it is compared with hierarchical topology. The results show that the best performance is achieved in a mesh topology and it provides remarkable improvement in handover performance and UDP packet loss or duplication for Hawaii protocol. The remainder of the paper is organized as follows. In section 2, an overview of Cellular IP, Hawaii and Hierarchical Mobile IP is presented. The simulation model for the Hierarchical and partial mesh topology is discussed in section 3. The performance results of all micromobility protocols in hierarchical and partial mesh topologies are reported in section 4 and finally the conclusions are given in section Micromobility protocols The primary role of micromobility protocol is to ensure that packets arriving from the Internet and addressed to the mobile hosts are forwarded to the appropriate wireless access point in an efficient manner. Existing proposals for micromobility can be broadly classified into two types: routing - based and tunnel-based schemes [6]. Routing based schemes aim to exploit the robustness of conventional IP forwarding. Here, a distributed mobile host location database is created and maintained within the network domain. The database consists of individual flat mobile specific address lookup tables and is maintained by all the mobility agents within the domain. These schemes are exemplified by the Cellular IP [7] and Hawaii [8] protocols, which differ from each other in the functionality of the nodes and the construction methods of the lookup tables. The tunnel-based schemes apply the concept of registration and encapsulation in a local or hierarchical fashion, thus creating a flexible concatenation of local tunnels. In the context of MIPv4, the Mobile IP regional registration proposal [9] falls in to this category. Hierarchical Mobile IP [1] plays a similar role in Ipv6 networks. GTP-based mobility management in GPRS and UMTS provides an early example of tunnel-based schemes Hawaii In this proposal [11], the micromobility domain is composed of Hawaii enabled IP routers. The gateway into the domain, which handles all inbound and outbound mobile traffic, is referred to as the domain root router. When a mobile host powers up within a domain, it is dynamically assigned an IP address. Outside the domain, this address is routed toward the domain root, while within the domain it is used for identification purpose only. If the mobile host is visiting a foreign domain, this address is used as a MIP care of address (CoA). Forwarding entries for mobile hosts are created and maintained using explicit signaling message initiated 2

3 by the hosts. When a host transmits such a message on power up or changes of location, it is relayed, along the optimal path, to the domain root in the form of a Hawaii signaling message. All routers receiving this message establish and update mobile specific entries for the reverse path packet forwarding. Several path setup schemes are defined, which may additionally allow, in the case of handoff, the routers on the former downlink path to be notified to forward (transient) incoming packets to the new location of the mobile node. The domain root necessarily maintains a flat address lookup table with forwarding metrics for all active mobile hosts in its domain, while each routing node is required to maintain a part of this table. The Hawaii four path setup schemes are classified into either forwarding or nonforwarding type, based on the way the packets are delivered to the mobile host during a handoff: in the first type, the packets are forwarded from the old base station to the new, whereas in the second, they are diverted at the crossover router Forwarding Schemes: In these path setup schemes, packets are first forwarded from the old base station to the new base station before they are diverted at the crossover router. Forwarding scheme is classified into two types viz; multiple stream forwarding (MSF) and single stream forwarding (SSF) [8] Non-Forwarding Schemes: In these path setup schemes, as the path setup message travels from the new base station to the old base station, data packets are diverted at the cross-over router to the new base station, resulting in no forwarding of packets from the old base stations. There are two types of nonforwarding schemes namely unicast nonforwarding (UNF) and Multicast nonforwarding (MNF) [8] Cellular IP The Cellular IP proposal [7] adopts a similar approach to mobility management based on a rooted domain, but uses a different signaling technique. Instead of sending and processing explicit message, the nodes have an ability to learn the source IP address of uplink data packet and map them to the corresponding downlink interfaces. The uplink path (i.e., the direction toward the domain root), or gateway, is inferred by each access point/ access router within the domain using the beacon packets periodically transmitted by the gateway. All the packets generated by the mobile hosts are forwarded toward the gateway using this uplink path. In addition, to refresh its forwarding cache entries, a host may explicitly transmit uplink route update packets. Two handoff schemes are supported. Hard handoff allows some packet loss while being efficient in the amount of signaling overhead and latency. Semi-soft handoff aims to minimize the transient packet loss, while exploiting the capability of a mobile to receive packet from both old and new Access Points (APs). Similar to the Hawaii protocol, the forwarding cache of the gateway contains entries for all active mobiles in the domain Hierarchical Mobile IP The Hierarchical Mobile IP protocol [1] from Ericsson and Nokia employs a hierarchy of Foreign Agents (FAs) to locally handle Mobile IP registration. In this protocol, the mobile host sends Mobile IP registration message (with appropriate extension) to update their respective location information. Registration messages establish tunnels between neighboring FAs along the path from the mobile host to a gateway FA (GFA). Packets addressed to the mobile host travel in this network of tunnels, which can be viewed as separate routing network overlay on top of IP. The use of tunnel makes it possible to employ the protocols in an IP network that carries non-mobile traffic as well. Typically one level of hierarchy is considered where all FAs are connected to the GFA. In this case, direct tunnel connects the GFA to FAs that are located at access points. Paging extensions for Hierarchical Mobile IP (HMIP) [9] allow idle mobile nodes to operate in a power saving mode while located within a paging area. The location of mobile host is known by Home Agents (HAs) and is represented by paging areas. After receiving a packet addressed to a mobile host located in a foreign network, the HA tunnels the packet to the paging FA, which then pages the mobile host to reestablish a path toward the current point of attachment. The paging system uses specific communication time slots in a paging area. 3. Simulation model In this section, we present the simulation model and the performance of Cellular IP, Hawaii and Hierarchical Mobile IP in partial mesh and hierarchical topology with respect to handoff quality, buffering time and end-end delay of UDP packets. S1 CH R3 MH R1 S2 S3 R4 R S4 R5 S5 S6 R2 S7 Fig.1. Hierarchical network topology R6 S8 3

4 The simulation study presented in this paper uses the Columbia IP micromobility software (CIMS) [12], which represents a micromobility extension for the ns-2 network simulation based on version 2.1b6 [13]. CIMS supports separate model for Cellular IP, Hawaii, and Hierarchical Mobile IP. These simulation models are briefly described in the following: The Hawaii simulation model is based on the description provided in [14]. We use UNF and MSF handoff schemes. Since the Hawaii access points need to implement Mobile IP foreign agent functionality without decapsulation capability and are responsible for generating Hawaii update message, we modified the ase Station Node object to include these features. Hawaii routers are implemented in special Hawaii Agent object that can process Hawaii message and perform protocol-specific operation. The Cellular IP simulation model is based on the description of the protocol [15]. We implemented hard handoff algorithms. Paging and security functions are not used in the simulations. The Hierarchical Mobile IP simulation model implements the two-level hierarchical version of the protocol where there is a single GFA and some FAs in each access point. To simulate this protocol, a GFA Agent object is added to the existing simulation model. The GFA is responsible for setting up tunnels to FAs and encapsulating downlink packets based on the appropriate visitor list entry. All simulations are performed using the hierarchical and partial mesh network topology. In Hawaii simulation, the R i and S i represent Hawaii enabled Routers and ase Stations (Ss) and R is the Domain Root Router (DRR). In Cellular IP simulation, the R i and Si correspond to Cellular IP nodes where R acts as a Gateway to the Internet. In Hierarchical Mobile IP, the R act as a GFA, while R i represent mobility unaware router. Since the topologies considered represent the home network of mobile hosts, the packets arrive from a corresponding host without encapsulation. Here each wired connection is modeled as 1M/s duplex link with 2ms delay. Mobile host connects to the access point (APs) using the NS-2 carrier senses multiple access with collision avoidance (CSMA/CA) wireless link model where each S operates on a different frequency band. Simulation results are obtained using a single mobile host, continuously moving between Ss at a speed that could be varied. Such a movement pattern ensures that MH always goes through the maximum overlapping region between two radio cells. Nodes are modeled without constraints on switching capacity or message processing speed. The simulation network accommodates UDP traffic. UDP probing traffic is directed from Correspondent Host (CH) to Mobile host, with a packet interarrival time of 1ms and a packet size of 21 bytes. During simulation, a MH travels periodically between neighboring access point with a speed of 2m/s. During such a simulation, MH has to perform seven handover to move from S 1 to S 8 as shown in Fig.1. The distance between two adjacent access routers is 2m, with a cell overlap of 3m. All the base stations are placed on a straight line. S1 CH R3 MH R1 S2 S3 R4 R S4 R5 S5 Suboptimal route Fig.2. Partial Mesh topology 4. Performance analysis The cross-over router is defined as the router closest to the Mobile host that is at the intersection of two paths, one between the domain root router and the old ase station and the second between the old base station and the new base station. The cross over distance is defined as the number of hops between the common cross over node and the new access point. The operation of Cellular and Hawaii is different when the network topology is not a tree, however in Hawaii, the path setup messages are directed toward the old access point. For non-tree topologies, this difference will often result in different nodes being used as the crossover point. In Hawaii, the crossover node lies at the intersection of the old and new access points. As a result, the new downlink path will not necessarily be the shortest path between the domain root router (i.e., gateway) and the new access point. We illustrate this problem using the simulation of partial mesh topology shown in Fig.2. If a MH, initially attached to the network at S1, moves between access point S2 and S5, the resulting downlink path between the DRR and the new access point will be suboptimal, as illustrated in the Fig.2. This suboptimal routing problem represents a generic trade-off associated with handoff control signaling in micromobility protocols. If the handoff control messages are always transmitted up to the gateway, then the Mobile Routing Points (MRPs) higher up in the hierarchy will have to deal with a S6 R2 S7 R6 S8 4

5 potentially large number of messages causing performance bottlenecks. Keeping routing update messaging close to the access point seems reasonable because in most cases the old and new downlink paths overlap, and routing entries do not have to be updated along the common section of the path. y discarding update message at the crossover MRP, the MRPs higher up the hierarchy do not have to process these messages, hence minimizing the signaling load at those nodes and also we are reducing the number of hops between the new S and the cross over nodes UDP Packet loss due to Handoff We first present simulation results for the packet loss due to the basic handoff in each micromobility protocol for hierarchical and partial mesh topologies. To obtain these results, the mobile node is allowed to move between base station and the UDP probing traffics transmitted between the CH and MH. Cellular IP does not use the meshes or extra links of the partial mesh topology, due to the fact that the updates are routed to the gateway via the links of the tree structure in both topologies. So, the packet loss during handoff and the paths used for route updates are same for hierarchical and partial mesh topology. The UDP packet loss with respect to crossover distance is plotted in Fig.3. The crossover distance is 1,2 and 3 hops when the mobile host travel between S1-S2, S2 S3 and S4-S5 respectively. Fig.3 shows the average UDP packet loss for Cellular IP in hierarchical and partial mesh topology. From the graph it is observed that the packet loss increases as the crossover distance increases. The increase in packet loss is due to increase in number of hops during handoff between the new S and the crossover node. In Hierarchical Mobile IP, the presence of extra links may result in multiple routes with the same hop count between the new access router and the domain Gateway Agent. This means that the time needed by a regional registration request to reach the gateway, is independent of the chosen path resulting in the same packet loss during handoff. In this case, the protocol will chose arbitrarily one of these paths. Hierarchical Mobile IP does not use the mesh links. The packet loss is the same for hierarchical and partial mesh topology as shown in Fig.4. In contrast to Cellular IP and Hawaii, the Hierarchical Mobile IP sends routing updates only when registration message reaches the GFA. Therefore, the HMIP cannot benefit from the fact that a crossover node is topologically close to the base station. This phenomenon can be observed from the Fig.4, where the packet loss for the HMIP is found to be almost independent of the crossover distance and it is also much higher when compared to cellular IP and Hawaii micromobility protocols. Packet Loss crossover distance Fig.3. Packet loss for CIP in Hierarchical and Partial Mesh Topologies Packet Loss Hiearchical Partial Mesh crossover distance Hiearchical Partial Mesh Fig.4. Packet loss for HMIP in Hierarchical and Partial Mesh Topologies In contrast to the other protocols, Hawaii sends path setup messages from the new access router to the old one and not towards the gateway. This protocol will use the extra meshes and links if those links help to realize a shortest path between the new and old base station. The UDP packet loss with respect to handoff is plotted in Fig.5. It shows the comparison of number of packet loss in hierarchical and partial mesh topologies for the Hawaii UNF protocol during handoff. From Fig.5, we observe that when the MH moves between S1 S2, the packet loss is low and small for both topologies. In Hawaii and Cellular IP micromobility protocols, the handoff delay is related to the packet delay between the access points and the crossover node. If the crossover distance is large, the handoff delay increases with extra packet delay of 2ms for each hop. It is clear from the Fig.5 that when the MH moves between S4-S5, the UDP packet loss in hierarchical topology is very high when compared to the partial mesh topology. The increase in packet loss is due 5

6 to increase in the number of hops during handoff between the new S and the crossover node. The crossover distance is 3 hops for hierarchical topology whereas that of mesh topology is 2 hops, since the handover mechanism in Hawaii results in the use of suboptimal routing. This phenomenon can be observed from the Fig.5, where the UDP packet loss is low in mesh topology when compared to hierarchical topology. Packet Loss Hiearchical Partial Mesh crossover distance Fig.5 UDP packet loss for HAWAII in Partial Mesh and Hierarchical topologies 4.2. UDP Packet loss and duplication in HAWAII MSF Hawaii MSF protocol operates after handoff. In this protocol, the packet arriving at the old access point after a MH loses its air channel due to handoff, are buffered and forwarded to it at its new point of attachment using the access network. Routing state is also updated at the same time so that the new downlink packets are directly forwarded to the new access point. The packets that are buffered and forwarded from old the access point may arrive at the new access point interleaved with new packets. This results in misorderded packets being delivered to mobile hosts. Which adversely affect both audio and TCP application. In the case of partial mesh topology, this phenomenon of the multiple misordered packets arriving at the new access point is avoided by forwarding the update message to the old router before sending it to the crossover router. Then, the old router changes the forwarding port to the MH and forwards the buffered packet first through the suboptimal route as shown in Fig.2. The MSF scheme works well if the link layer at old access point can determine which packets the MH not received. In such case, MSF can efficiently forward packets using IP. If this cannot be accommodated, the IP must store all the packets received for a certain period (T f ) and forward them to the new access point. If we keep increasing the T f, some packets, which are already successfully transmitted to the mobile host at the new access point, will also be forwarded from the old access point. This results in packet duplicate. Fig.6 shows the effect of packet loss due to handoff with respect to buffering time for Hawaii MSF protocol in hierarchical topology. We have plotted the average packet loss (in positive value) or duplication (negative value) as function of buffering time. The MH is allowed to move between the S1- S2, S3-S4 and S4-S5. In this case, the UDP probing traffic is sent from the MH to the CH while mobile host performs Hawaii handoff with packet inter arrival time of 2 ms. From the graph, it is observed that the packet loss increases as the number of hops to the cross over node increase for a given buffering time. Packet Duplication or Loss bet S1 and S2 bet S2 and S3 bet S4 and S uffering time (ms) Fig.6. Packet loss for Hawaii MSF scheme in Hierarchical topology The same observation can be made for Hawaii MSF protocol in hierarchical and partial mesh topologies as shown in Fig.7. Since the number of hops to the crossover node in both the topologies is same, when the mobile host moves between S1 S2 and S3-S4, hence the performance of the protocol for the mesh topology is same as that of hierarchical topology as shown in Fig.6. ut, there is a significant difference in the performance in partial mesh and the hierarchical topologies when the mobile host moves between the S4-S5, which is shown in Fig.7. From this graph, we can observe that the packet loss decreases with buffering time for partial mesh and hierarchical topologies. We also observe that beyond T f = 7 ms, there is no packet loss but only duplication in partial mesh topology whereas in hierarchical topology the packet losses continue to occur. The average number of packets lost is more in hierarchical topology when compared to the partial mesh topology. Thus the performance of partial mesh topology is better than that of the hierarchical topology. 6

7 Packet Duplication or loss hierarchical partial mesh uffering time (ms) Fig.7. Packet loss and duplication for Hawaii MSF in hierarchical and partial mesh topologies 4.3. Analysis of UDP end - to - end delay for hierarchical and mesh topologies One of the important performance measures of Mobile IP networks is the delay experienced to deliver a packet from source to destination. Each packet generated by a source is routed to the destination via a sequence of intermediate nodes. The end-to-end delay is, thus, the sum of the delays experienced at each hop on the way to the destination. Each such delay in turn consists of fixed and variable. The fixed component, which includes the transmission delay at a node and the propagation delay on the link to the next node and the variable component, includes the processing and queuing delays at the node. The Packets may be rejected at the intermediate nodes because of buffer overflow. Hence, another important characteristic of a mobile IP network is its packet loss rate. Understanding the behavior of end-to-end packet delay and loss rate behavior is important for the proper design of network algorithms such as routing and flow control algorithms (e.g. [16]), for the dimensioning of buffers and link capacity, and for choosing parameters in simulation and analytic studies. In the case of UDP application, it s of interest to measure the end-to-end delay of the UDP packet and delay jitter. The maximum, minimum and average endto-end delay of UDP packet is measured for Cellular IP, Hawaii and Hierarchical Mobile IP protocols by making the mobile host to move between the base stations S1- S2 in hierarchical and partial mesh topologies and the results are plotted in Fig.8 and Fig.9 respectively. From Figs.8 and 9, we observe that the average end-to-end delay is the lowest for Hierarchical Mobile IP compared to the other micromobility protocols whereas maximum delay is the highest for hierarchical mobile IP, which results in very high delay jitter. The drawback of the partial mesh topology is that the end-to-end delay is more than that of hierarchical topology for all micromobility protocols studied in this paper. End to End delay(ms) HMIP HAWAII Maximum Minimum Average Cellular IP Fig.8. UDP End-to-End delay for Hierarchical Topology End to End delay(ms) HMIP HAWAII Cellular IP Maximum Minimum Average Fig.9. UDP End-to-End delay for Partial Mesh Topology 5. CONCLUSION In this paper, the performance results of micromobility protocols such as Cellular IP, Hawaii and Hierarchical mobile IP in partial mesh topology is presented and compared with hierarchical topology. The simulation results show that the topologies influence the performance of micromobility protocols. The packet loss due to handoff depends on the crossover distance in Cellular IP and Hawaii whereas it depends only on the distance to the gateway in Hierarchical Mobile IP. In contrast to Cellular IP and Hierarchical Mobile IP, Hawaii takes an advantage of extra links in mesh topology to reduce the handoff latency and packet loss. In this paper, we have shown how Hawaii UNF and MSF handoff schemes perform better in the partial mesh topology than hierarchical topology. The routing mechanism of Hawaii uses suboptimal routes after several handovers, which results in a slightly more end to end delay of UDP packet in the case of partial mesh topology because it is traded against end - to- end delay to improve the overall loss performance. 7

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