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VE4KLM > JNOS2 15.08.90 13:36z 61 Lines 2108 Bytes #999 (0) @ WW
BID : C29RV_VE4KLM
Read: GUEST
Subj: Re: update on ax.25 v2.2 support
Path: SR1BSZ<EA2RCF<LU9DCE<VE3CGR<VE3MUS<VA3PJB<VE4KLM
Sent: 260815/1315z @:VE4KLM.#WPG.MB.CAN.NOAM [Winnipeg] $:C29RV_VE4KLM
Thanks Marco, appreciate that, so brief explanation of the advantages of
the ax25 v2.2 is given below, I am just parroting what I found online.
Parroting what I found online, for X.25 systems, on which AX.25 is based :
The SABME (Set Asynchronous Balanced Mode Extended) is a Layer 2 LAPB
command frame used to initialize a link using modulo 128 (extended)
sequence numbering instead of the standard modulo 8 (basic) mode.
The primary advantages of using SABME Modulo 128 include :
1. Significantly Higher Window Size
Modulo 8 (SABM) restricts the maximum number of outstanding,
unacknowledged frames to 7.
Modulo 128 (SABME) expands the control field to two octets,
increasing the maximum sliding window size up to 127 frames.
2. Drastic Throughput Optimization over High-Latency Links
In satellite, long-distance wide-area networks (WANs), or high
propagation delay connections, a small window size forces the
transmitter to pause and wait for acknowledgments (ACKs).
Modulo 128 allows the sender to transmit continuously without
stalling, achieving higher local and end-to-end throughput.
3. Better Efficiency on High-Bandwidth (Fat) Pipes
When combining high transmission speeds with high latency, high
Bandwidth Delay Product, standard modulo 8 cannot fill the physical
pipeline.
Modulo 128 ensures the line remains fully saturated, maximizing
network utilization.
4. Reduced Acknowledgement Overhead
Because the receiver can acknowledge a much larger block of frames
at once using the extended N(R) sequence number field, the total
frequency of explicit acknowledgement frames (RR/Receive Ready)
is minimized.
Summary Comparison
Feature SABM (Basic) SABME (Extended)
Sequence Numbering Modulo 8 Modulo 128
Max Window Size (k) 7 frames 127 frames
Control Field Size 1 octet 2 octets (for sequenced frames)
Best Suited For Low-delay High-delay, satellite
local lines or WAN links
Maiko / VE4KLM
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