In the most complex case, a DSL service could have up to eight separate components (including the end-user’s workstation):
CPE router runs PPPoE session with the NAS (Network Access Server) and sends Ethernet datagrams to the DSL modem.
The backhaul network (DSLAM to NAS) could be partly ATM based. The ATM cells could thus pass through several ATM switches.
Eventually the ATM cells have to be reassembled into PPPoE frames. In a worst-case scenario, an ATM-to-Ethernet switch would perform that function.
Finally, the bridged PPPoE frames arrive @ NAS which terminates the PPPoE session and emits the IP datagrams into the IP core network.
I sincerely hope no network is as complex as the above diagram. In most cases, the backhaul would be either completely ATM-based …
Am I missing anything important? I know you could deploy numerous additional devices (for example, Cisco is promoting the Service Exchange Framework and Service Control Engine), but these devices would be placed deeper into the IP core.
A few years ago someone decided that the devices should be darker green/grayish-blue and I really like those icons. Then they unfortunately reverted back to the light blue (the DSLAM in the diagram is the "new" icon) ... but I guess this is a question of personal taste.
Remember to include double VLAN tagging (QinQ) in the third scenario, between DSLAM and NAS. It adds a lot of "interesting" questions. How do QoS mechanisms consider both tags? Headers or payload? And what about the MTU?
What other options for L2 are there other than ATM, as im sure not all ISP's will continue supporting an ATM network. In your overview, the link im talking about is the DSL access.
As far as I understand, DSL access always uses ATM cells, but this does not impact the rest of the ISP network if the DSLAM has (Fast/Gig)Ethernet uplink.
The other change that could exist is that there is no PPPoE and the CPE gets assigned an address via DHCP or static config. In a DSL network I built in my last job our DSLAM was also the IP gateway for the CPE and acted as a router
I've seen DSL modems that also act as PPPoE clients and provide LAN services (DHCP/NAT). Likewise, DSLAM could include the PPPoE server/IP routing functionality and terminate PPPoE IP traffic, or it could use a different (simpler) protocol stack ... and I need to know about that :)
You might want to extend the drawing to include LAC/LNS session forwarding (which still are layer 2).
There are several IP-DSLAM's who are able to act as an IP edge device and terminate PPP sessions directly, Huawei's SmartAX MA5600 can do it as far as I know (is someone really using Huawei devices? ;-) ) and I'm pretty sure that other vendors have simmilar solutions. So you've mostly bare Ethernet and no ATM here...
Yet another scenario would be a network which is on the way to an NGN - you might have legacy ATM-based services & devices (Voice over ATM, ATM-based DSL-lines, ...) and newer IP/MPLS-based services & devices.
I know providers which use MPLS- or L2TPv3-tunneling to transport the legacy ATM traffic over their IP core to an ATM switch / LAC - not a good thing for troubleshooting...
DSL is available with "native" ethernet via EFM. All VDSL2 is native ethernet as far as I know. I estimate that most DSL deployments today would use ethernet backhauled, IP-aware hardware these days.
Implementations and Applications of DSL Technology by Golden et al. explains S and C vlan tagging options concisely. But basically, you can have multiple VLAN's stacked just like with atm PVI/VCI. You can have 1:1 or N:1 configurations. It also expands the maximum amount of VLAN's by providing two 12-bit address segments, for 4096*4096 vlans.
I thought that in newer 'IP' DSLAMs, such as ALU ISAMs, that you terminate the session on the DSLAM itself. I don't think that this is a good idea for IPVPN as you'd then need to run MP-BGP and MPLS on the DSLAM as well. It'd be OK for consumer-grade internet access. Not very good for LI.
Nope, in such a case you can just use the DSLAM as LAC and forward the VPN sessions to dedicated LNS systems unsing L2TP...
First our teenage daughter has brought her first car. Her brother is a bit more laid back and is still looking. Thank goodness as I do not think I could cope with both at the same time
She is still a learner driver and will be taking her test soon. So for the last few days she has cleaned, added air fresheners and customized her little black car
Well needless to say she is being quite frosty towards me last night. I am worried that her driving lessons are not teaching her the basics and she won't pass her test. Maybe when she has calmed down this morning we can talk it through. Or she may do the teenage thing and go off in a mega sulk!!
Well just a quick look at a piece of vintage jewellery that I brought on Ebay a few months ago now. I did not realize that W A P Watson also produced jewellery under the Mirella name as well as Exquisite. The seller had 2 cornflower brooches for sale but some one had already a bid on the one so, I went for the other only.
Very rare jewellery as I have not seen any other jewellery in the last 3 years that I have been actively looking. This is now in my own collection and not for resale.
Still to come in this blog: vintage jewellery book reviews, dating vintage jewellery and how to replace stones.