12.4 MAC Bridging

A home router sometimes can have more than one means to connect to PCs. A home router can have a built-in Ethernet hub and a built-in wireless Ethernet access point as shown in Figure 12.16. PCs connected to either Ethernet or wireless Ethernet can share not only the ADSL broadband access service but also files (over a peer-to-peer network), if the MAC bridge function (as defined by the IEEE 802.1D standards [7]) is included in the home router. All LANs (defined under the IEEE 802 standards document group) have similar but different frame formats. A MAC bridge reframes a received packet from one port of a LAN and forwards it to a destination node through another port connected to a different LAN. Sometimes the recalculation of FCS is also necessary because different LANs might use different FCS methods.

Figure 12.16. A Home Router with Ethernet Hub and Wireless Ethernet Access Point

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Figure 12.17 shows the basic structure of a MAC bridge, which consists of an Ethernet NIC, a wireless Ethernet NIC, and an address table. A MAC bridge can have more than two NICs and identifies MAC addresses of attached LANs by monitoring transmission traffic. A MAC bridge also forwards broadcast packets across different LANs to help nodes on different LANs discover each other. Once learned, a MAC bridge forwards received packets to another LAN according to its MAC address and makes boundaries between different LANs transparent. It does not have to be inside a home router and a home router can be connected to any particular LAN among a number of different LANs connected through a MAC bridge. IEEE 802.1D allows multiple MAC bridges to exist across a number of different LANs, and any potential loop is avoided through the execution of the Spanning Tree algorithm.

Figure 12.17. Components of a MAC Bridge

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Packets other than those of an IEEE LAN cannot be forwarded through a MAC bridge directly; however, they can be delivered across an Ethernet-based home network through proper encapsulation. Figure 12.18 shows the sharing of in-house electrical wiring with an Ethernet-based HomePlug home network and an X-10 home automation network. All three PCs have HomePlug NICs, and one PC can control some lights and appliances equipped with X-10 devices. The PC with an X-10 device can control these lights and appliances by sending proper X-10 packets generated by choosing the corresponding items through an on-screen display. Another similar screen interface can also be developed and implemented on PCs without X-10 devices to send Ethernet (via HomePlug) encapsulated X-10 packets to the PC with the X-10 device and ask the PC to deliver the packets for them. The PC delivering the X-10 packet performs a similar function as that by a MAC bridge. However, the PC might need to provide much more help if an X-10 device is interested in talking back to a PC that is not directly connected to the X-10 home automation network. The task might be classified as translation instead of bridging.

Figure 12.18. HomePlug Home Network with X-10 Home Automation

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A PC-based translation process can also be developed and implemented to interconnect PCs of an Ethernet to consumer electronics linked through a 1394 network. Figure 12.19 shows a Digital TV (DTV) is locally connected to a digital camera, a set-top box, and a DVD with short-distance 1394 shielded twisted-pair cables. The DTV is also connected to a remote DTV and a PC via these Category 5 twisted pair based 1394b links. The DTV is able to view live pictures from the digital camera over the 1394 connection. The remote DTV and the PC are also able to view these live pictures over these 1394b connections. If a proper translation process is developed and implemented on the PC with the 1394b connection, the other PC should be able to exchange some packets, which might not provide a live video link owing to the asynchronous nature, with the digital camera via the wireless Ethernet. This translation process might involve relaying wireless Ethernet encapsulated packets over the 1394b connection and properly encapsulating 1394b packets for wireless Ethernet transmission back to the PC without a 1394b connection.

Figure 12.19. Wireless Ethernet and 1394b

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Home Network Basis(c) Transmission Environments and Wired/Wireless Protocols
Home Networking Basis: Transmission Environments and Wired/Wireless Protocols
ISBN: 0130165115
EAN: 2147483647
Year: 2006
Pages: 97

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