A Hierarchical Architecture for QoS Guarantees and Routing in Wireless/Mobile Networks

Transcription

1 A Hierarchical Architecture for QoS Guarantees and Routing in Wireless/obile Networks Indu ahadevan and Krishna. Sivalingam 1 School of EECS, Washington State University, Pullman, WA This paper addresses the problem of providing quality of service (QoS) support and routing for wireless networks in the presence of user mobility. The proposed architecture is hierarchical where cells (the basic region of mobile coverage) are organized into QoS/routing domains. The QoS mechanism is based on our earlier work that follows the Integrated services (intserv) approach with RSVP (ReSerVation Protocol) and CBQ (Class Based Queueing) used for signaling and scheduling respectively. The routing mechanism proposed in this paper is integrated with the above QoS mechanism and uses a combination of obile IP, fast route table updates and proxy ARP (Address Resolution Protocol). The architecture and mechanisms have been implemented in a wireless and mobile testbed that uses Lucent WaveLAN cards. Experimental results from this testbed are presented to show the validity of the architecture and to discuss basic system performance. ½ Corresponding Author. Contact phone: (509) Part of the research was supported by Air Force Office of Scientific Research grants F and F

2 2 AHADEVAN AND SIVALINGA 1. INTRODUCTION In wireless and mobile networks, it will be desirable to provide quality of service (QoS) assurances to user connections in the presence of user mobility. There are two issues to be addressed: (i) QoS signaling and scheduling mechanisms, and (ii) obility triggered routing that considers QoS. In the past, these two problems have been addressed separately. The signaling aspect of QoS provisioning in mobile networks has been studied earlier in [16, 8], the scheduling aspect in [7, 12] and architectural considerations in [10]. However, these solutions do not fully consider the routing issues involved. On the other hand, obile IP is a recently proposed standard for routing in mobile networks [11]. However, obile IP appears better suited for long term mobility, but is not designed to support frequent mobility. Other researchers have recognized this limitation and have proposed better routing techniques for mobile hosts and related handoff schemes [5]. However, these solutions do not consider the QoS problem of wireless networking. This paper proposes a unified approach that addresses both problems, and describes an experimental implementation that validates the proposed approach. The network architecture studied in this paper is the (micro)-cellular architecture in which the network is divided into cells with each cell served by a base station. The proposed solution is hierarchical with cells organized into QoS/Routing domains. For QoS provisioning, we adapt the Integrated Services (intserv) [4] architecture to wireless networks. We have studied this earlier without the routing considerations and reported results in [8]. In this paper, we incorporate routing mechanisms on top of this previous work and study the resulting performance. For routing, we propose a strategy that combines local routing table update, proxy Address Resolution Protocol (ARP), and mobile IP mechanisms. The rest of the paper is organized as follows. The campus-based network architecture and the concepts of the proposed approach are considered in Section 2. The implementation details of the approach are presented in Section 3, and details of the experimental testbed and results are discussed in Section 4. Section 5 concludes the paper.

3 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS 3 2. CONCEPTS OF PROPOSED QOS/ROUTING ARCHITECTURE In this paper a campus-based architecture as shown in fig. 1 is considered. A microcellular environment is considered, with a cell being the basic unit of transmission. Each cell has a base station (BS) that serves all mobiles within its region and that is connected to the wired network. When a mobile moves to another cell, it is handed off to the BS serving that cell. Groups of base stations are organized into a subnet as shown in the figure. The subnets (subnet A, B, etc.) are then connected to the departmental network. The departmental network is connected to the campus network which is connected to the Internet. Internet Campus Network R1 Departmental Network R2 R3 Subnet A Subnet B Subnet C R4 BS BS BS BS BS BS FIG. 1. Campus-based network architecture considered in the paper QoS and Routing Architecture The proposed integrated architecture for QoS and routing is described in this section QoS Support echanism The QoS architecture has two major components: (i) The intserv model for QoS where the ReSerVation (RSVP) protocol and Class Based Queuing (CBQ) algorithms are adapted to wireless networks, and (ii) Passive reservations [8] with neighboring base stations for maintaining reservation during mobility. Passive reservations can be used by other mobiles

4 4 AHADEVAN AND SIVALINGA in the cells until the mobile in question moves into that cell [16]. ore details of the QoS architecture and the passive reservation mechanism can be found in [8]. The architecture uses the notion of QoS-domains where a QoS-domain is any management specified domain. Passive reservations are made only with neighboring base stations that are within the QoS domain of the current base station. However, there are a few concerns with the intserv architecture such as scalability and the inability of certain applications to express the QoS requirements. An alternative architecture called Differentiated services (diffserv) has been recently proposed [3]. In contrast to RSVP s per-flow orientation, diffserv networks classify packets to one of a small number of aggregated flows. Our work on adapting diffserv to wireless networks is reported in [9] Routing Support The proposed routing mechanism uses a combination of mobile IP, proxy ARP and routing table changes to create a hierarchical structure. obile IP [11] is an IETF standard for providing routing under mobility. obile IP uses the concept of a home agent (HA) and a home subnet for every mobile. If a mobile moves frequently, it is hard to communicate fast with the home agent on another subnet. Packets can be delayed or lost in the process. Hence, it is better suited for long term mobility but not well suited for short term mobility. One method of overcoming the latency problems of obile IP is to not contact the HA at every move. This can done by changing the routing table at all the hosts so that the location of the mobile is known. Though this is a better routing strategy for frequent mobility, it is not scalable. Alternatively, the proxy ARP (proxy Address Resolution Protocol) [15] may be used for local mobility [5]. Proxy ARP allows a system (base station in this case) to answer ARP requests for a mobile. Therefore, all hosts that want to send data to a mobile, send it initially to the base station. For a scalable solution, the architecture must strike a balance

5 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS 5 between the approaches by using routing updates or proxy ARP locally for faster handoffs and using obile IP for long range mobility. The proposed architecture defines routing-domains which is a collection of base s- tations and subnets. Different routing policies are specified for intra-domain and interdomain mobility. The routing scheme is strongly tied to the QoS architecture, i.e. during route computation we need to ensure that the path will have sufficient resources reserved. Since passive reservations from the QoS-domain are used for this purpose, it implies that the routing domain follows the QoS-domain. From now on, the names routing-domain and QoS-domain are used interchangeably in this paper. We refer to this as the QoS/Routing domain. obility is classified into the following cases: (i) Intra-domain mobility, (ii) Interdomain mobility within the same departmental, generalized to administrative, domain, and (iii) obility between administrative domains. Intra-Domain obility: Within a domain, passive reservations are made between current base stations and the neighboring base stations following the scheme of path extensions. The routing method is as follows. For all existing flows, routing table changes are done to update the new location of the mobile. Since the routing table changes are made in a few hosts within the routing-domain, this is scalable. For new flows, the current base station uses proxy ARP to determine the location of the mobile [5]. Inter-Domain obility: When the mobile moves between domains but within the same administrative domain, the router does a partial rebuild of routes. As before, all existing flows are handled by routing table update while all new flows use proxy ARP. If the movements of mobiles between the routing-domains also cross subnets, obile IP requires that the home agent be informed. This can be prevented by using a hierarchy of foreign agents as suggested in [5]. obility between Administrative Domains: obile IP is used for mobility between administrative domains. Using obile IP ensures that the routing tables are kept small.

6 6 AHADEVAN AND SIVALINGA In this section, our proposed architecture was discussed from the QoS support and routing point of view. In the next section, the details of implementation are discussed. 3. DETAILS OF ARCHITECTURE As mentioned earlier, the fundamental elements necessary for providing intserv services in a wireless network have been presented in [8]. Our paper adds the routing component to the intserv QoS architecture and studies the integrated performance. A summary of the QoS implementation details and a detailed description of routing are presented in this section Implementing QoS Aspects The RSVP protocol [18] is a network management setup protocol designed to communicate requests among participating applications. RSVP uses receiver initiated reservation requests conforming to the intserv flow specifications [17] to help applications reserve resources. These reservations are satisfied at the hosts and routers using a traffic scheduling mechanism. One such mechanism is Class Based Queueing (CBQ) [6] that is studied in this paper. It consists of: (i) Classifier, which classifies packets into a pre-defined class, (ii) Estimator, which estimates bandwidth usage of each class and (iii) Packet scheduler, which selects the next class to send a packet. The passive reservation mechanism has been implemented for both RSVP and CBQ as explained in [8]. This enables the mobile to have resources reserved as it moves between regions. Also, there are QoS parameters specific to wireless networks in addition to typical parameters such as packet delay, packet loss rate, delay jitter and minimum and maximum bandwidth. The wireless QoS parameters implemented in [8] are: Performance feedback factor to enable the mobile to recover from packet losses that result in reduction of allocated bandwidth. This helps the mobile to adapt to short-term losses. Rate reduction factor is used for re-negotiation of QoS parameters when packet losses result in reduced bandwidth for longer periods of time.

7 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS 7 Power profile that specifies the application s choices based on its battery power level. Such choices are dependent on the application. The mobile periodically transmits its power level that is used by the network to take action as specified in the profile. Loss profile that specifies the application s choice of a bursty or distributed loss when channel errors occur [13]. Probability of seamless communication [13] defines the nature of breaks that can be allowed in the service. This section summarized the key QoS related mechanisms used in our architecture. For complete details, the reader is referred to [8] Implementing the Routing Aspects This section presents the implementation details of the routing architecture. While characteristics like high losses, low bandwidth and power constraint affect the QoS architecture, the routing architecture is affected by the mobility pattern. with BSa Router Old Sender with BSb with BSc New Sender BSa BSb BSc BSc FIG. 2. (a) (b) Routing within a Routing-Domain. The label on each router indicates its routing table information for mobile obility within Administration Domain For user mobility within the same administration domain, the routing strategy is similar to the QoS reservation strategy: (i) path extension for mobility within the current QoS/routing domain, and (ii) partial re-building of path for mobility to a different QoS/routing domain.

8 8 AHADEVAN AND SIVALINGA As seen from fig. 2(a), a base station that has made passive reservations will follow the path extension method for mobility within the QoS-domain. Therefore local route table changes are used for routing. The mobile was initially with BSa and BSb, before moving to BSc. Since the existing reservations reflect this path, routing table changes need to be made to follow this path. On the other hand any new sender which has not yet made any passive or active reservations, need not follow the path. The new sender is informed of the mobile s location by letting base station BSc act as the proxy ARP server. Therefore all the new senders reach mobile through BSc (fig. 2(b)). Note that the route table change architecture is needed because of the underlying QoS requirements. To enable static route changes in the architecture, the following changes must be made to all intermediate nodes (may also be base stations): All nodes that are RSVP capable must be enabled to act as gateways. This allows them to forward packets to various systems. All nodes that are RSVP capable must turn the ICP re-direction capabilities off. This allows the source-based routing required because of the QoS-based architecture. Data follows the path on which reservation is made and is not redirected to a closer router with ICP redirect messages. When a mobile moves from one base station to another, say from BSa to BSb in fig. 2(a), there is a need for BSa to know the address of BSb to change its routing table. This can be done in two ways. After the mobile has moved into the region of BSb, BSb sends a message to BSa about the arrival of the mobile within its region. The disadvantage of this method is that the handoff process is slower. The advantage is that the message can be sent reliably because it is sent on the wired domain. The other way is to make the mobile send a signoff message to BSa before registering with BSb. The advantage of this method is that the handoff process is faster. While the mobile is

9 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS 9 registering with BSb, BSa can make the necessary routing table changes. The disadvantage of this method is that if the signal strength thresholds for handoff are not chosen right, the sign-off message from the mobile may not arrive at BSa. In this paper, we follow the second method and send two signoff messages to make the signal more reliable. Sender Sender HA HA FA R R FA A1 (a) A2 A1 (b) A2 FIG. 3. obility between Routing-Domains Using obile IP between Administrative-Domains Fig. 3(a) shows the two administrative domains A1 and A2 with the mobile initially in domain A1. The foreign agent (FA) is currently a router/host from A1. When the mobile moves to domain A2, the Home agent (HA) is informed of the move and another FA is chosen in A2 as shown in fig. 3(b). The use of obile IP for long-range mobility takes care of the latency problem associated with frequent mobility. A problem that arises here is the discovery of the foreign agent in the new administrative domain. Also, the path from the sender to the new FA must have adequate resources reserved. If the FA discovery is made after the mobile has moved into a region, resource reservation may take a while resulting in degradation of quality of service. To counter this, the mobile may instruct the possible future base station to do a FA discovery on the mobile s behalf. The mobile is then informed of the new FA and the sender makes the required reservations.

10 10 AHADEVAN AND SIVALINGA In this section, implementation of the routing aspects of the architecture was considered. 4. TESTBED AND RESULTS In this section the details of the experimental testbed and performance results are considered Experimental Testbed The experimental testbed has three Intel Pentium PC systems that operate as base stations. Each base station is equipped with an Ethernet card and a 2.4 GHz Lucent WaveLAN ISA card. The base stations are in adjacent cells and the different Network Identifiers (NwID) of the WaveLAN cards identify the cells. The testbed also has two mobiles which are equipped with 2.4 GHz PCCIA WaveLAN cards. All the systems run the FreeBSD operating system. The testbed uses RSVP code version 4.2a2 [2] and alternate queueing package version [1]. The testbed also uses: (i) FreeBSD WaveLAN driver for PC- CIA cards which supports roaming, (ii) Handoff mechanism based on beacons produced by the WaveLAN ISA driver, (iii) odified RSVP mechanism, (iv) odified CBQ mechanism and (v) Applications used to test our architecture. ore details on the testbed can be found at [14] Results The purposes of the experiments was to show that the QoS and routing schemes work correctly as implemented and to measure certain basic latencies. (A) Validation of QoS-Domains: To verify our implementation of QoS-domains, the setup shown in fig. 4 is used. In fig. 4(a), BS1 and BS2 are in the same QoS-domains and so passive reservation is done by path extension. In fig. 4(b), BS1 and BS2 are in different QoS-domains and so passive reservation is done by the router. The purpose of this experiment was to validate the QoS-Domain implementation. Control message flows for the QoS domains: Fig. 5 shows the ICP-based messages that are sent between a base station and QoS-domain router to determine whether the QoS-

11 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS 11 Sender Sender Router Router BS1 BS2 BS1 BS2 (a) (b) FIG. 4. Experimental Setup for QoS-Domains. domain router or the base station makes the passive reservations for a mobile. The addr message sent from a base station to the QoS-domain router has the addresses of all the neighboring base stations and the mobile s address. The reply message from the QoSdomain router contains a vector indicating the neighboring base stations in the same domain so that the current base station can do passive reservations. The QoS-domain router takes care of passive reservations with base stations in other domains. Base Station QoS-domain Router Addr: Neighbor BS address + mobile address reply: Vector of BS in same QoS domain FIG. 5. Signal Flow to Identify QoS-Domain of Neighbors (B) Processing Delays when Reservation Paths are Extended: The delays involved in setting up a reservation by extending the paths and by partial re-routing are considered next. The experimental setup for the path extension consists of a sender, three intermediate base stations and a mobile. This is compared to the performance when there is exactly one router between sender and the mobile due to a partial route re-build. The time taken for the

12 12 AHADEVAN AND SIVALINGA reservation to be set up and the round-trip time for data are shown in table 1. The results shown represent the average from repeating each experiment ten times. TABLE 1 Reservation Setup Time and Round Trip Delay QoS route type Reservation setup Round-trip delay Path extension 157 s 3,937 s Partial re-route 84 s 3,249 s (C) Resource Usage when Sender/Router Does Passive Reservation: Consider a setup where the sender is sending data to 10 mobiles. For each of these 10 mobiles, the sender has to make 6 passive reservations and maintain status for 60 reservations. The PATH and RESV messages of RSVP are approximately 200 bytes each and the memory requirement for the PATH and RESV state information is 200 and 140 bytes respectively. For a single mobile, the PATH message traffic sent in 1 second is ¾¼¼ ¾ ½ ¾Ã Ô (number of mobiles * number of passive reservations * packet size * the refresh interval). Here, refresh rate is 2 messages per second. The numbers shown here are lower bound values as there are also other state information maintained. The overhead is 192 Kbps and 1.92 bps for supporting 10 and 100 mobiles respectively. Thus, the overhead of reservations can be substantially high as number of mobiles increases. (D) Validation of Routing-Domains: Fig 6 shows the experimental setup for verifying the routing domains. In fig. 6(a), the path along which QoS reservations are made is indicated. Fig. 6(b) shows the alternate path where only mobile IP alone is used. As observed, this is not the same path along which QoS reservations were made. We therefore need to use routing table changes to reach the mobile. But as we see from fig. 6(a) and (b), the segment denoted (1) is common to both setups. Therefore, we propose to use obile IP in this segment and routing table update mechanism for other segments.

13 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS 13 HA HA (1) (1) FA FA (2) (2) BS1 (3) BS2 BS1 BS2 (a) (b) FIG. 6. Experimental Setup for Routing-Domains. (E) Address Table Change: When a mobile moves into a new cell, it registers with the base station in the new cell. The implementation of the handoff procedure is shown in fig. 7 where the mobile signs off from the old base station when it is going to move into a new base station. We measured the average handoff time to be 90 ms and the routing table update time to be 67 ms. The sensitivity to handoff times is dependent on the application. For a 64-kbps videoconferencing application, the average delay tolerance is 300 ms; and that for a 16-kbps compressed voice is 30 ms. Our implementation seems more suitable for the video application in this case. However, it is possible to improve performance using optimizations such as directly writing into the routing socket. Old basestation obile New basestation Beacon Stronger Beacon Address Table Change Signoff Signon FIG. 7. Intra-domain Handoff echanism. To summarize, the working of the QoS-domain and routing-domain architecture was discussed along with the implementation on our experimental testbed. The experiments were

14 14 AHADEVAN AND SIVALINGA mostly conducted to validate that the implementation was correct. However, substantially more work is needed to measure system and application level performance metrics such as latency and throughput. This requires a larger testbed and significant experimentation time and is reserved for future work. 5. CONCLUSIONS This paper described an architecture designed to support resource reservations and fast routing in a mobile environment. The concept of QoS/routing domains was used to create a hierarchical architecture which takes advantage of various methods of reservation capabilities and routing policies. An implementation of the proposed architecture on our experimental testbed was discussed. The methodologies and experiments used to validate the architecture include: (i) the need for an hierarchical architecture in terms of resource usage and processing delays, (ii) the working of the QoS-domains, and (iii) the working of the handoff mechanism and the address table changes for intra domain mobility. REFERENCES 1. Alternate queueing code altq RSVP code rel4.2a2. ftp://ftp.isi.edu/rsvp/release. 3. Steven Blake, David Black, ark Carlson, Elwyn Davies, Zheng Wang, and Walter Weiss. An Architecture for Differentiated Services. IETF draft, draft-ietf-diffserv-arch-01.txt, August R. Braden, L. Zhang, S. Berson, S. Herzog, and S. Jamin. Resource ReSerVation Protocol (RSVP) Version 1 Functional Specification. RFC 2205, September Ramon Caceres and Venkata N. Padmanabhan. Fast and Scalable Wireless Handoffs in Support of obile Internet Audio. AC/Baltzer Wireless Networks, November S. Floyd and V. Jacobson. Link-sharing and resource management models for packet networks. IEEE/AC Transactions on Networking, 3(4): , August Songwu Lu, Thyagarajan Nandagopal, and Vaduvur Bhargavan. A Wireless Fair Service Algorithm for Packet Cellular Networks. In Proc. AC obicom, pages 10 20, Dallas, TX, October 1998.

15 A HIERARCHICAL... FOR QOS AND ROUTING IN WIRELESS/OBILE NETWORKS Indu ahadevan and Krishna. Sivalingam. Architecture and Experimental Results for Quality of Service in obile Networks using RSVP and CBQ. AC/Baltzer Wireless Networks, July (Accepted for Publication). 9. Indu ahadevan and Krishna. Sivalingam. Quality of Service in Wireless Networks using Enhanced Differentiated Services Approach. In Proc. Intl Conference on Computer Communications and Networks, Boston, A, October Naghshineh and A. S. Acampora. Design and control of micro-cellular networks with QoS provisioning for data traffic. In Proc. International Conference on Communications (ICC), pages , Dallas, TX, June C. Perkins. IP obility Support. RFC 2002, October Parameswaran Ramanathan and Prathima Agrawal. Adapting Packet Fair Queuing Algorithms to Wireless Networks. In Proc. AC obicom, pages 1 9, Dallas, TX, October S. Singh. Quality of Service guarantees in mobile computing. Computer Communications, 19(4): , April Krishna Sivalingam. DAWN WSU Networking Research Laboratory. krishna/dawn.html, krishna@eecs.wsu.edu. 15. W. Richard Stevens. TCP/IP Illustrated, Volume 1: The Protocols. Addison-Wesley Professional Computing Series, Anup K. Talukdar, B. R. Badrinath, and Arup Acharya. RSVP: A Reservation Protocol for an Integrated Services Packet Network with obile Hosts. Tech report TR-337. Rutgers university. 17. J. Wroclawski. The Use of RSVP with IETF Integrated Services. RFC 2210, September Lixia Zhang, Steve Deering, Deborah Estrin, Scott Shenker, and Daniel Zappala. RSVP: A New Resource ReSerVation Protocol. IEEE Network, 7(5):8 18, September 1993.