On-Line Encyclopedia - B

The Maui Communications Networks
On-Line Encyclopedia

Alphabetical List Of Definitions For Technical Terms

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Backhaul Network
While not a specific technical term, "backhaul network" typically refers to a network (either wireless or wired) that is used to carry data from some primary set of network devices to some behind-the-scenes processing or communication device. For example, if a store used wireless, hand-held bar code or RFID readers for inventory management, the data collected by these wireless readers would have to be delivered to the store's database and inventory control system. In this situation it could be said that, "a Wi-Fi backhaul network was in place to connect the readers to the database." In a different situation, one could say, "We use the in-building Ethernet network as a backhaul for our point-of-sale terminals."

Bluetooth
Bluetooth is a short-distance wireless communication standard being defined in the 802.15 standard. The IEEE 802.15 working group develops standards to specify "Personal Area Networks" (PANs) providing connectivity for devices close proximity (less than roughly 10 or 20 feet) of each other. The working group is standardizing a technology called "Bluetooth" that had its origins in a group of manufacturers and related companies called the Bluetooth Special Interest Group. Bluetooth technology is quite popular for applications like wireless headsets and wireless connections between digital cameras and computers (for downloading pictures). Bluetooth is also prevalent in the cell phone and PDA (personal digital assistant) market for synchronizing telephone books, and notepads in hand-held devices to desktop computers. At issue relative to Wi-Fi is the fact that Bluetooth operates in the same frequency band as 802.11b and 802.11g. The presence of Bluetooth and Wi-Fi in the same room can cause interference that can disrupt either, or both, of the groups of communicating devices.

Block Codes
In a wireless network the receiver has no real-time contact with the transmitter. The receiver must recover an entire transmission and process it before it can send any information back to the transmitter. This "half duplex" method of communication makes it impossible for the transmitter to know, in real-time, whether or not the transmitted data was properly received. The receiver must be able to determine whether or not a received data frame was correctly recovered. To do this, an algorithmic method called Forward Error Correction (FEC) is used. FEC implements three well know methods:
  • Block Codes
  • Convolutional Codes
  • Interleaving
Block Codes use a preset algorithm to process a block of received bits. The algorithm adds a predetermined bit sequence to the transmitted data and the receiver reverses the algorithm to see if the same predetermined bit sequence is present. If it is present then the receiver knows the data was correctly transmitted.

Bridge
A bridge is an interconnection device that forwards packets of data based on the destination hardware device address in the packets. Technically, one would describe a bridge as an interconnect device operating at Layer 2 of the OSI 7-Layer Reference Model, the Data Link layer. The term bridge was commonly used until the mid-1990's when it became more prevalent to use the term "switch" to refer to the device that forwarded packets based on their Layer 2, hardware address. The key aspect of bridging that is relevant to 802.11 Wi-Fi is that the bridge receives a data packet, evaluates the destination hardware address, and then forwards the packet. An 802.11 access point operating in bridging mode will receive packets and then retransmit them. Unlike an Ethernet bridge (or switch) which always has at least 2 (and usually many more) ports to which cables are attached, the bridging access point has only one antenna (or one pair of antennas in the case of a diversity antenna device). Packets are received by the antenna, and then transmitted back out the same antenna. The Ethernet bridge receives packets from one port and forwards them out one or more other ports; never in-and-out the same port. An 802.11 bridge could be used to extend the coverage area for a Wi-Fi network. Consider a situation in which three access points were installed in a building. Access point "A" is on the first floor, "B" is on the third floor, and "C" is on the fifth floor. Assume that "A" and "B" are within range of each other, and "B" and "C" are also within range of each other, but "A" can not transmit signals to, or receive signals from "C" (because "A" is too far away from "C"). If access point "B" were operating as a bridge it could receive packets from "A" and forward them on to "C", but only if the intended destination for the packets was attached to the network at "C". This is different than a Wi-Fi repeater which, if it were device "B" in the example, would forward all packets, without making a forwarding decision based on the destination address.
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