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| The cable modem industry has recently converged on a standard protocol for cable modem systems, called DOCSIS (Data Over Cable Systems Interface Specification). The question arises whether the DOCSIS MAC can also be used for Broadband Wireless Access (BWA) applications. This paper clarifies several key elements of the MAC functions and why Aperto® Networks believes that DOCSIS is not a suitable choice for BWA systems. |
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DOCSIS uses static allocation of channel parameters.
Aperto can vary the channel parameters on a per subscriber basis. |
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| The cable channel has very different characteristics as compared to the wireless channel. Since the DOCSIS MAC was designed for cable, it lacks several features that are essential for wireless systems. A characteristic that is unique to wireless is that the channel behavior is a function of the interference level and location of the subscriber unit, relative to the base station, and hence varies from one subscriber unit to another. As a result, the channel parameters should be individually tailored to each subscriber for optimal performance. The DOCSIS MAC does not allow this to be done. Thus, the coverage from a base station is optimized for the set of subscribers whose channel characteristics for reliable communication fit the common set of link parameters. |
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| The Aperto MAC, on the other hand, is extremely flexible. It allows Aperto’s OptimaLink® to adjust up to ten different multi-layer link parameters per individual subscriber, on a dynamic basis. It does so by adopting Aperto’s RapidBurst®, a TDMA-based burst mode of operation in the upstream and the downstream, which allows the base station to control the parameters of each burst individually. The base station chooses the most optimal set of parameters for each subscriber based on the OptimaLink adaptive algorithm. As a result of this architecture, subscribers whose channel characteristics vary widely are able to co-exist, and the presence of units with less efficient (but more robust) link parameters does not affect the performance of units with more efficient link parameters. The benefits of this adaptive architecture are important to capacity and performance: |
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The DOCSIS MAC does not allow for re-transmission of downstream data and has an inefficient scheme for re-transmission of upstream data.
Aperto incorporates a powerful ARQ re-transmission scheme as an integral part of its MAC. |
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| DOCSIS does not have provisions for link layer re-transmissions since the cable plant, especially the downstream, is relatively clean. For wireless protocols it has long been recognized that link layer re-transmissions should be an essential part of the MAC protocol. This is due to the fact that the wireless channel is subject to several impairments that are of transient nature, such as those caused by co-channel interference or multipath. Consequently, the Aperto MAC protocol incorporates a powerful link layer packet re-transmission and recovery scheme (also known as ARQ) in both directions. The ARQ protocol design is supported by the Aperto MAC to dramatically increase performance, and it works in concert with the Aperto link adaptation algorithm – OptimaLink – to choose the best set of link parameters. |
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DOCSIS MAC does not allow fragmentation or concatenation of downstream traffic.
Aperto MAC provides fragmentation for improved performance. |
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| Given the bit error rate on a channel, larger packet sizes lead to bigger packet error rates. In the presence of ARQ, larger packet error rates lead to more frequent re-transmissions and hence are wasteful of link bandwidth. Hence it is optimal to reduce the packet size as the bit error rate increases and, conversely, to increase the packet size as the bit error rate decreases. In view of this, fragmentation/concatenation of packets into segments should be an essential part of the BWA MAC protocol. In the Aperto MAC, fragmentation in combination with ARQ-based re-transmissions of errored fragments form a powerful combination that allows the system to operate over severely impaired channels. Furthermore, the Aperto MAC protocol can vary the fragment size as a function of instantaneous link conditions. |
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DOCSIS contention request protocol does not scale very well in heavy traffic.
The Aperto contention request protocol seamlessly scales up in heavy traffic. |
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| Simulations carried out at Aperto Networks show that the DOCSIS upstream contention state machine generates an excessive number of contention requests, when the under heavy traffic. This behavior is intrinsic to the design of the state machine itself and cannot be improved by software alone. The Aperto upstream contention state machine has been designed to scale up seamlessly as the traffic load on the system increases. It does so by generating contention requests in proportion to the rate of upstream transactions, rather than to the rate at which upstream packets are being generated. The result can be dramatic: contention requests under heavy traffic can be reduced by up to 100 times. |
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Some non-cable, wireless-related link parameters are unknown to the DOCSIS MAC and therefore are not controllable.
The Aperto MAC is able to control several wireless specific link parameters. |
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| A good example of such parameters are antenna spatial and polarization diversity, which are concepts that do not exist in the cable world but whose use are absolutely vital in a wireless environment. The DOCSIS MAC cannot control transmission diversity, while the Aperto protocol is able to do so. This enables the Aperto system to use diversity as an important tool in reducing co-channel interference and multipath propagation related impairments. |
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