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[facsi]

Hi guyz,

what do you know about SDH or Synchronous Digital Hierarchy in comparison with ATM?
I was discussing in another forum about adsl and someone told me that my carrier uses SDH instead of ATM. I got confused and think that he might be wrong. He said that but I think that when he heard  that info, it was supposed to be understood as the ISP to internet part of the system, and not the isp-client one.


The deal is: he said I could not use PPPoA because my ISP uses SDH. Is this correct?

[roseway]

SDH is an underlying multiplexing system which is protocol neutral, and is widely used throughout the world for transmitting ATM frames. So I think your informant was wrong. See https://secure.wikimedia.org/wikipedia/en/wiki/Synchronous_Digital_Hierarchy

[kitz]

Sorry, Im unsure about networks outside the UK.
But here in the UK, BT uses SDH between its 21CN network nodes. In effect it 'wraps' the ATM packets.
http://www.kitz.co.uk/adsl/21cn_network.htm#SDH

What is much more likely is that your ISP simply uses 'Ethernet' rather than ATM for encapsulating data frames using PPP.  
This is common in some countries, such as the USA.

You need to be looking at PPP (Point to Point Protocol) and which method the ISP chooses to use.

Have a read of this which explains quite a lot and also has some diagrams to aid understanding.
http://en.wikipedia.org/wiki/Point-to-Point_Protocol_over_Ethernet

The transport protocol used on the telephone network is ATM. The DSL modem encapsulates PPP packets inside ATM cells and sends them over the WAN.
One way of doing this is to bridge the Ethernet packets containing the PPPoE packets over ATM, using the mechanism specified in RFC 2684; this is sometimes called PPPoEoA (PPP-over-Ethernet-over-ATM).
Another way is to convert PPPoE into PPPoA on AAL5 (RFC 2364).

[facsi]

Many thanks. both of you.

Do you actually recommend me trying to make my ISP change my connection to PPPoA for increase performance? or the difference is so small comparing both?
Is not that I need it, but i like these adventures. xD

In the beginning (2001-2002) my ISP started using pppoa but on 2003 started using pppoe. Old connections still remain oA and you can change them to oE. But once on oE you cannot go back to oA. Theorically. I have some arguments to make they change. (I hope them work :p)

[roseway]

Do you actually recommend me trying to make my ISP change my connection to PPPoA for increase performance?

I think you would be wasting your time. It seems that your ISP's policy is to move its customers from PPPoA to PPPoE, so it's unlikely that they would agree to moving you in the opposite direction. And any difference in performance would be very small.

[kitz]

I wouldnt bother, at the end of the day its just 2 different methods of doing the same thing.  Both methods have their plus and minus points.

By changing things you may in fact be emulating the different protocol, which will only increase the overheads and mean less useful data is transfered.
It may even cause MTU issues ala AOL(UK) when they started using different protocols on parts of their network, when BT Wholesale recommends and uses ATM. 
It possible that the reverse is true, and you could perhaps run into some minor issues if the Wholesaler (not the ISP) in your country recommends and uses the other protocol.
I should imagine there is a valid reason why your ISP is now recommending and using PPPoE rather than PPPoA.

[Mick]

Do you actually recommend me trying to make my ISP change my connection to PPPoA for increase performance? or the difference is so small comparing both?

PPPoA has a slight edge over PPPoE in terms of packet size (due to the PPPoE 8 byte header overhead).  The difference is infinitesimal and depends on the overall size of the payload and the number of packets it takes to transmit it.  Over a long period of time you may be able to measure a difference, but we're talking about a few additional packets here and there.  So theoretically, PPPoA is marginally more efficient because (depending on the size of the payload) it can transmit the same data using fewer packets than PPPoE.

On the other hand, if your router and the telephone exchange equipment support it you can configure your router to use 'baby giant frames' (with an mtu of  1508) so that the full 1500 byte packet can be transmitted with PPPoE without fragmentation.  If not, then you better set the mtu for your router and the PCs in your LAN to the default PPPoE size of 1492 bytes.

Personally, I think that a better quality, high throughput/low latency router will give you a much more noticeable difference in performance than a PPPoA Vs PPPoE connection ever will (as long as you have set up your mtu correctly on your network).

HTH.