Table 1:802.11 b data transfer rate of standard
802.11 Digital Link Layer 802.11 data link layer consists of two of the layers, the logical link layer LLC (Logic LinkControl) and media control layer MAC (Media AccessControl). 802.11 and 802.2 using the exact same LLC layer and the 802 protocol of 48-bit MAC address, which makes the bridge between wireless and wired is very convenient. But the MAC address of the only wireless LAN only. The MAC and 802.3
802.11 MAC protocol is very similar, are on a shared media support multiple users to share resources and data from the sender before sending the network availability. In the 802.3 protocol, is called CSMA / CD (CarrierSense Multiple Access with CollisionDetection) agreement to complete the adjustment, the agreement resolved all the workstations on the Ethernet cable on how to transfer the problem, use it to detect and Avoid if two or more network devices require data transfer network conflicts. In the 802.11 WLAN protocol, there is a certain conflict detection problem, the problem known as the “Near / Far” phenomenon, which is due to the detection of conflicts, equipment must be able to side while receiving data signals to transmit data signals, which in wireless systems is not possible.
Given the differences in 802.11 on the CSMA / CD has made some adjustments, using a new protocol CSMA / CA (CarrierSense Multiple Access with Collision Avoidance) or DCF (DistributedCoordination Function). CSMA / CA using ACK signals to avoid conflicts, that is, only when the client receives the ACK signal back to the network after the data has been sent to confirm the correct destination.
CSMA / CA protocol workflow is: a workstation like the wireless network to transmit data without being detected by the network to transmit data, the additional wait for a period of time, and then randomly select a time slice to detect, if the wireless network Road in still no activity, then the data will be sent. The receiving end of the workstation by the sender if the integrity of the data is sent back an ACK packet, if the ACK packet is received by the receiver, then the data sent is complete, if the sender does not receive the ACK packet, then either The data sent was not fully received, or send ACK signal failure, no matter the kind of phenomenon, the data reported in the transmitter to wait some time after the retransmission.
CSMA / CA in this way to provide shared access to wireless, this explicit ACK mechanism is very effective when dealing with wireless issues. However, whether or 802.3 to 802.11, this approach is the additional burden, so similar to the Ethernet network and 802.11 network performance comparison is always the rival.
Another problem is the wireless MAC layer “hiddennode” problem. Two opposite workstations using a central access point to connect the two stations are able to “listen” to the central access point of the existence of barriers to each other or the distance may be due to the reason can not perceive each other’s existence. To solve this problem, 802.11 MAC layer in the introduction of a new Send / CleartoSend (RTS / CTS) option, when this option is turned on, a sending station sends a RTS signal, then wait for the access point back to send RTS signal , since all workstations in your network can “listen” to the access point signals, the CTS will allow them to stop sending data, so the sender can send data and receive data ACK signal will not cause conflicts, which to indirectly solve the “hiddennode” problem. Because RTS / CTS network resources need to take up the additional burden on the network, usually only in those newspapers with large data (large packet retransmission will cost more).
Finally, 802.11MAC sub-layer provides the functions of two other strong, CRC checksum and packet fragmentation. In the 802.11 protocol, each wireless network data transmitted on the reports are attached parity bit to ensure that it does not appear when the transmission error, which through the upper and Ethernet TCP / IP protocol to verify the data different. Package allows a large fragment of the datagram in transmission time into smaller parts to be sent in batches. This network is congested or there is interference case (large data reported in this environment transmission is vulnerable to damage) is a very useful feature. This technology greatly reduces the number of cases, the data reported to the probability of retransmission, thus improving the overall performance of wireless networks. MAC sublayer is responsible for fragmentation received a large packet re-assembly, the upper layer protocol of this slice of the process is completely transparent.
Joint structure, cellular structure and roaming
802.11 of the MAC sub-layer is responsible for solving the client workstation and the connection between the access point. When an 802.11 client to enter one or more access point coverage, it will be according to signal strength and packet error rate to automatically select an access point to connect (this process is also known to include a basic service set BSS). Once an access point to accept, the client will accept the signal will be sent to the access point channel switching frequency. In the ensuing period, the client will periodically poll all the frequencies to detect whether there are other access points to provide higher service performance. If it is detected in the election, it will, and a new access point for consultation, and then switch to the new channels of service access point channel. (See Figure 4)
Figure 4: The access point roaming
This re-negotiation usually occurs in the wireless station out of its original access point connected to the scope of services, after the signal attenuation. Other circumstances also occur in the signal caused by changes in the building or just because the original access point of congestion. In the case of congestion, such re-negotiation to achieve a “load balancing” feature, it will be able to make the highest utilization of the wireless network.
This dynamic approach made in consultation connected network administrators can extend wireless network coverage, which is arranged in these areas through multiple access points overlapping coverage achieved. IT administrators must be noted that the 802.11 DSSS channel between the cover must comply with certain norms, the same channel between the adjacent cover for each other
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* Support Provisions of IEEE P1394b Revision 1.33+.
* Compliant with PCI-Express Revision 1.0a.
* Compliant with IEEE 1394 OpenHCI Specifications V1.0 and V1.1.
* Compliant with IEEE 1394-1995 for a High Performance Serial Bus.
* Compliant with IEEE 1394a-2000 Standard.
* Fully Backward Compatible to 1394a Specification.
* No Driver required (plug-and-play).
* Ports: 2x 1394a (9-pin, Bi-lingual), 1x 1394a (6-pin),
* Compatibility: Windows XP/Vista/7 (32/64 bit)
* Supports Data Transfer Rates of: 100Mbps/200Mbps/400Mbs/800Mbps
* Size: approx. 4.72″*3.93″*0.66″
1 x Controller Card
1 x Firewire Cable
- 1-Lane (x1) PCI-Express with transfer rate 2.5Gb/s full duplex channel
- 3x External Ports (2 * 6 Pin + 1 * 4 Pin)
- Compliant with PCI-Express Revision 1.0a. Support 400/200/100Mbps data transfer rate
- Compliant with IEEE 1394 OpenHCI Specifications V1.0 and V1.1
- Attention: Don’t Support hot-plugging. Before installation, Turn off the power and then plug in the power cord.