Low throughput caused by suboptimal utilisation of transmission time#
The most often advertised parameter of wireless comminication technology is throughtput of physical layer. However, the most important paremeter for the user - effective throughtput - rarely achieves such high values. It is caused by necessity of utilisation of transmission channel for purposes different from transmission of data.
IEEE 802.11 Standard assumes sharing of a medium by multiple stations. As a result some amount of time must be devoted for realization of mechanisms of multiple access and collision avoidance.
Due to relatively high susceptibility of radio communication to interference and resulting packet losses, system of acknowledgment of packets delivery was introduced in the Standard. It is beneficial for improvement of efficiency of transmission most protocols used in the Internet. Nevertheless, double transmission direction change is required in order to transmit short packet carrying only information about receipt of a portion of data. Thus, additional time is wasted.
Poor scalability with respect to distance#
Application of a single channel in a two-way transmission (half-duplex mode) requires pausing of sending until remote station stops transmission. In case of connections established at long distances, propagation time must be taken into account what leads to consequent decrease in effective throughput. What is more, when delays connected with propagation of signal become comparable to time intervals related to mechisms of access contention, probability of collision resulting from errorneous assessment of channel as free increases. That is why the efficiency of links in IEEE 802.11 technology falls as the distance increases, regardless of signal strength.
Lack of dynamic selection of all transmission parameters#
There exist implementations of IEEE 802.11 protocol selecting modulation and parameters of forward error correction coding on the basis of analysis of lost packet distributions. Such a technique is necessary for transmission speed adoption to external conditions. Modulation, however, is not the only parameter having impact on the quality of connection. Other important factors are also transmission power and packet size.
It is obvious that too low transmission power used on a link with high attenuation and interference may cause high packet losses. However, on a low-attenuation link too high transmission power is undesirable because this can cause limitation of efficincy due to packet losses.
Transmission of too short frames may unwillingly reduce link efficiency while transmission of too long ones may make communication dramatically diffcult in severe conditions.
Unfortunatelly, all devices accessible in the market working according to IEEE 802.11 Standard require manual setting of both parameters. Time-consuming testing is needed for optimal realization of this process. Quick reaction to changing conditions is practically impossible.
Solution - MRTP protocol#
As Miure Duo was designed for fast transmission of data between distant point-to-point links, a specialised transmission data protocol MRTP was created which no longer works on the basis of flow control applied in WiFi, enabling an achievement of maximum possible efficiency of radio interfaces used. It is remarkable within this class of devices. It was possible because of two independent radio transmission paths and provision of our own unique techniques:
- fraggregation - technique thanks to which the size of frames sent by radio are set apart from user traffic
- asynchronous retransmission - notification about frame loss realized by means of separate transmssion path
- optimization of transmission parameters - optimal selection of modulation, frame size and transmission power on the basis of countinuously performed link quality probing
One of the main assumptions established for development of MRTP protocol is that radio frame size is an important parameter which should be set adaptively to conditions in the transmission medium and should be independent of traffic delivered by the user.
Utilisation of radio frame sizes up to over 4000 bytes (double of the normally available) allows to increase the data transmission duty cycle, what positively contributes to achieved throughput. Ethernet frames are usually at most 1518 bytes long, therefore in order to construct large radio frames, they have to be aggregated.
On the other hand, if external conditions cause smaller frames to make better use of the link, there may turn up a need for fragmentation, i.e. splitting user-delivered packets into smaller pieces.
Fraggregation is new term which describes a process simultaneous fragmentation and aggregation of the packets. In contrary to separate use of these techniques, fraggregation allows to fill whole available frame space with user data. Thanks to this, if there is some free space remaining in the currently fraggregated frame, but the whole user packet cannot fit there, it is split into fragments. Then, one part fills the radio frame completely, while the remaining packet fragment goes to the next radio frame.
Short acknowledgement frames sent on the same channel as the data they refer to are replaced by adding the information of unsuccessful reception to the structure of the MRTP protocol. Besides decreasing of the overhead incurred by the retransmission, we have gained the possibility of sending them using different channel than the received data. This in turn enables to use each of these channels for transmission of data in different directions (full-duplex transmission), what eliminates the reason of low efficiency on long-range links.
Optimization of transmission parameters#
The implemented transmission parameter adaptation algorithm is the only solution available in the market, which is able to perform simultaneous selection of modulation scheme, frame size and transmission power in order to achieve the highest efficiency and reliability under suboptimal conditions.