Cell Relay Archive

[blckshaw@clark.net (Robert Blackshaw)]

"Mark S. Lewis" <msl@NetEFX.com> wrote:

>Netwizards:

>I've been researching the various encapsulations used on xDSL lines and
>looking their various benefits.  From what I have determined so far, these
>are the main variations:

>1. Ethernet encapsulation used mostly with transparent bridging.  This seems
>like the easiest to implement especially for an Ethernet based CPE.
>However, it is not a very smart way to use the link.  Many employ filtering
>to avoid sending unnecessary LAN traffic over the WAN.

>2. Frame relay (RFC1490) multiprotocol encapsulation can be used with
>routing.  This seems like a reasonable approach for data traffic.  Routing
>provides better use of the link.  Plus you get the benefits of frame relay,
>namely link management and multiplexing of logical connections.

>3. ATM using FUNI (RFC 1483) provides an interface between frame relay and
>ATM.  Since it is a variable length packet encapsulated like frame relay but
>with ATM addressing, it would seem to perform and be managed like frame
>relay.  Aside from providing an interface to ATM, is there any other
>particular benefit to this encapsulation over frame relay?

>4.  ATM using cell-based encapsulation (AAL5?) can be used to provide the
>additional ATM services for time-sensitive applications.  Not knowing much
>about ATM at this point, what other benefits are provided using this
>encapsulation?

>5.  PPP is being used over 2, 3, & 4 above.  This provides the benefit of
>dynamic link negotiation, including IP addresses, DNS and other values.  Of
>course it supports multiple protocols, and can also provide authentication.
>What other benefits are there of PPP?  Which link layer encapsulation would
>be preferable?

Probably not at the moment, but all five of your proposals could be valid
depending upon how the DSLAM peels off traffic and routes it to the
destination. Frex, it could come out of the DSLAM and drop onto a
LAN connection (or rather a hi-speed link to a LAN), it could be routed
to ATM, etc. So all of your scenarios could be perfectly valid.

Bob