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Author Topic: VDSL modemís protocol stack - ATM?  (Read 5636 times)


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Re: VDSL modemís protocol stack - ATM?
« Reply #15 on: September 17, 2016, 10:55:35 AM »

If you are sending a 1500 byte packet over ATM over AAL5, it'll still take the same amount of time to transfer all those 1500 bytes due to the bandwidth of the link!

Im afraid you've lost me.  All the documentation about ATM says that it allows for fast switching and the small packet size was used to prevent serialisation on the slower part of the link.   

Perhaps Ive got it wrong, but I thought AAL5 was an adaption layer -  which in effect 'handles the conversion' from 1500 bytes to 53 byte ATM cells. 
ie the AAL5 device is handed a 1500 byte packet, it then breaks it down to smaller 53 byte chunks (ATM) and is the wrapper to ensure safe receipt of the broken up chunks at the other end.
So what we have is say 1500 byte ethernet. ->  Then it goes to AAL5, which breaks it up and 'converts' to ATM cells. ->  Small 53 byte packets are used over the ATM part of the link (DSL) ->  Hits the other end where AAL5 re-assembles back to 1500 bytes and its wrapper ensures safe receipt.
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Re: VDSL modemís protocol stack - ATM?
« Reply #16 on: September 17, 2016, 12:01:53 PM »

I probably should have said AAL5 over ATM, or only AAL5, but that piece is not particularly important.

As far as I can tell, this "serialisation delay" is simply the time taken to transfer X number of bits at a speed of Y bits per second.
64 bytes and a rate of 64kbps
64*8 = 512 bits
512/64000 = 0.008 seconds = 8ms

The small cell size of ATM in general may well be for all the benefits of the small cell size, but specifically, for the ADSL link where the ATM cells are only going between the modem and DSLAM, I think most of the general features of ATM networks aren't used. Nothing using your Internet connection would even be aware that it's using ATM cells in that section of the link.

1500 bytes and a rate of 780kbps (my current upstream speed)
(1500*8)/780000 = 0.0153846153 seconds, or about 15ms

$ ping -c 10 -s 106
PING ( 106(134) bytes of data.
114 bytes from icmp_seq=1 ttl=59 time=25.8 ms
114 bytes from icmp_seq=2 ttl=59 time=25.5 ms
114 bytes from icmp_seq=3 ttl=59 time=25.9 ms
114 bytes from icmp_seq=4 ttl=59 time=26.2 ms
114 bytes from icmp_seq=5 ttl=59 time=25.2 ms
114 bytes from icmp_seq=6 ttl=59 time=25.1 ms
114 bytes from icmp_seq=7 ttl=59 time=25.4 ms
114 bytes from icmp_seq=8 ttl=59 time=24.8 ms
114 bytes from icmp_seq=9 ttl=59 time=25.0 ms
114 bytes from icmp_seq=10 ttl=59 time=25.2 ms

--- ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9011ms
rtt min/avg/max/mdev = 24.807/25.461/26.219/0.471 ms

$ ping -c 10 -s 1450
PING ( 1450(1478) bytes of data.
1458 bytes from icmp_seq=1 ttl=59 time=44.9 ms
1458 bytes from icmp_seq=2 ttl=59 time=45.3 ms
1458 bytes from icmp_seq=3 ttl=59 time=44.7 ms
1458 bytes from icmp_seq=4 ttl=59 time=44.2 ms
1458 bytes from icmp_seq=5 ttl=59 time=44.5 ms
1458 bytes from icmp_seq=6 ttl=59 time=44.0 ms
1458 bytes from icmp_seq=7 ttl=59 time=44.3 ms
1458 bytes from icmp_seq=8 ttl=59 time=43.9 ms
1458 bytes from icmp_seq=9 ttl=59 time=44.0 ms
1458 bytes from icmp_seq=10 ttl=59 time=44.3 ms

--- ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9012ms
rtt min/avg/max/mdev = 43.969/44.450/45.319/0.489 ms

Of course the ping times are still higher with larger packets, they will have been split up into ATM cells, but it still needs all the ATM cells to arrive at the other end before it can re-assemble the full packet. So I don't really think these small ATM cells have improved anything.

ADSL1 only had the choice of ATM or STM. But I can't really see any disadvantages of using PTM 64/65 octet encapsulation with ADSL2 for any amount of bandwidth, large or small.


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Re: VDSL modemís protocol stack - ATM?
« Reply #17 on: September 18, 2016, 06:27:49 AM »

The issue with serialisation delay arises when packets with different QoS classes hit the same  bottleneck.  With full size frames if the device has just started transmission of a big data packet at the time that a high priority (eg voice) packet arrives the voice needs to wait until all 1500 bytes of data have gone.   It can't jump the queue ahead of a packet in progress.   On point to point lines below 768k we used to use fragmentation and Interleaving so the voice could fit in between data fragments rather than whole data packets.       Not required for 1meg or above.

From memory the small fixed size of ATM cells was more about allowing switching in hardware.   I can't see the need to go nearly that small purely for reasons of latency/jitter.


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Re: VDSL modemís protocol stack - ATM?
« Reply #18 on: September 18, 2016, 06:47:20 AM »

The PTM used on VDSL2 apparently has the facility to pause sending a lower priority packet, and then send a higher priority packet, then resume sending the lower priority packet. That facility is described as "preemption". AAL5 does not have anything similar to allow higher priority ATM cells to be sent if it's in the middle of sending a large packet split into 30 or however many ATM cells.
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