This post can be considered a continuation of the previous one since it too is based on the observations and analyzes of the same monitoring file. Also in this case these are asynchronous calls but with the particularity of using three-way handshakes.
Although the common protocol exchange for 4G-ALE link establishment is the two-way handshake, it may occurs a traffic channel negotiation which requires a three-way handshake. When sending a link setup request, the caller is able to measure occupancy in its assigned sub-channels, but will not generally know whether the Confirm PDU from the called station will be strong enough to overcome low-level local interference in some sub-channels. However, after measuring the SNR of the Confirm PDU, the caller may compute that there will be adequate SNR on sub-channel(s) that it initially reported as occupied. In such a case, the caller should send a Caller Confirm PDU with Rx sub-channel vector bits set to indicate the wider available traffic channel (figure 1),
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| Fig. 1 - example 3-Way Computation of Traffic Channel (188-141D Figure G-19) |
However, as shown in figure 2, it's worh noting that in these samples no data traffic was recorded after the channel negotiations (the "responder" is here termed by me as "called").
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| Fig. 2 - contiguous 3-way handshakes |
Aside from the 3-way handshakes, it must be noted that both "Deep" and "Fast" WALE waveforms are used throughout the same handshake (see figure 3) conversely to what specfied by 188-141D: " Either of the waveforms may be used in any transmission, but the same waveform shall be used throughout a transmission" (#G.5.1). The transmissions could also be Point-To-Multipoint (PTM) Async link setups and consequently the third burst could be actually the Confirm PDU from a "second" called node, however the (deprecated) mix of waveforms still remains as well as the suspect that these are modified waveforms.
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| Fig. 3 - use of both Deep and Fast waveforms throughout the same handshakes |
As said, this sample is not a "spot" one but it's extracted from a quite large monitoring file that has already been (partially) analyzed previously, consequently it was logical to expect WALE PDUs with observed on-air durations different than the ones specified in the relevant 188-141D.
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| Fig. 4 |
As well as the not-compliant durations of Deep and Fast PDUs (due to their preambles length), in my opinion the lack of (expected) traffic after the 3-way channel negotiations and the "unusual" (if not deprecated) use of both Deep and Fast waveforms throughout a same transmission, are clues that reinforce the hypothesis of test transmissions aimed to verify the performance of modified 188-141D WALE waveforms and therefore of a "new" 4G HF modem... further analysis and investigations will follow in next posts.
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