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CSMA / CA |
| Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is the MAC protocol that is used by systems that conform to the various flavors of the IEEE 802.11 Indoor LAN Standard. This includes the older lower speed FH-SS and DS-SS based products as well as the newer 802.11a and 802.11b based products. Ever since the appearance of the first 802.11 products, there have been vendors who have taken this technology and applied it to outdoor use. These vendors are attempting to force fit a technology that was designed for short distances in enterprise networks, to large cells in public carrier networks. The results of this technology "mis-placement" have been mixed so far. The lower cost of the 802.11 based products has enabled many wireless operators to get into business, with a minimum amount of investment, but at the same time, the limitations of the technology have prevented them from scaling up their network and providing true carrier grade service. As a result, their networks are vulnerable to attacks from cable and DSL providers, and their wireless business is only viable in rural areas where these alternative technologies don't yet exist. On the other hand, wireless networks based on Aperto®’s product, are capable of holding their own and exceeding the performance of higher performance wired alternatives such as T1/E1 or fractional T1/E1 service. |
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| The CSMA/CA protocol was designed for use in LAN environments, where all the nodes are located within a few hundred feet from each other. The essence of the protocol is that each node listens in to the channel for a short time before it transmits. The main assumption that the protocol makes is, that for efficient operation, each node should be able to hear the transmission from every other station in the network. However, this basic assumption is violated in an access network. This results in several problems, the most troublesome one of which is the so-called hidden transmitter problem. This problem occurs when a wireless node cannot hear one or more of the other nodes, therefore the CSMA/CA protocol cannot function properly. When this happens, multiple nodes will attempt to transmit their data over the shared medium simultaneously, thus interfering with one another. This problem is especially serious at the upstream receiver in the BSU since every SU is trying to get to it, and can cause the overall performance to drop by 50% or more. The 802.11b standard addresses the hidden transmitter problem using a protocol called RTS/CTS. Before transmitting each packet, each node broadcasts an RTS packet to warn other nodes that it is about to send data, and after the transmission is complete, it broadcasts a CTS packet. This time and bandwidth consuming process is required for each and every transmission by each and every wireless node, and it still does not address the hidden transmitter problem fully since the RTS/CTS packets are also subject to repeated collisions. |
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| Aperto’s system does not suffer from the hidden transmitter problem, since all transmissions are controlled centrally by the BSU scheduler. The SU reserves data slots by a highly efficient contention resolution process using mini-slots and all upstream data transmissions are subsequently always sent in the reserved mode. |
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| Another area in which the 802.11 protocol performs poorly, is that of packet re-transmissions or ARQ. The ARQ protocol used by 802.11 is called the stop and wait protocol. Essentially, the transmitter sends the packet, and then waits for an ACK from the receiver before it tries to send the next packet. If the transmitting station is the BSU, this delay will impact all of the remote nodes because they will have to wait to send any transmissions destined for the BSU. If the stop and wait ARQ is combined with the RTS/CTS protocol, even each ACK will have to be preceded by an RTS and followed by a CTS, thus slowing down the network significantly. |
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| Aperto believes that ARQ is an important part of the broadband wireless protocol mix, however it has implemented an efficient ARQ protocol in which the receiver does not have to ACK each and every packet and a single ACK packet is sufficient to acknowledge all the data packets that have been received so far. The transmitter does not have to wait for the ACK packet to come back before it sends the next packet, and its can send up to 64 Kbytes of data without receiving an ACK. This reduces the ACK overhead significantly, and enables the system to scale to large number of nodes without affecting performance. |
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| Security is another area of weakness for 802.11 systems. Because of the wide availability of 802.11 compatible hardware and in-adequate encryption procedures, it is relatively easy to break into an 802.11 network. Aperto uses proprietary protocols and data scrambling and is also planning to introduce DES and 3-DES based hardware encryption to address the security issue. |
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