Virtual cell in mobile computer communications

Abstract

In this paper, we design and develop a virtual cell approach for the transmission of IP datagrams in mobile computer communications. A virtual cell consists of a group of physical cells whose base stations are implemented by remote bridges and interconnected via high-speed datagram packet-switched networks. Host mobility is supported at the data link layer by virtual cell protocol (VCP) using the distributed hierarchical location information of mobile hosts.

As far as the IP layer is concerned, it appears within a virtual cell as if the communication between two mobile hosts in different physical cells were taking place directly as in the same physical cell. Between virtual cells, a one-hop forwarding pointer from a mobile host's native virtual cell to its current virtual cell is maintained at the VCP layer. This means that the combination of the existing routing protocols of the IP layer and the forwarding pointer maintained at the VCP layer between virtual cells gives the same effect of having a fully duplicated location information among virtual cells for the global mobile network.

Thus, the virtual cell approach eliminates the necessity of IP-level mobile host protocols that may interfere with the conventional IP protocol in a practical sense. In addition, given mobility and communication patterns among physical cells, it achieves a logically flexible coverage area of a virtual cell so as to minimize the total communication cost for the global mobile network where inter-virtual cell communication is more expensive than intra-virtual cell communication (K. Lim and Y.-H. Lee, IEEE Personal, Indoor and Mobile Radio Communications PIMRC'94, pp. 1237–1241, Sep. 1994; K. Lim, Y.-H. Lim and Y.-H. Lee, IEEE ICC'95, pp. 1839–1843, June 1995). To demonstrate the correct operations of VCP and measure data which are used as parameters to a performance model for the virtual cell system, we implement VCP on a simulated environment. The performance analysis of the virtual cell system is out of scope in this paper and one is referred to K. Lim, Y.-H. Lim and Y.-H. Lee, Proc. 10th Int. Conf. Information Networking ICOIN-10, pp. 551–560, Jan. 1996.

Introduction

As computers become more powerful and portable with the appearance of high-speed wireless interfaces, there is an increasing demand on the provision of mobile computer communications in TCP/IP environments. A fixed host in internets is always assigned an IP address which not only serves a unique identifier used by the higher layer protocols but also represents the current location of it. An inherent problem from the transmission of IP datagrams in mobile computer communications is that when a mobile host (MH) migrates, its IP address is only valid as the identification information but not able to represent the current location information. To solve this problem, much research and development has focused on how to integrate the functionality of host mobility into the IP layer, preserving the compatibility with the conventional IP protocol 1, 2, 3, 4, 5, 6.

Although these mobile host protocols vary on how to represent and maintain location information for efficient tracking of MHs, the techniques of using the IP options and IP encapsulation have been considered. Typical examples of the first technique are virtual internet protocol (VIP) 2, 3 and the mobile host protocol using the IP loose source routing option [4]. IP-within-IP (IPIP) [1] and internet packet forwarding protocol [6] use the second technique. Even though the details of these mobile host protocols are different, we consider one representative mobile host protocol for each technique, VIP and IPIP, to illustrate common features and problems of integrating host mobility into the IP layer.

In VIP, the network layer is divided into two layers; the VIP layer resides on top of the normal IP layer. The VIP packet header is implemented as an option of the IP packet header. An MH keeps its permanent IP address used at the VIP layer for its identification and acquires a transient IP address used at the IP layer for its current location information when it migrates. If the MH is in its native network, both addresses are the same. After acquiring a transient IP address, the migrating MH sends its native network a notification packet whose header contains the permanent and transient IP addresses. As the packet propagates to the native network, every network entity including MHs, mobile support gateways (MSGs), and even intermediate gateways on the path snoops the header information and stores address conversion information to a cache. In the same way, the header information of all data packets in transit is also used by the network entities to maintain their caches. If the source has the cache entry for the destination, the source executes address conversion before sending a packet. The existing routing protocols of the IP layer can then correctly deliver this packet to the destination. Otherwise, the source assumes that the destination resides in its native network and sends the packet accordingly. As the packet traverses to the native network of the destination, if an intermediate gateway has the cache entry for the destination the gateway executes address conversion and forwards the packet to the current location of the destination.

Unlike VIP, intermediate gateways in IPIP are not involved in the support of host mobility but merely in transport service. The source MSG encapsulates a network control packet for mobility management or an IP datagram from an MH into another IP datagram whose source and destination addresses specify the communicating MSGs, and transmits it over an internet. The existing routing protocols of the IP layer then correctly deliver it to the destination MSG. Thus, MSGs consider internets as a full mesh of logical point-to-point links to interconnect them. A mobile network consists of a number of MSGs, each of which maintains the location information of its own MHs. Every MH in the mobile network is assigned a unique IP address but the network part of the IP address is the same. When the MH migrates, a forwarding pointer is set from the previous MSG to the new MSG for location tracking. If the MSG has no location information for a specific MH, it broadcasts an inquiry message to the other MSGs in the same mobile network asking who has the MH. However, IPIP also requires a transient IP address when the MH visits a foreign mobile network.

Regardless of which technique is used, the integration of host mobility into the IP layer reveals several implications. First, underlying networks differ widely in their network size, bandwidth, protocol, and packet size so that they or some of them may not meet performance needs for the support of host mobility such as rapid migration and tracking of MHs. Moreover, because they are usually under different administrative controls, efficient network management and optimization for the global mobile network may not be easy tasks.

Second, intermediate gateways may cause some performance problems no matter they are involved in the support of host mobility or not. If they are involved, as in VIP, they must be modified or replaced to understand VIP so that the benefit from using existing internets as transport networks is diminished. Furthermore, because intermediate gateways have to snoop every packet in transit to maintain location information, and examine every data packet in transit to try to perform address conversion, the protocol processing and memory loads at intermediate gateways may severely affect overall performance. Even in case that they are not involved, as in IPIP, because they usually implement packet switching operations in software, protocol processing time coupled with possible network delay between multiple hops of IP gateways may greatly affect cell switching latency.

Third, each physical cell administered by an MSG is in principle assigned an IP network address because every MH and intermediate gateway is a network entity in TCP/IP environments. It means that locating the MH in a different physical cell necessitates a different network address to represent its current location. Some undesirable features of IP-level mobile host protocols essentially comes from this fact. VIP requires a large amount of the transient IP address space which becomes a very scarce network resource as internets are rapidly growing. IPIP uses one permanent IP address for each MH but relies on the broadcast inquiry among MSGs when the location information is not available by a forwarding pointer, restricting the scalability of IPIP to a local area.

Fourth, although IP-level mobile host protocols based on options keep the compatibility with the conventional IP protocol in their specifications, in a practical sense they may interfere with it because most existing fixed hosts and intermediate gateways do not implement the IP options and their implementations to support the IP options may not be feasible in the near future. In addition, the current efforts to provide IP multicasting protocols and connection-oriented protocols require IP-level mobile host protocols to be compatible.

As high-speed, connectionless, packet-switched networks are emerging to extend LAN-like performance across a wide area, we believe that they can greatly affect the support of host mobility in mobile computer communications. Examples are ATM networks 7, 8 and switched multimegabit data services (SMDS) networks 9, 10, 11, 12 which are subnetworks providing an MAC service across a wide area in a large interconnected network. Considering the requirements of base station networks from two different viewpoints, application and mobility management, the virtual cell system takes advantage of high-speed datagram packet-switched networks with the multicast ability for base station networking. The base station networks interconnect remote bridge BSs, so as to preserve the interconnection level of physical cells at the data link layer. The current location of MHs is identified by the base station network address. It represents which BS can communicate with a particular MH. For efficient location tracking of MHs, the virtual cell system constructs a distributed hierarchical location information structure. Based on the distributed hierarchical location information, the virtual cell protocol for handoff, address resolution, and data transfer is designed. The handoff procedure can utilize the multicast ability of the base station network to achieve the consistency of the distributed location information. Since the IP network address of a migrating MH represents at least the near location information of the MH in the virtual cell system, the distributed location information coupled with the existing routing protocols of the IP layer give the same effect of maintaining a conceptually centralized server for the whole mobile network.

Given mobility and communication patterns among physical cells, suppose that an optimal partition of a cover of disjoint clusters of physical cells is obtained and the virtual cell system is deployed so that each cluster corresponds to a virtual cell. To evaluate the performance of the virtual cell system, we apply an open multiple class queueing network model. There are three types of messages entering or leaving the virtual cell system via BSs: the handoff message, the data message, and the address resolution message. The handoff messages are generated due to mobility patterns, and the data and address resolution messages are due to communication patterns. The mobility and communication patterns in conjunction with the topology of the deployed virtual cell system are used to determine service transition probabilities for each type of message in the queueing network model. By solving the traffic equations of the queueing network model, we obtain various performance measures such as the network response time for each type of message and the utilization of the base station networks and the backbone network of the virtual cell system.