A NEW ROUTING MECHANISM FOR INTEGRATING CELLULAR NETWORKS, WLAN HOT SPOTS AND MANETS

A NEW ROUTING MECHANISM FOR INTEGRATING CELLULAR NETWORKS, WLAN HOT SPOTS AND MANETS Dave Cavalcanti 1, Carlos Cordeiro 2, Anup Kumar 3, Dharma Agrawal 1 OBR Center for Distributed and Mobile Computing 1 Department of ECECS University of Cincinnati Cincinnati, OH (cavalcdt, dpa)@ececs.uc.edu Philips Research USA 2 Wireless Communications and Networking Department Briarcliff Manor, NY 151 Carlos.Cordeiro@philips.com Department of Computer Engineering and Computer Science 3 University of Louisville Louisville, KY ak@louisville.edu Abstract - A growing need for ubiquitous connectivity has motivated the integration of various wireless technologies such as cellular systems, WLANs and MANETs. In this paper we introduce the Integrated Routing Protocol (IRP) that exploits topology information obtained by cellular Base Stations and WLAN Access Points in the route discovery and maintenance in a heterogeneous wireless access network. IRP also provides connectivity to the cellular network and/or WLAN hotspots through the multi-hop routing by allowing out of coverage users to maintain routes to Gateway Nodes (GN). We provide a simulation study of IRP with two different link quality metrics, number of hops and a new integrated metric based on the Expected Transmission Time (ETT). Our results show that IRP improves the network coverage and capacity and allows connectivity alternatives that are not supported by other integrated solutions. Index terms heterogeneous wireless networks, integrated routing, multi-hop routing, link quality estimation. I. INTRODUCTION The availability of a multitude of wireless technologies such as, Wireless WANs (e.g., 2G, 2.5G, 3G, the proposed IEEE 82.2, etc.), and Wireless LANs (e.g., IEEE 82.11a/b/g and HiperLAN/2), integrated with the Internet will provide a ubiquitous computing environment, in which users with multi-interface terminals should be able to select appropriate interface and access network that meets the service level and cost requirements. Providing cost effective data services across heterogeneous networks is a challenging task. One of the steps towards future heterogeneous network is the integration of single-hop wireless networks (e.g. cellular systems and WLANs hot sports) with mobile ad hoc networks (MANETs), where users directly communicate in a peer-to-peer fashion [1]. Several architectures have been recently proposed (see [1],[2]-[6]) for integrating cellular systems and WLANs. However, to the best of our knowledge no existing solution exploits all connection possibilities during the route discovery and maintenance in a generic scenario with a cellular system, WLANs hot spots, MANETs and multiradio terminals. Most existing works consider specific networking scenarios and optimization goals, such as improving the performance in the downlink cellular channel (e.g. UCAN [5]) or providing load balance among neighboring cells (e.g. icar [2]). This limits the applicability of existing solutions to a generic environment where requirements of different types of users must be satisfied with the lowest cost for the system. The main contributions of this paper are twofold: (i) the Integrated Routing Protocol (IRP) that takes into account the availability of multiple technologies and multi-radio devices in the route discovery process; (ii) the performance analysis of a heterogeneous network considering two link quality metrics, namely number of hops and a new adaptation, for a heterogeneous scenario, of the Expected Transmission Time (ETT) [7]. Our integrated routing mechanism can be applied to any generic heterogeneous network with different cellular and WLAN technologies. But, in order to show practical applicability of IRP, we consider a single cellular CDMA/HDR [8] Base Station (BS) collocated with an IEEE 82.11 [9] WLAN Access Point (AP), Single Mode (SM) Mobile Stations (MS) equipped with one IEEE 82.11 interface and Dual Mode (DM) MSs with one cellular and one IEEE 82.11 interface. We assume a Core IP Network connecting the cellular network and WLAN APs [1]. IRP is based on two basic features, namely, topology discovery and gateway discovery. Topology discovery is the process by which BSs and APs obtain information about direct multi-hop links between the MSs within their coverage, and use such information to discover routes. The gateway discovery is the process of providing connectivity to out of coverage MSs to BSs and/or APs through multihop communication with Gateway Nodes (GN). Throughout this paper we use out of coverage to refer to MSs that are not able to directly connect to the BS (e.g. any Single Mode WLAN MS) or to the AP (e.g. any WLAN capable MS which is out of the communication range of the AP). In IRP, out of the coverage MSs maintain a route to a GN, which is responsible for providing connectivity to the BS or the AP. Our results show the impact of the topology discovery and the link quality metric on the network capacity and show how IRP supports connectivity alternatives not considered in other architectures. The remainder of this paper is organized as follows. In Section II we discuss some existing integrated architectures and give the motivation for our work. Then, we describe the proposed integrated routing mechanism in Section II and provide a simulation-based performance evaluation in Section IV. Finally, some concluding remarks are given in Section V. II. MOTIVATION AND RELATED WORK Providing WLAN services to cellular users, and vice versa, has motivated the design of 3G/WLAN integrated architectures [1][11]. But these integrated solutions do not consider MSs using their WLAN in the MANET mode. The idea of integrating MANETs with infrastructure-based (single-hop) networks has been motivated by the possibility of reducing the traffic load at BSs/APs, improving the overall cell throughput [5], and enhancing system s coverage [6]. Further, in a truly integrated and pervasive networking

environment, the MSs connected to the fixed infrastructure (e.g. the Internet or cellular network) must be able to reach the nodes in the MANET part through multi-hop routing and vice versa. Therefore, the design of an integrated routing solution must consider multi-hop routing and its related issues such as, frequent route changes and higher control overhead to discover and maintain valid routes. Several integrated architectures have been recently proposed [2]-[6], but most of them consider very specific networking scenarios and optimization goals. Please refer to [1] for comparison of integrated architectures. MIRAI [4], UCAN [5] and Two-Hop-Relay [6] consider the most generic scenarios, including cellular networks and WLANs. However, MIRAI concentrates on the integration of cellular systems and infrastructure based WLAN, and do not consider the multi-hop concept. UCAN and Two-Hop-Relay consider dual mode terminals that can also communicate through multi-hop routing, but the main goal is to improve the performance in the cellular channels. Also, they consider only the scenario where the mobile users initiate connections towards the BS or AP. They do not provide an integrated routing mechanism that allows virtual corresponding nodes in the cellular network/wlan to discover routes towards out-of-coverage MSs and do not consider peer-to-peer connections between MSs. As can be noted, none of the proposed integrated protocol is self-adaptive, in the sense that it can not efficiently operate in any heterogeneous scenario, i.e., with different combinations of cellular systems, WLANs, MANETs and multi-radio terminals. These factors highlight the need for new integrated solutions that can provide an adaptable networking environment, hiding the underlying heterogeneity from upper layers and exploiting all wireless technologies available in the end-toend path establishment. In addition, to the best of our knowledge, no analysis of an integrated routing process with different link quality metrics has been done. III. INTEGRATED ROUTING PROTOCOL The Integrated Routing Protocol (IRP) is a generic protocol that adapts to the current heterogeneous network. That is, it works just like a multi-hop routing protocol when the MS operates in an isolated MANET, and alternatively, it utilizes the fixed infrastructure (BS or AP) in an attempt to discover a route to the destination whenever connectivity to BS or AP is available. IRP follows the cross-layer design approach as it includes a link quality estimation procedure that provides information to the topology discovery and gateway discovery processes. A. Topology Discovery Topology discovery is incorporated into IRP by allowing every WLAN capable MS to periodically transmit Hello packets on the WLAN interfaces. MSs maintain a 1-hop neighbors table and associated costs to each WLAN link with a neighbor. Here, we consider that each MS i associates a generic link cost c ij to its neighbor j. The MSs send their neighbors table to the BS/AP as they are updated. The BSs and APs also maintain the quality (cost) of downlinks and uplinks with their associated MSs and combine this information with the neighbors tables received from the MSs to build a weighted connectivity graph of the integrated network. The integrated network graph is used by BSs and AP to execute a shortest path algorithm (e.g. Dijkstra s algorithm) to find the minimal cost route between two MSs as they receive a route request from a MS or need to find a MS to send downlink traffic. B. Gateway Discovery In IRP each out of coverage MS maintains a valid route to a GN. The MSs maintain the address of the current GN (GN_id) and the cost to reach the GN (Cost_to_GN) and these two fields are included in the Hello packets. All MSs under coverage of an AP/BS set themselves as GN and set the Cost_to_GN to zero. Otherwise, MSs set the GN_id to an invalid address and Cost_to_GN to a large value. Whenever any MS gets a Hello packet with a Cost_to_GN field smaller than its current value, it sets the GN_id received in the Hello packet as its current GN, updates the route to this new GN by setting the node from which it received the Hello packet as the next hop, and sets its current Cost_to_GN as the cost to reach the next hop plus the Cost_to_GN received in the Hello packet. Furthermore, whenever a MS detects that it has lost connectivity with the BS/AP (by not receiving an expected Beacon signal) it will reset its GN information, which will be propagated to other nodes through the Hello packets. Any MS receiving a Hello packet with an invalid GN_id from the MS that is the current next hop to the GN, will also reset its gateway information. C. Route Discovery IRP differentiates the route discovery procedure when the MS is within the coverage of a BS and/or AP from the case when no direct connectivity to BS or AP is available. The summary of the procedure to send a Route Request (RREQ) packet is given in Figure 1. 1. if (inside coverage && this is the first RREQ to dst) 2. send RREQ with TTL=1 to BS/AP; 3. else if (out-of-coverage && this is the first RREQ to dst) 4. if (there is a valid GN) 5. set out-of-coverage flag in the RREQ; 6. send RREQ to GN; 7. else 8. start ring search procedure in the MANET; 9. endif 1. else 11. start ring search procedure in the MANEt; 12. endif 13. endif Figure 1: Route discovery procedure. We use the single cell scenario shown in Figure 2 as an example to describe the operation of IRP. The dotted lines in Figure 2 represent the direct WLAN links between MSs. i) Inside Coverage: Consider the requesting MS is within coverage of a BS/AP and assume the number of hops as the routing metric. Suppose the Dual-Mode MS A has a packet to send to C.

Since A is under the BS coverage, it will send a RREQ packet with TTL=1 in both cellular and WLAN interfaces. B and L will not reply since they do not have a route to C. In this case, only the BS knows that C can be reached through G (using its topology information), and it will send back a Route Reply (RREP) including in its payload the route BS- >G->C, as it is the shortest route from A to C. When A receives the RREP, it will up date its routing table with the next hop in the route contained in the RREP and will send the first data packet including the source route. Also, A will set a flag in the data packet header, such that the MSs in the path can identify that it carries the source based route and up date their routing tables. Dual-Mode MS Single-Mode MS A L BS: Base Station AP: Access Point N M B E BS F J Figure 2: Example of a heterogeneous network. In case the MS does not receive a RREP before the RREQ timer expires (second RREQ for the same destination), it will send another RREQ with the TTL field set to the initial value of the ring search procedure (TTL_START) to use the MANET route discovery; because it assumes that there is no direct connectivity to BS or AP, or the BS/AP was not able to find a route. In this procedure the TTL is gradually incremented each time the RREQ time expired and the RREP is not received. ii) Out-of-Coverage and Gateway Operation In this case, the requesting MS firstly sends the RREQ to its current GN, which forwards the RREQ to its current BS and/or AP. In addition, a out-of-coverage flag is included in the RREQ packet to indicate that the source is currently out of coverage, such that, if a MS, within a BS/AP s coverage, receives a RREQ with this flag set, it will optionally to act as a GN to source MS. The GN will forward the RREQ with TTL=1, but replacing the source address in the RREQ by its own address. This replacement of the source address is done as it is more likely that the BS/AP can find a route from the GN to the destination, since the original source is out of coverage. The decision of a MS to act as GN may depend on factors, such authentication and power constraints. However, in this paper we will assume that MSs under coverage are always able to act as GNs to BS and/or APs. Once a GN changes the source address in the RREQ, it will receive only part of the end-to-end route from the BS/AP while the complete route is required at the source MS. Thus, the RREQs with the out of coverage flag set also carry a route from the source to the GN. Each intermediate C AP coverage area I G H AP O D MS forwarding the RREQ appends its address to the sourcegateway route, such that the GN can obtain the reverse route to the source. Therefore, when the GN receives the reply from the BS/AP it uses the reverse route to construct the complete route, which is added to the RREP sent back to the original source. In case the GN does not receive a RREP from the BS/AP before another RREQ from the same MS searching for the same destination is received, the GN assumes the BS/AP was not able to find a route for such destination and just forwards the second RREQ as any other intermediary MS that does not have a route for the destination. In case a RREP is not received at the source MS before the RREQ timer expiration, another RREQ is sent according to the ring search procedure described above. Consider the case when F needs a route to J, it will send the RREQ directly to G (assume G is the GN for F), with an out-of-coverage flag set and will carry the multi-hop route from F to G which is used by G to reconstruct the complete route. G will forward the RREQ to the BS, which will reply with the route BS->J. Then, G will include the overall source route in the RREP and send it back to F. Furthermore, although the RREQ is sent directly to the GN, as a unicast packet, the forwarding nodes in the path to GN can also generate RREPs, as long as they have valid routes to the destination. iii) Multiple Route Replies Several nodes can reply to a RREQ as long as they have valid routes to the destination. MS starts sending the data once it receives the first RREP and as other route replies for the same destination come in, the routing table is updated, only if the new route has a higher sequence number or, if the sequence number is equal but the new route has smaller cost than the current route. D. Route Maintenance Routes are maintained in the same way as in AODV [12]. Once a MS detects the next hop for a route to be unreachable, MS propagates a notification of broken link to all its active neighbors. This broken link notification is propagated back to the source, which can decide to start a new route discovery. IV. PERFORMANCE EVALUATION We have implemented a heterogeneous multi-hop network with the proposed routing scheme in ns-2. We have considered a single CDMA/HDR [8] cell with the BS placed at the center of an 886 x 886 m 2 area, and have set the BS s transmitting power such that it can cover the whole cell area. Further, we assume a WLAN AP collocated with the BS, Single-Mode and Dual-Mode MSs all equipped with IEEE 82.11b interfaces operating in DCF mode [9]. Other parameters of the simulated system are summarized in Table I. Although we consider a single cell, all ideas behind the proposed integrated routing protocol can be extended to a multiple cells scenario, by adding mobility management protocols on top of our integrated routing mechanism. This will be addressed in future works. Although IRP support different connectivity alternatives shown in Figure 2, due to

space limitation we analyze the performance of two scenarios, as described next. All results are averaged over 1 runs with random topologies. Table I: Parameters of the access technologies and routing. System Parameters Area 886 m x 886 m Simulation time 2 sec Downlink channel rate 2.4 Mbps (max rate) Scheduling Proportional Fairness Cellular system Uplink channel bandwidth 153 Kbps (shared) MAC (TDMA) slot = 1.67 msec Propagation effects Slow and fast fading Bandwidth 11 Mbps WLAN interfaces Communication range 15 m MAC 82.11 DCF Propagation/Reception Propagation model Two-Ray ground models SINR based reception threshold = 1 db Routing parameters Hello packets interval ETT estimation 1 sec S=512 bytes A. Link Quality Metrics We consider two link quality metrics, namely: number of hops (Hops) and an adaptation for heterogeneous scenarios of the Expected Transmission Time (ETT) in [7]. The ETT of a WLAN link is defined in [7] as: ETT = ETX ( S B), (1) where S is the packet size, ETX is the number of expected transmissions to send a unicast packet on the link, and B is the link s bandwidth. The ETX estimation method used in [7] increases the control overhead and can not be applied to the heterogeneous links in our scenario. Thus, we propose new definition and estimation method for the ETT in order to adapt it for a heterogeneous network. We define the ETT ij in the WLAN directional link from i to j is computed as in (1), but we estimate the ETX ij by allowing ach MS to keep a moving average of the ratio between number of collisions detected and number of bytes received in its WLAN interface. This information is included in the Hello packets periodically exchanged. Then, once i receives a Hello packet from j, it extracts j s average collision per bytes (AVG_Coll_bytes j ) and computes the ETX ij as follows: ETX = ( 1 AVG _ Coll _ bytes * S). (2) ij + j In the case of CDMA/HDR links, we define the ETT i-bs in the uplink channel between MS i and the BS as ETT i-bs = S/B up, where B up is the total uplink bandwidth divided by the number of users sharing the uplink channel. In the downlink channel, the service level perceived by each user varies in each time slot t with the allowed instantaneous data rate R i (t) and the queuing delay. Therefore, we allow the BS to maintain a moving average of the queuing delay (AVG_qdelay i ) and the downlink data rate (R DWi (t)) for each MS within its coverage and compute the ETT BS-i in the downlink with MS i for each time slot t as, ETTBS i ( t) = AVG _ qdelayi + S RDWi ( t). (3) B. Scenario 1: Network capacity In this scenario we consider the BS, 4 SM MSs and 4 and 1 FTP/TCP flows between random DM MS pairs. Initially, we considered a static network and measured the aggregate throughput with IRP using number of hops (Hops) and ETT as link quality metrics. The results are shown in Figure 3(a), which also depicts the aggregate throughput in the two extreme cases, i.e., when all connections are routed through the BS (Cellular/HDR) or through the MANET. IRP was able to find all possible routes in the simulated scenario and achieved a higher throughput than the cellular network and the MANET. The best result was obtained with ETT. ETT does not limit the hop length of multi-hop routes, which results in more connections through the MANET and less traffic at the BS, which explains the higher aggregate throughput. The route acquisition latency and the routing overhead are shown in Figure 3(b) and (c), respectively. As expected, the MANET has the highest route discovery delay and overhead. We have also analyzed the impact of mobility on this scenario by assuming the Random Way Point mobility model with a fixed pause time of 5 seconds and maximum moving speed varying from, 5, 1 and 15 m/s. The aggregate throughputs are given in Figure 3(d). The advantage of IRP and ETT over the other schemes decreases as the mobility level increases, due to the negative impact of mobility on the multi-hop connections. However, even under higher mobility, the integrated model performed better than the isolated networks. (Mbps).7.6.5.4.3.2.1 Aggregate Throughput (static network) (msec) Route Acquisition Latency (static network) 18 16 14 12 1 8 6 4 2.25.2.15.1.5 Routing Overhead (static network) (Mbps) Aggregate Throughput (Mobile network).7 HDR.6 IRP ETT IRP Hops.5 MANET.4.3.2.1 5 1 15 max speed (m/s) (a) (b) (c) (d) Figure 3: (a) Aggregate throughput, (b) Route Acquisition Latency, (c) Routing overhead in a static network, and (d) Throughput with mobility.

Mbps.16.12.8.4 Aggregated Uplink Throughput 16 12 (msec) 8 4 E2E de lay (sec) 2 1.6 1.2.8.4 Route Acquisition Latency.8.6.4.2 Routing Overhead 5 1 15 2 25 3 35 4 5 1 15 2 25 3 35 4 5 1 15 2 25 3 35 4 DM M Ss 5 1 15 2 25 3 35 4 (a) (b) (c) (d) Figure 4: (a) Aggregate throughput, (b) E2E delay, (c) Route Acquisition Latency, and (d) Routing Overhead as functions of the number of.. C. Scenario 2: Support to out of coverage MSs In order to study the impact of the number of and the link quality metric on the out of coverage MSs performance, we consider in this case the BS and the collocated AP, 4 SM MSs and different numbers of DM MSs (5, 1, 2, 3 and 4) without mobility. We simulate 5 CBR/UDP flows generating a total load of 153.7*1.1 Kbps from randomly selected SM MSs towards the BS, and 5 flows with the same load from disjoint randomly selected SM MSs towards the AP. We have also considered Hops and the ETT for WLAN links as metrics to define the Cost_to_GN field in the Hello packets. Then, we analyze the two types of connections and two different metrics, which results in four combinations: AP Hops, BS Hops, AP ETT and BS ETT. The effectiveness of multi-hop communication depends on the network connectivity. As expected the aggregated throughput increases with the number of (Figure 4(a)), and this effect is more visible in the case of connections to the AP, due to the limited communication range of the WLAN interfaces. On average, longer multi-hop routes are required to connect to the AP than to the BS, which explains the higher route discovery delay and routing overhead to obtain routes to the AP than to the BS (see Figure 4 (c) and (d)). The bandwidth difference between the WLAN interfaces and the uplink HDR channel is reflected on the average End-to-End (E2E) packet delay. As expected, MSs connecting to the AP experiences smaller E2E delay, than MSs connecting to the BS (see Figure 4(b)). Although connections to the AP had smaller delays in this example, it may not be always the case, as the traffic conditions vary in the MANET and at the WLAN AP. Differently from scenario 1, the two link quality metrics perform very similarly in this case because the topology information is not as useful as in the previous scenario, since the only route options are the direct links between the GNs and the BS or the AP. The main performance limiting factors here are the multi-hop connections between the source MSs and the GNs. Compared with other architectures that support scenario 2 (e.g. Two-Hop-Relay [6]) the link quality estimation used in IRP allows more multi-hop routes, supporting more users. Also, IRP can operate with other metrics and the more accurate the metric is, the higher the system capacity. V. CONCLUSIONS We have proposed an integrated routing protocol (IRP) for a heterogeneous network. 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