Ethernet frames sent over ADSL, using ATM Cells

[robmurphy]

Does anyone have a good example of how this works. I have been searching the web for information on this, but it is not that clear, at least to me.

I think that an ethernet frame is sent using LANE, and for a user the connection type will be UNI, not NNI.

From what I can see 2 bytes are added to the ethernet frame, these being the first 2 bytes in the first cell. The ethernet header then follows this.

This would mean a 200 byte frame would take 202 bytes. I would assume this would take 4 cells of 53 bytes to transmit, can any one confirm this.

Basically I'm looking to understand how much bandwidth of an ADSL link is used when sending IP traffic. I was surprised to find that most of the ethernet frame was sent. This means the source and destination MAC addresses are sent, and these are Layer 2. The source MAC address may well be the one of the router.

Does any one know if the ethernet FCS is sent in the last cell after the payload.

Rob.

 

[imagoon]

Been a long time but I think in LANE:

byte: 0 LE header (LEC ID) Destination MAC address
4 Destination MAC address
8 Source MAC address
12 Source MAC address Type/length
16 and up Information

So the Ethernet frame goes out at an almighty 37 bytes at a time. With efficiency around 70% This made sense when same size cells could move all kinds of protocols "fast" on slow routers but not so much now. The 70% ignores the control frames also.

ATM over DSL is rare now (PPPoA). Most often it is PPPoE.

 

[ScottMac]

DSL does not use LANE. As a strict point-to-point it is basically just Ethernet chopped into cells and pumped down a VCC (0,35 is popular ...) i.e., Segmentation and Reassembly - SAR and some rudimentary signaling.

There is no need for LAN Emulation (LANE), as there is no broadcast to emulate, and no need for CSMA/CD emulation - it's a point to point.

 

[robmurphy]

I'm pretty sure in the UK the DSL still uses the ATM cell structure. That is what I've been told by the DSL providers.

Is there any of telling if a link is PPPoA or PPPoE. The 2 are shown together on most of the routers I have seen. If it helps the VPI is 0 and the VCI is 38.

Rob.