Abstract
Recently, virtual LAN (VLAN) has attracted much attention due to its capability of allowing a LAN segment (a subnet) to span across networks without being bounded by the actual physical location. This increases flexibility when designing networks and reduces configuration/reconfiguration costs associated with LAN stations moves, adds and changes via VLAN management software. This concept would also allow the reusability of vast base of communication applications what are widely used in traditional LANs, and then speed up the deployment of emerging networks. Our research deals with the design of supporting virtual LAN services over so-called nonbroadcast multiple access (NBMA) media like asynchronous transfer mode (ATM) and wavelength division multiplexing (WDM) networks which are currently believed to be the most promising network technologies for the future broadband networks. Supporting (virtual) LANs over NBMA networks poses a set of challenging design problems due to the mismatches between their networking models, e.g., the connectionless and the connection-oriented access methods, and the diverse bandwidth requirements from (different LANs) end users and the fixed channel capacity provided by the network like WDM. Also, broadcast LAN segments are connected together with bridges and routers. ATM (WDM) networks are joined together using ATM switches (optical packet switches). To provide most of the advantages of connection-oriented media, a new networking model is needed to use high-performance switching fabric to emulate connectionless (broadcast) media. We first present a framework on how the networking model, developed by the ATM Forum, to emulate legacy LANs (Ethernets and token rings) on an ATM network. The networking model is based on a client-server architecture. The LAN emulation (LANE) concept provides a powerful instrument to build VLAN segments, because the membership of a station in a VLAN is primarily determined by the VLAN server to which it is connected, and not by its physical attachment to the ATM network. The VLAN, in our opinion, is potentially more rewarding in a mobile environment where the constant moving of mobile stations is highly anticipated. This is a challenging issue given the ATM network inherently does not handle the mobility very well owing to its connection-oriented nature. We investigate the design of a virtual LAN based on the ATM Forum LAN emulation standard to be employed in a mobile environment. The proposed VLAN architecture can efficiently manage multiple VLANs given the topology of each VLAN is constantly changing due to the movement of mobile stations. This architecture exploits a hierarchical structure to interconnect multiple VLANs which 1) ensures that frames between a mobile station and any station, either static or mobile, that belongs to the same VLAN can be exchanged transparently; and 2) handles excessive inter-VLAN traffic efficiently. In addition, on occurring a handoff, the ATM network has to maintain network connections and reroute data to the new location of mobile stations. Combined with the conventional cellular handoff schemes, the data rerouting is performed at the base station. As a result, a communication path between two mobile stations may become inefficient when the station moves, e.g., the path elongates and incurs extra processing overheads. To overcome this problem, we suggest maintaining a separate connection for each pair of mobile stations instead of each pair of LANs (as defined in the ATM Forum). Following on this suggestion, an extensive cellular handoff scheme is proposed to reroute user connection(s) from old location to new one which 1) maintains the data continuity without frame loss during a handoff; 2) preserves in-sequence ATM transmission for bi-directional traffic as stations move during a communication; 3) minimizes the handoff time; and 4) keeps the rerouting path as short as possible. Following the LAN-over-NBMA networking model successfully used in the ATM environment, we propose a feasible VLAN architecture on a WDM network. The proposed solution still relies on the client-server model, and exploits a server-based multicast tree connection to maintain the VLAN membership and to deliver broadcast/multicast MAC frames within the VLAN. As to unicast frames, they are sent via a mesh of point-to-point connections. To offer finer granularity of wavelength usage and further enable the possibility of bandwidth on-demand (BoD) support for a given VLAN, the proposed approach attempts to exploit the strengths of both optics and electronics, in which packets (frames) are either routed over a lightpath transparently, or forwarded from wavelength to wavelength by electronic switching for merging/splitting or replicating optical packets. Furthermore, in order to establish a multicast connection or a unicast connection, the issue of how to select the best wavelength to route the given multicast/unicast connection is also investigated.