虛擬區域網路 (Virtual local area network, VLAN) 近來不論在學
術界或產業界均引起廣泛的重視與討論. 它 (1) 將傳統區域網路 (LAN)

局限在一"特定區域"內 (bus,ring) 的使用,擴大至"任何區域"

(mesh).(2) 將區域'網路"廣播式媒介"(broadcast media) 的資訊存

取方式,套用在"非廣播式媒介"(nonbroadcast multiple access, NBMA)

的網路上. 如此一來, 大量應用在區域網路上的通訊軟體可再度使用

(re-use) 在新型態網路 (尤指 NBMA 網路的發展)上, 加速其成長速

度與使用率.

本研究是探討如何在 NBMA 網路提供 (虛擬) 區域網路界面.所熟知非

同步傳輸模式(asynchronous transfer mode, ATM) 與波長分割多工

(wavelength division multiplexing, WDM) 網路是兩個典型 NBMA 網

路. 基本上, LAN 與 NBMA 網路有著以下的差異點: LAN 存取網路資

料是屬於免接式 (connectionless);而 NBMA 網路是屬於預接式

(connection-oriented).另一方面, LAN 之間是透過橋接器 (bridge)

或路由器 (router) 相接成一較大的網路;然而, ATM 或 WDM 網路是

由交換器 (switch) 點對點相接成一網路. 如何善用高效能交換器的

封包轉送能力來達成區域網路仿傚 (LAN emulation) 或虛擬區域網路

(VLAN) 的目標是一重要課題.

首先, 本研究探討在 ATM 網路上如何達成區域網路仿傚. 依據 ATM

Forum 所定義的操作模式: 區域網路仿傚是屬於 client-server 的架

構. 這將提供一有效且直接的網路平台來建構虛擬區域網路. 這是因

為工作站的網路屬性 (membership) 取決於相連的伺服器 (VLAN

server) 而非工作站本身的放置地點. 此研究進而探討在 ATM 網路上

如何提供"移動式"工作站與所屬虛擬區域網路間連續的通訊. 基本上,

我們將不同的 VLAN 伺服器藉由階層結構(如 tree) 將它們互連在一

起. 此互連架構負責追?移動式工作站的位置 (location management)

及負責轉送不同 VLAN 的封包資料 (traffic forwarding).如此一來,

移動式工作站可和所屬虛擬區域網路的任何工作站 (不論是靜止的或

移動式) 通訊.

沿用 ATM Forum LAN-over-NBMA 的操作模式, 本研究探討在 WDM 網

路上提供虛擬區域網路界面的做法. 為了能有效的使用網路頻寬, 我

們特別強調隨意頻寬 (bandwidth on-demand, BOD) 虛擬區域網路的

可行性. 為達此目的,我們提出具備光 / 電交換能力

(optical/electronic switching) 的路由器 (wavelength router) 架

構. 此意味著, 封包可依據波長透過 optical switch 作為選徑的依

據, 或著封包可由 electronic switch 由一波長轉送至另一波長, 以

達到隸屬不同連線 (virtual connection) 封包的合併或分離, 以及

複製封包.本研究也將探討如何選定適當的波長以建立單播/群播

(unicast/multicast) 連線.

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.

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