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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Wireless Local BridgesNetwork bridges are an important part of any networkthey interconnect multiple LANs at the Medium Access Control (MAC) layer to produce a single logical network. The MAC layer, which provides medium access functions, is part of IEEEs architecture describing LANs. Chapter 8, Designing a Wireless Network, describes this architecture. Bridges interface LANs together, such as ethernet-to-ethernet or ethernet-to-token ring, and also provide a filtering of packets based on their MAC layer address. This allows an organization to create segments within an enterprise network. If a networked station sends a packet to another station located on the same segment, the bridge will not forward the packet to other segments or the enterprise backbone. If the packets destination is on a different segment, however, the bridge will allow the packet to pass through to the destination segment. Thus, bridges ensure that packets do not wander into parts of the network where they are not needed. This process, known as segmentation, makes better use of network bandwidth and increases overall performance. There are two types of bridgeslocal and remote. Local bridges connect LANs within close proximity, and remote bridges interconnect sites that are separated by distances greater than the LAN protocols can support. Figure 2.7 illustrates the differences between local and remote network bridges. Traditionally, organizations have used leased digital circuits, such as T1 and 56 Kbps, to facilitate the connections between a pair of remote bridges. Chapter 3, Wireless Metropolitan Area Networks, describes wireless remote network bridging that uses radio and infrared light as the medium.
Most companies that build wireless LAN NICs also sell a wireless local bridge. Proxim, for example, sells a bridge called RangeLAN2 7500 Access Point that interfaces the wireless RangeLAN2 family products to an IEEE 802.3 ethernet network. RangeLAN2 7500 operates in the 2.42.4835 GHz ISM band using frequency-hopping spread spectrum. RangeLAN2 7500 optimizes the networks performance and reliability by filtering local packets and only forwarding packets meant for other network segments. RangeLAN2 7500 automatically learns source addresses by monitoring network traffic. Once identified, the address information is stored and forwarded within the network, resulting in an overall reduction in network traffic. RangeLAN2 7500 enables roaming, allowing mobile computing users to move seamlessly from one RangeLAN2 wired access point to another without losing their network connection. The filtering process of a local bridge is critical in maintaining a network configuration that minimizes unnecessary data traffic. WavePOINT, a wireless local network bridge that is part of the WaveLAN product family, has a filter table that contains MAC layer addresses mapped to either the WaveLAN or ethernet side of the bridge. When the bridge receives a packet, the bridge creates a record containing the MAC address and the port it receives the frame on in a dynamic table. WavePOINT also enables you to enter static associations between addresses and ports in the Static Filter Table. These entries cannot be overwritten by the WavePOINT. When the bridge receives a frame, the bridge looks at its destination MAC address, then checks both the dynamic and static filter tables. The following situations may occur:
Radio-based Wireless LAN ConfigurationsThe combination of wireless NICs and bridges gives network managers and engineers the ability to create a variety of network configurations. There are two main configurations a wireless LAN can assumesingle cell and multiple cell. Single-Cell Wireless LANsFor small single-floor offices or factories, a single-cell wireless LAN covering roughly a million square feet may suffice. Single-cell wireless LANs only require wireless NICs in devices connected to the network, as shown in figure 2.8. Access points are not necessary. For example, Xircoms Netwave wireless LAN product, as with most other wireless LAN products, allows several configurations. You can create a spontaneous LAN easily using Netwave equipped portables without the use of any access points to form a peer-to-peer network. Any time two or more PCMCIA adapters are within range of each other, they can establish a peer-to-peer network. This allows an organization to form an ad hoc network for temporary use.
With Xircoms Netwave products, the area covered by stations within a peer-to-peer network is called a Basic Service Area (BSA). A BSA covers approximately 150 feet between all units in a typical office environment (650 feet in open areas). A single radio-based wireless LAN segment, such as the BSA, can support 625 users and still keep network access delays at an acceptable level. These networks require no administration or preconfiguration. With Netwave, there are two types of peer-to-peer networkspublic and named. Public networks use a default domain, allowing any Netwave user within range to join the network. If you need more privacy, you can create a named network by having users configure the Netwave adapters with a specific Domain ID. With this configuration, only stations having the same Domain ID can join the network. The optional data scrambling feature requires participants to know the scrambling key used by the network to decode data packets from other stations on the network. In addition, the Domain ID specifies a unique hopping code that minimizes interference between adjacent wireless networks. Most vendors utilize similar approaches.
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