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All MIMO devices of the R5000 product family could be used with Polling technology (by default), or with TDMA technology. Polling firmware version described as “MINT”, TDMA firmware as “TDMA”. Both software versions are available on the official InfiNet Wireless FTP server: ftphttps://ftp.infinet.ru/pub/Firmware/.

Firmware upgrade fully replaces previous firmware, configuration kept with no modifications after upgrade. Upgrade could be done during any free from service provision technological cycle with short-time wireless network link interruption (device reboot is required for firmware upgrade).

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titleFigure - TDMA operation scheme

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TDMA uses continuous byte stream transmission instead of packet transmission used in Polling technology. This is done specially in order to fully utilize limited time devoted for transmission. TDMA byte stream is divided into multiple portions, but these portions are cut according to time size, not by length. Therefore, since total size of time frame is fixed, then in case of insufficient data to fill time frame completely – the unloaded part of time frame would be sent in vain. Usage of shorter time frames size helps to amend such drawback and at the same time decrease latency, however the short time frame size leads to decrease proportion of customer traffic payload within the time frame and decrease of maximum throughput.

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titleFigure - TDMA time frames structure in PtMP with subslots

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In such circumstance, system can send and receive data from several CPEs during one time frame. Such subslots usage significantly decreases latency in highly loaded by subscribers PtMP, however throughput of the whole PtMP decreases too due to higher amount of the service data used and decrease of payload data size. The system use only one subslot in each direction, then average latency for the subscribers (Round-Trip, in case all subscribers are equally active) would equals number of subscriber stations multiple on time frame size and index "2.5". Usage of several subslots decreases latency proportionally to number of subslots. Meanwhile, the more number of subslots is used, the less would be the part of customer payload within the Uplink time frame. Hence with ratio DL/UL 50%, throughput in Uplink part, would be much less than Downlink. It stands to reason Downlink is dedicated only for transmission of data by the base station, however Uplink is shared among all subscriber stations – consequentially, the more number of subscriber stations, the less amount of Uplink is being provided to each subscriber terminal. Accordingly, it is required to ensure symmetrical sector load – then DL/UL ratio should be lower (for example, 45%). Moreover, DL/UL ratio more than 60% with relatively big number of connected subscriber terminals, does not result in increase of throughput Downlink, because decrease of subslots number drastically increases the latency (whereas throughput decrease and each subscriber unit experience output queue size increase), in consequence, ARQ mechanism does not function correctly, because acknowledgement of data requires a lot of time.

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titleFigure - Example of packet delay variation under different link conditions

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In case TDMA technology application, packets latency is stable and almost does not depends on the channel load (retries are not present). In case of retries, latency would increase because each retried packet would be sent in a new time frame and would occupy the same time as the original packet. However, this delay will not affect service of the subscribers who have no problems with data transmission.

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