[cisco-bba] Re: [c-nsp] IP DSLAM ( Fully Distributed BRAS)

Mikael Abrahamsson swmike at swm.pp.se
Tue Aug 9 04:57:23 EDT 2005

On Fri, 5 Aug 2005, Vusi Ndebele wrote:

> I wanted to get some views and experiences of the much touted 'IP DSLAM'. As
> far as I can tell, many of the DSLAMs on the market right now are more
> Ethernet DSLAMS effectively bridging with a bit of fancy snooping etc for
> multicast. I call an IP DSLAM a box that has encapsulates subscriber traffic

We use the DSLAM as a media converter. It does the same job as a Cisco 
2950 or 3550 would do in a ETTH environment. Private vlan (or one vlan per 
customer) for security, IGMP snooping, L2+ access lists for spoofing 
protection, DHCP snooping to populate the access lists, etc. Routing is 
done upstream from it.

The DSLAM does conversion from ethernet uplink (tagged vlans) to untagged 
ethernet over ATM. The customer can then terminate it as bridged IP over 
ethernet to one or more ports in their CPE, or as route/bridged in their 
CPE and do NAT in the CPE.

Personally I would like to completely do away with all the ATM stuff, it 
does nothing for me. I want Ethernet over DSL directly without the ATM. 
Prioritization can be done using .1p or some other queueing mechanism.

Too bad this only happened on VDSL, I wish there were more ADSL2+ CPEs 
that would do it.


IP over Ethernet over ATM means high probability that even the smallest 
packet will use three ATM cells instead of two (as in IP over ATM). Bad 
for low-bandwidth high density VOIP (many phones in one place). Not so 
much a problem for residential as they usually only have 1-2 calls at any 
given time and uplink seldom is less than 800kilobit/s on physical line.

Mikael Abrahamsson    email: swmike at swm.pp.se

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